Jfrahm

Hi all,
Any good calculators out there to relate compression, boost and timing? I have a Supercharged 968 with the stock 10.9:1 compression and I wonder if power would go up with less timing pullout if the CR was reduced. I can add a tall head gasket and maybe take .3-.5mm off the rods by resizing (not sure about this.)

Power is good now and I am planning to add water injection too, which I have had great results with on my Saab. However I plan to move the supercharger over to my black car and put in some forged rods I just scored so I have the opportunity to play with the CR a little bit. I might get it down to 10:1 without too much hassle. I've seen so much power fade away with timing pullout on other cars that I wonder if the engine would be happer with lower CR.

I suppose I can test the car as is with and without water injection and see if I get some timing back.

Cheers,
-Joel.

V2Rocket

why not just get lower-compression pistons

fast951

The only way to answer your question is to tune it on the dyno. The WI will help, I had to use it on my supercharged 914 in order to add any ignition. In a SC application, heat is on the high side, unless you intercool it.
The 16V head is very good when it comes to detonation. However starting with a static compression of 10.9:1 then adding boost, your dynamic compression will be fairly high. So the fuel octane will be a issue.

The 968 uses knock sensors, so it might be already pulling lots of timing to keep it under control. This uncontrolled ignition retard will not help performance, it is designed to try and save the engine if possible.

Reducing the static compression is a good idea, you might get away with the higher CR if you run E85. Regardless, you should tune your AFR and ignition on the dyno. This is the safest and the only way to get the most out of your engine. Tuning can be done with a PiggyBack.

Another thing to be concerned with is the DME itself. There is lots of code in the DME that will get you once you add forced induction.. The DME could go into a limp mode, AFRs will be fine then lean.. Again, many factors are in play, just something to keep track of.

gt37vgt

i would keep the engine together play with water injection E 85 or LPG (105 octane ) if its in your area measure inlet temps engine temps listen get something to help you listen for a knock like and amp or safeguard ..another guy on here managed 10 psi and 400 or so hp from a standard S2 engine with a turbo
i think this is pretty realistic . it will cost you at least $2000 to do anything with the bottom end it always does
don't go near the con rods stay focused on tuning it carefully get good wide band and take heaps of temps when your head gasket blows buy a cometec .080" head gasket .
the other rough guide to compression threshold is to multiply boost pressure but compression ratio and compare .
so say some one happily runs 2.5 bar (absolute) in a 8.5:1 2.5x 8.5 is like 21.25:1 "effective compression"
so then 21.25/10.9 is 1.949 so take of one bar to go from absolute to boost pressure and you have .95bar

so in summery going .95 bar is about as ambitious as going 1.5 bar on a 8.5:1 engine ..

Rogue_Ant

Also, keep in mind that we have ~3psi less atmosphere up here... makes a big difference.

JDS968

I don't know whether you've decided on any specifics yet, but do you have power targets and target boost levels? FI systems are generally better developed working towards achieving specific goals rather than just by adding parts or modifications. 50:50 water/methanol injection goes a long way, as does an efficient intercooler, so if the whole system is designed efficiently, it should be able to flow well enough to make a lot of power on low to moderate boost.

Jfrahm

Sure, I'll ride my unicorn out and get some right away. :-)

Jfrahm

Yeah that's what I'm wondering about. It runs fine now but I suspect I'm getting timing pullout, or will when it gets warm out. There are lots of boosted engines with pretty high static CR these days but 10.9 is a bunch.

E85 is interesting but not right now.

On other cars I've seen boost go up but power stay the same due to timing pullout, I figure there is a sweet spot but with 10.9CR I'm well North of it at this point. Since my black car motor has the early 968 rods and I have other reasons to monkey with it I could lower CR while I'm in there messing with it.

-Joel.

2bridges

In other high compression supercharged applications it is common to run meth injection.

Have a blown LS6 with 11.1 compression running 8.5 psi boost on pump gas. This setup is twin intercooled + Meth injection is triggered at 4psi. In this application meth is just an extra measure of safety, but many are running BIG boost with meth and high compression. Meth makes a world of difference, just have to tune for it.

Rogue_Ant

Joel, how much boost currently?

gt37vgt

I'll never understand why people would prefer to change pistons I'd always prefer to change fuel type ..

Chris White

I have Unicorns available....

http://www.thinkgeek.com/stuff/41/un...shtml?cpg=128H

I also can get alusil compatible pistons in any configuration you can come up with...

Chris White

E85 is not easy to get around here....if it were I would design an engine that would really use its potential – and it would need new pistons – 10.5 : 1 is not high enough compression…

So – if you change fuel type you should still change pistons if you want to get the most out of the potential of the new fuel!

Euro951

Could you add overlap by adjusting the cams somehow and add ignition timing? Doesn't overlap reduce cylinder pressure a bit? But I guess that would put the powerband even higher up in the rpm zone?.

Chris White

Overlapping the cams (less lobe separation) is a way to ‘fool’ a high compression NA motor into acting like it has less compression– but it has a lot more down side on a turbo motor – the back pressure in the exhaust is far greater and any significant overlap will cause a lot of reversion (that’s a bad thing).