Carburetor Sizing

[dereklola]

I've got an ex-Indy Lights (non-turbo) Stage II engine for road racing. Ultimately I'll use the unique cast magnesium intake trumpets (one per cylinder) and high pressure fuel injection but for starters and to make life easier I'm thinking of using a Holley 4bbl same as or similar to the NASCAR V6s. Once the car is running OK we can fiddle with the injection and mapping etc.

Anyone know what carb they used for the NASCAR V6s?

I just visited the Holley web site and after plugging in my parameters they recommended a 4150HP series - in one of two specifications - one rated at 390 cfm, the other at 600 cfm, and the range goes up to 1000 cfm. That's quite a range!! How does one choose a flow rate? I calculate 252 CID at 7200rpm red line to be 1021 cfm. Why would I not need a carb that flows that amount?

Any help or advice appreciated - Derek

[750H.P.V6]

Derek,

I'm not sure what the CFM rating was on Busch GN carbs. If I was in your shoes I'd put on a Holley 4BBL approx 800-850 CFM. Any smaller would be a restriction. You could drop Ron Tarabori a line and see what he runs on his dirt track car.

Neal

Originally Posted by dereklola

I've got an ex-Indy Lights (non-turbo) Stage II engine for road racing. Ultimately I'll use the unique cast magnesium intake trumpets (one per cylinder) and high pressure fuel injection but for starters and to make life easier I'm thinking of using a Holley 4bbl same as or similar to the NASCAR V6s. Once the car is running OK we can fiddle with the injection and mapping etc.

Anyone know what carb they used for the NASCAR V6s?

I just visited the Holley web site and after plugging in my parameters they recommended a 4150HP series - in one of two specifications - one rated at 390 cfm, the other at 600 cfm, and the range goes up to 1000 cfm. That's quite a range!! How does one choose a flow rate? I calculate 252 CID at 7200rpm red line to be 1021 cfm. Why would I not need a carb that flows that amount?

Any help or advice appreciated - Derek

[Jay Wicks]

The Busch series were limited to 550 CFM by class rules, not what you would want. Check out the Holley HP series, great bang for the buck, very tunable,but, most of the time work very well out of the box.

[JDEstill]

? I ran through the calculation and got about 470 acfm for 252 cid @ 7200 rpm, assuming a 90% VE?

As for rated CFM, I believe that is assuming a 0.5" water pressure drop. So if you have an engine sucking in 600 cfm with a "600 cfm" carb, the manifold pressure will be 0.5" w.c. lower than the pressure above the carb. If you put that 600 cfm carb on an engine that wants 800 cfm, it might take 1" w.c. to flow that amount. It'll flow what the engine wants, it just takes a little more pressure to make it happen, and you'll get a lower manifold pressure as a result.

The higher the manifold pressure you have the more air flow you have and the more power you can make, so you can measure how much vacuum you are pulling at WOT and if it's a couple of inches of water you know you need a bigger carb. If it's only 1", or 0.75", then you can be confident that the carb isn't that big a restriction and going bigger probably isn't going to improve your air flow much.

Of course there is always the air/fuel mix quality issue, and I guess velocity issues and wet flow and all that. But I'm just talking about on a pure air flow basis.

So anyway, that's probably why the Holley site recommended a 390 or a 600, since if they got 470 cfm like I did they probably saw the choice as being a little undersized for good low end or oversized for good WOT.

Just my thoughts, hope it's worth something.

John

[chevyII]

650 works well for most SBC V8 so I cant imagine going much bigger

[750H.P.V6]

I was thinking he was going to get his money's worth out of the Stage II heads and run the RPM up to the 8500 range. At that point a larger carb will pay dividends. I have a friend that had and old Indy Lights car with a ex-Busch engine in it and he was running a carb. I'll drop him a line and see what he was running.

Neal

Originally Posted by chevyII

650 works well for most SBC V8 so I cant imagine going much bigger

[chevyII]

yes with the rpms in the upper over say 5000 most of the time your right. I think it might depend on that more than anything else. Good call.

[dereklola]

Hey thanks guys - sorry for my late response - I was waiting for an email to tell me someone had replied but didn't get one.

All of what you said makes sense and I'd thank John in particular for explaining just what the flow 'ratings' mean and the relationship to manifold pressure drop. Also I made a mistake in the engine flow because I forgot to divide by two for the 4-cycle engine - doh! On a pure arithmetic basis it comes out to 510cfm @ 7000, 583 @8000 and 656 @ 9000 should I wish to explore those limits. The original Indy Lights series was limited to 7000 (spec engine) but I agree the engine will go higher - whether I want to explore the added power and speed from those RPM is another matter! The car will already be double the power of my current car and don't forget road racing requires on-the-limit braking and corner just as much as drag racing down the straights. And I'm already 68!

Operating RPM range while on track should be between 5000 and 7000 - or 5800 and 8000 if I choose a higher red line.

I already picked up a used 600 double pumper on eBay to do a physical mock-up and will probably buy a 600 or 650 HP to run. I see Proform makes a Holley 'knock-off" for less money. Anyone got any views on these? Are they any good?

Thx - Derek

[unboostedV6]

I have a 650 on my 272" which runs well to 7000 rpm, then runs out of air. The very same carb on my 8445 headed 256" was more than enough for 7000. Bill Chaffin runs a Dominator on his super stocker. V6's seem to like really big carbs...may have something to do with more pronounced intake pulsing in the manifold than a V8. I'll be moving up to an 850 or 950 soon.. GB

[dereklola]

Greg - interesting comments - thx - Derek

[Ron Tarabori]

OK I'm late getting to this but here is my two cents worth. I run a carbed odd fire motor on 1/3 to 1/2 mile dirt tracks. Our competition is a field of big dollar small block chevy and fords with a 360 cube limit. We are limited to a 500 cfm Holley two barrel carb. My motor will pull out of the corners with the best of the v8s and usually pull away about half way down the chute when the motor gets around 7 grand. My rpm range is 4500 to 8300. When we go to other tracks that allow it we put on a 750 carb (because I have one) and it will add 1000 rpm to the motor so we have to take some gear out. My point is these motors will run well with a small carb but really like a lot of air. Next year I'm going to go to a 950. Bob Mustic has a carb on his odd fire motor that flows around 1050 and his car is street driven and raced. It is a *****cat on the street even with that big carb. Hope that helps. Ron

[dereklola]

Ron - thx - all good, useful info. Out of interest when you use 8300 and 9000 RPM how many hours between engine rebuilds?

Now all I've got to do is decide whether I want good driveability or max power. Should be simple! At least you guys have confirmed that the engine will run with a wide variety of carb sizes.

Thx all.