generic question about ITB's

Uncle Bob · Post by **Uncle Bob** » Mon May 15, 2006 8:58 am

I know this has been beat to death, but I have researched this a lot on this site and have found many posts but no exact detail info that I'd hope someone could expound apon.

Everyone that goes to ITB's find that speed density is difficult to deal with, and some try/goto alpha-N to ease the tuning. Some don't like alpha-N though and go back to speed density.

The question is, what problems do people run into with ITB's and alpha-N? It would seem to me, that it could be setup up very effectively....but then I consider if boost is in the equation and realise that boost will change at boost-capable RPM's drastically....is it mainly a problem with turbo applications?

I'm still debating to build my system with a single TB or use ITB's, but its a turbo application, and I'd like to find the best setup that will give me the least amount of headaches....but best performance. I know, I'm asking a lot

I'd rather go with ITB's....right now it runs pretty good with ITB carbs, but jetting with boost is a pain in the bum.....I don't want to run into a simular situation where I have to compromise the build just to get the engine to run properly....which is what I have found with carbs.

Jim · Post by **Jim** » Mon May 15, 2006 3:27 pm

The trick with Individual Throttle Bodies is averaging the vacuum/boost
signal from all 4 intake ports , some factory setups have an arrangement
for this since the stock computer had to have a MAP Sensor vacuum port ,
since you have carbs this connection may not exsist between your ITBs ,
in that case you will have to create your own vacuum/boost manifold to
average the pulses from all intake ports before connecting to your MAP Sensor .

On the other hand , yes ITBs are sexy looking , but they won't make any
extra power over a good tuned common plenum manifold when running
a blower or turbo .

The best setup is a single throttle body on the inlet of your compressor ,
this runs the compressor in a vacuum when the throttle is closed ,
this lets the compressor freewheel to very high rpm at cruise
making for quicker boost response and eliminates the need for a
blow-off valve on the outlet of the compressor .
This setup also tends to unload the exhaust turbine
which relieves some exhaust restriction at cruise and reduces temps on everything .
This "may" tend to make some turbos leak oil into the intake , ask the
manufacturer for recommendations .

Don't go nuts on the size of the TB , you will get far better driveability
and only sacrifice a few HP with a stock sized TB .

An aftermarket fuel pressure regulator with
1/1 vacuum/boost pressure compensation is nessesary .

Alpha-N is crude at best ,
and as far as I'm concerned , it is strictly for race cars .

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

Uncle Bob · Post by **Uncle Bob** » Mon May 15, 2006 5:04 pm

this is for a motorcycle, and due to spacing issues, there is no way I'll be putting the turbo next to the intake (ie, draw thru) and I wouldn't be happy with that much plenum between the intake and the TB.

The main attraction to ITB's isn't their looks (not for me anyway), but response. Properly tuned, the respond very quickly, and have a tiny bit more peak capability (which I'm not terribly worried about). And since this is a bike, its already setup for bolt-in ITB's off another bike with little to no modification.

There is also the simplicity with ITB's, since they are a complete unit, ready to go. All I would have to change is the injector size, where as a single TB would require a lot more fabrication.

But those are my specific issues....I'd still like to get more feedback about plus/minus of the two systems, and hear peoples experiences. I may make up my mind for completely unrelated reasons, but I'm sure if nothing else, this thread would be a valuable resource for others that have had the exact same question rolling around in their head

Mike_Robert · Post by **Mike_Robert** » Mon May 15, 2006 5:50 pm

Ryan, try to remember that most of the people who have had excellent experiences with their ITB installs don't post about it. I have personally been involved with in one way or another at least a dozen SD/ITB installs and they all went well, especially when the owner/tuner has an open mind on the differences between them and plenum based machines. SD on most ITBs is not only possible but probably the best solution. For the others there's MAF and for a very few others there's AN. IMHO, of course! Thanks for bringing this up for discussion.

-Mike

Mike_Robert · Post by **Mike_Robert** » Mon May 15, 2006 5:59 pm

Oh, the response thing. In my opinion, they SEEM to have better response because a small throttle opening at low rpm/load is about equal to WOT due to the usual large size of the TB in relation to the engine size. I have other takes on this issue as well but believe that there is not a huge real world "response" difference solely attributed to the fact that that they're ITBs. There's flow inertia, tract tuning, etc. involved as well. A well designed intake of any description will always perform better than a mediocre design of another type.

-Mike

Uncle Bob · Post by **Uncle Bob** » Mon May 15, 2006 6:36 pm

I won't argue that design is a major factor, but ITB's allow for very small amount of intake volume between the valves and the throttle bodies. The more volume, the longer between transition. Granted, its not much.

But I'm not trying to discuss the positives and negatives to ITB's by themselves....to me thats not even really an issue. I'm very confident I could make either work flawlessly with enough effort. Which one has the least amount of pitfalls waiting to find.....both fabrication and tuning wise....

Mike_Robert · Post by **Mike_Robert** » Tue May 16, 2006 3:52 am

Ryan, I would be inclined to think that whatever is easiest for you to fab up will work just fine. You obviously have the mindset, knowledge and research abilities to make it work, whatever "it" turns out to be. Let us know what you decide to fab up (and pics would be nice!).

-Mike

muythaibxr · Post by **muythaibxr** » Tue May 16, 2006 5:58 am

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.

whittlebeast · Post by **whittlebeast** » Tue May 16, 2006 6:27 am

Some of the responses appear to take into account that this is a TURBO application. Jim is correct in that the best why to throttle a turbo motor is before the turbo compresses the air. ITBs will be a nightmare with high pressure air banging off the front side of the throttles. A blow off valve would be required as well as a wastegate. Youe get all sorts of strange reversion going on in the turbo on shifts. With a turbo application, a MAF system (Ms1-Extra only ant this point) or SD are your only two options. AN is just not a good choice.

Hope this helps

AW

Uncle Bob · Post by **Uncle Bob** » Tue May 16, 2006 7:27 pm

whittlebeast wrote:Some of the responses appear to take into account that this is a TURBO application. Jim is correct in that the best why to throttle a turbo motor is before the turbo compresses the air. ITBs will be a nightmare with high pressure air banging off the front side of the throttles. A blow off valve would be required as well as a wastegate. Youe get all sorts of strange reversion going on in the turbo on shifts. With a turbo application, a MAF system (Ms1-Extra only ant this point) or SD are your only two options. AN is just not a good choice.

Hope this helps

AW

I'm more from the motorcycle world of things, so I don't know what is common belief in the car world on this level of tuning....but in bike land, its very common (I'd say single TB is unheard of) when it comes to EFI blow thru turbo setups.

Its possible to bend butterflies, I have heard of some extremely high strung race bikes having problems with this, but its usually coupled with some other catastrophic event....but these are 1 liter engines making 500+ HP to the wheel, just to give you an idea of how "common" that issue is.....