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do I need a tune if...

Old 08-29-2018, 09:41 AM
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That's always the determining factor
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Old 08-29-2018, 09:45 AM
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Originally Posted by clove86 View Post
That's always the determining factor
That is the real question, isn't it? If you want to spend a couple hundred, probably won't get far enough to do much so it is pissing money away. Intakes, headers and a tune will run maybe $5000 or less. Less if you do the work. Otherwise, probably not worth it to spend money. Drive it stock.
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Old 08-29-2018, 10:24 AM
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If youre patient enough you can get around 30-40 cfm more than stock it will take a lot of time beats paying someone
No need to give up on it. OR sell that one buy a converted LT1?

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Old 08-29-2018, 11:04 AM
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Originally Posted by aklim View Post
That is the real question, isn't it? If you want to spend a couple hundred, probably won't get far enough to do much so it is pissing money away. Intakes, headers and a tune will run maybe $5000 or less. Less if you do the work. Otherwise, probably not worth it to spend money. Drive it stock.
Not trying to jack anyone up but intake, headers, and tune should be $2000 and if you look for used and do the work youself less than $1000.

Buy a set of used 113 heads off a 1988 or newer car, intake, headers and tune and you will have 275 HP to 300 HP for $1000 to $1500 if you do the work yourself. Craiglist and swap meets are your best friends.

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Old 08-29-2018, 11:16 AM
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Real nice pair of freshly rebuilt and ported 113s in the parts section for 800 thats a good start.
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Old 08-29-2018, 11:30 AM
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Originally Posted by bjankuski View Post
Not trying to jack anyone up but intake, headers, and tune should be $2000 and if you look for used and do the work youself less than $1000.

Buy a set of used 113 heads off a 1988 or newer car, intake, headers and tune and you will have 275 HP to 300 HP for $1000 to $1500 if you do the work yourself. Craiglist and swap meets are your best friends.
I was just going off what TPIS offers. $1000 for headers, $1000 for intakes and say $$600 to $800 for a dyno tune. Add labor to install and misc stuff and it comes to $5000. But like you said, if you buy used and DIY, it could be cheaper.
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Old 08-29-2018, 07:12 PM
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Originally Posted by bjankuski View Post
Not trying to jack anyone up but intake, headers, and tune should be $2000 and if you look for used and do the work youself less than $1000.

Buy a set of used 113 heads off a 1988 or newer car, intake, headers and tune and you will have 275 HP to 300 HP for $1000 to $1500 if you do the work yourself. Craiglist and swap meets are your best friends.
Not trying to jack anyone up but the OP's 1991 car ALREADY HAS 113 heads on it (contrary to the Clove86 user handle)! LOL That's why I asked the question about speed density that you replied to earlier in this thread! (BTW...Not sure I understand that reply because I was under the impression MAF cars couldn't vary more than 15% from the stored (MAF) table data. So, like SD cars, the base point from which 15% can be varied seems the same -- pre-programmed table data. If 15% can be adjusted from trims, why couldn't that extend more and more -- as trim data is adjusted?)

Clove86:
Cuisinartvette (Ron) has done as much with porting as anyone in this forum (having done multiple sets over the years). What he says about porting 113's should be noted (though Bjankuski has built several killer track cars too!) I looked at the heads mentioned in our 4Sale forum. They are the SAME heads as on your car but ported and rebuilt with some nice upgrades. They'll provide more lift (for a cam change) and/or more air flow. Depending on your 91's mileage AND your goals, this is a worthwhile option. TPIS used to charge $800-$900 for 113 porting 10 yrs ago. (Can't remember if that includes springs/valves -- but it was a long time ago anyway). Don't know if they still do it but maybe that qualifies as "Frankenstein"? Personally, I would suggest (even on a 27-yr-old car) that springs, valve guides, and especially valve seals are worth considering -- which makes rebuilding heads a consideration -- for the serious longterm owner. Hence Brian/Ron's suggestion of the (cheaper) ported set in 4Sale. Subtract what you can get for your current heads and you've got a nice set of heads on the cheap.

I won't talk about cams other than to say a cam can nearly double what the intake (and even headers) gives you because it "opens the door for air travel". Get a bigger door, more air will get in. Hold it open longer/wider and the effect is amplified. If you were to consider heads, factor in a cam and tuning.

