Is my carb overkill??
Thread Starter
Instructor
Joined: Jun 2008
Posts: 105
Likes: 0
Is my carb overkill??
So I just recently got my car somewhat tuned and drivable since I bought it and did a cam swap. The car is a 75 vette with ported heads here are the specs on the cam
Intake Duration at 050 inch Lift: 244
Exhaust Duration at 050 inch Lift: 254
Duration at 050 inch Lift: 244 int./254 exh.
Advertised Intake Duration: 302
Advertised Exhaust Duration: 312
Advertised Duration: 302 int./312 exh.
Intake Valve Lift with Factory Rocker Arm Ratio: 0.510 in.
Exhaust Valve Lift with Factory Rocker Arm Ratio: 0.533 in.
Valve Lift with Factory Rocker Arm Ratio: 0.510 int./0.533 exh. lift
Lobe Separation (degrees): 114
A performer intake and a holley 750 vacuum secondary carb
I want to throw on some AFR's in the future and I wanted to only buy one carb. But the car doesn’t seem to have the power I thought It would. It might be a gearing issue but I was just wondering if its to much carb
Intake Duration at 050 inch Lift: 244
Exhaust Duration at 050 inch Lift: 254
Duration at 050 inch Lift: 244 int./254 exh.
Advertised Intake Duration: 302
Advertised Exhaust Duration: 312
Advertised Duration: 302 int./312 exh.
Intake Valve Lift with Factory Rocker Arm Ratio: 0.510 in.
Exhaust Valve Lift with Factory Rocker Arm Ratio: 0.533 in.
Valve Lift with Factory Rocker Arm Ratio: 0.510 int./0.533 exh. lift
Lobe Separation (degrees): 114
A performer intake and a holley 750 vacuum secondary carb
I want to throw on some AFR's in the future and I wanted to only buy one carb. But the car doesn’t seem to have the power I thought It would. It might be a gearing issue but I was just wondering if its to much carb
Race Director
Joined: Dec 1999
Posts: 19,610
Likes: 778
From: Forked River NJ
Until you put on the AFR heads and raise the compression ratio the cam is to big for your car. If your gears are in the low 3s that cam is to much. You need at least 3.55 better yet 3.70s or better with that cam. The intake is going to also be a restriction with that cam. That is a big cam for your combo.
Race Director
Joined: Dec 1999
Posts: 19,610
Likes: 778
From: Forked River NJ
The Victor Jr. is the same height as the Performer RPM which has fit under my buddys stock 73 hood. I run a Victor Jr. on mine but I have the LT1 hood.
Race Director
Joined: Apr 2000
Posts: 17,676
Likes: 201
From: Who says "Nothing is impossible" ? I've been doing nothing for years.
Yeah you need the whole package not just bit and pieces. If you want to stay with that cam get some AFR 195 Eliminators and bump up the compression to 11:1 and with the Vic Jr you are looking at 400-450 HP 
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Tech Contributor
Joined: Aug 1999
Posts: 14,380
Likes: 6,391
From: At my Bar drinking and wrenching in Lafayette Colorado
The carb will run just fine - the stock Q-Jet is 750 cfm. With the vacuum secondary Holley, you can err on the big side and not have any drivability issues. Once you get the whole package put together you're going to want to put the light, white spring in the secondary side to get those secondaries fully open. If you're running the stock spring in the carb you're probably barely cracking the secondaries open at all right now, so your carb is likely providing about 500 cfm on a good day...
Lars
Lars
Racer
Joined: Aug 2005
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From: Pace FL
Instructor
Joined: Jun 2008
Posts: 132
Likes: 9
From: California
Until you get some compression you will have a dog
No matter what carb or intake. Without the heads a victor intake will make things worse not better. You need more compression, more head flow and more rear end gear to match that cam. Worring about the carb or secondary spring at this time when the combo is all wrong is silly?
Thread Starter
Instructor
Joined: Jun 2008
Posts: 105
Likes: 0
Ya i suppose it is. The carb was used so i was just trying to get it running right so when i do put the heads on it would run right.. I thought the ported heads would have helped alittle bit but i suppose its in the compession. Btw where is the stamp with the code for the rear end cause i have no idea whats out back.. Im quessing like 308's
Instructor
Joined: Jun 2008
Posts: 132
Likes: 9
From: California
Compression
Ya i suppose it is. The carb was used so i was just trying to get it running right so when i do put the heads on it would run right.. I thought the ported heads would have helped alittle bit but i suppose its in the compession. Btw where is the stamp with the code for the rear end cause i have no idea whats out back.. Im quessing like 308's
as a basic rule of thumb as the cam duration grows the amount of overlap of the opening of the intake and exhaust valves increases and the bleeding off of cylinder pressure gives you a lower dynamic compression ratio. Bigger cam = a need for more static cylinder comression. That is why the cam manufacturer gives you a recomended static comression ratio for each cam (ususally). Now the flow of the heads determines how efficient you pump is (the engine is a pump of sorts). The more CFM potential in the heads the more hp potential. Based on these flow numbers you select a cam to match (the size of the motor factors in here) and dial in the comression to the package. It isn't really as simple as just picking some parts. That is why you see some guys making way more hp with a correct 350 than other guys do with a wrong 383 for example. A forum like this is a good starting point since many people has done the research and come up with some good combos. There are probably SBC specific engine forums where the guys really could dial you in.
Oh yea I have no idea what gears you have but I suspect a vette that year has some freeway flyers like 308 in it.