What would be the most compression (scr) that should be run assuming 92 octane gas and a tune?

 

Its an easy question to answer for you,
But given the wrong advice your engine will have preignition & detonation problems too.

A Corvette owner has absolutley no way of knowing what octane gasoline they are actually filling their gas tank with 100% of the time.

You only know if you purchase your fuel in 55 gallon barrels at a time & store them at home in your garage.
Race Fuel or Methanol Alcohol.

I have seen guys run 13.0 : 1 static compression ratio on 93 octane pump gas.
Tried to run very long duration cams with them.
worked for a while.
Eventually blew apart.

Detonation won.

9.0 to 10.0 : 1 plenty safe with modern Aluminum heads.

11.0 to 12.0: 1 possible.
Depends upon how talented you are building engines & how good of a tuner you are.

Even the best engines fail from detonation.

BR

 

Most, if not all the guys running above 11:1 SCR are doing so with bigger cams. Or, with alternative fuels.

Builders will tell you to stay under 10:1 -- especially if they do the build. Some will go 10.5:1. Few will go higher and guarantee it. I've also seen SCRs in the 11's on pump gas (with bigger cams).

Getting the quench right will help maximize the SCR you can run. Operating temps help too. (Vizzard made a chart on this topic. Try to find it.)

Consider that fuels are supposed to degrade as time goes on. I picked 10.25 with my high(er) compression cam. If you're build is good, I think 10.5 is a safe number to shoot for. Some will say 10:1 (as posted above).

 

I would ask your engine builder. So many thing come into play here.

 

Good point...

Mines at 10.5:1.
In order to run smoothly and yeild the max performance with the max advance, it needs 93oct min and has a max op temp of around 220 (to avoid knocking) Depending on ambient temp. Under these conditions it can rev to the redline under load and survive.

Less oct 89-91 requires one OR both lower op-temp or less ign advance or ambient temp.

Cheap gas (87) means stock advance, op temps of less than 200 and no stress driving habits.

All gasoline is created equally. Its all the same product with a different name on the bottle...exxon, shell, whoever. The difference in the grades is in the addmixture to increase Octane, which is just a measure of time. How long does the explosion last? The longer it does, the higher the Octane.
Basically, hi-oct fuel burns slower, providing more consistant pressure on the piston dome, and pressure that last longer to hold the tension on the piston longer so that it is not free to flop around in the cylinder. Its not more pressure, its longer lasting pressure that increases as the volume of the cylinder is increasing with the piston moving downwards.

Knocking is poor or incomplete combustion thats happens quick, so the bang is over before the piston has moved thru its entire stroke. This is where the cam timing comes into play. As long as that ex valve is closed, that explosion should be ongoing. This calculation is well beyond my ability, but you get the idea.

Lower compression engines are designed to utilize the low oct fuels in order to maintain that precious 14.7:1 air to fuel ratio. Less thermal efficiency from the fuel, less air required to maintain that ratio.

 

Some of your info is true & others is not.

Main not true statement I will point out,

Engines do not make maximum Torque & HP at 14.7: 1 AFR.
The factory catalytic converter works at 100% efficiency cleaning up unburned hydrocarbons at 14.7:1.

Engines have been buit with static compression ratios as high as 12.0:1 running on 87 octane **** water gas & raced hard.

How good of an engine builder are you ?
Can you really tune?
Do You have $$$ ?

BR

 

What someone is going to find in their GM made engine, I'll stand by my statements.

Sure, a custom tune/build can be run on white wine or cheap paint thinner...even 87 gasoline thats been cut 15% with some form of alcohol. The "tune" as you note, (as I stated) involves how much and how far the ign timing advance curve can go.

I do not know where you;re getting that info about Cats and the air/fuel ratio, as the stoichiometric ratio has absolutely nothing to do with cats. The ratio calculation is about combustion, not emissions.
I DO agree that more power can be generated at a slightly richer ratio. Thats NOT the optimum ratio though.
It is etched in stone that gasoline has a stoichiometric ratio of 14.7:1 and thats never going to change. Its chemistry. If pure performance was the goal, and longevity, reliability and drivability were NOT an issue, then lets all go back to carbonators and see how much $8 a gallon 106 fuel we can burn up driving to work....

