Difference in orque/horsepower curves after fitting ITB's?

[MechaniX_034]

Hi guys,

I've been mulling over building up a new plenum lately for my 4G63T, after seeing some positive results with an SR20DET(non-RNN14 variant, which came with ITB's stock) I'd like to ask if anyone has any experience running ITB's with a turbocharged engine, specifically:

-If the throttled volume is reduced to the runners proper(allowing perhaps a longer runner?) does the plenum volume remain as significant, or does it become negligable(is "bigger better" in such a scenario?)

-What might the differences in torque and power delivery forseeably be(ie. would such a setup a more linear torque graph?) I'm not after top-end power by any means(from what I understand, this would be negligable) the goal of the design exercise is to reduce lag and to bring the boost threshold forwards while enhancing throttle response.

Thanks so much for that, gents!

[AU N EGL]

Smaller diameter runners, higher air velocity = greater torque

larger diamerter or longer runners, higher air volume = greater HP

The question is where is the optimal runner diameter size for highest air velocity AND air volume ?


Now cam shafts would also have to selected to maximium potential of the air intake size runners.

[ian_w]

The throttle bodies should give you improved transient response - i.e. will come on boost faster. When the throttles are shut the pressure in the manifold is significantly below atmospheric pressure, when you open the throttles the manifold takes a finite amount of time to reach atmospheric pressure and then on to positive boost pressure. As the volume between the engine and the throttles is a lot less with the port throttles then this filling time is greatly reduced. As well as the SR20, the RB26 Skyline engine also has port throttles.

Dependant on how you setup your boost control, at part throttle you can run a small amount of pre-throttle boost pressure. This means that when you open the throttle you will already have some boost available, hence great transient response. You should avoid going too mad with this or the throttle response will be very twichy, it also reduces your part throttle fuel ecomony ( if that is of any interest to you? ).

On an N/A engine, typically the plenum volume wants to be as large as possible to make maximum power. Its less important on a turbo as you are forcing the air in but still, the more efficient you can make the engine the more power you will make with the same boost.

Selecting a port diameter, length and plenum volume is not really something you can figure out by simple equations as its a very complex system - best left to expensive computer simulation programs. I imagine your port throttled inlet length will be less than the production plenum. It will also probably be a larger diameter which effectively makes it shorter as well - overall therefore I would guess it will tend to more your torque curve up the speed range.

A friend of mine builds a lot of Evo, Impreza and Skyline engines. A month or so back he was discussing build a 4G63 with Skyline throttles and a specially ported 'down draft' head. You may want to give him a shout - his website is below, tell him Ian Whiteside recommended you.

http://www.whitfieldraceengines.co.uk/index.html

[MechaniX_034]

Hi Ian,

Thanks so much for that. Part-throttle fuel economy isn't a concern as the car is used for sprints only, and yes, the ALS is enabled through a MOTEC P&P unit. The engine's already running an aftermarket single-throttle plenum(80mm) but I am looking for whatever I can(turbo size and engine displacement are to remain fixed, as the combination has worked very well thus far) We're looking for as much response as we can get out of it while spreading the curve over as much of the graph as possible.

I'll shoot Whitfield an email on Monday or Tuesday, and get the input from them as well.

Again, thank you so much for your reply!

[ckiesz]

I've been looking into the same thing for my 2jzgte powered project, using hayabusa throttle bodies. In therory, I would think that the runner length formulas for a naturally aspirated engine would still apply, only the number for the wave speed (speed of sound) would need to be adjusted for the higher manifold pressure.

[ian_w]

The N/A intake tuning equations will still work on a turbo. Speed of sound is not affected by air pressure but it is strongly dependant on air temperature. The higher air temps on the turbo will give a faster speed of sound and therefore shorter intake than an N/A.