Hitchcocks Forum

Colleagues, I've got a techie question for you. Why does changing the exhaust system require you to rejet the carburettor, or remap the fuel injection system? I don't mean a vague, hand-waving explanation, I mean exactly why? As I see it, the carburettor and the FI don't have any direct "knowledge" of what's on the other side of the exhaust valve. All they know about is how wide the throttle is, and how hard the engine is sucking on the inlet stroke. Their job is to measure the amount of air going into the engine, and make sure that just the right amount of petrol is mixed with it to give the correct air-fuel ratio. So, even if a free-flowing exhaust system allows it to "suck" more air in (because of better exhaust scavenging), shouldn't the carb or FI system automatically add more fuel accordingly? The only reason I can think of is if the FI system doesn't have any way of measuring the amount of air going in (i.e. an air mass sensor), but just uses a throttle position sensor, from which it extrapolates the amount of air going in. Does the EFi have an air mass sensor (typically hot-wire sensors on cars, these days)? Can anyone explain why changing the exhaust often requires a change in the jetting/mapping?

In a nutshell, the jets in carbs can only pass a finite amount of petrol dependent upon the size of the hole in the jet. More air flowing through the throat of the carb requires more petrol to maintain the approx 14.1 fuel/air ratio at any given throttle position. Bigger jets, ie.bigger holes, allow more fuel through to maintain the correct mixture of fuel/air.

oh boy, here we go again.

norm,............. norm come in please.

Here's a man who knows how to kick a hornets nest, don't you Thack?.

I expect many varied answers to this one a few of which may make sense, most probably won't!!!!!.

What is going on here, the following paragraph dissapeard from my last post.

A carb as you say only knows what is going in and any increase in airflow will gave a corresponding increase in petrol flow, in over fifty years of riding and maintaining what are now called "classic bikes" I have never had to make any carb adjustment when changing an exhaust system.

Mauri, when I see big long posts like that I usually don't bother reading them.

Getting that way myself Norm, life's too short

But Norm, think of all those wonderful contributions from me you'll be missing!

Induction, compression, power, exhaust. Why would any change in the exhaust need to be compensated for in the induction mixture? The inlet valve has long since closed and the mixture burned before the exhaust is expelled into whatever resistance the pipe offers. Only thing I can think of is it may run hotter in a free flowing exhaust and therefore needs a richer mixture to cool things down. Perhaps I miss understand.

Back pressure and gas flow are I believe the fundamental issues with the change in exhaust systems. Suffice it to say the exhaust system is as fundamental to the ingestion of air/fuel as ones own terminal orifice is to our ability to chuck fried egg and chips into our intake portal.