cam timing and compression and ign timing

[efhughes3]

Yes, it is the 3.2 in my '84, which I'd rebuilt 2 years ago reusing my p's and c's which got a clean bill of health by Ollies.

[Gunter]

The wrist pins in Carrera 95 mm pistons are 23 mm.
Are the new 98 mm pistons prepped for that?

[efhughes3]

These are 98mm pistons spec'd by Andial, made by Mahle for a 3.2 bolt in. A 3.2 to 3.4L conversion isn't that rare...

[911st]

Ed,

Are you going to upgrade the rods?

A well tuned and cam'd 3.4 can make a lot of HP past the point where the stock ones put you at risk. Even with good rod bolts it seems.

Sounds fun.

[efhughes3]

I've already got ARP rod bolts, Supertec studs, Ti retainers, racing springs, cams, twin plug heads, etc, etc.

I spent a lot of money building it, with the only "shortcut" being the re-use of my p's and c's. I have a new thread going in the rebuild forum calling it "Ruby's rebuild Rev1" or something similar.

This is a pretty common upgrade, IMO. I've not heard of any rod failures with 3.4l motors still running Motronic.

[911st]

I was of the thought that rod bolts would take us to 7k and a bit beyond but the real expert's that make money building Porsche race motors seem to advise otherwise.

I also do not know if 3.4 pistons weigh more or less than stock 3.2's. This might be a factor.

Thus, if say 6800rpm is not going to be exceeded probably no issue but with the right cam and exhaust usable HP can be made past that point, even with Motronic's. Our AFM's are used on cars that make up to 285/300hp (M5) so that dose not seem to be a big limitation though it seems to be costing us +/- 10hp.

Sounds like another nice addition to an already great car.

[efhughes3]

This isn't a full race motor, and I will probably stay with a 6800 RPM limit. Again, I'm not trailblazing here, this modification has been done for years on more than a few 3.2's. If I had super radical cams and ITB's or carbs and wanted to get into the 7000+ rpm range, then certainly Pauter rods or similar (and a number of case mods) would be warranted. Other than careful checking of deck height and finally powering the bottom set of plugs, I shouldn't have any issues. This will give me a great little street motor, and will finish what I started two years ago, IMO. That's my story, and I'm sticking to it.

Part of my build sheet is a new chip from Steve W (I think he's sick of upgrading my original chip from him!), and I hope some time on the dyno with he and his laptop present.

[T77911S]

Originally Posted by Gunter

Reducing valve clearance from 0.004" increases the risk of burning valves, mostly the exhaust valves.

yes i know, i only asked to find out the effect valve adjustment has on cam timing. when we do a valve adjustment, can we really tell if it runs any different or is it just in our heads that it should run better.

[T77911S]

Originally Posted by mca

I always heard the opposite.

However, my 964 cams are timed at 1.4mm (1.26 is the "norm" so mine are advanced) and I have noticed that my power isn't dropping at high rmps nearing redline. It honestly feels like it want to keep going and going. Doesn't run out of breath.

Yet I haven't noticed any change in low rpm power.

Anyhow, just thought I would share some off topic, non-scientific data.

hey craig,
you may try advancing the cam and see if the low end gets better, or it comes in at a lower rpm, without losing the top end since you say it feels like it wants to keep going.

[911st]

I believe once the valve is at about 40-50% lift it is at its max flow rate on our motors. After that the curtain area under the valve seats quickly exceeds the capacity of our ports.

More lift dose not mean more flow. Keeping the valve open longer ngets us more flow.

Lift is a byproduct of duration, not necessary a goal although the higher the lift the steeper the ramps on the lobe and the faster the valve will be opened or closed.

Thus, tightening up the valve clearance is probably not going to keep the valve open any significant amount longer.

[mca]

Originally Posted by T77911S

hey craig,
you may try advancing the cam and see if the low end gets better, or it comes in at a lower rpm, without losing the top end since you say it feels like it wants to keep going.

My cams are advanced - set at 1.4mm. I was told to set them at 1.26mm.

But then I saw Gunter's comments around the different timing specs for 80-83 SCs. Now I am wondering if I should go more advanced?

Either way, I am happy with my set up. Really enjoying it.

[mca]

Originally Posted by Gunter

SC-engines have essentially the same engine and cam and yet, the '78-'79 non-Lambda overlap is 0.9 - 1.1 mm but the '80-'83 with Lambda is 1.4 - 1.7 mm.

Gunter,

Thanks for sharing this info. I had NO IDEA that there were different cam timing specs for Lambda SCs!

I have 964 cams set at 1.4mm (82 SC with Lamda - O2 sensor disconnected) ... does this mean that my cam timing is retarded and not advanced? I was told to set them at 1.26mm.

Thanks,
Craig

[911st]

The 3.0 CIS motor at different times advanced and retarded cam timing. With this peak HP moved from 5500 to 6000rpm. With the 3.2 Carrera it kind of split the differance.

The 3.2 Carrera and 964 used different cams with the 964 increasing duration or time the valves are open about 5 deg. This moved HP peak from 5900 to 6100rpm on the 3.6.

Of course intake and exhaust construction can effect breathing and move peak TQ & HP around some also.

Valve to piston clearance determines how much one can move an existing cam around.

If one puts a 964 cam in a 3.0 or 3.2 the window one has to play with cam timing should narrow as it has more lift.

[911st]

If playing with different cam timing one should test there valve to piston clearance. I belive this can be done by taking the slack out of the valve clearance and turning it one to one and a half turns. This should equal about .040 to .060. If the piston still clears the valve then there is enough clearance.

I would double check this before trying.