Harm of overcarburetion?

[EvilBert]

First up, this is my first post, and I just want to say what a fantastic resource this site is! I've spent many hours lurking here when I should have been working!

I'm soon to join the smallish contingent of NZ Cobra owners - my rolling chassis should be ready mid-2007 - and I'm now trying to get to grips with various engine options.

My question: what is the worst that can happen if a carb's flow-rating exceeds what is required given a specific displacement? One of the engines I'm looking at is the Engine Factory's 351W, which has an Edelbrock Thunder 800 cfm included. From what I've read, this seems like more carb than is necessary - but, even if it is, what is the harm?

Cheers,
Mark

[mrmustang]

Too much fuel and a carb not set up properly could wash your cylinders down with fuel. At that point the mixture of fuel in your oil will certainly cause your engine to prematurely wear out.

Bill S.

[Ron61]

Bill hit the main problem with to much fuel, plus the car won't run as well and your gas bill will increase from the wasted gas. That being said, if you are getting the engine from a reputable builder, they should have the carb matched to the engine so it will be ok. You may have to do some minor tweaking depending on altitude and so forth.

Ron

[mj_duell]

Originally Posted by mrmustang

Too much fuel and a carb not set up properly could wash your cylinders down with fuel. At that point the mixture of fuel in your oil will certainly cause your engine to prematurely wear out.

Bill S.

This is dead on and has happened to some forum members before. Be very careful not to flood your oil with fuel. It's not just increased wear, if the oil degrades enough you could fry a bearing or two. Sell the 800cfm and get a carb matched to the engine set-up.



--Mike / Turn Ten Racing, Inc.

[Wayne Maybury]

As already stated, you will probably gas wash your cylinders and possibly contaminate your oil with gas. From a performance view point, the velocity of the air passing through an over sized carb will be lower than ideal, possibly resulting in poorer (is that a word?) throttle response.

I have a 351W, .030" oversize for a displacement of 357 cu. in., producing 472 HP. I have a Holley 750 DP which is certainly adaquate for this engine. I couldn't see any reason to go bigger than the 750.

Wayne

[scottj]

Too large of a cfm rating, i.e. "over-carbing", will result in a small gain in top end at the expense of poor low speed throttle response- refering to engine speed. Over-carbing doesn't mean that the engine will run rich; it just means that overall drivability will be compromised. Rich/lean is a calibration issue. A carb of anysize, if improperly calibrated for the engine, could cause a rich (or lean) condition. A common mistake made is to install a large carb, calibrated for a racing engine, on a street/performance engine. A race-calibrated carb will run too rich at idle and low engine speeds on a mildly cammed engine.

Scott

[vettestr]

Scottj,
I was trying to remember proper spelling for (stoichiometric) think it is correct = air-fuel ratio and getting TOO detailed when I saw your post pop in. Ya what you said.
A carb is a BOX that adds a mixture of fuel to parts of air = units to units within its designed limits to the signal it sees. Ya what you said.

[Chaplin]

Originally Posted by scottj

Too large of a cfm rating, i.e. "over-carbing", will result in a small gain in top end at the expense of poor low speed throttle response- refering to engine speed. Over-carbing doesn't mean that the engine will run rich; it just means that overall drivability will be compromised. Rich/lean is a calibration issue. A carb of anysize, if improperly calibrated for the engine, could cause a rich (or lean) condition. A common mistake made is to install a large carb, calibrated for a racing engine, on a street/performance engine. A race-calibrated carb will run too rich at idle and low engine speeds on a mildly cammed engine.

Scott

Assactly correct. You could wash the cylinders with a 600 cfm if it is not jetted properly.

[Poolw]

The formula for the correct carburator size is - Cubic inches x RPM divided by 3456. Then you may want to multiply this answer by 90% if you have a mild cam. A wild cam you will multiply by 1.10%.

351 x 5000/3456 = 507.8 x .90% = 457.
351 x 5500/3456 = 558.6 x 90% = 502

[EvilBert]

Thanks all - very helpful.

[Anthony]

read this

http://www.popularhotrodding.com/tec...boosters_tech/