750 Holley too big? will less CFM be better

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vanrah

G-Body Guru
Apr 16, 2013
868
93
Near Afton, Wisconsin
Greetings Myles & all; This is just a short note, more late. I have some carbs that may work for you that I'm selling for a friend + one of mine. Here are the pictures. Later Ole' Bob.
 

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bracketchev1221

Royal Smart Person
Jan 18, 2018
1,079
113
750 is not too big for a healthy 355. I ran a 750 4779 Holley on my 465 hp 355. It was an animal off idle. Would blow the tires off into 3rd gear.
 
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Kwik_Cutty88

Moderator
Moderator
Nov 22, 2011
1,167
113
Coastal North Carolina
You are kinda answering your own question. You have a miss match engine combo w/ low comp and large heads big cam which is killing the runner velocity which, as mentioned, your not pulling air on the bottom end and getting a poor carb signal. Your clean pass shows you can mask the problem. As mentioned above a wide band tuning and the aid of a down track recorder tells you exactly whats happening
and where (no guessing)

I also agree. Does your camshaft have a lot of overlap that is bleeding off compression? In addition to your dished pistons, I'd be curious to know exactly what the static/dynamic compression ratio is.

An AFR gauge will put you in the right direction to get the engine to run right. But in my opinion you need to ask yourself a few more questions about this engine combination.

What are your static/dynamic compression ratios?
What is the operating range of your camshaft?
What is the operating range of the intake?
Do you really need a 215cc head for this engine combination?

Evaluating the engine at this level may seem like re-inventing the wheel when you're just trying to get it to run properly, but I think it will give you some important direction on the approach to tuning this combination.
 
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gnvair

G-Body Guru
Sep 1, 2018
876
93
Southern New Jersey near Philly
The low compression with a lot of duration kills cylinder pressure. You should try to get the compression up to use with that cam and those heads. If that is not possible then go with a smaller carburetor. Bigger is not always better with carburetors.
 
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lilbowtie

Comic Book Super Hero
Jan 7, 2006
3,460
113
Canton Mi
A 750 isn't too big for that engine if set up properly. I know you probably just threw the engine together w/ what you had, but you need to know what your ultimate goal is going to be and choose the proper engine - drive train components so they will complement each other. My 350 is set up pretty good can leave at any RPM w/o a problem - even at idle.

 
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rogue_ryder

Greasemonkey
Oct 27, 2017
206
63
Colorado
You might want to contact:

Eric Jackson is one of the best Holley Carb builders there is, if you give him your setup he can probably rebuild your carb and get it to a baseline that will get it to run right or he'll tell you that you need to do cam swap to get the thing to run right.

Here's a link to some good info he posted about regarding what to do before buying a new carb: https://www.yenko.net/forum/showthread.php?t=84742

There's no way any of us can accurately diagnose or tune your carb from the internet, it's all just guessing.

If it were my car first thing I'd do is make sure the carb is good, a 750cfm at sea level jetted correctly (GET A WIDEBAND O2 sensor and AFR guage ASAP) on a 355 should make more power at WOT than a 650cfm; if it were a street car yeah go with a smaller carb as you'll get better driveability on the street; but for a 1/4 mile car you shouldn't necessarily need to go smaller than a 750 cfm on a 355, I'd be pissed if I dropped $500 for a 650cfm just to see my ET go up and MPH down. Here's a link to Decent Dyno Test Article of Carb Sizes on SuperChevy. There's so many variables, jetting, timing etc. that nobody is going to be able to accurately solve your problem without being with the car and having the appropriate diagnostic tools (Vac Gauge, AFR, Timing light etc.)
 

gnvair

G-Body Guru
Sep 1, 2018
876
93
Southern New Jersey near Philly
The other thing people don't consider is the cfm sizing for a given engine combo can vary. Especially consider that you can get away with a larger vacuum secondary carburetor than you can with a mechanical secondary. A mechanical secondary carburetor requires more precise sizing than a vacuum secondary carburetor and is also the reason they are typically sold in sizes starting at 600 cfm with larger sizes available in 50 cfm increments (i.e. 650, 700, 750, 800, 850). Then of course there are the HP series and 4500 series with even larger sides than 850 cfm. But for most small blocks you can get away with a 750 vacuum.
 

64nailhead

Goat Herder
Supporting Member
Dec 1, 2014
3,799
113
Upstate NY
You don't have a mismatched motor if you intend to consistently spin it over 6000 rpm's. Your heads and carb will support 450-550hp without issue. The question is how you intend to use this. For a daily driver - yikes, this sucks - a set of stock Vortecs will outperform it in low rpm ranges. It comes down to an end goal IMHO. What is yours?
 
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Csilvy2000

Master Mechanic
Mar 14, 2012
250
28
Texarkana, AR
Try any 650 or 750 vac sec. carb.
 

Max Headroom

Master Mechanic
Sep 8, 2011
398
63
I'm not sure I agree about the AFR. Even if it is right on, the motor could run like crap because of weak charge density and velocity due to the mismatched parts.
If I'm understanding what I read, you have a low compression motor with dish pistons and large runners in the heads. I don't think a smaller carb will make any difference. Low compression means thin intake air charge. This means the venturi doesn't pull a lot of fuel. Large diameter runners in the head reduce what velocity the piston travel might generate, especially at low rpm. Your intake charge is reduced (both fuel and air) by the weak pull generated and the lethargic velocity. I think going to bigger squirts, jets etc. will make no difference at best and at worst make things way crappy down low. On the exhaust side, a big cam needs the cylinder cleared of spent gas quickly or a lot of exhaust mixture gets left behind when the valves change state. (open/closed) Once again that means velocity, which 215 cc runners arwe not going to provide.

In my opinion, put a set of stock heads on and the launch should get significantly better. Everything else will fall on its face but you will know what your problem is.
 
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