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Chris T

Rev limit for hot 3.4?

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Ladies and Gents, not wanting to divert 911Hillclimber's thread (like I have on DDK!)......

 Next week my 3.4 converted 3.2 goes off to be dyno tuned. It has a Canems ECU so I can choose the rev limit. Spec is Std 3.2 crank and rods but with ARP rod bolts. 98mm (3.4) bore with JE 10.5:1 pistons, cams are 993SS items with std valves and retainers but race springs. It will also have a 993 plenum inlet, SSIs and twin sparks but the internal stuff is what determines a safe rev limit.

So, stock 6400? 7000? 7250? 7500?

Thoughts please. Chris.

 

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My limiter kicks in somewhere around 6750 if that helps.

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Yes, but I think it’s the rod bolts that fail not the rods them selves. Your ARPs should see you good 

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Get it on the dyno, set it at 7k and see what the graph shows you

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Hi Chris,

Good to hear that the car is nearing completion, should be mega.

I believe a standard CS is 6840, so as Shirish says, I think it should be good for 7k. 

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I asked the question to Nick Fulljames who supplied pistons, cylinders and advice. He reckons power will be dropping off by 7250 although the engine will happily rev to 8000(!). As such he recommends a limit of 7500. Sounds a bit high to me, but he knows these engines better than most folk....

Chris.

 

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My old engine is probably closest in spec to yours without being 3.4.  The dyno will provide part of the answer, Nick gives you a technical answer and the rest is your preference.  I will explain.

1. The dyno will find peak hp and tq - there is likely a graph on my hot rod thread somewhere that would be a very good guide.  The cams will hold tq longer than std but on the dyno numbers alone, there is likely no point in a rev limit higher than about 6750rpm.  Even at 6750rpm you will likely be at/past peak power and past peak tq which IIRC was around 5400rpm or thereabouts.  Maybe you are good power to 7250rpm as Nick says, but I am sceptical.  Anyway, IIRC, 6750rpm is where Wayne set my limit initially based solely on the dyno as there was no point going higher by that criteria alone, though I changed that limit later (see point 3).

2. Nick is also giving you something close to the technical safe rpm limit based on components.  Your valve train sounds like its built for 7000+rpm.  You are relying on the race valve springs to keep the top end in shape, but I see that as more of a mis-shift insurance policy than a licence to set the redline at say 7500rpm.  You are relying on the ARP bolts for the bottom end.  I defer to Nick, but I have never heard that 7500rpm is sensible on ARPs.  Sounds like an upper limit as your bottom end is not lightened, just strengthened a bit.  Whatever, it certainly gives you licence to play in the 7000rpm area IMHO.

3. Preference comes into it on the basis that a higher rpm limit is useful given the somewhat lengthy 3.2 gearing.  Your 3.4 litres, plenum + res flap and resultant tq will help this, but having more rpms to play with is also useful sometimes if you just want to hold a gear a bit longer.  Particularly in say the Alps, once altitude, sharp corners and steep roads come into play.  I set my limiter at 7200rpm in the end, though most of the time I never went that high as you can feel the power is tapering off.

HTH

[Edit - peak tq was almost 250lbft at 5300rpm and holding almost flat to 6750rpm and peak hp was c280hp at about 6700rpm.  Both tapered off quickly thereafter.  Will re post the graph.]

Edited by Richard Bernau

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That should be 280hp at 6250rpm (not 6750rpm) and holds flat to 6750rpm.

Lower curves are std 3.2 - though in hindsight, the scale on those looks way off.  But, the shape of the tq curve is the important thing to takeaway.  Cut to the chase, I think 6750rpm is too low and 7250rpm is fairly safe and is all you will need.  I would be more worried about fuelling and mis-shifts as potential causes of disaster.

 

bernauaftervsstockCarrera32_zps258cfc31.

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I like graphs , and I'm told mine is a 3.4 ...😎 you can see mine peaking the same as RB's around 6200 and max torque at 5200ish , although mine runs the standard barn door

P1010210.jpg

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Thank you gents for taking the time to post such detailed replies, much appreciated. Sounds like Nick's answer is indeed the mechanical maximum possible, beyond what may be required and beyond the engine's best. When the dyno plot is done I'll look at a sensible place to put the rev limiter with perhaps a 'soft' limit at 7000 and a 'hard' limit at 7250. 

All of this is immaterial unless I get the engine built and back in before the day. so I'm back up to my garage right now! 

Cheers, Chris.

 

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I dont think the engine will particularily like or hold for a long time if repeatedly stressed to 7500 RPM.

Anything much above 7000 will strain things and you will need a reinforced drivetrain for Porsche Reliability. 

that 993SS is probably all done by 7000 as well. 

Id put the rev limiter at 6700-6800 tbh.

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Axel, I'll see what the graphs say and set a limit accordingly. I take your point about power trailing off towards 7000 but I also respect BR's point about occasionally wanting to stretch it just a bit further before braking..... I have a hard and soft limiters as well as shift light facility, so shift light at around 6750, soft limit at 6900 hard at 7200 or something like that.....

Thanks again all, I'll report back! Chris.

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will be interesting to see the graph!

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18 hours ago, Chris T said:

Axel, I'll see what the graphs say and set a limit accordingly. I take your point about power trailing off towards 7000 but I also respect BR's point about occasionally wanting to stretch it just a bit further before braking..... I have a hard and soft limiters as well as shift light facility, so shift light at around 6750, soft limit at 6900 hard at 7200 or something like that.....

Thanks again all, I'll report back! Chris.

Sounds like a sensible approach Chris. 

When are you planning on getting the car on the rollers?

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