the bummer is that it is pretty much impossible to intake tune without an A\FR meter equipped Dyno....

the good news is that i do have a dyno... :) ....and i'm all for sharing info with you guys....

the topic has been touched on before, but there is a BUTTLOAD more behind intake tract tuning than what CFM an intake flows..... velocity and relative straightness are very inportant.... but length and volume are BIG factors that need to be fine tuned once the mass cfm requirements are met....

the intake tract\ tuning is dominated by harmonic pressure phenomena..... reflected pressure waves within the system..... primarily caused by the opening and closing of the intake valves.... and as waves are known to do, they travel back and forth within confined spaces and reverse their pressure sign when they meet change in the volume they travel in.....

at some rpm you can have the wave travel many times through a carb within each engine cycle...and that would effectively over carburate with a rich mixture, since the carb doesn't care which way the signal is traveling, or if it has already been through the carb body once or twice..... every pass of a negative pressure will get more fuel added to it.....

and some rpms will see the intake valves opening while a wave is traveling towards the air filter...and creating a negative pressure (vacuum) in the intake tract @ the exact opening of the intake valves..... which means there will be a delay in the influx of the intake charge since the downward traveling piston will have to move far enough create a vacuum greater than the vacuum present in the intake tract before any cylinder filling will begin......

the best tuning situation of all occurs when the prerssure wave is holding a slightly positive pressure of correctly mixed fuel\air just behind the valves at the moment of valve opening...... a small "boost" pressure if you will... pushing in fresh mix before the piston is creating any of its own vacuum....

even more magical would be, if , at that moment of cam overlap (exhaust valves not quite closed\ intake valves just opening), the exhaust system was delivering a vacuum..... so a pull from the tail of the exhaust event mated with a push from the intake tract....pure magic in terms of engine effectiveness!! and BTW.... exhaust pressure pulses, both positive and negative, find their way into the intake tract...and add another level of complexity to all the pressure wave phenomena....

all sorts of factors influence the intake tuning..... since the engine isn't a bunch of separate components, but rather the sum of all its parts.... :
air box...filter...intake..carb size...cam selection... porting...compression...bore size...exhaust system..etc..
all of those can, and do, affect the tuning of the intake tract ..... easier to build your engine component package completely...and then tune in the intake at the end......

and..one intake may be just perfect on one engine setup...and terrible on another... gotta test to find out...

anyways... my current YFZ setup has shown a VERY dramatic improvement in A\FR & power graphing.... i have seen bigger changes in power... but NOT in A\FR

and these are the SAME intake....with the only difference being overall length being shortened by 1.25"...and it has worked the same with cam & displacement changes since it was modified...! nice...

first graph is from the first tuning set of modded yfz with the stock cams... the only difference is the length of the intake tube....

while the jetting would still be in the normal\ fine fluctuation range on the un-cut version... pretty normal for carbs, and if i can keep A\FR fluctuation within 1~1.5 points i'm usually satisfied....

but what IS going on other than a\fr jetting, is negative pressure wave timing hurting cylinder filling effectiveness....once the intake was shortened, everything became better timed & jetted....

blue is with full length intake
red is intake cut by 1.25"

Click to view attachment

and the rest of the jetting\ graphing has been perfectly consistent during the rest of the testing

Click to view attachment

what a good read.. i sure learn alot from your posts. thanks for all your testing and teaching

Whats the OEM rev limit at on a stock YFZ?

From looking at the graphs, the shorter intake does better at 8K or better RPMs. The original has a more consistent line up to about 7K RPMs.

Just my 2 pesos!

Good read!

i think the stock limiter is hitting at 10,500 rpm

the shorter intake is the red line in the first graph.... and the blue line only bumps above the red line in a couple points on the scale....

both lines in the second graph are with the shortened ESR...

thanks for the thanks....!

always glad to share

Yup!


I think I have mixed emotions on this mod!

For the duner/drag racer...........no brainer!

Great info John!

Amazing how the exhaust expansion wave and the intake wave can really help VE!
I have been doing some research on this myself.....good stuff!

Ride the wave baby.........first incoming wave is to long Ill wait for the second wave, lol.

yep steve..... plenty of waves coming along... you can time intakes for which "order of wave reflection" you want...lol... sometimes distances wont work for the first reflection, so tuning for the second or third harmonic is the way to go.....

timing the wave events from intake and exhaust to arrive during cam overlap @ the desired boost rpm is a like finding free horsepower..... sort of like supercharging with pressure waves....

interesting stuff for sure...

Excellent testing as always John. I think you have found my cure in my HP graph. I've always had that dip, and I think a little cutting is in order.

Since I know at some point you'll read this...

Call Me Big Guy!!

Thanks for the great info.. I am new to this forum and only joined because Teresa told me you post all your information on this site. I have spent about 2 weeks reading a lot of the posts and can't believe how much I have learned. I plan on sending my stuff to you real soon to get ready for 08 D14 hare scrambles. I know you are probably real busy and it would take more typing but you might benefit from posting some of this info. on Michigan Dirt Slingers also. Hope I can get that much power for my YFZ. Keep up the great posts.

As always good info John, now if we all had a dyno and spare parts!

i have always called you eric... or terseJr....but if you prefer to be called Big Guy....Big Guy it is...!

bump... just because the A\FR lines are SOOOOOO perfect for a carbureted machine... :)

2 B pinned