Unless you really want a polished intake, swapping to what you've purchased is really a waste of time (for the effort involved). Stock TPI long-tube runners are a much bigger restriction than the mild porting on that stock, polished base. I wouldn't call intake, headers, and cam a "Frankenstein build". If you really want to notice the difference in a build, that's what it takes to meaningfully modify an L98. Headers are somewhat helpful on their own but topside, it really helps to match intake modifications to the cam installed. If you JUST want to get a bit more air topside, get bigger tubes and save yourself the headache of cracking the base from the block/heads. You can refresh injectors while in there and probably get by without much/any retuning.
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Old 08-29-2018, 07:23 PM
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Very good explanation.
That was my feelings. I needed someone to verify.

I'm going to continue forward with installing injectors and consider a cam in the future.
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Old 08-29-2018, 08:47 PM
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Originally Posted by clove86 View Post
Very good explanation.
That was my feelings. I needed someone to verify.

I'm going to continue forward with installing injectors and consider a cam in the future.
Injectors will allow you to fuel your car at any place. I would consider the whole package before going any further than the injectors. You should consider all the factors today so you can plan your SYSTEM as opposed to getting this can and forcing it to work with that head and some other intakes with lousy headers. It has to work as a team not a Frankenstein as you mentioned
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Old 08-29-2018, 11:37 PM
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Originally Posted by GREGGPENN View Post
Not trying to jack anyone up but the OP's 1991 car ALREADY HAS 113 heads on it (contrary to the Clove86 user handle)! LOL That's why I asked the question about speed density that you replied to earlier in this thread! (BTW...Not sure I understand that reply because I was under the impression MAF cars couldn't vary more than 15% from the stored (MAF) table data. So, like SD cars, the base point from which 15% can be varied seems the same -- pre-programmed table data. If 15% can be adjusted from trims, why couldn't that extend more and more -- as trim data is adjusted?)
Sorry, apparantly I was confused by the 86 handle and forgot that he had a 1991 vette, so the 113 heads are already on your car. As far as the comment on MAF vs SD the maf actually measures the amount of air entering the engine so unless you have an air leak or you modify the maf and change the actual measred values the maf will read the correct values and the tune will be mostly correct even on a modified car. If you change the maf and the values are off or you have an air leak after the maf the tune could be thrown off by more then 15% and a retune or fix will be required. (In other words a maf car can tolerate changes that are much greater than 15% and still run well) On a SD car if you modify the engine and the changes tto airflow exceed 15% at a given position a check engine light will come on and the fueling will be off, minor modifications can force a retune on a SD car.
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Old 08-30-2018, 09:06 AM
  #31  
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Originally Posted by aklim View Post
Injectors will allow you to fuel your car at any place. I would consider the whole package before going any further than the injectors. You should consider all the factors today so you can plan your SYSTEM as opposed to getting this can and forcing it to work with that head and some other intakes with lousy headers. It has to work as a team not a Frankenstein as you mentioned
Yes and no. Again, as I've mentioned a couple of times recently, batch-fire engine (like the L98) can throw some serious fuel with smallish injectors. Because it can "inject" fuel on each crank revolution, you effectively double the perceived injector size. Years ago, member Corky claimed AT LEAST a 500chp ceiling on 24lb SVO injectors (25.5 in our SBCs). Most importantly, his was a claim based on significant modifications to his car -- with monitoring. Plus, it makes perfect sense. Since injectors can also be programmed with a small enough minimum PW, the 24's proposed by the OP can handle anything other than a radical modification including a stock setup. That means they'll work now.

So, if the car is in NEED of injectors, go ahead an install them. Even if they are 24lb in a SBC fuel rail (vs Ford 24 SVOs), they'll support most upgrades. If you only "kinda" want to mod your car, look for some large longtubes and install those at the same time. They are easy enough to pull and resell if you're not impressed and/or never want to go futher....or want to restore it to stock. During the coming months/years, you can consider head/cam/headers.

If you don't NEED injectors now, waiting until you develop a longterm plan is good advice.
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Old 08-30-2018, 10:28 AM
  #32  
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Originally Posted by GREGGPENN View Post
Yes and no. Again, as I've mentioned a couple of times recently, batch-fire engine (like the L98) can throw some serious fuel with smallish injectors. Because it can "inject" fuel on each crank revolution, you effectively double the perceived injector size. Years ago, member Corky claimed AT LEAST a 500chp ceiling on 24lb SVO injectors (25.5 in our SBCs). Most importantly, his was a claim based on significant modifications to his car -- with monitoring. Plus, it makes perfect sense. Since injectors can also be programmed with a small enough minimum PW, the 24's proposed by the OP can handle anything other than a radical modification including a stock setup. That means they'll work now.

