new Carburator size?
#21
Race Director
Just one last note. A 670 Avenger will work fine on his existing motor, although for reasons I don't want to get into I don't like those carbs. The 4779 Holley 750 will work fine too on his existing motor and will work fine on the motor he plans to build with a roller cam and Brodix heads, so he only has to buy one carb
#22
Race Director
100 CFM over recomended is a good rule of thumb. Also agree with using a wideband A/F Meter in real driving conditions for best tuning. Not taking anything away from the Quadrajet but I have always preferred a Holley. All GM's high performance engines used Holley carbs.
I have a 4778 Holley Mechanical secondary DP in the garage taking up space. It is a 700 CFM. It will need a kit and tuning but it is nice and all there. For a 350 with a hydraulic roller which will limit you to around 6000 RPM, a 336 gear and automatic, I think it would be perfect. $75.
I have a 4778 Holley Mechanical secondary DP in the garage taking up space. It is a 700 CFM. It will need a kit and tuning but it is nice and all there. For a 350 with a hydraulic roller which will limit you to around 6000 RPM, a 336 gear and automatic, I think it would be perfect. $75.
#24
Race Director
I have a 4778 Holley Mechanical secondary DP in the garage taking up space. It is a 700 CFM. It will need a kit and tuning but it is nice and all there. For a 350 with a hydraulic roller which will limit you to around 6000 RPM, a 336 gear and automatic, I think it would be perfect. $75.
#26
Race Director
Also annular booster are better for the street, you can buy them separate if the carb has spun in boosters. Hope this helps.
#27
Le Mans Master
I'm not saying anything about air flow or using charts or reading magazines. And the last thing I would ever do is phone Holley tech line. The guys who designed and know how to tune these carbs are long gone.
When this forum was only around for a year or two I bolted a Holley 750DP with mechanical secondaries on Victor Jr single plane intake, and a solid flat tappet cam on my mild 355ci. Everyone thought that was crazy.
A forum member by the name of MountainMotor told me that carb would be the best for my combination and he was right. This guy forgot more than most know about motors.
So you can argue all the theoretical points about this that you want, proof is in the pudding as they say
Chassis dyno
I am talking from personal experience and have use a 750 Holley DP on a mild 355ci to a wild 406ci without any problems. A little tuning is needed as would be with any carb you bolt on.
You should never take a carb, no matter what is recommended and bolt it on your motor and expect it to run perfect. These are aftermarket carbs and have to be tuned to your specific motor. A wideband O2 sensor like the LM-1 is required to do this properly
When this forum was only around for a year or two I bolted a Holley 750DP with mechanical secondaries on Victor Jr single plane intake, and a solid flat tappet cam on my mild 355ci. Everyone thought that was crazy.
A forum member by the name of MountainMotor told me that carb would be the best for my combination and he was right. This guy forgot more than most know about motors.
So you can argue all the theoretical points about this that you want, proof is in the pudding as they say
Chassis dyno
I am talking from personal experience and have use a 750 Holley DP on a mild 355ci to a wild 406ci without any problems. A little tuning is needed as would be with any carb you bolt on.
You should never take a carb, no matter what is recommended and bolt it on your motor and expect it to run perfect. These are aftermarket carbs and have to be tuned to your specific motor. A wideband O2 sensor like the LM-1 is required to do this properly
BTW, Summit Racing just came out with their own line of carbs. They look to be compatible with Holly DP stuff but have no gaskets beneath the fuel level. They have 600 and 750 CFM vac and mechanical sec models and retail for ~$275.
#28
However, this is only going to happen in a small range of rpm, and not at the top of the curve. ED69ray said he had 100.5% ve at 4600 rpm, but I can virtually guarantee you he did not have that at 6000 rpm. (Now, with a different cam it is possible that he could, but we don't have race engines, so it isn't going to happen on the street.) ED, if you are still following this thread, I'd like to see the air flow rate (or VE) graph vs. engine speed.
you are bang on, I'll post a scan tomorrow - only at 4500-4900rpm was I at 100% VE or greater
VE was highest at max torque 4600rpm (100.5 % and 448tq)
VE was 96.4% at max hp 5700rpm and 444hp
at 6000rpm she hits the wall:
5800rpm.............442hp/400tq.............VE% 95.5
5900rpm.............403hp/359tq.............VE% 92.2
6000rpm.............372hp/326tq.............VE% 88.4
#29
Race Director
#30
Drifting
Member Since: Apr 2006
Location: gold coast queensland
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i had a stock 81. original everything .stock carb, heads, 2.78 gears the lot !!! . bought all the parts i needed for my rebuild which was GM fastburns,demon 750 , air gap.roller cam, transmission ,bla bla bla but before i did the swap i stuck on the demon 750 DP just to see what it did. ran like a big fat pig. way too much fuel getting in. it drove but man it was a slug.
bolted on all my other bits and pieces and been hammering ever since. if your budgeting for less then 280hp at wheels stick with the 600-700 carbs. if you have 280+ at the wheels go the 750 . thats my opinion.
bolted on all my other bits and pieces and been hammering ever since. if your budgeting for less then 280hp at wheels stick with the 600-700 carbs. if you have 280+ at the wheels go the 750 . thats my opinion.
