924Board.org

[BIMMERBOY]

Here goes. I have been gettin input in the 924 portion but the normaly aspirated people don't seem to have a strong answer to my question. I am experimenting with a very high boost turbo system. The fuel side is a 928 CIS distributor (two injectors per cyl on my one point eight 4 ) This CIS unit has a TB attached pre-metering plate. I had planed to remove it and use the stock throttle on my intake manifold. Then i asked: Why have it there? You can read the dialogue in the non-turbo forum if you don't know what i am talking about.
Does anyone know about throttle before turbo vs throttle after turbo. More interesting still has anyone seen a before-and-after like on f1 engines? Thanks much!

[John H]

What model of CIS is the 928 version. If you're using an early model unit that relies on airflow to set volume of fuel if you have a blow off vale in the system you will find the motor will overfuel at the time of overboosting and this can result in excess fuel into the turbine side of the turbo. This shoudl be avoided unless you wish to rebuild the turbo after every race meeting The TAG turbo F1 motors in the 80's used to dump fuel in the turbine area to help keep the turbo spooled up when changing gear etc.

Your using a garret turbo so if you want to keep the turbo turning over when low boost in the manifold you can do what Bentley do /did on the Mulsanne turbo - feed boosted air back to the other side of the compressor wheel - this kept the turbo ticking over and reduced lag.
Most of the quick turbo's I've seen over here (New Zealand) had the throttle body after the intercooler.
Not sure if this is what you want to hear.

[BIMMERBOY]

The '79 928 used the mass-air plate mechanical CIS. You are correct about the BOV if it vented to the atmosphere. I cannot understand why anyone would do that anyway however because venting into the vaccume behind the turbo would be more effective anyway.
The exhaust fuel injection and rich blowby setups I discovered when I was looking into this idea. My setup is to far along and on to short a budget to include such a setup.
(one idea that occured to me was to add a diesel injector in my turbo adapter plate using the high pressure, nearly 100 psi, of the cis setup)
My idea however was yet another thing. I have found a few people who put two throttles on the intake here is a diagram:

HH<------- filter
||
KZ#<------ CIS PLATE
||
H<---------Throttle 1
\
{G}<------ turbo
||
|
|W|
|W|<------ Intercooler
|/
||
H<-------Throttle 2
\
/\
//\<----- Int. Manifold
HHHH

This setup was used by Mazda and a popular mod. was to remove the throttle on the manifold. This is how I understand it to work. As you press the throttle the throttle on the manifold flies open. Then ,after it is mostly open the throttle before the turbo opens allowing full flow. In this way the turbo cannot force high boost between it and the engine as the turbo would never have both a source of air and no place to put it. With this setup there is no need for a BOV to equalize pressure before and behind the turbo also, when not near WOT the turbo spins in vaccume (in a traditional setup the turbo always spins in close to atmospheric conditions).
Tricky but, on a large turbo like the T04, anything to decrease resistance on the impeller thus increase the ratio of turbo revs to exhaust flow. Also it makes stalling the turbo impossible. Detriments+complexities =unknown. I hope this helps you understand my question better now.




the turbine area to help keep the turbo spooled up when
changing gear etc.

Your using a garret turbo so if you want to keep the turbo turning over when low
boost in the manifold you can do what Bentley do /did on the Mulsanne turbo -
feed boosted air back to the other side of the compressor wheel - this kept the
turbo ticking over and reduced lag.
Most of the quick turbo's I've seen over here (New Zealand) had the throttle body
after the intercooler.
Not sure if this is what you want to hear.

[Peter_in_AU]

just out of interest, why are you building a space shuttle and using mechanical injection?

Is it the class you're running ?

[larso]

good question..

then again, why did the carrera GT use mech injection...bmws and datsuns and volvos had electronic injection in 80s, all bosch, so WTF???

ANd how come the 944 doesn't use MAf, And how come porsche doesn't use wishbone on 911s until the late 90s!!!

[ This Message was edited by: larso on 2001-08-29 10:53 ]

[Peter_in_AU]

the Carrera GT probably used the mechanical injection from the 928 because Porsche had a lot of them lying around and it probably meant the least number of changes to the 931 systems.

The rest is probably all about cost, tooling and training. Remember, Porsche have never really been at the pointy end as far as technology is concerned.

A lot of the drag racing classes do not allow electronic injection. Perhaps that's what bimmerboy is up to.

[ This Message was edited by: Peter_in_AU on 2001-08-29 12:16 ]

[Rick MacLaren]

Lars: Maybe Porsche prefers a conservative strategy on new technology, preferring to wait until it's proven, because they themselves haven't developed it. I know if I had a car company, I wouldn't be jumping on the bandwagon for new technology created by someone else if I had a formula already that worked satisfactorily.

[larso]

ok sounds good but wishbone is what was on formula cars (and formula cars are proven to handle well!) and isn't that new, and they used it on the 928 in 78, but not the 911, anyway back to the TB thingy, what is this car being used for????? track??? street???