Originally Posted by usa3990
Hey guys, I am having a difficult time trying to decide what size cfm carburetor I should install on my 1990 chevy small block 350. The carb I have on there right now is to small(backfiring through the carb, all that good stuff), so I'm looking to buy a larger one, but I dont know what size to go with. First off the cam is a Edelbrock 7102, ( Duration (in degrees): 308/318, lift: 0.325/0.340 (int/exh)). It also has a Edelbrock performer intake and headers to a 2.5 in dual exhaust. Last it has a 4 speed so I usually shift at higher rpm's than a normal automatic. This car is for street use. I was thinking a 750 cfm's but was reading and most people were saying 650 is better. Let me know what you guys think. Thanks, Dustin.
Dustin, I guess things just pop out at me because of the years of doing this stuff. I might be reading you wrong, but it's just the way your post is worded that tells me you have not built this motor around the cam, but have just stabbed the biggest cam you could into an otherwise stock 350 Chevy. Additionally, you have used the wrong intake manifold.
I don't think Edelbrock is doing anyone any favors by describing the camshaft you're using as making power from 1500-6500. Matter of fact, I personally think it is downright criminal. There is no camshaft on the planet that will make power through a 5000 rpm range and they know it. Seems to me they are just trying to sell more camshafts by fudging on the figures and I think that is just wrong. The best that the 7102 is going to do in my opinion is to make power from 3000 to 6500. They say in their description that it needs a Performer RPM intake manifold to work best. Maybe you didn't read that part.
The other part of this debacle and one that you can't be faulted for if you haven't that much experience or didn't counsel with someone with more experience before purchasing the cam is that you need between 10.5:1 and 12.0:1 static compression ratio to make this cam work. Since you didn't say anything about changing pistons or heads on your motor, I have to assume that the long block is otherwise stock and that you have added the cam, intake manifold, carb and headers and left everything else as it came from the factory. Now, I don't know for sure what the static compression ratio of your motor is, but I'd guess somewhere around 8.5:1, which would leave you about 2 full points shy of having enough squeeze to make the cam work. If you were all set on buying an Edelbrock cam, you would have been much better of using a 2102.
Now, for the rest of you guys who may not agree with me at first look, here are the specs on the cam. Decide for yourself.
308/318 degrees duration advertised
234/244 degrees duration @0.050" tappet lift
0.488"/0.510" valve lift
Timing events 10/44/59/5
IC 107, EC 117, LSA 112
Like I said, I could be wrong, but it looks like to me to be another case of choosing the wrong cam in the first place and nothing you do will make it right. You need less cam or more static compression ratio.