ITBs and Tubocharging. Issues?

[RarlyL8]

I have seen a few high end twin turbo jobs with individual throttle bodies intalled. I have also read that it is not a good idea to have ITBs with turbocharging.

What are the dynamics involved? The high end shops have done it; were they wrong to do so and why?

[911teo]

subscribing...

[RarlyL8]

You haven't subscribed to much just yet. Ha!
I'll give 'er a bump for the West Coast.

[Rob Channell]

Here is my guess and I'll wait for the smart people to jump in and tell me what I don't know.

Here is my basic hypothesis. "Seems to me if you can figure out how to get a constant 1 bar boost to the outside of the throttle body that the throttle response will be just as good as a normally aspirated ITB setup."

I read a comment saying it(spending $$$ on ITB setup) was a waste because the throttle response is going to suffer due to the long path from the intake to the throttle bodies. The air has a longer path to get to them and the power will only come on when the boost has risen. This could be a moot point if you are keeping the revs up and maintaining boost pressure.

I then started wondering about a two stage boost setup. The turbo boosts the air pressure to say 5 bar or maybe even more. This air is in a large pressure vessel(whatever spare room you can find). Then there is a pressure reduction to maybe 1 bar or a little over of boost. This is the air pressure that is maintained through the intercooler and after, all the way to the ITBs. That way when the ITBs open up there is instant 1 bar boost to go. This would probably only give you a second or two of boost, but maybe by then the turbo would be picking back up.

It's probably a stupid idea, because someone would have already designed a system this way. Maybe the large primary chamber takes too long to get presurrized the first time, making even more turbo lag....

I'll shut up and listen now.

[930turbo]

I don't think there would be any advantage to isolated runner or ITB intake design with a turbocharger. In order to keep the boost up between gear shifts and reduce the risk of compressor damage, you need a source of vacuum to assist in opening a pop-off valve(recirculating in the case of CIS) . The only way to accomplish this is to access individual sources from each isolated runner down-stream of the throttle bodies and tie them together before the valve. Your efforts would probably be better spent in another area of the engine build. The common plenum design is tried, true and effective. The idea of a reserve boost accumulator probabaly wouldn't work due to the consumption exceeding the reserve upon initial throttle deflection. The volume necessary to be effective would be so large it would take a noticable amount of time to recharge the tank. I believ all it would do is delay the lag to immediately following the surge from your reserve boost - followed by tank repressurization lag. Just a thought.

[RarlyL8]

I have seen ITBs on high end twin turbo conversions. There must be something to it. My thinking is that ANY reduction in the length from the throttle body to the intake valve is a good thing, even in a turbocharged application.
I would like to know if there are any negatives to using ITBs with a turbocharged system. I think Goran (BeepBeep) touched on the subject somewhere in his It'sFun2 thread, unfortunately I'd rather not read 1000 lines of posts again to find that sentence.
By negatives I am thinking about such things as "how do you bypass the throttles during closed-throttle events" and "is there a signifcant air flow interruption in the design needed to house these ITBs in a turbocharged application".

As far as having a continuous 1bar of boost available at all times - that would be a good job for a supercharger. I could see some REAL possibilities there.

EDIT - Looks like one of my questions was answered while I was typing. Throttle bypass would be a problem.

[mnmike]

you see the E36 M3 (95-99) turbocharged a lot. Id find one of the companies that does those, call and ask. They could explain it pretty well i would think.

[Jim2]

Think back to the days of ITBs on carbureted applications, side draft/ down draft webers etc. At partial throttle openings you get a higher inlet charge velocity (down-stream of the butterfly) which gives more mid range torque due to ram effect of the charge, whereas a single throttle body at the end of a plenum sees the entire column of charge in the runner moving very slow when at low and mid power demands.

However, if you look at old aggresive race cars with big power (individual runner style) you will see very large diamerter butterflys, this was not only for accomodating a large choke, but also to diminish the restriction which is caused by the butterfly and shaft at WOT, but then catch 22 comes into play and mid range torque drops off due to the large runner and butterfly dia. The trick equipment used slide plate throttle bodies which eliminated the butterfly and shaft, but kept the throat diameter a bit tighter sustaining a higher inlet charge velocity.

