So I have the engine disassembled and cleaned and once I get some side projects, I am planning on starting the rebuild. One question that is bugging me but I think I already know the answer is....

Did I just buy way to big a CFM carb?

When I started planning the top end rebuild (my plan here
http://forums.corvetteforum.com/c3-tech-performance/2424114-engine-build-)up-feedback-fall-edition-long.html
I did some research, read a bunch of magazine articles and noticed that without fail 383 builds used a 750 cfm. Ryans Car Page even summarizes a lot of these builds:
http://www.ryanscarpage.50megs.com/combos1.html

So when I needed a new carb a year ago, I went ahead and bought the speed demon 750. On the current / old engine build, this was def too much carb but will it work on the new configuration? Or am I still oversized? I know the volume calculations say yes but I am hoping with the more aggressive cam and higher flowing heads I will be more in line with the capabilities of the carb. My hope is, at worst, I simply sacrificing throttle response.

Trying not to add to the already blown out of the water budget I put together for the rebuild so hoping I can make this carb work for the new set-up.

Thoughts / feedback?
Thanks
Shane

 

I have a 400 HP SBC and I installed the 750 Speed Demon. It is not too big for your configuration, but you will need to tweak it and get the correct jet size for it. It's a good carb, but you will have to tune it. My two cents...install a Holley Double Pumper instead. Less tuning and way more tech help at Holley. For crisper response a 650 would work better, with just a little difference in performance.

 

if you keep the 750, you will have to jet it down to make it work with the small block. the advantage of a higher flowing carb is less air restriction at very high RPM's. the advantage of a smaller carb, like the 650 is higher air velocities at lower RPM's which better atomizes the gas and mixes it better with the air. This results in more lower end torque but can result in increased air restriction at very high rpm's. So the real question is, where is your engine going to spend most of it's time? Cranking high rpm's or normal street driving? You can jet both a 750 and a 650 to work with your engine. Many people think bigger is better and stick 750's on small blocks. However, with carbs, bigger is not always better. When I stick a 650 on my 427 BBC, the throttle response is wild, but it seriously limits the upper end.

Hope this helps...

 

You will be fine with that carb, just tune it properly

 

Thanks. I had rejetted and adjusted the carb for the old config and in preparation for the new engine build, I once again followed Lars' paper and also reset to mfg spec.

As a test, I went through the Holley Carb Selector on their web site and it actually recommended a 750cfm 4150 (part #82751).

Everything I hear is BG is good but the ore I read and learn it sounds like the Holey is just superior from an ease standpoint.
Shane

 

I don't see any reason why a BG carb is any harder to tune than a Holley carb, I had them both on my 406ci

 

There is no difference in ease of tuning or tuning methods between a Holley and BG. You can get my BG and Holley Tuning and Installation papers by e-mailing me:

V8FastCars@msn.com

I run 750's on performance engines over 302 CI with no problems. It's all in the tuning.

You do not "jet down" a bigger carb to run right on a smaller engine. In fact, a big carb installed on a small engine receives less of a metering signal than a smaller carb, so it actually runs lean. You need to richen up a big carb on a smaller engine - not lean it out. If you lean out a big carb on a small engine, it will run like crap, thus the bad name for the big carb/small engine combo.

I run a lot of different carbs and carb sizes on my test engine (357 smallblock with heads and a cam). I've run carbs as big as 1050 on this engine with no problems at all if the carb is set up right. Here's the deal: A small carb will be easy to tune, because it will run "OK", even with a lot of tuning problems. A "right sized" carb needs to be set up right, which most people can't do. An "oversized" carb will not give you any more horsepower, but it will not be your limiting factor. An "oversized" carb will run just fine if it is properly tuned on a smaller engine. I run 1050 and 850 carbs on my small blocks with no problems at all. I don't advocate this for the average street car, but I make the point: A carb that's "too big" is probably not your real problem when it comes to tuning - it's the overall setup and state of tune of your engine and carb. If you don't know what you're doing, get a small carb. If you understand carb setup and tuning principles, err on the big side.

I'm going on the dyno with a small block next week. I have 2 carbs to run on it: a 750 and an 850. I won't be running a 650/670. I'll post the results...

Lars

 

Hey Lars
I actually have your paper and have used it as my bible each time I take the carb off, etc for any reason. In preparation for the rebuild, I have actually reset to mfg spec, replaced all gaskets and reset based on your paper.

In terms of ability to tune, I definitely cut my teeth while tweaking the engine the last year and a half since I bought the BG. I am feeling much more comfortable with it and the addition of a wideband AF gauge is helping as well.

If you think the carb is workable then I feeling pretty good I could get it reasonably close.
Thanks
Shane

 

sorry for the incorrect info Lars, thanks for clearing that up.... it all makes sense. And thanks for your great tuning papers I got from you!