generic question about ITB's

Uncle Bob · Post by **Uncle Bob** » Tue May 16, 2006 7:28 pm

muythaibxr wrote:Speed density, depending on the size of the throttle bodies, will give you the best control of AFR at lower throttle openings. Once you open up more though, it may not give you the best control, as the MAP reading will only change by 1 or 2 kPA once you open the throttles a certain amount.

This is why some people go to alpha-n. Alpha-n has no contribution to the amt of fuel given to it by load... so unless you're always driving on a level surface, the AFR will not be consistant at low throttle openings (and to a point even at higher throttle openings.)

I'm working on tuning with a combination of the two. I tune with SD below about 20-25% throttle, then above that, I switch to alpha-n. I did this with some code I wrote for msns-extra. When I have it tuned and I'm happy with it, I'll let everyone know the results.

thank you, this is some good info

Jim · Post by **Jim** » Wed May 17, 2006 1:18 am

Why be normal ?

The only difference between Bikes and Cars is that the Bikes do
everything twice as fast and half as big .

Take a look at what some of the Bike Mfg. are doing with fuel injection ,
Huge tuned plenums with long runners , they don't do this just to
keep the intake noise down , it makes power .

...................Jim

Jim · Post by **Jim** » Wed May 17, 2006 1:30 am

Muythaibxr ,

The reason that the MAP changes so little is because there is very little
change in the volume of air flowing into the engine , this is most obvious
when an engine has an oversized throttle body , the last half of the throttle
travel makes little or no change in flow , you could completely remove the
throttle butterfly and would not get any more air flow so adding more fuel
without a coresponding increase in air will just make the engine very rich and
you will actually loose power .

...................Jim

whittlebeast · Post by **whittlebeast** » Wed May 17, 2006 5:12 am

Most likely the reason that dragrace motorcycles get away with having the turbo before the throttles is that they realy don't lift on the throttle to shift. The issue only comes up when the turbo, is at full rpm (say 175000 RPM) and the throttle, that is down stream of the turbo is slamed shut. This problem gets even worse when the waistgate is plumbed to the back side of the throttles and the waistgate can not even help blead the exhaust. At this point the real problem is in the momentum of the turbo wheel and the compressed air has nowhare to ge except back thru the compressor. I would think that suddenly lifting out of the throttle on a 500 hp motorcycle at 160 MPH is also one of those things that is avoided. Like dragracing in a driving rain with slicks, they dont have a real feel for hydroplaning like the autocrossers do.

AW

Karl_Skewes · Post by **Karl_Skewes** » Wed May 17, 2006 9:15 pm

SD does seem pretty good. I like combining all runners and using dampener in the map line to give a good signal.

vacuum leaks will really mess with itb setup though.

you don't have as much vacuum to work with, this is a little like cams, but cams only make a sudden ve jump with rpm.

however, as muythaibxr said, you end up having map rows 2kpa apart and if you haven't got your signal well conditioned your afr won't be very stable.

I like the idea of speed density/alphan hybrid of some sort... maybe on my new motor with itb's and 300 deg cams?.

Uncle Bob · Post by **Uncle Bob** » Wed May 17, 2006 9:23 pm

guys....I'm not asking for turbo theory. I appreciate the by-play, but really, some of you are suggesting stuff that is kinda silly. Blow thru's aren't uncommon in car land or bike land, so I can promise you, its been well hashed. My bike currently is a blow thru ITB carb setup, so you'll have to tell it how badly it works so it stops acting so normal-like

As for long runners on bikes.....haven't seen a long intake track on a bike in 20 years. Which one are you refering to? Air ram systems? Thats a whole nother subject. Most high performance bikes these days have radically short intake tracks...sorry, air ram ducting doesn't count as part of the intake track

So if you are making a ITB turbo blow thru setup, and you have to deal with a very large range of pressures from idle to say 30psi of boost, wouldn't this compromise the necessary fine tuning around 100KPA ?

Karl_Skewes · Post by **Karl_Skewes** » Wed May 17, 2006 9:33 pm

come to think of it, depending how much power you're pushing and what type of ignition control you require, you may find that tuning with big injectors at very low load to be a little troublesome.

You can either just run it a little rich (12:1) and live with it at these points, or use ms2 chip for extra res....

Uncle Bob · Post by **Uncle Bob** » Wed May 17, 2006 9:48 pm

Karl_Skewes wrote:come to think of it, depending how much power you're pushing and what type of ignition control you require, you may find that tuning with big injectors at very low load to be a little troublesome.

You can either just run it a little rich (12:1) and live with it at these points, or use ms2 chip for extra res....

I was depating on that, I was thinking about running a second set of injectors ....but I'm not sure how necessary it would be. I'm aiming for pretty high output, so if it was single injectors, they would be very large.

Jim · Post by **Jim** » Thu May 18, 2006 1:36 am

There are fuel pressure regulators that have
adjustable vacuum/boost pressure compensation ,
instead of the generic 1/1 ratio they can be adjusted for a 2/1 ratio ,
this eliminates some of the problems with humongo injectors ,
especially idle problems .

This allows your fuel pressure to vary from say , 20 to 80 psi and keeps the
injector pulsewidth in an adjustable range at idle .

...................Jim