Which Carburetor?

[nascardriver0723]

Which is better, a 2 barrel or a 4 barrel carb. Which one of these would you pick a 350 cfm 2 barrel carb or a 500 cfm 2 barrel carb for a 358 sbc engine and why?

[Trev]

Isnt there a formulae for working out the size of a carby, or a vacumm guage reading

[OldSStroker]

Which is better, a 2 barrel or a 4 barrel carb. Which one of these would you pick a 350 cfm 2 barrel carb or a 500 cfm 2 barrel carb for a 358 sbc engine and why?

The easy answer is the larger one. If you are trying to build a 400+ fwhp engine for a class that requires a 2 bbl, the carb will be the limiting factor. IOW, it's equivalent to a restrictor plate.

If the organization will allow a 4bbl, even a 390 CFM rated one, it actually flows more air than a "500 CFM" 2 bbl because the 2 bbl was rated at a higher vacuum (3.0 in hg vs 1.5 for the 4 bbl). To compare 2 bbl to 4 bbl multiply the 2 bbl rating by .707 (square root of 1.5/3.0).

So the 500 cfm 2 bbl should flow .707 * 500 or about 354 CFM when tested lke a 4 bbl.

Of course when you use a 500 2 bbl or a 390 4 bbl on a 358 engine producing lots of power, the carb is really working at a high manifold vacuum and flows a LOT more than 354 or 390 CFM just because the vacuum is so high. It still restricts the max power obtainable, however.

Up to a point, bigger is better, lots better.

[nascardriver0723]

so a 4bbl carb is bigger and better than a 2bbl carb and the 4bbl has more air flow and less vaccum than a 2bbl? The rules are you can run any 4 barrel carb, but the 2 barrel carbs are limited by weight...

Or better yet..Say you had a SBC engine 358 CI.. You could run any 4 barrel carb or a 500cfm 2 barrel carb. What carb would you run? for a 1/2 mile or less track

[OldSStroker]

so a 4bbl carb is bigger and better than a 2bbl carb and the 4bbl has more air flow and less vaccum than a 2bbl? The rules are you can run any 4 barrel carb, but the 2 barrel carbs are limited by weight...

Or better yet..Say you had a SBC engine 358 CI.. You could run any 4 barrel carb or a 500cfm 2 barrel carb. What carb would you run? for a 1/2 mile or less track

What's the total car minimum weight with a 2 bbl? How about with a 4 bbl?
Asphalt? Dirt? What tires?

If track surface and tires limit traction, which I suspect in this case, a lot of extra hp from a 4 bbl might not be faster in a heavier car.

Do the fastest (winningest) cars run 2 bbl or 4 bbl? That should give a clue.

Let's say you need a certain amount of rearwheel torque to be a top runner. You can get it easier with a more powerful (and therefore more torque in the usable rpm range) with the 4 bbl. However, with a car weight penalty, this will be a larger number than the 2 bbl car needs, AND, if you build the 2 bbl for higher rpm, you can use more gear, and get the needed rear wheel torque (which accelerates the vehicle) with the lower powered 2 bbl.

Hypothetical examples:

4 bbl engine makes 450 fw hp @ 6000 rpm max engine speed. Let's say overall gear is 5.00:1 and car reaches a given mph@ the 6000 redline. On this short track, the 6000 engine is at 3500 when driver hits the gas coming out of the corner. This is below 4500 rpm torque peak.

2 bbl engine makes less MAXIMUM power/torque, but up to maybe 4000-4500 it has about the same torque as the 4bbl, and it winds to 7200 at same mph and same point on the track. Overall gear needs to be 7200/6000 x 5.00 or about 6.00. Coming out of the corner at the same mph, the engine is at about 4200 where it's closer to torque peak and still within the rpm /power range where the 2 bbl can provide adequate air.

The 2 bbl has more torque at the slow point PLUS that torque is multiplied 20% more due to the 6:1 vs 5:1 gear. Plus the 2 bbl car is lighter so is easier to accelerate and it has more tire per lb. of car.

At the end of the straight let's say the 4bbl is up 40 hp to the 2 bbl. The 450@6000 4bbl has about 1970 lb-ft at the rear wheels, while the 410 @ 7200 2 bbl only has 1795 lb-ft at the wheels. If we assume a 2350 lb. 2 bbl and a 150 lb penalty for the 4 bbl, the torque/weight ratio (which is proportional to thrust force/weight ratio) is .788 for the 4 bbl and .764 for the 2 bbl. or about 3% less. It was better for the 2 bbl at corner exit, so the 4 bbl is probably catching the 2 bbl at the end of the straight. Catching isn't passing, however.

Obviously we should really compare actual engine output and car weights, etc, but if you buy the assumptions that rear wheel torque and it's resultant thrust force at the track surface is what accelerates the vehicle, and F=Ma (per Newton), the 2 bbl might be quicker.

The down side is that the 7200 2 bbl engine is more costly to build, and each hp or lb-ft will cost more and not last as long as the slower turning 4 bbl. So, it's cheaper to run the 4 bbl. Of course if you turned up the 4 bbl ($$) as high as the 2 bbl, you'd be quicker, but poorer. If traction is limited, the turned (and geared) up 4 bbl might just overpower the tires. I'm betting this is the case.

I'm not trying to lose you in the numbers, just proposing why the rules allow 2bbl and 4 bbl carbs. A copy of the rules sure would help!

Your results may vary, and no guarantees are expressed or implied. Just my $.02 on what I think your rules allow. Others WILL have differing opinions.

[nascardriver0723]

[ A copy of the rules sure would help! [/quote]
http://www.mansfield-speedway.com/rules_latemodels.htm

[OldSStroker]

[ A copy of the rules sure would help!

http://www.mansfield-speedway.com/rules_latemodels.htm[/quote]

THANKS!

They sure give you lots of choices, don't they?

The GM crate engine 88958604 is about $5000 less carb, and if they really enforce the "sealed engine" rule, it might be a good way to go. Do cars with these engines win? Do they finish in the top 5? top 10?

To make more power, and you can make LOTS more within the rules, you'll need to spend considerably more money. Maybe twice as much or more. "How fast do you want to go?"

A lot depends on your budget and competitive level. If you have a winning chassis, a top 5 driver and the car will put down a lot more than the 400 hp crate engine provides, build a 550+ hp 23* aluminum head 4-bbl engine and go to it. I'd want to know how much power the chassis and tires will put down.

If, on the other hand, you are on a limited budget, do not have a son who builds race engines, have a mid-pack driver, and the crate rules are enforced, the crate engine should get you a lot of fun. It appears that you can play with different 4-bbl carbs, and gearing. Chassis setup would seem to me to be the secret here, because your crate engine should be pretty much bulletproof and require very little attention. Hire the best chassis guy you can afford to setup and adjust the car.

FWIW, if traction (forward bite) is a problem, REAR SUSPENSION, Rule 3 would get my attention. The good systems are very hard to find on the car.

My guess is that the REALLY fast cars don't use the crate engines, and they spend tons of money and probably violate Rule 3.

My $.02

[nascardriver0723]

I dunno, i've never raced there. But they say rules are strickly enforced for safety. Actually im just starting racing late mods this year. So, knowing the rules with a 358 ci engine with steel heads, what carb would you choose? 4bbl or 2bbl.. and if you want to take it farther which CFM