When you click on links to various merchants on this site and make a purchase, this can result in this site earning a commission. Affiliate programs and affiliations include, but are not limited to, the eBay Partner Network.
especially when you compare how tight the 928 tolerances are compared to other tried and proven performance engines.
Funny you should mention that. Last night I went looking for the assembly clearances for the 928 big end and main bearings. While I could not find those specifically stated, it seems that 2-3 thou on a 2" bearing is a good number.
Also, the 928 engine is often talked about as "20th century design", but given your comments re the clearances, how does it compare, in absolute terms, with other engines, from then and now, in it's bearing design?
....Also, the 928 engine is often talked about as "20th century design" ...
When considering most cars built in the first decade of this century might it be more appropriate to say it is "21st century technology designed/built in the 20th Century"?
I think that adding a second oil pressure sensor after the mains, perhaps in the heads, might make sense and detect any problems earlier. However, there's the effect that pressure in a relatively slow flowing fluid is going to equate quickly, and I'd be interested in understanding the scenarios in which there are big differences in the oil pressure between different parts of the galleys.
Another thing to think about is the amount of air in the oil. Since the 928 oil pump has very high capacity and is designed to bypass a lot of oil at high rpms, it's conceivable that even if there's some air in the supply the pump can compress the air to a high enough pressure (and bypass less) such that the pressure in the oil galleys remains high. Since air doesn't lubricate very well, this would be a problem; and more importantly, a problem that is not necessarily revealed by the oil pressure reading.
One thing that would be very useful measurement would the cycling of the oil pump bypass valve. If the oil bypass valve starts bypassing less of whatever is in the system, that's an indication of the pump pumping compressible fluid -- there's a lot of air in the system. The bypass valve position, oil pressure, and rpm could together be used as a simple table that would give the amount of air in the oil. High rpm, low oil pressure, and bypass valve not bypassing would be an indication of a lot of air in the circuit. But, important, high rpm and bypass valve not bypassing would still indicate a problem even if the pressure in the oil galleys is high -- it's partly pressurized air in the galleys.
On the rod side clearances, here's my opinion. The rod side clearances in most engines, and by memory in 928 S4 engine are so wide that they don't directly influence the oil flow rate from the simple leak area perspective.
The reason why one might want to open up the rod side clearances in a high rpm engine is that the crank and the rods bend and flex. Larger bearing clearances and large side clearances allow for these changes in shape without creating metal to metal contact with a high load. The problems created by too tight rod side clearance may look like oil supply problems, but I believe that the underlying cause is different. This is just my belief, not pretending to be an expert. Just pointing out that from the simple geometry perspective, the side clearance shouldn't impact oil pressure in any way as long as the parts don't change shape.
One more thing relating to some earlier comments in the thread. I don't understand why, say, 4 bar oil pressure at 2000 rpm would ever be a problem. The required oil supply pressure at the mains is proportional to the square of the rpm. At 2000 rpm, the main need only about 10% of the oil pressure that they need at 6700 rpm. 4 bar of oil pressure at 2000 rpm should be plenty, if there are no actual problems with the engine (and thus the 4 bar oil pressure isn't a symptom of those other problems).
A smarter oil pressure warning system would have a table of the warning light coming on at different pressures depending on the engine rpm. This could again be accommodated by an ECU that would take the bypass valve position, rpm, and the oil pressure as inputs.
Originally Posted by Hacker-Pschorr
It has been discussed and something I'm very surprised doesn't get more air time around here, the one perceived downside is you will see lower oil pressure.
The engine sitting in the corner destined for my 79 track car is only coming apart so Turbo Todd can machine the connecting rods to open up that clearance. I'm also doing the same for the 944S engine I'm putting together. Even though it's a 100% street car.
The specifications for every stroker I've seen had wider side clearances than a stock engine.
Originally Posted by Hacker-Pschorr
IMHO it's a no-brainier to increase oil flow across the rod bearings, especially when you compare how tight the 928 tolerances are compared to other tried and proven performance engines.
A while ago I talked to Todd about adding an oil pressure sender after the rod bearings like the GM LS motors have. I think that would be very useful in a track car.
It becomes a problem in stop-start freeway driving in the L.A. basin in the summer. Which I try never to do, but occasionally get caught in. You can sit and watch the oil pressure nudge downward. I am adding a virtual oil temp gauge into the flush console build so I can get a better feel for oil temps under various driving conditions. (I have a set of these hoses but pathetically have not yet installed them...)
about as hot as it gets out here in vegas..stop and go. Measured via a sender in the drain plug for the oil and on the crossover next to the stock sender for the water..
Id like to add an external cooler and ditch the radiator. car has a hard enough time staying cool let a lone cooling enigne oil also.
you also see why i added an aftermarket temp gauge.
My oil pressure gauge wasnt working at the time of this picture...it is now so i will be curious to see what the pressure shows. Never had the oil pressure light come on however.
I essentially never see 5 bars of pressure on my stroker unless I'm way up in the rev range. Cruising around at 3K rpm, I'm typically below 4 bars. The first road trip I called Greg shortly after I took this picture. He laughed and said, 'Yeah, they all do that'.
And there shouldn't be anything wrong with 4 bars at 3000 rpm, as long as it's not an indication of some unrelated problem. The crankshaft mains require 1/5th of the oil pressure at 3000 rpm compared to the oil pressure that they require at 6700 rpm.
Someone who knows their electronics should put an extra logic piece in the instrument cluster that would turn on the oil pressure warning light at different pressure at different rpms. 2 bars at 2000 rpm is probably enough, while 5 bars at 6700 rpm might indicate a problem.
.... Id like to add an external cooler and ditch the radiator. car has a hard enough time staying cool let alone cooling engine oil also ...
I have the same view. I have removed the radiator oil 'heat exchanger' in my 82 and fitted an independent oil cooler. I can't even see the benefit of the rad connection to warm the oil, considering the thermostat won't allow it to circulate until it's up to temp anyway.
In a decade of racing I always saw oil temps skyrocket before coolant temp went up ... not the other way around. Bigger oil cooler and/or more air to cooler kept engine temps under control, as long as water coolant system was functioning correctly.
My question to you all is that has anyone installed a larger, dual pass oil cooler to the stock location? With dual pass, one could make the core much deeper as the tube behind the cooler would be no longer be required. The larger core would greatly increase the capacity of the core as a heat sink and reduce any oil temperature spikes. A simpler shroud could also be fabricated. Finally, if this were a kit, hose to run the oil in series thru the end tank and external coolers would make sense.
04-30-2015, 08:11 AM
