427V8

While I know it happens, I don't understand why changing the exhaust on my car would affect the jetting on my carb.
( it's been a while, but the carb gets richer with better flowing exhaust, right)

People tell me it's because the engine breathes better.
To me this means there is more airflow through the engine.
How could more airflow change the mixture of the carb?

As I rev my engine the carb keeps the mixture the same even though the higher I rev it the more flow it sees...
So how does the carb 'know' the difference between reving the engine and changing the exhaust??

The only thing I can imagine is that with poorly flowing exhaust not all of the combustion gasses are extracted from the clyinder, and not all of the oxygen is consumed by combustion.
So the next charge has a fraction of unburnt gasses in the chamber and more fuel is needed to keep the mixture constant?

Maybe I'm thinking too much :crazy:

moosie982

Freer flowing exhaust = engine running leaner,dont overthink it. :seeya
Peace, Moosie

UKPaul

I used to know this, but it's been a long time....
I think it's because during the overlap period the exhaust, being (usually) less restrictive, allows more scavenging of exhaust & unburn't gasses. More charge is then required to fill the cylinder on the next intake and the carb needs to be able to supply just a bit more fuel to keep the mixture correct. It may also be because some of the intake charge also exits via the exhaust during overlap, but it's all a bit too far in the past to remember (but I'd love to be reminded!).
:cheers:

V8yunkie

The engine "breathes" easier; E.g. with the bad exhaust at 500 rpm it flows 500cfm of air, with the better exhaust, it flows 600 cfm.

To retain a constant, optimum mixture of fuel and air with the better exhaust, the carb should bring in more fuel to more air. Therefore, main- and/or secondary jets and/or metering rods (on the Q-jets and Carter-carbs) should be adapted to enable the carb to supply more fuel.

In some cases, that might not be necessary for a simple reason: The carb setting was too rich from the beginning (already with the bad exhaust). In that case if bigger jets come in, it even might have an adverse effect: The mixture is too rich again which results in a "lazy" (and smelly) engine. People then usually say: Well, that exhaust did not bring anything in...

But given that the mixture was OK with the bad exhaust, the good exhaust needs requires more fuel to maintain the good mixture.

Thomas

robzr

OK here are my (not guaranteed correct) theories on this.

First, if you gain a more efficient exhaust system, then by the time your intake stroke comes around, there will be less pressure in the cylinders. This translates into more intake vacuum. Intake vacuum determines the speed of the air through the venturis at a given throttle opening, as well as the air density (key here! this would be why the proportions change). Speed throught the venturis dictates the magnitude of the low pressure zone, which creates the pressure differential, and therefor the speed/force of the fuel entering the intake stream. I think this would have the effect of richening the mixture (when going to a freer flowing exhaust) slightly, but an effect nonetheless.

Also, when the intake stroke begins you will have some residual exhaust left in the cylinder. The intake charge (a/f) will be diluted by this already-burnt gas, therefore leaning your mixture. When you go to a freer flowing exhaust, their will be less residual exhaust left, therefore less dilution of the intake charge. This would have the effect of richening the mixture as well.

Yet for some reason when you go to a freer flowing exhaust you need to richen the mixture. So apparently my theories are wrong. Oh well :)

Rob

Pete79L82

You are all close, but forgot one consideration.

Air/fuel ratios are calculated by weight not volume.

With a good exhaust system all the burnt(hot) gases are scavanged from the cylinder. They are then replaced by fresh (cooler) intake gases. Cooler air is more dense (heavier) then hot air. So to correct the air/fuel ratio it requires you to richen the carb.

But then again, maybe I am the one full of hot air :D

Jon69L46

So does that mean you always have to rejet. What are the obvious symptoms, because ruining the coating on the new headers.

I will be going to 1 5/8 headers and 2 1/2" exhaust soon. How would I determine what is needed.
Thanks

Double Aught

I am no expert here, but when I smogged my 78 L82, (it has heddman hedders, hiflow cat and 2chamber FM), the tech had to fiddle with the carb to get it to pass. He said there was a lean condition inside the carb. After reading all of these posts, it would seem to me that a free flowing exhaust will lean out a carb. With that said, it is going to the Dyno shop on Monday for them to trick out my carb and ignition.

UKPaul

Umm, as more air is flowing through the carb, then maybe the jets can't supply enough fuel (they're only metering devices at the end of the day)? But, open pipes definately cause a lean mixture (assuming it wasn't over rich to start with). Sometimes a 5% increase in main jet size is all that is required, or maybe raising the needle one position, or a different size of needle or needle jet (depending on what type of carb it is). Pistons have frequently been holed in high performance engines due to exhaust changes without the required carb mods.
:cheers: