924Board.org

[Dave951M]

I recently bought Al's 931, he graciously threw in a short block from an 81 into the deal. Question is, are there any differences between the 81 and 82 block? Current plan is to build the 81 block, swap it in later. Since this is an Audi motor, any ideas about increasing compression ratios, say to about 8.5:1? I'm thinking that along with shot-peening and radius the rods, balance and knife edge the crank (should I crossdrill too?) align bore, polish crank, Total Seal rings, bore (how much over is safe?), new pistons. The rest of the system will get the 930 wastegate upgrade, MSD ignition, freeflow exhaust, polish&port, portmatch, intercooler. What kind of hp would this possibly make and still be streetable? Any suggestions here are welcome, I'm new to this motor, but not engine building. TIA

[924 turbo]

Differences between '81 and '82 block:
None

Increasing compression ratio:
Use european or custom 8.5:1 pistons. The european '81 and '82 had 8.5:1 pistons. The Carrera GT used forged 8.5:1 pistons. I suggest keeping the stock 8.0:1 ratio and just running more boost. Total Seal rings would be fine.

Crank:
Knife-edging would be fine. The stock bottom end on a 924/turbo is extremely strong and well designed. Bearing failures are very rare. I wouldn't worry about the bottom end too much.

Wastegate:
The 931 shares it's wastegate with the 930. The upgrade you are speaking of is probably the wastegate spring, for running more boost. I suggest keeping the boost under 0.8 bar, as the stock fuel system only adds fuel up to that point. Running 1.0 bar causes detonation, even when running 91 octane gas. You could also use a boost controller instead, and keep the stock spring.

MSD ignition:
I have no experience here, but I would say that the stock system on the '81-'82 cars (digitally controlled) is adequate. I would upgrade the coil to a slightly hotter unit, however.

Exhaust:
The stock system with a gutted cat is more than adequate except in very high boost applications.

Intercooler:
Use a top-mount Carrera GT style intercooler and a hood scoop for an easy installation. Front-mounts require lots more pipe and cutting. Unless you're running very high boost, the top-mount is adequate.

Bore:
2mm over is fine, but I would just keep the block unbored if possible. There is very little to be gained by boring it. If you can keep the stock pistons, I suggest doing so. All other options are quite expensive, for very little gain.

An 8.5:1 engine, with a gutted cat (or straight-pipe) with 0.8 bar boost, intercooled should make around 215-225hp.

[larso]

The performance upgrades mentioned are good, but you'll never want to drive your 931 if you have a 951! Just kidding.

But I don't agree about bearing failures being rare;

1. I've got a buddy here where I live that spun a bearing just driving the car
2.I parted out a car, looked at bearings and they were worn,
3.I've looked at several 924 and 931 blocks and they all have worn bearings,
4.the oil drained from 924s seem to have more metal in it than all other cars I've looked at when draining oil,
5.all the oil pressure problems in 924s are usually do to worn bearings, lots of 924s blow head gaskets causing bearing failure (oil becomes acid),
6.I've never seen a 924 with good oil pressure (unless it's new, like Rick M.'s engine), the 924s are loose engines (making bearing failure common),
7.oil temperatures and hence viscosities fluctuate more in the 924 (so using thicker oil is good, but when cold, bearing wear is bad). This means that if you are not doing highway driving, 931 engines are prone to bearing failure, since temperatures fluctuate so much, and no oil is made for fluctating.
8.931s run hot, heat causes bearing failure
9. 931s and 924s have flickering oil pressure sensor lights. The owners manual says this is normal, I think they are bullshitters, this is definately not good and shows a worn engine.
10. 931 engines are noisier than other engines when worn, even when valves are adjusted properly, signifying bearing wear.
11. Bearings in 924s seem to need to be replaced a lot more regularily than other engines...sometimes at 50,000 miles, sometimes at 25,000, sometimes at 100,000
12. I've never seen a guy replace an oil pump, or cam elbow, or oil pump valve, or oil filter and have oil pressure cured...it's almost always the bearings.

.... I would never drive a 924 without an oil gauge, but I'd drive a honda without one and never think twice about it...I personally think that's why porsche puts oil gauges in their cars, cause they know oil pressure is a problem hehehe. I think bearing failure is very common in these cars.

[John H]

I sort of disagree with you lars.

My 931(937 replica) has done 260,000 miles we repl;aced the big end shells at 80,000 miles due to a blown headgasket, oil pressure is normally around 8 to 10 bar on start up (cold) and drops to 5 to 6 bar while running. I've also runa mechanical guage to check that the Porsche supplied electrical one is accurate and while it changes a bit when the lights come on it is pretty accurate.
I also run twin oil coolers which helps keep the pressure up.
I have also seen the a couple of 931's spin bearings so some have good pressure, others don't.
I beleive it come sdown to what the first owners used and how the car has been maintained.
Mine was owned by BP Oil initialy and did heaps of miles in the first two years and had regular oil/ filter changes (more often than recommended) the next owner had it converted to Carrera Gt specs and it was well maintained.
I continued the trend by changing both oil and filter at 5000 mile interval using a good quality oil. As I was running in a race series sponsored by an oil company I got buckets load of oil. In fact I ran inteh series for four / five years and still had enough oil left over to carry on the regular changes for the next three years.

[Dave951M]

This is where my thoughts are at the moment, I checked the short block this afternoon, no cylinder ridge so I'm thinking just a hone and tank for the block (more later)

To increase compression ratio-
Deck the block by .030 or so, anybody tried this? What is the clearance valve to piston, or what is the lift on a stock 931 cam? I would like forged pistons, but if all it really needs is a tank and new bearings, I can increase compression by dropping the deck.

Crank-
Knife edge and balance are high up. What about any potential oiling problems like the infamous #2 rod on the 951? From everything I've read, it will take some very serious hp to break it.

Wastegate-
MBC is a must, as is an adjustable FPR. Will the stock fuel pump supply it? By increasing fuel pressure, assuming the injectors don't reach saturation on the duty cycle, the stock system can be fooled into making much more hp than intended by changing the fpr. This trick is done fairly often on the 951s. As for octane, I have no aversion to running 10% toluene at all times, it sure helps with the 951s needing higher octane fuel. (I saw the other posts about toluene, don't bother with a flamethrower, I ain't listening to any treehuggers.)

MSD-
I happen to like the MSD and the Jacobs second, just wondered if anyone here has tried it.

Exhaust-
Cat has no guts now, already done. I'm thinking a very free flow muffler or resonator here other than the cat. Anyone with experience with a 3" system?

Intercooler-
Where do I get a top mount intercooler and what is it's capacity as far as heat soak? Hood scoop is no problem. For boost, I'm thinking in the 15psi range with a possible peak at about 18psi in certain situations.


Bore-
Currently thinking about just a hone job, and deck.

Has anyone tried nitrous along with increased boost? Gotta ask, I'm an adrenaline addict.

I started inspecting the 81 short block this afternoon, it looks to be in great shape other than what I've mentioned above. My goal is to get to about 250hp and lighten the car by about 300+ lbs. To that end, the a/c will go, wheels will be changed, insulation will be history, Sparco seats or similar installed, rear seat removed, lexan front and rear glass. In addition, what is being done to help with cooling? Has anyone tried a larger radiator?

[John H]

dave
a few answers to your question
Block/head
No need to deck the block – to get the better reliability out of a high horsepower 924 a lower compression ration is better. The factory raced the 924 Turbo at Le Mans and using a 924 block, kugelfisher injection, big turbo, low compression ratio (about 6.5 to 1) and high boost they got around 370 hp. If you deck the block you will need to go for either an offset keyway in the cam on an adjustable cam sprocket. I run an adjustable sprocket ,ade by Kent Cams in the. More for playing with the cam timing rather than because the head or block has been decked.

Crank
924 / 924 turbo moors are reasonably strong motors and there doesn’t seem to be any inheriant design faults. Balance of the rotating parts is crucial to getting the high horsepower reliably.

Waste gate-

to utilize a fully adjustable waste gate the best way of doing this is by removing the vent at the top of the waste gate and adding a line into the engine bay. You can then feed boosted air into the top giving better adjustment than restricting the flow to the bottom. Another trick is to put a non-return valve in the control line to limit the waste gate from opening till you reach a set pressure. The 931 waste gate starts opening around the 5lb mark.

Ignition – if the car is a digital controlled unit no real advantage to go the MSD way.

Exhaust-
I run a stainless steel system than is around 75mm dia (almost 3 inch). Has intermediate muffler and a Porsche 931 (euro spec) rear muffler (also stainless). My car is euro spec so no cat. Inter cooler-
I can make you a unit – however I’m having problems sourcing the secondhand cores at the moment. I have a genuine GT inter cooler on my car that I use to fabricate copies. Rick MacLaren has one of my units on his series I.


Running an inter cooler, having a fresh motor and running 1 bar boost you should be able to get at least 230 hp. Many years ago 1988/9 when I had replaced the head gasket, valve guides rings and bearings I put the car on a rolling road and ran 1.2 bar boost – we got approximately 260hp out of the motor. Not to 100% accurate as we had to calculate the drag of the transmission. But it was showing more hp at the back wheels than a 220hp 951 (944 Turbo) showed early that morning. Also on the track later that week it smoked the 951 till I ran out of brakes.

For cooling the standard radiator is OK as long as you add a secondary oil cooler. The engine under racing conditions doesn’t over heat . But without the secondary cooler running 1 bar boost it used to run real hot.

[Dave951M]

Good info on the block, that seems to nail down one variable. An adjustable cam sprocket was thought of, what's your take on it's effectiveness?

The wastegate changes look pretty good. I would still like to know if anyone knows the hp, air/fuel ratio where the stock injectors reach over 80% duty cycle. Should I look at changing injectors as well? What about possible stand alone engine management?

Ignition, I agree with you that a higher voltage coil might be all that's needed.

The crank will be knife-edged and polished. Are there any known oiling problems like the 951 #2 rod bearing? Is a higher pressure oil pump called for or any pan baffles needed?

Engine Work Firmly Decided on-
Clean and tank
Balance
shot peen rods
lighten, match, radius rods
new bearings, seals, rod bolts
hone cylinders, clean pistons, reinstall with total seal rings.
align bore

Head work- undecided other than adjustable cam sprocket and port matching.

Like I've mentioned in my other thread, this car is a project that can take any form from here on out, I just have to decide what to do and start doing it. Thanks for the info guys, any other comments are welcome.

Dave Miller

[Zuffen]

The metal you are seeing on most worn 924 engines with low pressure are from the head and cam.

dirty oil eats the upper end before it does real damage to the lower end.

Keep in mind that metal does pack into the lower end eventually.

I rebuilt the engine on a 81 931 from a spun bearing, the wear wasn't appreciable on the other rods. The bearing in question was starved of oil and began the tear down cycle which quickly balloons into critical failure. Keep in mind the failure was catasrophic because the PO owner or a representative beat the hell out the engine on more than one occasion after the initial problem.

I've seen bearing failures that stopped short of crank damage, becuase the owner could tell that the car wasn't sounding right and then began to baby the engine until tear down.

_________________
Bob Dodd - 924turbo@cox.net
931 1982, 944 1982 euro, 924S 1988SE, 93 968 tip 06 Silver Cayenne S, 06 Black Cayenne S

I have Way too many cars, parts for the 931,944 and 951

[Rick MacLaren]

You asked: "The crank will be knife-edged and polished. Are there any known oiling problems like the 951 #2 rod bearing? Is a higher pressure oil pump called for or any pan baffles needed?"

I don't think we're succeptible, with our cars, to the same dreaded problem as the #2 with the 951. Still, in retrospect, camphering your crank oil outlet holes couldn't hurt, I s'pose. More oil around there is always good, I think. I blew a rod bearing on, coincidentally, the first piston near the front of the engine. Not typical. You'd expect the piston nearest the rear of the engine to be most affected by heat, but not so in my particular case.

I haven't installed a higher pressure pump on mine and things are moving along ok, with about 15K on the rebuild. I change my oil every 1500 to 2000 KM. Worrywart. Still putting in around 10 PSI and the car goes like stink.

You know, I've done the rebuild, including total balancing, and I was hesitant to knife edge the crank. Maybe I should have done so, but I'll give you my reasoning.

Seems to me that the basic principle is this: You knife edge the crank to achieve (a)weight loss and (b) smoother flow of the counterweights through the oil - sort of like reducing parasitic loss by sharpening the medium that has to move through the oil.

Those principles are sound. Nobody needs extra parasitic loss.

To handle the first matter, the weight issue, I questioned whether or not I needed to knife edge my crank because (a) I could achieve weight loss by shaving the flywheel about 1/10" and (b) loss of weight through the balancing act done with the rods and the rest of the assembly.

So weight was handled. Regarding the second issue, parasitic loss, my bias was cautionary: Didn't want to second guess the Porsche engineers, who may well have desired this sloshing and actually desired this parasitic loss for the sake of lubrication.

I felt there was a second important benefit to having big counter weights, namely, they seemed to be there to slosh around oil under the system. I assumed, maybe rightly or wrongly, that the 'sloshing' of the counterweights was actually beneficial to preserving the longevity of the bottom end. In a word, I kept my counterweights stock, not knife-edged, because I wanted all that slosh. I wanted the oil to get churned up.

To this day, I don't know if it was the right decision, but I'm not lamenting NOT having knife-edged the crank. I think I'd worry every day whether or not I was getting enough oil spilled around down there, and would be all anxious that it'd blow up again. So I'm glad I decided to keep it stock.

Glad to hear you're balancing and shot-peening the rods, that won't hurt. And you'll appreciate the confidence it brings.

You said: "I would still like to know if anyone knows the hp, air/fuel ratio where the stock injectors reach over 80% duty cycle. Should I look at changing injectors as well? What about possible stand alone engine management?"

I haven't gone for larger injectors just yet and doubt whether I'll need to do so. Once you lighten everything up, the flywheel, etc, you'll be surprised at how much air and fuel your car can handle. Ten or eleven PSI is 'about' the limit with our fuel systems, and it's really quite fast.

The best stand-alone system, for the buck, I can find, is the Links system. They use it on 944 turbos and it piggybacks on top of your existing system. Easy to use, easy to install, and you can map your fuel/air mix to achieve a constant 'flat' line across your RPM range, with no appreciable dips or surges in gas, so you won't either lean out or thin your oil from too lean or too rich a mixture.

Head should get all the valves checked to be within spec, absolutely new guides, and maybe consider new valve springs. These turbos need excellent valve springs, and this is far more important than any porting, matching, or 'three angle' grind jobs.

That's my $0.02. YMMV.

CAUTION:

IF you are doing this rebuild, and spending ALL this money, do yourself a BIG favour - CHECK ALL YOUR OIL LINES and have them blown out with air, oil, whatever you can to discharge any metal from previous 'errors'. I found a piece of RING in an oil line POST rebuild, just before the engine was installed.(!) Check your turbocharger lines, the lines to your oil cooler, all the gallies and anywhere metal can get stuck. It's a very small check but it can save your rebuild from nasty damage later on. And there's no point in doing an engine with dirty peripherals. Same with the vaccuum system. Replace ALL damaged rubber, or any elements in the system that can destroy air or fuel delivery during the break in period.

You probably know this junk, but it never hurts to mention it.

[ This Message was edited by: Rick MacLaren on 2002-07-03 22:19 ]

[numbers]

I agree with John and Bobby. You don't need to do a lot to either the bottom or the top. Save your money. As for perfect circle rings, I have not had good luck with them. They are so hard, that they tend to not seat very well. It is true that they will last for a long time, and not leaving a gap should decrease blowby, but my experience is that your oil consumption will increase. This could be a particular problem if you are not going to bore and install new pistons. Stock rings will go for 100,000 miles plus, so once again, I say save your money. Same with camfering the crank. Unless you have a starvation problem, all you will gain by having the crank camfered is lower oil pressure, and increased oil use. I would just put everything back stock with good quality parts, and balance it. Run your boost as high as you are comfortable with and go like hell. Running race parts on a street engine, is like running premium in a low compression engine. There is no gain, and it costs a lot more. Don't tell the highschool kids this, it would ruin the speed shops business.

[Dave951M]

As far as knife edging the crank, when we built domestic V8s back in the day, knife edge, balance, and chamfer the oil holes were the first things on the list. Next was magnaflux and nitride. I understand your theory about the oil, but parasitic loss can be substantial with low displacement engines, not to mention that as rpms increase, the very action of the crank can cause cavitation in the oil pan. That leads to air entrained in the oil that is pumped into the engine. I was thinking of a windage tray, they're fairly easy to make and help to keep the oil where it belongs without crank induced foaming. The reason for a slight overfill of the pan before "enthusiastic" driving is because of oil sloshing away from the pickup. Poor oil pressure can be caused by several things, wrong viscosity, loose mechanical tolerances, weak pump. Has anyone tried to increase pump pressure?

I thought Audi designed this engine for trucks, am I mislead here? That would explain the massive journals on the crank and rods. The mass of the crank and rods makes for a lot of rotational and reciprocating mass. I still think it could benefit from a diet. The comments about removing mass from the flywheel reinforces that. Just where and how is it done on this engine and did it materially affect the pressure plate engagement?

Crap in the lines-- you bet. Don't ever trust even your most trusted engine machine shop. We ALWAYS cleared every line on the engine before any reassembly, even if the lines were only part of a subassembly removed for service. There could always be some trash in the line that caused a major failure later, leading to a very unhappy customer.

Fuel delivery, if I'm understanding this right, the stock system will handle up to about 10psi, correct? If you change to a higher pressure fuel regulator, the higher pressure will flow more fuel because the higher pressure in a given diameter line will mean faster flow, ergo, more fuel from the stock system. This trick is used extensively on lightly modded 951s as an interim to more heavy duty changes. Wouldn't the same apply to the 931? What is the stock fuel pressure, wouldn't bumping it up by say, 15% lead to more fuel injected?

I'm just musing over the possible avenues to take, this car will go on a diet and get somewhat pumped up. Just trying to get a plan before calling the machine shop.

Dave

[John H]

[Rick MacLaren]

John, you said "As I mentioned befor ethe same CIS system was use don the 931, Carrera GT and the Carera GTS and the GTS definitely ran 1 bar boost."

Yes. These later cars have the crank-fired ignition, don't they? If so, maybe that's how they've managed to handle higher boost.

I shouldn't ball-park when discussions are serious like this...But, anyways, the figure '10' (or '11') I've encoded in my head is what I've heard is the upper limit on the amount of boost you can effectively use GIVEN restrictions on the amount of fuel we can pump out of our injectors on a 1980 Series I. Where I got that figure escapes me right now - correct me if I'm off on the exact number. But my car was I think 7 PSI stock from the factory. So I've been assuming that at about 10 (or 11) PSI (can you see me getting on uncertain territory here?), we're maximizing our burn - we're not too rich and not too lean. I've been assuming that at 12 and up, we're inevitably burning lean, at least, this is the assumption I've been making with my Series I 1980 931.

Maybe the later cars, '81 and 82, that have more advanced digital ignitions will correct for limitations in fuel delivery. But I'm not sure my early system will adjust properly across a wide RPM curve. This safe figure, 10 or 11, I've taken on because I'm pretty sure my ignition doesn't work as well across the whole range of RPM's as the later series cars.

You can also kind of feel it. I've seen my car, pre-rebuild, at 12 PSI really NOT benefitting from the additional 3 PSI to 15 PSI. Even now, I'm not sure I'd get as much UUUMPH going from 10 to 13 as I got from 7 PSI to 10 PSI. Don't know if that was because the car was already baffed, or if it's because the mixture wasn't correct and optimal. Anyways, at 10, with my old ignition system, it seems about as fast as that system will allow. Links has to be next.

After the digital Links system, maybe a higher producing fuel pressure regulator is in order...

[ This Message was edited by: Rick MacLaren on 2002-07-04 12:42 ]

[Dave951M]

I see what you mean by the limitations of the "Stock" fuel system at 10psi. I'm proposing to use a 951 trick to get more fuel through the injectors by increasing the pressure in the system after the fuel pressure regulator. Higher pressure means more fuel volume through the injectors, within the limits of physics of course. If the GT cars can make 300+hp with CIS, then it stands to reason that larger injectors and a higher pressure at the injectors will supply the needed amount of fuel. CIS is a very simple system that we love to hate, but it's simplicity is a strength in this case. There's not much of a computer to deal with like in the 951. That's why I'm still looking for an answer from someone who has tried it on a 931. The results should be quite interesting.

Dave

[John H]

The actual track cars used a Kugel fischer system rather than the bosch CIS system and these intially gave trouble with running too lean after a few hours - hence the note that two of the GTR in the 80 Le Mans finsihed with burnt valve. The tird car they managed to catch in time and alter the misture.
As I ststed I'm not sure what the differneces are Euro to US but I know for a fact thatthe Euro digital cars can run the higher boost with no problems.
What I do and many of my acquanitnaces here do is run O2 sensor which illuminates the state of the exhaust. While not 100% accurate none of the guys have denotated their motors and all show to be running at the opitium.
Also by means of shimming the regulator inthe meterign head you can increase the pressure in the lines from the metering head to the injectors - which is what i think Dave is talking about.
I use dto notice the difference between 10 and 12 psi boost but after a whiel you don't notice it. But it does show up on the track. The difference between 10 psi my normal road boost and 18 pounds boost which I sometimes run on the track is really noticable.