To this day I remain surprised that this concept of an accerlerated charge works because a partially open butterfly seems like an extreame turbulence causing restriction, but it does works.

Get out your credit card and order those TWMs!

Jim

EDIT, I have never heard of a "downside to ITBs" when going turbo. Just a myth like lots of other crap out there.

Now for a myth rant with regards to aspirating the inlet ducting on a turbo application, get out a claculator and compute the inlet side volume using some rough diameters and tubing lengths, and say your intercooler is about 50% volume for the charge core, or heck, just use 100%. Now divide the pumping volume of your turbo engine's displacement and see how many revolutions (cycles) it takes to move one inlet charge volume, then calculate the amount of time it takes to move this inlet volume at say, 3000 rpm. I bet the reults will be 100ths, or perhaps 10ths of seconds, certainly less time than it takes to remove your foot from the gas pedal. end [myth rant]
Jim

[Garen]

Call TWM induction in Goleta, CA, and ask. They have two different setups for 911s, and also have many non-Porsche applications as well.

And, you are right. Many $$$ setups do run them. Pat Williams has a complete twin-turbo build on his website with some trick ITB's, the 935/956/962/911GT1 all run ITB's, and so do Champ cars (actually, I think I remember reading somewhere that Champ cars have ITB's, and also a single TB at the plenum - sounds weird, but I remember reading it somewhere).

Downside, too much $$$, possibly hard to get a consistent vacuum signal (poor balancing between each cyl due to inadequate balance bar capabilities), TPS setup is more difficult, and in the case of our cars, no known "bolt-on" setup like the 3.2 Carrera intake.

FWIW, I am considering a custom setup using some old 911S ITB's and some custom injector blocks... $$$ permitting...

-Garen

[TonyG]

The advantage to ITB's with turbo charging are to a large degree the same as with a N/A car, which is.... throttle response.

The better the throttle response, the more power that is being made at a given moment in time in response to a throttle movement.

The more power that is generated at that moment in time, the more exhaust gas generated.

The more exhaust gas generated, the faster and more resonsive the turbine (apples-to-apples).

In short, ITB's in a turbo application will make the turbo more responsive off-boost and come on boost sooner.


TonyG


PS> Just went to ITB's in my 3.6L 911SC with TEC3r because I'm going to be turbo charging myself real soon.

[TonyG]

RarlyL8

>>>By negatives I am thinking about such things as "how do you bypass the throttles during closed-throttle events" and "is there a signifcant air flow interruption in the design needed to house these ITBs in a turbocharged application".
<<<

I'm presuming that you are refering to a CBV (compressor bypass valve).

You don't bypass the throttle bodies. You bypass the compressor and intercooler.

ITB's have nothing to do with this since the bypass is not behind the throttle plates.

The vacuum source is built into the throttle bodies (which is exactly where you get your MAP source).

It's a trivial affair...

TonyG

[RarlyL8]

That's right. I was thinking in terms of the mechanics involved in the throttle bypass valve - they are not the same as with the blow off valve.

The direct question I have is "How do you handle pressure equalization in off-throttle events when ITBs are used?"

[Jim2]

The direct question I have is "How do you handle pressure equalization in off-throttle events when ITBs are used?"

Do you mean vacuum equalization? example: obtaining a good vacuum signal from down stream of the butterfly when off throttle?

[TonyG]

The throttle bodies, in the case of the TWM 3003, have 1 vacuum port per bank. Each 1 vacuum port is connected to 3 cylinders.

You would then "T" them together and run them to a CBV. Most aftermarket CBV's are adjustable, meaning that you can adjust the point at which they open.

A bypass valve is the same as a blow-off valve, with the exception that one does not vent to atmosphere. Instead, it simply "blows off" to the turbo inlet pipe.


And when it is open, under all light load conditions, air is drawn in the opposite direction, into the engine through the CBV (around the compressor, intercooler, and associated plumbing).

