Theoretical Rev Limit

[ObrHaus]

Good day,

I’ve recently rebuilt the motor in my 935 tribute to be a bit more robust and I hoping someone can give me an idea of how hard it can safely be pushed. Currently I have my redline set at 7200 rpm in MS3x, but ideally I’d like to be able to run to 7500 briefly. When I do track the car (short tight track) I need everything I can get out of second gear due to the tighter corners. For reference this is a street legal 935 bodied car that’s going to an 8/41 ring and pinion with tall rear tires 345/35r19. Currently the motor is eunning an efi setup, 3.2 intake manifold with all the hardware to support it, no fueling issues, running at 1 bar street/track, and configured as follows;

Gt3 oil pump
Turbo oil squirters
3.3 turbo crank std/std no mods
Pauter straight cut intermediate gears
Dry film coated main, nose, and rod bearings
Pauter rods w/ ARP bolts
Mahle 98mm Pistons and cylinders
ARP head studs
Rebuilt turbo heads, intake ported to 38mm, twin plugged,
Flame ringed
Aasco valve springs with titanium retainers
Pauter rocker shafts
DRC 993 Sprint cam

I think my biggest fear is oiling (or lack of), but I don’t run above 7000 rpm for more than a second or two, just when going through the gears. I have several 911 engine builds under my belt now (turbo, non turbo, stock, and heavily modified), but my knowledge is still quite limited around this. Should I keep the rpm’s in the 7000-7200 range for safety, or am I being paranoid?

Thanks,
Andrew

[Tippy]

So no cross-drilled crank? I’d think 7500 is nothing for the rods, but without cross-drilling, middle journals run risk of oil starvation.

[ObrHaus]

That’s what I’m worried about, wasn’t sure if gt3 pump would help at higher rpm without cross drilling or solely just better for scavenging.

[JoeMag]

Though the GT3 oil pumps better, remember the oil pressure regulation is based on the oil pressure regulator piston/spring, not on the pump (assumption being oil pump can keep up). Assuming you're seeing better oil pressure at lower RPM than you'd typically see?

[ObrHaus]

I can’t remember exact pressure off hand (car is parked for the winter), but I do recall it being very strong oil pressure wise.

[Ollies930]

GT3 oil pump has same pressure side as 930 oil pump, so no difference in oil supply. But std 930 crankshaft will benefit from cross drilling (GT3 crankshaft factory cross drilled, for instance) at higher rpm. You might also want to shim the oil pressure control springs a bit, to get higher pressure (more flow) at higher rpm.

[JoeMag]

I had a long time well known race engine builder recommend I run 75-80 psi oil pressure in my race motor. It was recommended to use 6mm washers stacked in the plug to raise pressure.

I decided to modify the plug that holds in the spring/piston into an adjustable regulator so I do not need to keep removing plug to change pressure. ...it's sealed up using 3bond 1184.

Based on measurements here's what I've determined:
- oil press piston area 0.271 in2
- spring constant of spring 13.6 #/in (determined by compressing spring on a kitchen scale)
- Working the math 0.02" = 1 psi
- A typical 6mm washer is ~0.040", so raise oil pressure ~2psi/washer

[ObrHaus]

Thank you for the advice and way to turn the pressure up, I think I’m going to keep my rev limit at 7200 for now and next time I open up the case I’ll get the crank cross drilled.

Thanks,
Andrew

[PeteKz]

You'll be fine. Shimming the relief spring is common and easy, but messy, because the oil will drain out of the hole while you're doing it. So have something to stuff in the hole or another plug ready.