Besides, my post was describing MY engine and how it relates to its octane requirements under different conditions for the original poster to use as a reference.
Since the physical engine does not change when the grade of fuel does, those actions/reactions are 'normal' for my very well built motor with close to 200K, no oil consumption and the ability to continue good wheel spin after the upshift into 2nd...(auto) so I'd say that I did well. No idea what the hp & t are now since the quiet exh was installed. Enjoying the car is more my goal than racing these days.

 

Did not mean to upset you.

Not everything is etched in stone.

There is alot more power to be had than just running a V8 engine at 14.7:1 AFR.

I have not done it 1sthand by EFI tuning.

But I have done it 100's of times with a Carburetor, jetting, stagger jetting, changing airbleeds, modifying emulsion systems.

At times correctly tuned you may see over a 40 to 50 + horsepower difference peak.

That is only part of the story.
What really counts is the torque curve generated from camshaft beginning powerband till it reaches peak & slightly over.

The more RPM's without the engine blowing apart & not running out of air, the better for me.

You want at least 100 % volumetric efficiency too if it can be had.

You can reach over 130 -140 % volumetric efficiency in a very good normally aspirated racing engine.

I am torque, Horsepower, & speed crazy.

Where are ideas & differences don't agree.

 

Forget about $2.50 gallon a gas anymore.

Those days are gone.

Good race gas starts at $15 a gallon.

Really good high octane gas is getting close to $20 per gallon.

Just getting from point A to point B is expensive anymore.

Racing is more costly $$.

 

Well for a little more info, what I am looking at would be a 383 with either 12 or 5 cc pistons.

That being said, with a 58 cc head, the compression would be 11.4 and with 54 cc head it would be 11.98. (5cc pistons)

With a 12cc piston it would be 10.5 with a 58 cc or 11 with a 54 cc.

Now all of this is assuming a .020 deck height and a .039 head gasket.

Now the heads were supposed to have been sized to 58 cc when I purchased them used, and the local machine shop had milled them to clean them up. So I am going to have to cc them to see what they actually are.

This is all going in a LT1 that had a factory compression of 10.4 and as we know the LT4 came with 10.8

I know as a general rule, the bigger the cam, the more you can get away with a higher SCR. I just didnt know what the limits would be with a hotcam.

 

Personally, I would be nervous going above 10.5 with .059" quench. Even then, you'll have to make sure you don't have any sharp edges in the cylinders and control temps VERY well. If you zero deck it, you could be better off. Quench is about the spacing on the perimeter of the chamber. That keeps the charge condensed in the middle.

 

I run 0.040" quench and 11.2 SCR with 98 octane RON. I belive that
should be 93-94 AKI, by US octane rating.

Zero problems. But on the other hand, I run bigger cam, and therefore
I'm still in "pump gas" DCR range.

DCR is much more important factor than SCR. Until you know your DCR,
you can toss arround SCR numbers as you like, but it doesn't mean much.

 

I think you can get away with close to 11.0 with the hot cam,I am running close to that with my lt4 heads and hot cam,heads are at 54cc with slight clean up.

I don't agree with 87vette that more power with 14.7,that is perfect for emissions not for power,more like in the range of 12.5-13.0.

 

Reprinted from another site regarding power and various AFRs....

AFR -- Comment
6.0 --- rich burn limit (fully warm engine)
9.0 --- black smoke/low power
11.5 -- approx rich best torque at wide-open throttle
12.2 -- safe best power at wide-open throttle
13.3 -- approx lean best torque
14.7 -- stoichiometric ideal for gasoline
15.5 -- lean cruise
16.5 -- usual best economy
18.0 -- carbureted lean burn limit
22.+ EEC / EFI lean burn limit

More importantly, the difference between 10.5 and 11:1 in compression will net about 5hp at the most. Could mean a whole .05 seconds in the quarter mile. As yourself if that's worth it OR if being safe (and not having to redo the heads) is safer.

