Can a 3.0 safely rev to 7,000 RPM on a consistent basis?

[dd74]

All the talk about the strength of the 3.0 liter motor makes me wonder if it can safely rev, on a consistent basis, to 7,000 RPM.

Well, the question is, can it? And with every shift through the gears?

My feeling is it can't. So with that in mind, what modifications would be needed to rev a 3.0 liter engine to 7,000 RPMs on a constant basis, that is with stock or 20/21 cams?

Thanks for your input.

[Matt Holcomb]

David,

You'd probably want to drop in a set of racing springs and titanium retainers.

Everything else should be tough enough.


Matt Holcomb
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1974 911 Carrera 2.7 (RoW)
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[Early_S_Man]

Late ROW SC engines had a redline of 7000 rpm, so the factory had no real concerns about reliability! There was an abundance of safety margins built into all pre-84 engines, and the rev limit was often related to the practical torque curve and not to mechanical limitations of the bottom-end or top-end components! Even the 68 thru '71 'T' engine was perfectly safe to 7000 rpm ... not so with the 3.2 or 3.3 engines with their 9 mm rod bolts!

[dd74]

So then...

In the event that 20/21 cams can rev the SC engine to 7,000 RPMs, there really should be no reliability problems, correct?

I have considered titanium springs and such, but I wonder about their necessity.

[aigel]

What will you get above 6500rpm? It won't make you faster, unless you have more umph up there too whis isn't the case on a stock engine. It's asking for trouble in my eyes. At that high an rpm, the piston chases the exhaust valves when it is closing. I'd be afraid that the piston wins the race. BAM!

Build the engine with the right tolerances, ARP rod bolts, aftermarket rods, stronger (quicker return) valve springs and cam / intake / exhaust that produce the high rpm power and you are all set.

Don't rev the snot out of a stock engine. It doesn't make sense, unless you are an engine builder in need of business. My $.02

Cheers, George

[Bill Verburg]

The following are the factory max permissible rpm

• '76/77 Carrera 3.0 7000
• '76/77 930 7200
• '78-81 SC & 930 7000
• '82/83 SC 6700
• '82/83 930 7000
• '84-89 Carrera 6520
• 964/964RS 6700&#177 20
• '94-95 993 6700&#177 20
• '95 993RS 6840

In some cases the rev limiter is set lower than max permissible rpm.

[dd74]

Originally posted by Bill Verburg
The following are the factory max permissible rpm

• '76/77 Carrera 3.0 7000
• '76/77 930 7200
• '78-81 SC & 930 7000
• '82/83 SC 6700
• '82/83 930 7000
• '84-89 Carrera 6520
• 964/964RS 6700&#177 20
• '94-95 993 6700&#177 20
• '95 993RS 6840

In some cases the rev limiter is set lower than max permissible rpm.

Very interesting. Thanks, Bill.

What would attest to the 300 permissible rpm difference between the '82/83 SC and the '78/81 SC? Does it have anything to do with the intakes on the '82/83 being smaller, for example?

[cstreit]

Remember with a 20/21 cam you are also reducing your valve clearances... A mis-shift could cause you some heartache, and with 300 RPM more you are just narrowing your margin...

[cowtown]

Originally posted by Bill Verburg
The following are the factory max permissible rpm

• '76/77 Carrera 3.0 7000
• '76/77 930 7200
• '78-81 SC & 930 7000
• '82/83 SC 6700
• '82/83 930 7000
• '84-89 Carrera 6520
• 964/964RS 6700&#177 20
• '94-95 993 6700&#177 20
• '95 993RS 6840

In some cases the rev limiter is set lower than max permissible rpm.

Wow - Bill are you sure the 76/77 930 is 7200 and not 6200??

I've got my Crane spark cutoff set to 6000 right now as I make sure the 930 engine is ok, but I'm ready to reset it to factory redline now. I thought it was 6250 before I read this!

Anyone got a second source to confirm?

