600 HP LS engine carb section

[hymey]

I currently have a LS2 previously mentioned in other threads,366ci it has large port L92 heads 11.9:1 compression, camshaft 248-254 106 hydro roller, Lightened balanced internals, GMPP ported intake, 750 HP carb. No dyno time yet. I have the jetting and timing close, this combos spins up and is still making power at 7500 rpm shift point 8000rpm. I want to spend some time with it on the dyno, but I feel it is undercarbed. It is making excellent power, maybe there is more power with a larger carb, I have also room for more headwork the motor has potential for 600hp on pump. Without to much concern on drivability would a 850 cfm larger venturi carb be better for my application. I really feel the small venturi/1.75 bore HP ultra 950 while a nice carb would be a waste of $$. I seen a test on the comparisons of these two carbs not much in it like 1 hp. However I do have more comp/more cam more cyl head then there combo. I was thinking a properly calibrated 850cfm. If there is little difference in power I could do some more with my carb and spend the $1000.00 elsewhere on making the car run faster as ultimately it is what I want. Is 750cfm capable of supporting a real 550-600hp at 7500rpm on a small block? I have read of combos make north of 600 with 750's but that is magazine jargon.

Cheers

Joel

[swatson454]

I wonder how much a 1 inch, open spacer would help, assuming you're not already running one.

[hymey]

Hey there, I have a 3/4 spacer on it at the moment. Maybe I should try a 2 inch open spacer?

Cheers

Joel

[MadBill]

In general, the better it responds to open spacer height, the more it wants more CFM...

[Matt Gruber]

how much vacuum does it pull WOT at 7500?

[Troy Patterson]

Having been down this road many, many times I can tell you more carburetor is going to benefit your combination tremendously. If it were mine, I'd be running nothing less than a well built 1000 (+) cfm carburetor on it. You can call or email me thru my website.

Troy Patterson blog tmpcarbs.blogspot website TMPCarbs.net Engine Professionals Magazine's TMP Tuning article, pg. 44

[dave brode]

Joel,

I hope that you don't mind the only loosely related post.

On the subject of carbs, you mentioned the HP carbs. Afaik, the normal "HP" 950 has the same venturi and throttle plates as the old 4779. And, the HP 1000 has same venturi and throttle plate as a 4781 850. I realize that the main body is a little slippery on them, but I don't know how they can justify rating them as they do.

You mentioned an HP "ultra". Is it a 1.75" throttle, but 4779 venturi size?

That said, I'm no expert, but I would bet a 4500 would pick it up. I have seen them pick lesser engines up.

Dave

[bigjoe1]

I have a good friend with a very similar engine combo as you describe. We have run his engine on the dyno many times ( 50 runs ) He kept going to a larger CFM carburetor, and the power also kept going up== He is up to 1000 CFM ( 750 started ) it is making 648 HP at 7000 RPM. It revs right up to 7600, then the valve train goes crazy .It picked up over 20 HP from the 750 to the 1000 CFM carb


JOE SHERMAN RACING ENGINES

[hymey]

Thanks guys,

Looks like all you experts are suggesting 1000cfm plus, including a 4500. I will look into it, thanks.

Joel

[hymey]

I have a good friend with a very similar engine combo as you describe. We have run his engine on the dyno many times ( 50 runs ) He kept going to a larger CFM carburetor, and the power also kept going up== He is up to 1000 CFM ( 750 started ) it is making 648 HP at 7000 RPM. It revs right up to 7600, then the valve train goes crazy .It picked up over 20 HP from the 750 to the 1000 CFM carb


JOE SHERMAN RACING ENGINES

Joe, what camshaft/manifold combination is your friend using. Is he using a 4500 or 4150? A cnc'd large venturi body with properly setup metering blocks would do well!

Joel

[bigjoe1]

The cam is a custom hyd roller from comp cam I had made up espeically for his engine. The carb he ended up with was a very highly modified 850 that we got to flow 995 CFM= I think the intake was an Edelbrock victor with an 850 carb pad. We plated with spacers and so forth. It started out at 600 Hp and we were able to tune it up to 647.This guy is a young kid ( by my standards ) and he does not have lots of money to play with. In his 1970 Nova, he has got it to run 9.97 at 133 the last I heard


JOE SHERMAN RACING ENGINES

[hymey]

Im still a young kid (32) . But dont have a lot of coin to play with but thanks for sharing your experiences!

[hymey]

Joel,

I hope that you don't mind the only loosely related post.

On the subject of carbs, you mentioned the HP carbs. Afaik, the normal "HP" 950 has the same venturi and throttle plates as the old 4779. And, the HP 1000 has same venturi and throttle plate as a 4781 850. I realize that the main body is a little slippery on them, but I don't know how they can justify rating them as they do.

You mentioned an HP "ultra". Is it a 1.75" throttle, but 4779 venturi size?

That said, I'm no expert, but I would bet a 4500 would pick it up. I have seen them pick lesser engines up.

Dave

Hi Dave, The HP ultra has the 4779 body with the 850 base plate and evidently flows 820-830cfm. Garry Hunt runs them on 366ci super sedans that make 700-720 hp on methanol and he has bettered some modified 850 shop built carbs by 10 hp, they have good adjustability but generally only change jet sizes up or down 1 or 2. He uses them quite a bit now. I have a feeling a modified 850 will work. Demon carbs get a bad rap, but there is also 1.800 inch bases and 1.6 venturi combos. like troy and joe said bigger is better!

[hymey]

Just done some more research. A guy on ls1tech had a fast L92 combo in a mustang 2990 pound not including driver (near 250 pound lol). He had a modified 750 HP and was running 9.9@ 138 mph with that combo shifting at 7000 rpm. 780 cfm pro systems carb. Thats not to bad.

[JDR Performance]

We've got the 1.8 throttle bore and 1.6 venturi in a 4150 if you're wanting to stay with a 4150 intake and a big carb.