Intake tuning for your quads

[rthy]

By request I am posting this and I know its over-simplified and uses generalisatons but here goes anyway. Also of note these sort of calculations should only be used as guides, nothing beats a dyno.


Basic pricibles of harmonic resonance in the intake:

When the intake valve of an engine opens a low pressure wave travels up the intake untill it hits the end of the trumpet/plenum entry and will send a high pressure wave back down the pipe. During which time the engine has gone though its combustion. If the pressure pulse has taken the right amount of time it will hit the intake valve just as it is opening sending a possitive pressure wave into the intake of the engine. This however will only occur at its greatest at one point in the rev range (variable cam timing exempted). If an engine is at the right rev range for this to happen it is having a harmonic resonance in the intake.

To adjust the rev range at which the resonance will happen the length of the intake can be changed, for owners of 4a engines with 20v throttles this is relatively easy. Basic theory is that the longer the trumpet the longer it will take the pulse to reach the end and come back, this will work for a lower rev range because the engine at lower revs will take longer to go through its 4 strokes and hence the time delay needs to be longer to get the right resonant frequency.

So as a rough rule:

Longer trumpets than standard = more of a low end power boost
Shorter = more of a top end power boost

all in theory anyway


This can be worked out with the graph or forular as shown on Mr Accoustics webpage:



Standard 20v trumpets are not equal lengths however the rough tuning point is 5000 rpm according to Mr Accoustic

Also fo note having a harmonic resonace does not mean the maxim power will be there, the best way to think of it is it shifts your power curve in either direction depending on your tuning.

Just for the record F1 cars use this princible to the max and have variale length trumpets, and from what I hear they often have possitive pressure inside the chamber: ie boost without air being artificially pushed in.

I will try and add more information shortly when I have more time.

[ae86 drifter]

good write up sam im interested to see what else u can find out

[Gilly]

sounds like i want long ones for daily driving and short ones for track work

indeed a good write up Sam and something i had heard about but never looked into.

[dr1ft-pig]

by the looks of this i think i will go with shorter trumpets, will experiment when i got the cash for dyno runs and more trumpets

[Oly AE86]

Principal works indeed.

My Original trumpets inside and airbox were very short about 20mm and top end was Nuts, but bottom end was lacking.

Current trumpets are about 70mm long and bottom end has increased, definitely better up hills and driving the streets. More practical for day 2 day.

Oly

[riojin]

so for a worked late model 7afe with ITB's shorter trumpets are better?

i saw one time in a street machine (i dont read the bogan mag much anymore) a 351 with ITB's but the trumpets were all different lenghts. how would this work, would it have much potential?

it was a show car and i assumed it was just for looks

[Konakid]

Very good post, have always thought about how the length would alter the power band.

[Gilly]

i'd be interested to see pics of peoples setups, especially the longer items as there isn't a great deal of room on number 4 throttle due to the clutch master being in the way. I remember when Biggo's trumpets were being made the last one had to be made shorter

does inlet diameter/taper mean anything Sam?? i also remember reading something about the bellmouths themselves having an effect, with the lips that curl right over them selves being the best choice of all (much like the T3 items)

[starni_boy]

from what i have read, the bellmouths that curl right round themselves (eg. T3 items) have a less restriction for air travel up the throttle bodies.

in a drift battle magazine, it has a full article on 4age engines, i have it here somewhere, and it talks about the trumpets..

[rthy]

wow this topic got a whole lot more interest than what I expected. I am a bit short on time right now to reply properly but I will see what I can do tonight