Tim Murphy

Double post, sorry.

onebad928s

when i pulled out the spark plugs they were a white color(detenation) which i knew happened but it was after the fact it was a kit purchased and said to be proven to work as supplied. i guess to say dont trust everything you here. Ohwell next project.

Lou
83 928 5speed

GoRideSno

Cory
Tim is the man (Lag & Z too) and you can't beat centrifugals for high HP, especially on the top end. I've said it before and will say it again, I would have gone this route if I had a 5 speed (more gears, easier to control rpms).

The centrfugal is not superior however for all range power. Check out a thread titled "Dyno Battle". My first time at a dyno ever, with a brand new design, installed only hours before and with no tuning whatsoever I was able to make around a 40% power increase through the ENTIRE rpm range not just at low rpms. I was making ONLY 7Psi, and using only a small intercooler. On that thread you will see dyno charts for 2 centrifugal blown 928 5 speeds. You will notice that the positive displacement automatic design makes the same HP and torque at 3000 rpms as the centrifugal 5 speeds s and one is running 14 psi, yes twice the max boost as my positive displacement setup.
Now if you took my % increase at 7 psi, did a little math and came up with my %increase at 14 psi (provided that I use a larger intercooler, which I can) you would find that at 3000 rpms I should have 70 more HP (around 33% more for the positive displacement) than the centrifugals . The centrifugals would catch up at around 4500 rpms. The centrifugals will have about 60 more max HP (about 13% more for the centrifugal) at the top (5500rpm for me, 6000rpms for centrifugals). Now if I had a 5 speed I could add 3-5% to my HP. Using 4% the advantages would then be around 40% more on the low end for the positive displacement and 5% topend for the centrifugal.

Clearly the positive displacement design is superior for all range power.

This pertains mainly to automatic cars, 5 speeds can stay in the higher rpms better.

The dyno run was with a twin-screw but a roots would be very similar below 10psi or so.

The Euro intake probably won't be worth your effort/time/money (you choose) if you are going with an SC. An increase in manifold volume will likely hinder lowend performance gains from the SC. IF you have PET take a look at the 930 manifold, flat as a pancake and very small volume.



Andy K

Z

The centrifugal isn't making that 14psi at that 3,000 RPM though. It's making less than that down there, and is probably making more like around the amount of boost that you were there.

That's assuming that you can increase the boost to 14psi at 3,000 RPM, and can keep making that much boost. It's also assuming that the centrifugals don't increase their boost at 3,000 RPM like you would be.

Captain Carl started with his original engine when he first supercharged, and then put on Euro heads and cams, and I think maybe a Euro intake too. If you check with him he'd probably be able to give you some information on what the effects of the change were on a supercharged car.

hacker-pschorr

10.4:1 compression?

1977 - 1979 8.5:1 (CIS)
1980 - 1982 9.0:1
1983 - 1984 9.3:1
1985 - 1986 10.0:1 (32 valve)

If this data is wrong, please let me know.

Tims test car is an 81.

hacker-pschorr

Since we are on the topic of 2-valve SCing I'll toss this question out there. What were the differences between the design of the 944 Turbo combustions chamber and the 2-valve 928's?

John..

I suggest anybody playing with a 4.5 liter seriously watch the ignition advance. These cars have roughly the same combustion chamber as the 944 turbos and those cars do not run full ignition advance at the higher boost pressures. There are a few easy tricks to assure a margin of safety.

I ccd my heads and stock pistons and came up with a touch over 8:1, these cars are not 8.5:1 or 9:1 like some literature states. My math could be wrong, but I think I am fairly close.

Now, who is interested in my MAF conversion for the L-Jetronic system? I can even supply a prorgam to make the car run up to 8 lbs without a rising rate regulator....hint, use 914 1.7 liter injectors... 8 lbs on that engine should be no problem for short bursts, but you have to cover all the bases, that means pay close attention to fuel and ignition.

Good points on the centrifugal vs. positive displacement, but if you want great mid range and top end the best method is with the turbo. As of today I am running about 12-13 lbs on high boost mode (not yet dialed in so it is very infrequent), but the system will develop over 10 lbs at about 4500 RPM and I can have 5-6 lbs as early as about 2800 RPM in the right gear. I will probably run the car most of the time at about 11 lbs, as it seems to run really well right there.

I only run about 20 degrees total advance on the boost...until I get the car checked out that is where it will stay.

John..

For those who are interested.....take a look at this:

http://www.flash.net/~joeao/greg/verybadthings.html

Weissach1982

Is there a MAF conversion for the L-Jets? What kind of work and money would that take?
Also, since i'm buying these Euro Runners, Plenum and TB, why would there be a problem with SCin as i'm heard, wouldn't better the airflow be better with the biggerTB pluenum and runners to make power etc...?
Thanks,
Cory J
82 Weissach

bcdavis

It's all about getting *extra* fuel into the engine to compensate for all that extra air you are shoving in there...

Fuel fuel fuel...

(and charge temp, and CR.)
Charge temp can be reduced with an intercooler, and CR is set, unless you rebuild the engine.
So to prevent detonation, and a lean condition, it is all about fuel...

Getting a supercharger bolted up, intercooler in line, and the plumbing installed is not that hard.
It is getting the extra fuel that is so important.
That is what is taking the SC folks a long time for each model of 928.
Different fuel delivery systems on each iteration of the 928...
Each one needs to be tested, to see how the stock system handles the extra air.
Apparently the CIS cars adapt quite well.
Other cars need adjustments, larger injectors, rising rate fprs, etc...

Normy

Hacker- I have an '85 S2, and these have 10.4-1 compression with two valves/cylinder. No knock sensor, however...

Roots Vs Centrifugal: Normally, I'd think the Roots type would be better, since they make full boost at very low RPM's, and these engines generally seem to be weak on torque at low tach speeds. Roots-type blowers need bigger intercoolers generally, since they heat the air more for each pound of boost produced.

The reason that the centrifugal type of compressors used by Tim and John are better for these cars is that lots of low RPM torque simply means wheel spin and axle-hop with an independent rear suspension.

N!
'85 S2 5 Speed

ViribusUnits

It seems to me that the L-jet should be easy to modify.

Get eighter 8 of the 914 injectors, or the S4 fuel pressure regs. Then adjust the main spring in the AFM to compensate. Tune the car up with the stock Porsche intake set up. Upgrade the fuel pump at this time as well.

Now install your supercharger. Install the AFM infrount of the blower, add a jumper to get from the stock AFM location to the new one.

Since it's already tuned to the new fuel injectors or regulator, you should be just fine when you add the blower. Just a little final tuneing, and your good to go, no?

Boy I wish I had some cash...

hacker-pschorr

So are you saying my info is off or do you have a modified US car or a Euro car?

John..

To my knowledge I am the first to do an MAF conversion on the 928s L-Jetronic.

Weissach1982

What was invovled in the MAF converison on your car John, since its a tid bit different engine setup than my L-jet 82.
Thanks,
Cory J