I've read the the 944 oil pump cavitates at high RPMs... assuming that's valid, wouldn't thicker oil make that worse? It's harder to pump and is easier to cavitate.

 

I had the FR Wilk race chip in my stock engine before I went to the NA Tune. It moved the rev limiter to 7k if I recall. Never had an issue and I did hit the limiter more than a few times.

 

If I tear down the engine and find that the bores are damaged, I'm pretty sure I won't be able to resist the temptation to bore them out a bit more. Or maybe even if they aren't...

Why would longer rods be necessary if I'm only changing the bore, though? I'm not really interested in increasing stroke (to keep the high-revving nature of the motor intact).

 

That's my thinking, too. I've wondered if it would be possible (or even advisable) to run smaller clearances in the engine to allow the use of thinner oil and help protect against cavitation and air entrainment.

 

I've been rolling this over in my head and it makes sense to me that a thinner oil will resist cavitation and air entrainment better than a thicker oil. The research I've done in the last few days agrees with me.

Is there any reason a 944 engine can't be built with tighter bearing clearances? I've seen engine build literature that suggests that it's no real issue to do this, it's just a matter of proper fitting and precise machining. But I figure that with rod bearing failures being a somewhat common issue and the fact that these cars have been around longer than I've been alive, someone has probably already at least considered this possibility, if not actually done it before.

Are there other aspects in the engine besides just bearing clearances that affect what oil viscosity is appropriate? What I've read so far deals only with bearing clearances, but I know that's not the only place oil flows in an engine.

 

A big thing is the temperature that the oil is at, here are a couple Mobil 1 oils plotted across 0 to 100* C.





I'm not sure if our engine oil gets hotter than average cars, but I do know many other German cars (especially from that era) also run thick oil.

 

944 on race track runs about 205F oil temp, close to 100C. Graph shows very low viscosity at that temperature, good reason not to run over 6500 rpm, and only seconds at 7000 rpm

 

because if you used S2/968 pistons on a 2.5 crank with stock rods, the pistons will be 4.5mm below deck.

 

Couple of thoughts.

The Firestone Firehawk series 944 S2s had their rev limiter set at 7200 using a stock bottom end and valve train. The 16V motors can take 7200. The 944S exhaust cam is hotter than the S2 cam and the intake cams are the same. It's very noticeable on the 944S when it comes on the cams compared to the S2.

A good friend's 944 S2 made 199hp at the wheels with the following modifications: 968 intake manifold, cat delete, 951 cat-back and MSD ignition coil. He has since added a SciVision MAF which made a significant difference on the butt dyno but hasn't had it dyno'd yet.

My 944S has the same modifications minus the cat-back and is considerably quicker than it was with the AFM and 944S intake manifold. Freeing up the intake side appears to make a much larger difference in power output than opening up the exhaust (but it does sound great on WOT!).

If you're interested in building a new motor that will rev freely and really be a kick in the pants, I completely agree that you should lighten the rotating assembly by replacing the con rods and flywheel but you might not get much out of cutting down the crank. Also, you can safely go up to about 11.9:1 compression with new pistons and it should really make a difference. Lightweight lifters are available from a few sources at around $16/ea but realistically you'll only spend a few seconds at/around 7k so you might be better served spending your money elsewhere.

 

Ah, didn't know there was a difference between the 2.5 and 3.0 pistons besides the larger diameter. Thanks for the info.

 

Thanks for the input, I'm glad a bunch of people are providing feedback on this.

The crank machining is for reduced weight but mainly for reduced windage via knife-edging to help control oiling issues. It may be overkill on a car that will probably never see more than 25% track use, but it seems like a half-effort to me to lighten the reciprocating parts without also lightening the rotating parts.

I currently have an S2 intake on my S and I think it was worth a boost in throttle response and mid-upper range power, though that's only a butt-dyno reading. I wonder if there is a big difference with the 968 intake?

 

+1,000,000
This should be step 0 in anyone's list of adding performance - ditching the AFM setup, by any means necessary.

 

On the STAHL website (headers & other racing goodies in NY, USA) they have an archived article in hour revving past peak HP has advantages in the go fast department.

 

Its sort of a gearing thing vs power band.
You want to make power high enough and shift at a certain point that the next gear ratio will land you in the meat of the powerband so you're not bogging waiting for the cam/turbo...

 

In regards to bearing clearances, you're on the right track, but there's only one bearing available for the stock rods/stock crank, so there's only a few options to tighten the clearances. One of the best options is to send your rods to Michael Mount and have him machine them to fit aftermarket rod bearings. The crank main bearing clearance can be tightened by finding the rare and expensive .25mm undersize repair bearings, then have a crankshaft repair shop resize the mains to match the new bearings and if they're good they can set the dimensions to tighten the clearances.

Another thing to consider is the areas of the engine that rely on splash lubrication, such as the cam lobes against the lifters.