How does Porsche determine redline RPM on various engines?

[supersignman]

I was wondering if anyone here with engine building experience knows how and/or what determines the redline RPM Porsche designates for their various air-cooled engine combinations? I've had extensive experience building small block Chevrolet stock car racing engines but I'm a newbie with my first air-cooled 1986 3.2. I have to say it's the smoothest, most well-balanced engine I've ever felt by far though. I've built 600 hp SBC engines with shaft mounted rocker arms, forged pistons, racing valves/springs, etc. and our self-determined redline numbers were determined mostly from talking with other racers running similar engine combinations. It was commonly known that you didn't really want to turn the engine combination we were using more than about 8200 RPM for very long or you were going to have problems but we never blew one up due to over revving. In fact, we had a problem on one dyno run and an engine went to 9000 but we never had any problems out of that engine.

One of the reasons I'm curious about this is because it seems like a lot of people have the top end of their engines done around 100K - 150K and I'm in that range. I'm pretty sure my valve guides are worn because I see a little blue smoke from time to time and I'll go through about a quart of oil every 300 miles of normal driving or 100 miles of hard driving. With my engine building experience that amount of oil usage doesn't concern me because we used to run without any valve seals at all and would easily use 1 - 2 qts in a 100 miles of racing but if I could get a little more performance a top end rebuild at this point might be justifiable.

It feels like my 3.2 just loves RPM. It's quite a disappointment to have to shift at 6250. The engine seems like it really just "gets on the cam" at about 5000 and feels like it could pull hard to at least 6750 if I was to take it that far. I guess I'm wondering if valve springs and/or valves are the main RPM limiting factor and if having my heads rebuilt with some performance springs and/or valves would allow me to safely move my redline up? From the limited knowledge I have so far, it seems like some of the early 70's RSR engines have a redline quite a bit higher than mine.

[GH85Carrera]

If you are an engine builder you understand the valve springs can't move the large valves in the 911 head really fast. The stock rod bolts can stretch if they are over stressed.

Part of the issue is the breathing inside the case. If you do a total rebuild, and do some fly-cutting on the case, and stronger connecting rod bolts, and other expensive things like titanium valves and such, you can get the revs up. Just open your wallet and empty it out.

[supersignman]

I can certainly see where rod bolts might be the weak link. Do the higher revving earlier engines have better bolts? With my limited experience on these engines so far I can see where a wallet can empty very quickly without a lot of HP gain if you don't use discretion. Titanium valves aren't out of the question though if I could move the redline up with just a head rebuild.

[kevingross]

From my limited exposure to engine building, I believe the answer is that it depends on a lot of things such as valve float, lubrication delivery, cooling, loading on moving parts (rods, valves, etc.), and so forth.

[darrin]

careful with the worn valve guides too -- as built/spec'ed, the valve to guide clearance helps dissipate heat from the valves to the head --too big a gap between the worn guide (not seal) and valve can allow the valve to overheat and (worst case) drop a head --

[crashmy911]

I think rod bolts first thing to do then titanium stuff and race springs. A chip and you should be able to rev to 7 grand

[RWebb]

Level 1: Ti valve keepers; ''race' valve springs; ARP rod bolts on stock rods; pistons if you want

- I am at level 1 (good to 8k spins) & have it twin-plugged too

windage, etc. is not very imp. on a dry sump motor

* * *

Level 87: electro- valve "train"

[supersignman]

Thanks for the input! I'm thinking now I may have asked the wrong questions though. I may should have asked first if the stock intake runners, valve size, cam, etc. will actually allow the engine to keep pulling hard after 6250 or does it fall flat? If it's just an illusion that it wants to keep pulling then I probably don't need to waste the money on eliminating the weak link in the valvetrain or rotating assembly.

[Trackrash]

3,2s can gain quite a bit from an exhaust upgrade, usually meaning headers, and a chip to match. Spinning it faster is a completely different ball game.

