993turbo

I have a 90' GT which Im currently in the process of tuning with my ST2.

Its a std 90' GT M28/47 engine freshly built. Pistons / Rods ''within a gram''. Times Cams etc. X-pipe, No cats, ''Sportsfilter''. Airpump / AC delete

The org. rev limit is 6800 if I remember correctly.

How much can I SAFELY go up to?

Is there anything to gain above this level?

Any experiences here?

dprantl

You can go up safely by about -400RPM, maybe more even. Yes, that's a negative number. Unless you address the high RPM oiling issues of the engine, you don't want to be running anything more than ~6,500RPM or so.

Dan
'91 928GT S/C 475hp/460lb.ft

Cheburator

There is nothing to gain - in fact at 6900rpm you will get valve float, which does not sound good... Without better cams/bigger valves/better flowing intake/more fuel/better exhaust you will not gain anything by raising the rev limit.

AO

To answer your question, I think you can push up to about 7200 RPM or higher. I remember talking with Todd T. from Green Bay about this very subject. He felt that the GT (in stock form) was fully capable of going as high as 7500 without issue. But I think it is a pretty well documented fact that the valve springs are maybe not up to the task and you could get valve float.

As for the oiling issue... That's a can of worms there IMO. I cannot say if it is an issue or not. I will say, that i would not run a 928 engine at high RPMs for a sustained period without looking in to oiling. Brief journeys in those areas of the rev range are probably okay, but one man's brief, is another's eternity, so I will leave it at that.

I will say that even with my exhaust and boost, HP peaks at about 6k. So I have no reason to venture into the stratosphere. SCer doesn't like it way up there anyway.

ptuomov

Before you read below: Caveat lector, caveat utilitor!

There's three considerations really: Valvetrain, main rotating assembly, and oiling.

Valvetrain:

The valvetrain is set to float normally aspirated before the revlimit. I can't prove that, but the simulator says the valves will float and the marks on the cam lobes also may suggest that. These engines don't make more power up there anyway without bigger cams. Therefore, you can install beehive springs and lightweight hydraulic lifters at the same time you install the bigger cams and cure the float problem exactly to your desire.

Main rotating assembly:

The basic rotating assembly of GT and S4 are the same weight and strength per year, I think. Therefore, the limit is the same for GT and S4 built the same year. Later years they went from the more expensive powder forged to cheaper cast rods. The later cast rods for the weight ought to be weaker, but I can't prove that. One person who owns an engineering company and understands engines at a completely different level than I do computed the critical limit for the stock rotating assembly to be 6900 rpm. However, I believe that this is with a very conservative accepted failure probability. An additional problem with the rotating assembly is that there are no cheap and durable rod bearings for the stock journal size. One might want to turn down the crank for a more common rod journal size, but each step opens new expensive problems if you go down that route.

The oiling has many problems:

The problem that is the hardest to cure is the large main journal size, which requires very high oil pressure. One needs to make some computations to make sure that the 8 bar oil pressure is enough for the planner rpm, and there are some friction etc. assumptions in those computations that make them difficult. If the crankshaft is drilled in a way that the oil passages stay in the outer circles of the crank, then the problem caused by the large main journal size is somewhat mitigated -- unfortunately the 928 crank oil passages go smack right thru the middle of the crank centerline.

The problem that causes the most failures is the oil pickup uncovering. The pickup uncovers if the rpms are high or the cornering forces are high or, especially, both. Oil pan is shallow which directly drains the oil out of the deep part of the sump and in addition increases the probability of crank hitting the oil. When crank hits the oil, 4000g centrifugal forces or whatnot are involved vs 1g for gravity. The oil ricochets off the walls and oil in the sump back to the cranks, which is like ping pong ***** in the lotto machine. (The 4000g makes sure that it's not "clinging" to the crank, all the action is in the bounce of the oil from the crankcase or oil surface.) The 1g turn effectively places the engine on it's side making the oil drain channels from the heads level so the puny 1g gravity acceleration goes to 0g. The pressure differential between crankcase (high) and the heads (low) creates oil plugs in the drain channels, completely stopping the oil drain from the heads, literally suspending the oil in the drain channels even after the turn.

I am not going to venture a guess about the oiling solutions here, use the search and decide your self which solutions are snake oil and which are the real deal.

Don't believe a word of what I wrote above without thinking it thru yourself.

WallyP

My only comment - 1G gravity plus 1G turn equals 45 degrees.

ptuomov

And 1/2 of the 90 degree V8 angle is...?

Of course you are right about the engine position. I was simply referring to the oil drain channels form the heads that are in approximately 45 degree angle since this is a 90 degree V8. I think they effectively at horizontal level in a 1g turn, right?