[Bill Verburg]

I'm hurt,

The following are all from the manual '76/'77
930 rev limiter 7000 &#177 200
max power is at 5500 where the mean piston speed is 12.9m/s

C3 max power is at 6000 where the mean piston speed is 14.1m/s
rev limiter 6800 &#177 200

2.7 rev limit 6500 &#177 200, max power is at 5800 mean piston speed at max power 13.6m/s

[cowtown]

Originally posted by Bill Verburg
I'm hurt,

The following are all from the manual '76/'77
930 rev limiter 7000 &#177 200
max power is at 5500 where the mean piston speed is 12.9m/s

C3 max power is at 6000 where the men piston speed is 14.1m/s
revlimiter 6800 &#177 200

2.7 rev limit 6500 &3177 200, max power is at 5800 mean piston speed aat max power 13.6m/s

I didn't mean to question your accuracy by any means. It's just that we've all seen inconsistencies across different literature on power ratings, fastener torque values, etc.

It's a high-stakes kind of question - I just wanted to be sure!

Thanks Bill.

[Bill Verburg]

Just kidding, the manual could still be wrong

[petevb]

Hold on a minute here. 6520 for my poor 3.2 is the lowest of the lot. That can't be right... Seriously, I'd heard that 6800 was the recommended "shift or you'll break it" point for those cars. Is this contradictory, or a more conservative limit? The weak point is still those rod end bolts, yes?

[dd74]

Well, I don't know. If Bill's information is correct, then we should be shifting our cars at a much higher RPM. That is, if the rev limiter doesn't come into play first.

Makes for a faster 911 at any rate.

[aigel]

Originally posted by dd74
Well, I don't know. If Bill's information is correct, then we should be shifting our cars at a much higher RPM. That is, if the rev limiter doesn't come into play first.

Makes for a faster 911 at any rate.

Thats BS. How can you generalize and say that shifting late will make you "faster" on a stock engine?

On a factory engine, shift where the factory put the redline or before then. The P factory isn't going to keep you from revving the engine just because.

The power curve declines rather steep at that high an rpm and all you can gain is engine failure, not speed. You also will put yourself back at a high rpm in the next gear after the shift, and you will not be covering the highest hp and torque band that you can find.

I'll stop now, what do I really care? If you guys want to blow up your engines, go a ahead, I am sure the engine rebuild forum will be happy to help once the pistons hit the valves.

George

[1fastredsc]

But if you change your cams, as dd has already mentioned to say, 20/21 cams, then your still pulling hard with power when you hit the redline. Hence the bases of the question to push up the redline safely if there's a reason to, without changing things such as the rod bolts, springs, and retainers.

[aigel]

fastredsc, I got you. Changing valve springs is mandatory, along with retainers, especially since you are in there already changing cams. That will help the exhaust valve to close and will keep the valves from wanting to float at high rpms.

I still would play it safe and shift at factory recommended points because your bottom end is still factory. The rod bolts can stretch. If they stretch once because you over-revved, they may have yielded and then the bottom end can come apart on you later becasue the nuts may work themselves loose.

The key to building a reliable engine is to leave appropriate safety margins. If you put everything to the absolue limit, chances are that you will have premature failure. Not only Murphy and his family haunt us, but you significantly increase the likelyhood of catastrophic failure with every component that you operate at its limit.

If you want to go fast, it will cost you money to build a solid racing engine. You can't go fast with quick fixes. If it would be that easy, the factory would have done it already. I know, a lot can be improved over the factory's work just by ignoring emissions (which the factory couldn't), but still, ther is quite a limit of knobs to turn on a stock 3.0.

George

[dd74]

Good points, George. Thanks.

[Bernd Andritzky]

Hi,

weīve had an similar discussion concerning the rev limiter of the 3.2 engine in our german forum (http://www.forumworld.tv/cgi-bin/ultimatebb.cgi?ubb=get_topic&f=2&t=002274
) with the (my) conclusion, that the Porsche engineers had to make the experience that the valve springs get "softer" after many years/miles. They didnīt know that in the early 80īs and so the SC-engines were allowed to rev 7000rpm and more.
Knowing that the rev limit gets lower at high ages/miles they decided to put the rev limiter of the 3.2-engines to 6520rpm.
There surely is room to lift the rev limiter about 200rpm without any other modifications, but if You want to put the rev limiter to 7000rpm the first thing to do is using new (original) valve springs.
Stronger springs can cause some other trouble because of too high forces damaging the cams and/or the lever arms. The whole system (valves, springs, lever arms,...) has to match, so please no experiments without really experienced help.

Bernd

[Paul Franssen]

Danke, Bernd, sehr interessant!