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Oysteinrb · Joined: 21 Oct 2006 Posts: 47 Location: Norway

well i am ready to get a truckload of "do this instead, buy another car, that wont work" and stuff like that, but here i go anyway..

i am wondering what the relationship between compression, boosting and knocking etc etc is, and if there is a "rule" on how to canculate THEORETICAL hp, tq and all that (say if you change compression for example).

i am asking because i just got hold of a supercharger and i want to install it. i know there will be issues, and i know it might be cheaper to buy another car. but i also know that it is DOABLE, and that i will learn ALOT from trying.. get me?

anyway, the supercharger is an Eaton m90, and the engine i am "installing it on" is the 2.5 porsche engine from a (euro) 86 944 with about 80k kms on it. this engine is going into my 76 924 when it is finished (see the thread in the "engine transplants" section for the swap itself). superchargers are generally rare over here, and i know you guys probably sit on quite a bit of knowledge regarding the issue.

so here is the plan so far:
overhaul of the engine
making a bracket for the supercharger (it seems to fit quite nicely infact)
calculating the pulley size i will be using for the boost level i am going to run
machining a intake manifold/adapter (not sure what throttlebody i will be using yet)
installing a complete megasquirt 2 system
installing an intercooler (kinda optional)

so this is what i will need help to do since i am not THAT skilled in boosting..
what amounts of boost will my engine tackle in stock form detonation wise (i mean, if enough fuel and all that is correct).
what compression would be "the desired" if i wanted to go for 1bar (about 14,7 psi..?)
what will the 2.5 bottom ends limit be? (tq/hp)
what will the drivetrain limit be? (tq/hp)

i know these questions have no absolute answer, but can anyone of you make an educated guess? what are your experiences with boosting the 924S/944?

i would of course document the complete process and tell you my findindings and stuff, but i will need some info from you more experienced guys..

can anyone of you boostgurus help me out?

thanks a million!

Oeystein

Rasta Monsta · Joined: 12 Jul 2006 Posts: 11733 Location: PacNW

You may wish to consider upgrading your head gasket.
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Toofah King Bad

WeiBe (1987 924S 2.5t) - 931 S3

sequential · Joined: 18 Nov 2005 Posts: 500 Location: BANNED

Hmmm,
Few words !
Technically , you are going to have to upgrade more than Just the head gasket , look at the DIFFERENCES BETWEEN 44t/44N/A AND YOU WILL GET THE IDEA.
1. the clutch is a must
2. fuel injectors
3. fuel pump
4. a good ECU
5. oil cooler ( maybe not , norway , but recommended )
6 Head studs .
7. Intercooler

14 psi willl net Avg 150 bhp/TQ overstock, not sure if stock internals will handles this , BUT I BET IT WILL !!!
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928 gts prototype
baby blue engine block
steam in 1,2,3,4 sometimes
cold star issues while on stands
112 whp with new 4 valve head and MIS 2

Khal · Posted: Thu Jan 11, 2007 10:01 am Post subject: Re: Compression and Compressors

Oysteinrb · Joined: 21 Oct 2006 Posts: 47 Location: Norway

great! keep it coming guys

about 150 over stock..? that was very close tomy own calculations aswell then.

sequential, the ecu, pump, and injectors WILL be changed. i would rather go overkill on that side before i overboost... as i mentioned, i am planning MS2....

will stronger headstuds be problematic to aquire..?
how much tq/hp will my stock clutch take?

Oysteinrb · Joined: 21 Oct 2006 Posts: 47 Location: Norway

great! keep it coming guys

about 150 over stock..? that was very close tomy own calculations aswell then.

sequential, the ecu, pump, and injectors WILL be changed. i would rather go overkill on that side before i overboost... as i mentioned, i am planning MS2....

will stronger headstuds be problematic to aquire..?
how much tq/hp will my stock clutch take?

Smoothie · Posted: Thu Jan 11, 2007 11:30 am Post subject:

Head studs - http://www.raceware-fasteners.com/photo.htm

I don't know how much HP the 924NA clutch will take, but for comparison, here are the NA and 931 "contact pressure" specs -
924NA - 4810...5490 N
931 - 7200...7900 N
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"..it's made in Germany. You know the Germans always make good stuff."

'82 924T, US version, dark green metallic, 5 speed Audi 016G gearbox

simsport · Joined: 06 Nov 2002 Posts: 573 Location: UK Warrington

The M90 is a bit big for your needs, the M62 is better matched to your engine. Given you have the M90 then you will need to set the boost to work with either low comp pistons (8.5:1) or have it much lower to run with std pistons.

The M90 will struggle to give you much more than about 10-12 psi without a lot of heat. Better to run it cooler and have usable power with less hassle holding it all together.

The rule of thumb is about 40-50% increase in power for a decent supercharger setup, so about 240-260 bhp max.

Cheers
Simon
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Blown is always best!

endwrench · Joined: 07 Dec 2002 Posts: 1631 Location: Victor, Montana

Sounds like a fun project!