So, if the car is in NEED of injectors, go ahead an install them. Even if they are 24lb in a SBC fuel rail (vs Ford 24 SVOs), they'll support most upgrades. If you only "kinda" want to mod your car, look for some large longtubes and install those at the same time. They are easy enough to pull and resell if you're not impressed and/or never want to go futher....or want to restore it to stock. During the coming months/years, you can consider head/cam/headers.

If you don't NEED injectors now, waiting until you develop a longterm plan is good advice.
thank you again this is all I wanted to hear.
I have the original injectors on the motor now. These injectors are over 27 years old. They operate perfectly fine, no issues at all.
With all the forum reading I've done, everybody recommends replacing the injectors. I scored a great deal on 24 lb injectors, that I am very happy with.
this is a great first step until I can this is a great first step until I can put a bigger plan together.
Sorry for my handle, that's my birth year and I use it with all my handles. I understand that can be confusing. It is a 1991 automatic. I want to be smart and go into this car with a strategic plan. I do not want to Frankenstein part by part without an overall cohesive goal.

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Old 08-30-2018, 11:07 AM
  #33  
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Originally Posted by GREGGPENN View Post
Yes and no. Again, as I've mentioned a couple of times recently, batch-fire engine (like the L98) can throw some serious fuel with smallish injectors. Because it can "inject" fuel on each crank revolution, you effectively double the perceived injector size. Years ago, member Corky claimed AT LEAST a 500chp ceiling on 24lb SVO injectors (25.5 in our SBCs). Most importantly, his was a claim based on significant modifications to his car -- with monitoring. Plus, it makes perfect sense. Since injectors can also be programmed with a small enough minimum PW, the 24's proposed by the OP can handle anything other than a radical modification including a stock setup. That means they'll work now.

So, if the car is in NEED of injectors, go ahead an install them. Even if they are 24lb in a SBC fuel rail (vs Ford 24 SVOs), they'll support most upgrades. If you only "kinda" want to mod your car, look for some large longtubes and install those at the same time. They are easy enough to pull and resell if you're not impressed and/or never want to go futher....or want to restore it to stock. During the coming months/years, you can consider head/cam/headers.

If you don't NEED injectors now, waiting until you develop a longterm plan is good advice.
I wasn't worried about the size. I was more worried about the idea that they are Multecs and would limit where he can fill up.
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Old 08-30-2018, 11:11 AM
  #34  
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Originally Posted by clove86 View Post
thank you again this is all I wanted to hear.
I have the original injectors on the motor now. These injectors are over 27 years old. They operate perfectly fine, no issues at all.
With all the forum reading I've done, everybody recommends replacing the injectors. I scored a great deal on 24 lb injectors, that I am very happy with.
this is a great first step until I can this is a great first step until I can put a bigger plan together.
Sorry for my handle, that's my birth year and I use it with all my handles. I understand that can be confusing. It is a 1991 automatic. I want to be smart and go into this car with a strategic plan. I do not want to Frankenstein part by part without an overall cohesive goal.
But how long will they operate fine with ethanol? If you have no ethanol where you are, life is good. Most stations around me have ethanol and the injectors are fuel cooled and the ethanol eats at the windings so sooner or later, they are failing. My choices are to only fill up at gas stations where there is NO ethanol which can be inconvenient or run it till it breaks and replace at an inconvenient time. I choose to replace them now and forget the whole thing.
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Old 08-30-2018, 11:40 AM
  #35  
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Originally Posted by GREGGPENN View Post
Yes and no. Again, as I've mentioned a couple of times recently, batch-fire engine (like the L98) can throw some serious fuel with smallish injectors. Because it can "inject" fuel on each crank revolution, you effectively double the perceived injector size. Years ago, member Corky claimed AT LEAST a 500chp ceiling on 24lb SVO injectors (25.5 in our SBCs). Most importantly, his was a claim based on significant modifications to his car -- with monitoring. Plus, it makes perfect sense. Since injectors can also be programmed with a small enough minimum PW, the 24's proposed by the OP can handle anything other than a radical modification including a stock setup. That means they'll work now.
Greg,
I do not want to get into a tuning discussion for this post but what you are saying here is incorrect. Double fire means the injector pulse width is cut in 1/2 and the injectors are fired every revolution. Single fire means the injector is pulsed once every two revolutions. In either case the same amount of fuel is delivered. When a 1991 SD car is running it work is double fire mode until you reach 50% load and then it automatically switches to single fire mode so the injector can stay open longer deliver enough fuel to the engine. When an injector is rated at 22lbs of fuel at 100% duty cycle, that means 22lbs per hour if the injector was on for the full hour. Any pulsing will reduce the amount of fuel, so a double fire mode will actually reduce the amount of fuel that can be supplied vs a single fire mode. (On/off cycle time)