#31
Le Mans Master
I thought the point you were making was that you needed 750 cfm to make those power levels, and that is just not the case.
#32
you are bang on, I'll post a scan tomorrow - only at 4500-4900rpm was I at 100% VE or greater
VE was highest at max torque 4600rpm (100.5 % and 448tq)
VE was 96.4% at max hp 5700rpm and 444hp
at 6000rpm she hits the wall:
5800rpm.............442hp/400tq.............VE% 95.5
5900rpm.............403hp/359tq.............VE% 92.2
6000rpm.............372hp/326tq.............VE% 88.4
VE was highest at max torque 4600rpm (100.5 % and 448tq)
VE was 96.4% at max hp 5700rpm and 444hp
at 6000rpm she hits the wall:
5800rpm.............442hp/400tq.............VE% 95.5
5900rpm.............403hp/359tq.............VE% 92.2
6000rpm.............372hp/326tq.............VE% 88.4
#33
Safety Car
Not exactly their own. It's their version of the old Holley 4010/4011 (long discontinued), which was a knock off of the Autolite 4100. They're not bad carbs by most standards and are very well suited to anyone seeking "out of the box" performance.
#34
Race Director
Yeah two completely different motors and you compare only the carbs, makes sense to me. Yours probably would have made more power with the 750 Holley
#35
if the 750 was too big you would see it with a hit in the midrange - not so as I'm making 400+ tq at 3200rpm.
I don't believe that I'd have made 444/448 with a 600cfm carb.
At some point you have to put the calculator down and go with what works
Good Luck, Ed
#36
You might not believe it, but just because you think something doesn't make it true You never tested with a 600DP (or 650, 700, 800, etc).
And again, this is my point. You say go with what works... but you don't know that a 600 or other carb won't work. Clearly a 600 has the capability of flowing as much air as you need. So why doesn't it work? You just don't know if it works.
#37
I have just invested in several carb books to teach me how to tune my Holley... and in every book they say the same thing... bigger is not always better... Additionally, they spend a lot of time dispelling racer conventional wisdom...
I think we might be seeing the "If 650 is good, then 750 must be better right?" syndrome. If your engine only pulls 600 cfm... then a 750 will not help right? Won't you just have to jet the carb down a whole lot?
Again, despite any other engine stats, if your engine uses, say 561cfm... anything over is a waste right?
I think we might be seeing the "If 650 is good, then 750 must be better right?" syndrome. If your engine only pulls 600 cfm... then a 750 will not help right? Won't you just have to jet the carb down a whole lot?
Again, despite any other engine stats, if your engine uses, say 561cfm... anything over is a waste right?
#38
Drifting
Sometimes racer conventional wisdom comes from actually trying an idea and seeing what makes the car go faster.
You can work all of the math equations that you want, but if the car goes faster with a "too big" carb then you need to reevaluate your theory.
Tell me this, what does the carb cfm formula do to account for pumping losses caused by high vacuum levels in the intake at WOT?
Small carbs are fine on your daily driver, but if you are going racing you are giving up a little ET.
You can work all of the math equations that you want, but if the car goes faster with a "too big" carb then you need to reevaluate your theory.
Tell me this, what does the carb cfm formula do to account for pumping losses caused by high vacuum levels in the intake at WOT?
Small carbs are fine on your daily driver, but if you are going racing you are giving up a little ET.
#39
Race Director
Wrong. Volumetric efficiency can be higher over a wider range with a slightly larger carb than required due to less air flow restriction at the only entry point for air coming into the motor. That is where the rule of thumb 100 CFM bigger than required comes in. An engine is like a pump. The more efficent the flow in and flow out the more power that can be made. That is why headers make more power than iron manifolds. Less restriction of flow. That is why big flowing heads have more capacity to make HP than smaller heads. If you go too big it will reduce atomization of the fuel, it will drop out of the mix. Same with a carb. That is why racers run a little bigger carb than required. A street used engine might use a slightly smaller carb to keep the fuel suspended in the A/F mix better. All the Q-jets are 750 CFM. The Holley carbs installed on the factory high performance, solid lifter, high RPM motors were all 750 CFM or bigger. Even the 302 Chevy in 1969 used a 780 CFM holley. The early dual Quad corvettes used 2- 400 or 450 CFM carbs for a total CFM of 800 or more on a 283. They were just jetted down. Why would you put a 600 CFM carb on a performance build? It will probably work fine with a 327 hydraulic cam motor but a 350 and up all had 750's installed by GM even during the gas crunch when they were looking for economy.
#40
ok then, show me a performance built 350 or 383 with a 600cfm carb
as Mako said, why would you put a 600cfm carb on a performance motor? Because your calculator told you so?
When I said go with what works I meant go with what has been proven - personally I have yet to see a 450hp 350 or 383 with a 600cfm carb.