Simons advice is sound but I personally think it is a little conservitive, as it should be for a track car or a daily driver. I assume this will be neither. This is just gonna be a fun street car? I can't see why the M90 wont work very well for you. I'm running a 96cid twin screw on a 2.0L NA motor at only a 50% overdrive. Mine is pretty well worn so I doubt the efficiancy is any better than a roots.

I'm pumping 10-11 psi on 9.5:1 compression with no real problems so far. The oulet temps are getting pretty hot but I haven't measured them yet. My intercooler has kept my intake temps within 25°F of ambiant. My IC is the same size as my radiator but 3" thick. This has been my biggest proble so far. My engine wants to run warm on hot (90°+) days because of the IC. This should not be a difficult problem to address but my second problem is. The clutch is starting to show some weakness at this boost level. I am running a stock NA clutch.

I have 2 suggestions when you do your calculations. First, pulley size. If your calcs say you need 50% overdrive for your desired boost, go 70-75%. It just seems to work out this way. Next, I'm not sure others will agree or are having the same problem I am but when you calculate your injector size, add 25% to it. I'm running 44lb injectors and they are at there max and I only rap to 6200rpm!

Get Corky Bells book "Supercharged". It will answer a great number of your questions. Well worth it.

Oh ya, the IC is NOT optional at 14psi!

Todd
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'79 924NA. Rebuilt 9.5:1, MSDS header, Mega Squirt Injection, MJLJ-EDIS Ignition, 1.6L Whipple Charger and Intercooler, 10lbs Boost, 944 Trans, Custom HD Clutch.
"simsport" said....superchargers are better than turbos its official!....

Min · Posted: Thu Jan 11, 2007 4:15 pm Post subject:

I'd personally recommend o-ringing the block, and installing raceware headstuds.

Other than that, I'm glad there is another person on this board who's joining the crusade to make some horsepower out of our motors.

Min
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Custom means it didn't come from a box.
1980 n/a with EDIS and Megasquirt II Injection. 7 different colors and counting.

Oysteinrb · Joined: 21 Oct 2006 Posts: 47 Location: Norway

wow! love the info

as i expected, you guys sit on a lot of useful information...
so, it seems the essential mod here is firstly, new head studs. then, an intercooler..

i am not quite sure on the compression/boost i am going to run, but of course, i want the best of both worlds and not too much work. changing pistons is something i will do if it is necessary, but i would rather use the stock ones if it is possible. however, i guess it will come to a 944t piston exchange.

what mods would you reckon i need to do to reach 300hp?

thank you again for your replies!

simsport · Joined: 06 Nov 2002 Posts: 573 Location: UK Warrington

Well yes perhaps a little conservative,

On the cooling thing.....its interesting as my race car had the same problem until I fitted the IC smack up aganst the front of the rad.

I had thought it would be better to leave a little space so that the two cores didnt just warm each other. Wrong!

The air pressure drop between the two meant the airflow was too weak at the rad. As soon as I ran the two close together cooling was very much better.

I also reccomend an electric water pump as you can take out the stat and yet still control coolant flow via regulated pump speed.

Oh as an aside to get 300bhp?.....use an 944s2 or 968 engine with a rotrex supercharger...9M have seen well over 300bhp.

Cheers
Simon
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Blown is always best!

Lizard · Posted: Fri Jan 12, 2007 2:39 pm Post subject: Re: Mods

endwrench · Joined: 07 Dec 2002 Posts: 1631 Location: Victor, Montana

I don't want to hijack this thread but my answer may help Oysteinrb.

My IC is sealed to the radiator so no air can go around. I think part of my problem is the thickness of the IC. Only the bottem third is directly exposed and the 6" airflow path to the top 2\3rds has been narrowed to 2 1\2". This is plenty of room to move enough air but I can't get any natural "ram air" flow due to the contorted path.

I think I can fix this thru a multitude of ways wich I plan to implement before summer. First, I already installed a second blow-off valve before the IC to help bleed off heat generated at cruise. Twin Screws still generate heat in off boost conditions. Next, I will install a 944 radiator and a pair of high performance fans and then open up some air flow thru the stock IC ducting in the front cowl. If this fails to satisfy then I will look into the electric waterpump idea. I may look at it anyway just to help keep head temps down.

I don't know much about the 2.5L motors but if I remeber correctly they are 10:1 compression motor. This may be pushing things a bit. You may investigate the possibilty of just machining your piston tops to match the shape of the combustion chamber. I don't know but I would think you could get down close to 9:1 or so. I'm also sure this would weaken the pistons but would it be enough to matter in this case? Wanna live on the edge?

Todd
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'79 924NA. Rebuilt 9.5:1, MSDS header, Mega Squirt Injection, MJLJ-EDIS Ignition, 1.6L Whipple Charger and Intercooler, 10lbs Boost, 944 Trans, Custom HD Clutch.
"simsport" said....superchargers are better than turbos its official!....

Lizard · Posted: Fri Jan 12, 2007 3:37 pm Post subject:

Todd, check out Spal fans for high flow fans
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