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Old 08-30-2018, 07:09 PM
  #36  
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Originally Posted by bjankuski View Post
Double fire means the injector pulse width is cut in 1/2 and the injectors are fired every revolution. Single fire means the injector is pulsed once every two revolutions. In either case the same amount of fuel is delivered. When a 1991 SD car is running it work is double fire mode until you reach 50% load and then it automatically switches to single fire mode so the injector can stay open longer deliver enough fuel to the engine.
Hopefully, the OP's question basically answered so I'll post this versus taking it "offline"....

Brian...you are a smart guy with more experience than me so I have to volley back..... Corky was (is?) an equally accomplished member who posted about FORD 24 SVO's being able to support 500hp. Double-fire mode was the only explanation to would have explained the power levels he acheived. IIRC, he owned a MAF car....maybe an 85/86?....if that makes a difference. I know their ECMs/coding were also kind of unique. Actually, I think Corky claimed 500 REAR WHEEL HP out of Ford 24 SVO's though I didn't want to set the bar THAT high without being able to find that 8-yr-old post which included Jon from FIC). Can't remember how fast his car was but [at least] in the 10's?

The thread in question may have discussed WHERE the need for 30lb/hr injector started -- vs FORD 24lb/hr SVOs? Which, IF I could remember, might aide in my thread search? What I'm certain of is that Corky's power levels far exceeded the ability of his injector size to general that much power. It was a spirited discussion! LOL

Intuitively, FI is more efficient when running the smallest injector at the highest pressure -- for atomization. Obviously, injector ratings wouldn't account for double-fire mode (firing twice per 4-stroke cycle). They would be rated for sequential (single) injection. Having the "ability" to fire injectors twice -- while only running them at 50% makes sense if engineers wanted to make SURE you NEVER exceed (or get close to) maximum PW. If injectors were always LIMITED injectors to 50% PW MAXIMUM -- in DFM, why were we always cautioning each other to insure PW didn't exceed 85% if it's impossible to get there? (85% would mean 42.5% in DFM.....if they were really running @ 50%). I dunno....

Wouldn't it make more sense -- in the initial days of FI, to have double-fire mode available when injectors weren't made in sizes like 80lbs/hr? Wouldn't it also make more sense to use smaller injectors and run them up to their respective PW limits (i.e., 85-90%)? If you limit an injector to 50% operation, you need bigger injectors which seems counter-intuitive to the (atomization) efficiency goals of FI in general.

Fair disclosure: I don't think anyone PROVED the explanation of double-fire mode I brought forward here....but it became the only theory that made sense for Corky's claims....and even for [email protected] whose business is still selling injectors. I think we ended that thread concluding DFM was the only possibility/explanation. Plus, I'm not I've seen any significant (high) BPW reading in my datalogs which made me believe our historical conclusion was correct.

Is what you've posted gleaned this from experience...or do you have a link that explains double vs single fire mode? It was a big enough deal -- back then -- I wish we could have had you in on that discussion! I also wonder if DFM changed from 89 (MAF) to 90 (SD) operation?