[Lyle O]

Cam selection is also important. The stock 3.2 cams make peak power (in a stock motor) @ 6100 rpm I think (maybe less); 964 or 20/21 cams, with longer duration, move the peak higher (~6500+). Revving above 7000 really matters most if you have power to go with it.

[RWebb]

yup, cams & exhaust, Steve Wong chip

what state are you in? what emissions regs. do they have?

just be glad you don't have CIS

[supersignman]

RW Webb... I'm in Alabama. We're 49th in everything... thank God for Mississippi. The good thing about this though is that we don't have any emissions or inspections. Run what you brung and hope you brung enough.

[Locker537]

“Speed costs. How fast do you want to spend?”
- Donsco

[kent olsen]

Interesting responses.

Lets say your tach shows a redline of 6250. My guess is the engine makes peak power around 6000. So anything past that your losing power so why not shift. Now if your racing and your in the middle of a turn then your might stay and let it over rev some. As mentioned that comes back to how strong the engine is built.

For example I have a twin plug 3.0L in my 72T. Built to turn over 8000. I had a special cam cut for track day use. The peak power is at 6900 so I set the rev limiter to 7200. My gearing is such that when I shift a 7000 it drops to 4000 and the peak torque is 4500. Torque is what pulls you out of a turn not rpm above the peak where your losing power.

When I built the engine (with Mike Bruns help in Florida) I wanted to be able to run the daylights out of it and once a year change the oil and adjust the valve. Several years ago I had a missed shift. I saw the tach go past 8000. Everything was fine, no damage and I've put another 10,000 miles on her. Thank you Mike.

[RWebb]

Originally Posted by supersignman

RW Webb... I'm in Alabama. We're 49th in everything... thank God for Mississippi. The good thing about this though is that we don't have any emissions or inspections. Run what you brung and hope you brung enough.

I do believe you are confusing Alabama with Louisiana - and you stole our state motto.

But feel free to duke it out with Arky'saw for 48th...


anyway you can either put on dual free flow cats or straight SSIs to muffler (depending on your level of personal care for smog contributions; or plant some trees to mitigate)

[WP0ZZZ]

Originally Posted by kent olsen

Lets say your tach shows a redline of 6250. My guess is the engine makes peak power around 6000. So anything past that your losing power so why not shift.

If you are upshifting for maximum acceleration, the ideal upshift point is clearly beyond the peak power point.

Ideally, the upshift should take place when power in the new gear is as high as power in the old gear.

An easy way to see why this is the case consists in writing down the formula for car acceleration as a function of power and see what's the car acceleration if you upshift at peak power or if you keep the old gear for a little longer.

[supersignman]

RWebb... I actually know a guy with a Tesla... will that cancel out the carbon footprint of my oil-burning 911 that is usually found with my right foot planted firmly on the floorboard? If that's not enough maybe I could start recycling like my neighbors. That should help a lot to make up for my contribution to mild winters. They put out their little basket of plastic bottles and milk cartons every Tuesday morning for the 50,000 lb. diesel-burning-hydraulic-monster to roar up and collect. I've avoided the conversation about the reality of how that truck uses more energy sitting there idling for 30 seconds while picking up their plastic bottles than they ever contribute. I think I'll go hug a tree now to make up for my terrible attitude.

[supersignman]

WPOZZ... That makes sense. I think I've read something about "power under the curve" before where it talks about maximizing the area under the dyno curve to determine the best gearing and shift points. If most dyno curves look like a mountain peak then your maximum area under the curve (which would equate to the most hp) would be evenly distributed before and after the peak.

[Josh D]

The cams are on a 113* lobe center for emissions reasons. There just isn't enough valve overlap for breathing above 6700 rpm.

[stownsen914]

Agreed on the power under the curve philosophy on redline. I’ve understood that a 3.2 can breathe well enough to rev to 7000 if you put headers and a good performance muffler. The stock valvetrain and bottom end will both tolerate 7000. That does put you into a riskier zone if you miss a shift of course - you’ll be that much higher and more likely to bend valves. Better rod bolts are a great investment if you have the engine apart. A top end rebuild will make that accessible.