Extra Filter => Better airflow == Rejetting the carb ???

[peterpsg]

I've just ordered one of these for the GTR, which has a 750 holley on a 383ci SBC.

www.knfilters.com/search/product.aspx?Prod=66-1401XP

The guy in the shop commented that I'd probably need to rejet the carb on the rolling road to deal with the extra airflow!

Is this realistic - I can see the argument but...???

Cheers,
Pete

[stig]

I ran a XStream on the GTR and have one on my Can-Am. Doubt if you'd need a re-jet though???

[peterpsg]

I just spoke to Peter Knight, who built the engine, and as I suspected, the carb will self meter the extra fuel required... more air = more suction in the venturi = more fuel, so just a false alarm caused by a salesman with only a rudimentry knowledge of carbs!


Stig, did you go for the shiney chrome surround one, or the blue one?

[stig]

peterpsg said:

I just spoke to Peter Knight, who built the engine, and as I suspected, the carb will self meter the extra fuel required... more air = more suction in the venturi = more fuel, so just a false alarm caused by a salesman with only a rudimentry knowledge of carbs! Stig, did you go for the shiney chrome surround one, or the blue one?

I hada chrome one in the GTR but the Can-Am one is black?!?

[peterpsg]

stig said:

I had a chrome one in the GTR but the Can-Am one is black?!?

Gutted! Here's the one (blue) you really want for your car!
www.knfilters.com/search/product.aspx?Prod=66-1401XB

Guess you can always paint it to match!

[thevet]

I am running one of these filters on my GTR, but due to all the changes that have gone on with the engine I can't say if the filter made the engine need re-jetting. I've set up an air fuel ratio meter running to a lambda sensor in each exhaust bank which has enabled me to rejet my carb which was a 750 Holley into a blown 406 and so far have increased the primary from 70 to 78 and the secondary from 80 to 88. It really was so easy that it defies belief. That's not to say that it won't all go wrong on the track.............dry days are coming though the weather forecasters have promised, just like they did for the winter of the century.

[eliot]

Well given they all seem to run pig-rich anways, I wouldn't worry about it. Your'e probably only talking about a couple of percent difference. Do a plug chop at WOT or get your self a wide-band (actually dont get a wideband, because you will want to dump the carb shortly afterwards!)

[GreenV8S]

Not for a filter change, surely. I thought the main need for rejetting was when the pulse characteristics have been changed eg from a cam change or something affecting the exhaust back pressure. Something that just reduces the upstream restriction slightly without changing any of the other characteristics, you wouldn't expect to affect the carb would you?

[trackcar]

Well the fuel flow is affected by the local air pressure varying with air speed squared up to the point at which the jet won't physically flow any more fuel .. so if anything the car would run richer (all else being equal) unless the jet was limiting the outright fuel flow already?

If we assume that the less restrictive filter then reduces any non air speed related pressure reduction downstream (assuming float chamber vented to atmosphere and not to the filter casing) then you would expect both effects to be roughly self cancelling ..

so the short anser would be "no".

I think.