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Old 08-31-2018, 12:08 AM
  #37  
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Originally Posted by GREGGPENN View Post
Actually, I think Corky claimed 500 REAR WHEEL HP out of Ford 24 SVO's

Intuitively, FI is more efficient when running the smallest injector at the highest pressure -- for atomization. Obviously, injector ratings wouldn't account for double-fire mode (firing twice per 4-stroke cycle). They would be rated for sequential (single) injection. Having the "ability" to fire injectors twice -- while only running them at 50% makes sense if engineers wanted to make SURE you NEVER exceed (or get close to) maximum PW. If injectors were always LIMITED injectors to 50% PW MAXIMUM -- in DFM, why were we always cautioning each other to insure PW didn't exceed 85% if it's impossible to get there? (85% would mean 42.5% in DFM.....if they were really running @ 50%). I dunno....

Wouldn't it make more sense -- in the initial days of FI, to have double-fire mode available when injectors weren't made in sizes like 80lbs/hr? Wouldn't it also make more sense to use smaller injectors and run them up to their respective PW limits (i.e., 85-90%)? If you limit an injector to 50% operation, you need bigger injectors which seems counter-intuitive to the (atomization) efficiency goals of FI in general.

Fair disclosure: I don't think anyone PROVED the explanation of double-fire mode I brought forward here....but it became the only theory that made sense for Corky's claims....and even for [email protected] whose business is still selling injectors. I think we ended that thread concluding DFM was the only possibility/explanation. Plus, I'm not I've seen any significant (high) BPW reading in my datalogs which made me believe our historical conclusion was correct.

Is what you've posted gleaned this from experience...or do you have a link that explains double vs single fire mode? It was a big enough deal -- back then -- I wish we could have had you in on that discussion! I also wonder if DFM changed from 89 (MAF) to 90 (SD) operation?
According to the calculators, at 95% duty cycle, pushing 80 psi, 24pph injectors will work

I might be wrong but the way I am understanding it is that if you put the injector at 100% duty cycle, IOW, you don't pulse it at all and just turn it on and leave it on for an hour, it dumps 24 pounds of fuel assuming you have 43.5 psi. Also, if you increase the duty cycle to 100%, you never have the injectors closing at all and if it goes static, you lose control of the delivery and it becomes just an constant flow which is where you get your 24 pounds of fuel. If you increase the fuel pressure to say 80 psi and it is at 95% duty cycle, you will get your 500 crank HP.
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Old 08-31-2018, 12:55 AM
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Originally Posted by aklim View Post
According to the calculators, at 95% duty cycle, pushing 80 psi, 24pph injectors will work

I might be wrong but the way I am understanding it is that if you put the injector at 100% duty cycle, IOW, you don't pulse it at all and just turn it on and leave it on for an hour, it dumps 24 pounds of fuel assuming you have 43.5 psi. Also, if you increase the duty cycle to 100%, you never have the injectors closing at all and if it goes static, you lose control of the delivery and it becomes just an constant flow which is where you get your 24 pounds of fuel. If you increase the fuel pressure to say 80 psi and it is at 95% duty cycle, you will get your 500 crank HP.
Extraordinarily high FP was not involved when Corky made the claim/observation. I don't think Jon, Corky, nor myself started that thread so finding it has been very difficult. To run a 10.5 sec quarter with a 3k lb car, you need those ponies at the wheels -- which is about where Corky ran IIRC. Anyone else remember it?

EDIT: I stand partially corrected. Can't believe it, but I found the thread. It was CORKVETTE1 and he actually ran 10.27 (~600hp) from Ford 24 SVOs. So CORKVETTE1 seemed to (empirically) confirm that double-fire mode actually does deliver twice the fuel. I think it's post #7. Interestingly, Brian WAS an active member back then. Should have realized that since he helped me a year later!!! The partially corrected part is that I can't find reference to what pressure CV1 was running in his various setups -- though there is SOME implication of the fueling system being stock (except the injectors). At least he didn't MENTION running 200psi!!! LOL I wonder if I could still get a response from him on that issue??? He hasn't posted in 4 yrs but I sent him an email...in case he might be able to clarify what FP he ran.

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Old 08-31-2018, 08:56 AM
  #39  
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Originally Posted by GREGGPENN View Post
Extraordinarily high FP was not involved when Corky made the claim/observation. I don't think Jon, Corky, nor myself started that thread so finding it has been very difficult. To run a 10.5 sec quarter with a 3k lb car, you need those ponies at the wheels -- which is about where Corky ran IIRC. Anyone else remember it?

EDIT: I stand partially corrected. Can't believe it, but I found the thread. It was CORKVETTE1 and he actually ran 10.27 (~600hp) from Ford 24 SVOs. So CORKVETTE1 seemed to (empirically) confirm that double-fire mode actually does deliver twice the fuel. I think it's post #7. Interestingly, Brian WAS an active member back then. Should have realized that since he helped me a year later!!! The partially corrected part is that I can't find reference to what pressure CV1 was running in his various setups -- though there is SOME implication of the fueling system being stock (except the injectors). At least he didn't MENTION running 200psi!!! LOL I wonder if I could still get a response from him on that issue??? He hasn't posted in 4 yrs but I sent him an email...in case he might be able to clarify what FP he ran.
Greg,
This is just a math problem and I will keep this short and to the point. Most injector caculations to be safe are done at 80% duty cycle and .5 BSFC which means a 25.5 injector can support (25.5/.5 X .8 X 8 = 326 HP) But in real world examples running at 100% duty cycle in drag race situations or dyno runs is done all the time. Also a high perfromance engine can run BSFC numbers in the mid .35 range but lets use .4 for this example (25.5/.4 X 1 x 8 = 510 HP) This is in single fire applications with a SVO 24 lb injector runjning at 43.5 PSI. If you up the fuel pressure or have a lower BSFC the injector will support even more power. (Who knows what Corky had for number, I am just showing you how it can be done)

The 1991 cars run in single fire mode at WOT, and double fire mode at idle and light cruise

The 1986 through 1989 operate in double fire mode.

What this means is that at WOT the 1991 car will only fire the injector once per power stroke or stated differently once per 2 revs of the engine. This means it will keep the injector open twice as long as a double fire system, but it also has twice the time available to keep the injector firing. A double fire system fires once per engine revolution so it fires the injector twice per power stroke but each pulse is 1/2 the time of the single fire system, and it only has 1/2 the time available since it is firing twice per power stroke. The truth is that a single fire system can support slightly more power then a double fire system since it can supply more fuel since it does not have to account for the additional on/off time of the injector firing twice per power stroke instead of just once. That one additional cycle of the injector takes time that could have been used to provide fuel.

The math an engine running at 6000 RPM is operating at 100 revs per second, at 100 revs a second that is 20 ms total time available to inject fuel into the engine at 100% duty cycle. An injector typically has about a 1ms response time so that means if the injector cycles at all you only have 19ms of time to inject fuel into the engine if you are operating at 6000 RPM. This would be a 95% duty cycle, and this would be in single fire mode. In double fire mode you have the same 20ms of time but now you have two pulses of the injector and each pulse only has 10ms of time availabe to it and 1ms is burned up from injector cycle time so you only have 9ms availabe in every 10ms of operation. That means you have a total of 18ms of injection time per power stroke or 90% duty cycle in double fire mode. That being said if the injectors go to 100% duty cycle and are not cycled both sytems will be the same.

This is verified by looking at your scan data, the injector pulse width on a 1986 through 1989 car is shown at 1/2 of what you would expect because it is operating in double fire mode. So when you are looking at scan data to determine if your injector duty cycle you need to double the pulse width and then divide it by the total time per power stroke to get you true duty cycle. In the example above a scan data value of 10ms for at 6000 RPM would be 100% duty cycle.

I hope this was helpful. It looks like I failed and short and to the point!

Last edited by bjankuski; 08-31-2018 at 09:03 AM.
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Old 09-03-2018, 01:24 AM
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Originally Posted by bjankuski View Post
Greg,
This is just a math problem and I will keep this short and to the point. Most injector caculations to be safe are done at 80% duty cycle and .5 BSFC which means a 25.5 injector can support (25.5/.5 X .8 X 8 = 326 HP) But in real world examples running at 100% duty cycle in drag race situations or dyno runs is done all the time. Also a high perfromance engine can run BSFC numbers in the mid .35 range but lets use .4 for this example (25.5/.4 X 1 x 8 = 510 HP) This is in single fire applications with a SVO 24 lb injector runjning at 43.5 PSI. If you up the fuel pressure or have a lower BSFC the injector will support even more power. (Who knows what Corky had for number, I am just showing you how it can be done)

The 1991 cars run in single fire mode at WOT, and double fire mode at idle and light cruise

The 1986 through 1989 operate in double fire mode.

