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Why rejet after exhaust change?

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I've heard people rejet after an exhaust change, some after just changing the tail pipe.

My question is why would it be necessary?

I understand the physics behind it (I think). The engine is basically a pump so if the exhaust is freer flowing then it will be able to expel it's exhaust gasses easier = less pumping resistance or loss.

Also it would mean that slightly less exhaust fumes will be left in the chamber after the valves close which would mean that the chamber is nearer atmospheric pressure as the inlet opens.

The vacuum may now be slightly higher earlier on in the stroke so it could pull more mixture in.

This mixture is still metered by the carb but the flow may now be slightly higher than it was before (if the above is true).

The carb is still controlling the mixture so would just supply a little more of it.

So I don't understand why a rejet would be needed for any modification made to the engine. I can understand why it may be needed for altitude, air box or filter mods though, just not for anything after the carb.

Can anybody enlighten me?

Cheers, Rich.:thumbsup:

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because now there is so much more air flowing through, the jets arnt big enough to supply that extra amount of fuel to go with the air

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okay, so....

Normal set up and we'll say the carb is at a set position eg 1/2 throttle. You have a set sized orifice. Fuel comes out of the float bowl through the main jet mainly due to venturi effect?

It's the air velocity across the jet which sets fuelling rate and is limited by the needle valve.

So then you fit an after market pipe which increases the flow through the engine. At the same conditions as above this increased flow rate would generate and increased flow velocity which should draw in more fuel. That is unless that jet is already at the max fuel flow for it's needle height.

If so I understand:ride: - if not I don't:banghead:

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I'll take a stab at it seeing I took industrial instrumentation. The relationship between flow through an orifice and pressure applied, or this case lack of pressure(vacuum) at the orifice is not a linear one. For arguments sake lets say your flow increased 2x at a certain opening of the throttle. This doesn't result in twice the amount of fuel.

As an example,

Vacuum Flow Through Orifices

air flows in SCFM (Standard cubic feet / minute)

Orifice Diameter (In)" 1/64"

Vacuum in inches of mercury(difference flow through orifice in brackets)

0" .000

2" .018(.018)

4" .026(.009)

6" .032(.007)

8" .037(.005)

10" .041(.004)

12" .045(.004)

14" .048(.003)

Even the pressure drop versus flow through a venturi isn't a linear one if you want to take it a step further.

Edited by D0T-C0M

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Yeh, I'm with you now.

I just needed to read up a little on venturi effect to get my head around it. That fact it's the intake vacuum created by the venturi which draws the fuel in and not the air velocity across the jet.


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