In short, the blow-off as well as the bypass operation in an ITB engine is performed exactly the same as a common plenum/single throttle body situation. No difference.

TonyG

[RarlyL8]

Makes sense. My only experience with turbo systems is the CIS that my engine uses. I guess that for a non-CIS system the return pressure would not have to be recycled to the turbo inlet but simply vented to the atmosphere.
Can I assume that all other vacuum needs would be handled the same as in a N/A motor? On my old '73T MFI engine there was only 1 vacuum port per bank of 3 throttle bodies. Any reason why these would have to be tied together or would they work the same pressurized as they would when normally aspirated?

[TonyG]

RarlyL8

>>>Can I assume that all other vacuum needs would be handled the same as in a N/A motor?<<<

Not necessarily all. The fuel pressure regulator might have to be different because you have to raise fuel pressure as you raise boost. Also, the vacuum assist on the brakes will need a check valve (at a minimum) so that you don't pressurize it.



>>>On my old '73T MFI engine there was only 1 vacuum port per bank of 3 throttle bodies.<<<

Same deal on the TWM 3003 series throttle bodies.


>>>Any reason why these would have to be tied together or would they work the same pressurized as they would when normally aspirated?<<<

What do you mean "tied together"? Do you mean both left and right banks? Or do you mean all 3 runners per bank?

In any case, they all need to be tied together so that you get a "smooth" signal to any electronics (such as a MAP sensor) that are measuring engine load (you need all 6 pulses to be smooth), and so that you can get enough vacuum to operate your power brakes, and if you have an IAC (idle air controller)... so you can feed air into the system equally to all cylinders.


TonyG

[RarlyL8]

So all ITBs should be tied together for a common vacuum signal. How does TWM do this? What do the high dollar tuners do about the brakes on a 930? That's a pretty large ID hose, maybe 1/2".

[DW SD]

Originally posted by RarlyL8
So all ITBs should be tied together for a common vacuum signal. How does TWM do this? What do the high dollar tuners do about the brakes on a 930? That's a pretty large ID hose, maybe 1/2".

I don't know for sure. I would guess: vacuum accumulator tank fed by common vacuum signal as Tony suggested. Even the 993 3.6 has an accumulator tank. On the '95 it is under the spider manifold and about the size of a can of baked beans.

I've seen electric vacuum pumps w/ a small accumulator tank, too.
Good questions.

Doug

[TonyG]

RarlyL8

>>>So all ITBs should be tied together for a common vacuum signal. How does TWM do this?<<<

All 3 runners are tied together internally within the casting. Then a simple port nipple taps in.

Here's a pic which shows exactly how all 3 ports are tied together:



...
...

SHOWN WITH THE VACUUM PORT HIGHLIGHTED


....

FROM ANOTHER ANGLE




>>>What do the high dollar tuners do about the brakes on a 930?<<<

The same as everyone else! DW SD said it....

You tap the same vacuum source. But you have a check valve installed so you don't pressurize the vacuum booster.

Also, if you don't have enough vacuum to power the brakes, or if you want to have a "reserve" of vacuum, you have a "vacuum resovior" T into the line, behind the check valve. This sort of thing is standard issue on turbo cars since they can only build vacuum when the car is off boost (actually.... you can use a venturi to get vacuum on boost, which 944 turbos do... but you don't need it and it's often removed).


In the case of my 911SC with the 3.6L and the TWM ITB's, I have found that there is enough vacuum w/o the resovior (tapping into the vacuum source shown in the pictures).


TonyG

[TimT]

On this engine we used ITBs with 2 injectors/cylinder. For vacuum we made a small manifild that sits on top on of the breather tower on the engine case. This manifold accepts a vacuum hose from each barrel (6) and has a port for the power brake hose.

Typically you are only braking when off throttle, butterflies closed, therefore high manifold vacuum for the brake booster

If you look closely at the pic under the FPR you can just see the manifold and brake booster hose connection. I never took a pic of the manifold seperately