What this means is that at WOT the 1991 car will only fire the injector once per power stroke or stated differently once per 2 revs of the engine. This means it will keep the injector open twice as long as a double fire system, but it also has twice the time available to keep the injector firing. A double fire system fires once per engine revolution so it fires the injector twice per power stroke but each pulse is 1/2 the time of the single fire system, and it only has 1/2 the time available since it is firing twice per power stroke. The truth is that a single fire system can support slightly more power then a double fire system since it can supply more fuel since it does not have to account for the additional on/off time of the injector firing twice per power stroke instead of just once. That one additional cycle of the injector takes time that could have been used to provide fuel.

The math an engine running at 6000 RPM is operating at 100 revs per second, at 100 revs a second that is 20 ms total time available to inject fuel into the engine at 100% duty cycle. An injector typically has about a 1ms response time so that means if the injector cycles at all you only have 19ms of time to inject fuel into the engine if you are operating at 6000 RPM. This would be a 95% duty cycle, and this would be in single fire mode. In double fire mode you have the same 20ms of time but now you have two pulses of the injector and each pulse only has 10ms of time availabe to it and 1ms is burned up from injector cycle time so you only have 9ms availabe in every 10ms of operation. That means you have a total of 18ms of injection time per power stroke or 90% duty cycle in double fire mode. That being said if the injectors go to 100% duty cycle and are not cycled both sytems will be the same.

This is verified by looking at your scan data, the injector pulse width on a 1986 through 1989 car is shown at 1/2 of what you would expect because it is operating in double fire mode. So when you are looking at scan data to determine if your injector duty cycle you need to double the pulse width and then divide it by the total time per power stroke to get you true duty cycle. In the example above a scan data value of 10ms for at 6000 RPM would be 100% duty cycle.

I hope this was helpful. It looks like I failed and short and to the point!
Thanks Brian. I had understood injector calculations...including duty cycle and BSFC since the thread I linked. Though, I've never confirmed whether it's wheel or crank horsepower people talk about when equating time slips. For example, whether it's 500rwhp that's necessary to run a 10.50...or whatever. That said, from what I knew then (and now), it didn't seem possible that 10.25 time slips were MATHMATICALLY possible with 24 SVOs (25.5 @ GM pressure). And, really, that it was so far off you'd have to double FP to get into that "ballpark". That said, I've always had a beginner's level knowledge of BFSC which is often stated in the .45-.55 range for NA ICE's. Suggesting lower .35 and .4 are realistic is kind of a suprise. Certainly, no one suggested the necessary elements to resolve the mystery back then.

Considering my (TunerPro) BIN showed locations for BOTH double and single-fire modes, I thought both were possible in 89. At the time of my tuning single-fire was considered "default" by anyone I remember working with (wink). I think it was suggested that the same values should be entered in double-file buckets "just to be sure". But maybe I remember wrong. More than one person was nice enough to contribute to my learning back then.

Despite tons of respectable help, there's always more to learn. What you presented here is clear/understandable....though it doesn't necessarily resolve why (that which is supposed to be) an 89 TunerPro BIN would include double-fire mode -- if not utilized until later. Or....why both were leveraged until sequential appeared on the scene? I might guess there are hardware differences (referenced to the crank/cam) that "allowed" double vs single modes? Obviously, the thread I linked showed an inability to determine/connect what was/is possible for the ECM to "fire" based on circuitry. I could see that interconnected to CPS or other means. Maybe, double-fire mode is simply a function of a hardware limitation that "breaks" connection once per crank revolution? IDK.

Whatever, the case, the information you communicate -- assuming it's 100% correct (and I have no reason to believe otherwise), explains quite a bit and forces holes in my "education" toward the correct conclusions of what had to be going on in CORVETTE1's car...or any other which may have questions on injector sizing requirements. Thanks for that. In general, it sounds reasonable to assume that most (if not all) "NA SBC gen I/II 350/383 street builds" aren't likely to NEED injectors as big at 30lbs. And, that 24's or 24 SVO's would be sufficent until you get "radical". My personal observations have been that 400rwhp is about the limit of gen 1 builds unless a builder exceeds 400ci and/or incorporates power-adders. As such, I've always advised people they probably don't need those big injectors ... like 30's, 36's, etc... OTOH, maybe they work a bit better than I'd except in single-fire systems -- where pulse widths are longer? Guess I should also wonder if double-fire systems help maintain more even FP by "shutting the door" more often to allow pressure to rise? So to speak.
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