FWIW, When close, builder's can remove a couple CCs from the heads before a build.

NOTE: To be clear, the chart above is for pure gasoline. E10, which is the most common fuel in use today, has a stoich point in the 14.3:1 range. So, it might be reasonable to assume all of the numbers above are ~ 1/2 point higher than E10 would require. (Thanks Brian for pointing that out.)

 

Not sure your math (or calculator) is right. I get 11:1 SCR (8.3DCR) with 54cc and 5cc flat tops. With 58cc and 12cc reliefs, I get only 9.7 SCR (7.3DCR).

You didn't mention if the block is bored, so I used 4" bore. With the numbers I get, either option is acceptable. If you put in the 5cc dish just make sure your temps are kept under 200 and you have very good pistons w/o sharp edges. Better quality pistons have smoothed reliefs. Or, your builder can bevel edges for you.

With the 12cc reliefs, you'll be around stock compression levels and O.K. with just about anything you do. The higher compression option should be in the 10-15hp range (TQ) and a snappier motor off idle.

 

Greg,
After going back and reading my post, I noticed I left out that this was going to be a 383 motor. Sorry about that.
Also, I pretty much agree with you on the AFR charts. I remember when Alvin at pcmforless was dyno tuning mine. He suggested I get bigger injectors for any future mods. The factory, 93 injector, (22's) were maxed out and he was at 12.8 wot. I believe 14.7 is ideal for cruise and 12.5 was his target for wot.
BTW, in your opinion what should be the max. DCR for a street motor. Also, keep in mind this is not a daily driver. I put about 2,000 miles a year on it. Mostly like to go and play at Test and Tunes, etc with it.

 

Hey Rich

LTX are quite a bit more tollerant of high compression on pump gas but everything has it's limit.

My LTX has 9.1DCR and only run it on Sunnoco or Shell 92oct. I have found that to be the practical limit on my combination - BUT it's not an absolute for anything because there are too many other factors that come into play (plug heat range, combustion chamber shape, quench, cyl head temp....)

DCR is just a made up number that only gives the people who use it a way to compare different engines using the Intake Valve closing points and known Static Compression Ratios. I put only a little stock in it and only use it as one of many very rough guide lines.

If your going to leave the block uncut run the Victor 5898, or Mr. Gasket 5716 head gasket. This will put the quench at a respectable .046 and they are reasonable priced at only $17.00 each.

I'd recommend you run the 12cc piston. The flat tops are a bit steep and you have to give up decent quench by running that .039 gasket to get it reasonable at 58cc and then at 54cc's it's too much with the Hot Cam...

With the 12cc piston, thin .026 gasket and 54cc head you'll be at 11.34SCR and with the cam degreed in on 109 you'll be at 8.82DCR.

With a 58cc head you'll be at 10.82SCR and 8.42DCR.

Either will run fine and make good power IMO.
Will

 

Did a read on the DCR man there sure are a bunch of things assumed. If a valve is a few thou off the seat it is more like a dripping faucet than zero compression that is only the begining of what does not make sense in my head. Here is a very simple test to get where I am coming from.. Get a bicycle tire pump and put a small restrictor on the outlet and pump it slow, note resistance now pump it very fast and note again. The net folow is not 100% then and certainly has substancial compression taking place. With a well tuned intake there is positive pressure in the intake port that exceeds the cylinder pressure so the air is going in not out when the piston is rising. There is a bunch more and hard for me to use the word dynamic in any way as dynamic assumes a running condition and this model does not use rpm. A true dynamic situation would need rpm valve size rocker ratio cam profile head flow data..etc..

 

I agree wit what Will said. DCR should be used as rough guideline only. But SCR is even less important factor.

One thing that should allways be considered, if you go with very high SCR and big cam, there is no easy way back. You can't just drop in milder cam. Well, you can, but you risk too high DCR and driveability on pump gas.

But we are getting offtopic here. I believe that Will anwsered the original question.

 

The above is probably the best advice anyone has given you so far... Just for reference, Im running a cam almost identical to the hot cam in my 383 with 10.25 to 1 compression with no problems on 92 octane gas..WW