tee-boy

iTrader: (8)

If the best cat flows 600 CFM, our cars have 2 cats, so I guess they can support up to 1200 CFM. Is that correct? I'm guessing if you had an engine that sucked in 1201 CFM of air thru intake, you would be restricted on the exhaust, or is 1200 CFM on intake different than 1200 CFM on exhaust?

LS1curious

Whats gonna screw you is exhuast oygen content on a heavy breathing motor and heat build up in the catalyst.Generally they won;t hurt power but they will burn up and clog with a large camshaft.

tee-boy

iTrader: (8)

Good info, but Well my question had more to do w/ whether or not CFM on the intake is equivalent to CFM on the exhaust.

LS1curious

Generally you need 75% of your intake flow on the exhuast side. personally I prefer to error on the side of more then enough.

GOaT Cheese

There is alot less volume in the exhaust after the combustion process, the problem is that what volume is there has been expanded a whole lot from the heat of said combustion.

3.4camaro

iTrader: (1)

I think that intake flowing more than exhaust is ok because the engine is sucking air in and blowing air out. Negative pressure does not move air as efficiently as positive pressure pushing air out.

I think that's why intake valves are bigger than exhaust valves.

tee-boy

iTrader: (8)

This makes good sense.

Ed Wright

iTrader: (1)

If you flow the intake side with the intake manifold and throttle body installed, and the exhaust side with a pipe bolted on, you will find the intake/exhaust ratio closes up a lot. My LT1 super Stock heads flowed with a radiused entry on the intake side, and no pipe on the exhaust port has a ratio I/E of 75%. Add the intake, throttle body (with all the other ports sealed off) and a section of pipe on the exhaust port and it goes to 96%.

dano73327

Not really. Pressure differential is pressure differential, you just have an area of higher pressure trying to move to an area of lesser pressure to equalize.
There is not really any such thing as negative pressure.

Intake valves are bigger than exhaust valves for a couple major reasons.
1. Power goes up much more with increasing intake valve size and flow than
it does with increasing exhaust size and flow.
2. Air converges much more efficiently than it diverges, and the hot gasses
are less dense and flow more efficiently.

engineermike

Mass in = mass out (conservation of mass)
Volume in << volume out (P=pRT)

You see, exhaust gas is roughly the same pressure as intake air (unless you have a turbo or restrictive exhaust), but much greater temperature.

Therefore:
CFM out >> CFM in

So, if your engine draws in 1000 cfm, then 1200 cfm converters will cause a restriction.

Now, to drill down a bit deeper. . . just because a throttle body, intake port, or catalytic converter is rated at a given cfm doesn't mean that it won't flow more than that. These various items are rated at a given flow at a given dP, or restriction. Carburetors are rated at 1.5" mercury and heads at 28" water dP. This is the equivalent of 0.74 psi and 1.0 psi, respectively. So, a port that flow 300 cfm doesn't even flow the same as a carburetor that flows 300 cfm because they flow them at different dP's. In order to correct them to the same depression, you would use the following formula:

Flow at desired dP = Flow at advertised dP * (desired dP / advertised dP)^.5

By this logic, a 600 cfm carb can actually flow 1200 cfm, but instead of experiencing 0.74 psi dP, it would have 2.96 psi dP, which is a huge restriction on the intake side.

To drill down deeper still. . . restrictions affect power differently on the intake versus the exhaust side. I dare say that restrictions on the intake side are much more detrimental to power than those on the exhaust side. For instance, if you experience a 5 psi loss as the air passes through the intake plumbing, then the power will be roughly 2/3 of what it could be with a better inlet system. That could be 100 hp loss in a 300 hp engine. However, adding 5 psi exhaust backpressure to an engine will not have such a dramatic effect. Lingenfelter's rule of thumb was to keep exhaust backpressure under 5 psi. Some stock street exhaust systems cause 10-15 psi backpressure, while turbo's cause 40 - 60 psi. If your camshaft has alot of overlap and exhaust reversion into the intake port is a concern, then a little exhaust pressure can be very detrimental to power, so much less than 5 psi would be best.

Mike

tee-boy

iTrader: (8)

"So, if your engine draws in 1000 cfm, then 1200 cfm converters will cause a restriction." Since CFM is CFM, you are making this statement based on argument that converter flow is measured at different pressure than say throttle body flow?

Formula_Power

iTrader: (18)

Intake valves are also larger because there is more air comming into the combustion from the intake charge. After combustion alot of that potenal energy is tranfered to the piston and into heat so the is just less air that has to flow out of the exhaust yet another reason for the exhaust valve being smaller

tee-boy

iTrader: (8)

Wrong. The mass going in should be equal to mass going out. However, the volume is larger on the way out b/c the mass has more energy.

tee-boy

iTrader: (8)

Let's try this again..

"So, if your engine draws in 1000 cfm, then 1200 cfm converters will cause a restriction." Since CFM is CFM, you are making this statement based on argument that converter flow is measured at different pressure than say throttle body flow?

3.4camaro

iTrader: (1)

it MAY be a restriction because the air going out has been made hot, giving it a larger volume. So a cubic foot of air coming in has more mass than a cubic foot of air going out because its hotter.

tee-boy

iTrader: (8)

good point. so if mass is equal in and out, and volume is greater on way out, there will be more volume out needed in order to expel all of the mass. That makes sense.

3.4camaro

iTrader: (1)