Too high compression on 964 89mod 3.8L

[safe]

Originally Posted by piggdekk

Hi Sigwil,
I would have thought 8mm is quite a lot of material. taking 2mm off would still leave 6 mm, wouldn't it?

Me too. I would have guessed that 5 mm would be enough, might depend on piston diameter too. But I'm just guessing without research....

[dannobee]

Is there any chance that you can return these pistons and get the correct ones?

[sigwil]

There is no return on these parts ... bought them 2 years ago

https://www.fvd.net/de-en/10010303808/mahle-upgrade-38l-pistoncylinder-964-1181-109mm-wide-rod.html

There is now a new description of my 3.8L. set that wasn't there when I bought the set 2 years ago.

There is not much to get in my 964 cylinder head since there are large valves and 2 spark plugs

This is a lot about theory as everything is new versus my engine from 1989, sad but true.

I don't think it's possible to buy other pistons with lower compression.

Now it is a very good engine builder with long experience looking at my case so for I see what we arrive at but as it looks now needs 1mm total
between cylinder and block or cylinder and cylinder head

[r lane]

Shimming the cylinder in this case is a bad band aid and as stated can cause detonation when the deck height becomes extended. Don't know what a safe dome thickness would be, but 6mm which is a 1/4 of an inch I would think would be more than sufficient. Contact the piston manufacturer and get specs on what is the safe zone. But the valve to piston distance is the first thing that has to be considered. You have no choice but to cut the valve pockets if that distance is not correct. Bob

[racing97]

At risk of messing things up I ordered some Pistons from Mahle Motorsport for a 935 restoration project and they sent me a print of the old 3.2 long stroke turbo piston from the archives and I noted that the crown had .200 5mm of material and I was concerned. Their response was they would take it to .180 or 4.5mm if they needed too. of course this needs to be consistent where the pocket joins the crown on the inside that measurement must be accounted for this would be on the underside of the Piston.


regards

[sigwil]

It won't be easy and work 100% correctly 6 times ..you only have one chance

[mepstein]

Buy the correct pistons and sell the old ones.

[sigwil]

If we do not find a good solution, we will contact Mahle and hear if they have a solution
for there must be several who have bought this not just me...

[racing97]

Here is the app guide and fortunately they have started calculating the ratio at 1.mm below cylinder deck and identifying that that they do so.

http://www.us.mahle.com/media/usa/2019-mahle-motorsport-porsche-ap-guide.pdf

regards

[r lane]

I think we are all still guessing as the volume of the dome of the piston has not been determined other than here say. Not difficult to determine. Put top ring in the piston, heavy grease around that area and put it into the cylinder and it can sit at any height in the cylinder. With a pipet, measure the volume of fluid to fill the void above the piston up to a clear plex lid that you can make. Subtract that from the volume above the piston as if the piston were flat, and that gives you the dome volume. Measure the volume of the heads, they look like they have been cut, and then do the math with stroke and cyl diameter.

[trond]

get different pistons and sell these ones to someone who can use them for racing ? Don't compromise on squish clearance by shimming anything

[sigwil]

Calculation of compression is done when ALT is mounted on the engine also it is filled with water in spark plug holes and I got space for 55.2 ml wather but for and be sure all parts are measured separately for and see if the total will be the same (12 , 25) in compression.

Are there stamps that can be used (Mahle) then I buy them.

[safe]

Originally Posted by sigwil

Calculation of compression is done when ALT is mounted on the engine also it is filled with water in spark plug holes and I got space for 55.2 ml wather but for and be sure all parts are measured separately for and see if the total will be the same (12 , 25) in compression.

Are there stamps that can be used (Mahle) then I buy them.

What does FVD say? If they are advertised and sold as 11.8 and measure out to 12.2?
They are made for a 964 3.6 engine like you have not for something else?

[sigwil]

I haven't talked to FVD or Mahle .......... I'll fix this myself

Har du en rsr kamaksel med god overlapp kan dette stemme... i teorien.

Next week there will be machining of pistons too we will see how far below 12.0 we get in compression.

[piggdekk]

I knew an excellent machine shop halfway between Hvalstad and Asker, they had all kinds of CNC mills, and were very knowledgeable. It would take someone like them very little time to machine the pistons in the right way. Too bad I do not recall the name! but it was more than 10 yeras ago

[dannobee]

For those wondering about reducing compression by adjusting deck height, here's how the manufacturers are using it on the fly. Obviously squish isn't a very high priority.

https://www.roadandtrack.com/new-cars/car-technology/a15942295/heres-exactly-how-infinitis-variable-compression-engine-works-and-why/

And guess who now has a patent on variable length connecting rods? Obviously it doesn't matter as much as we thought.

https://www.enginelabs.com/news/porsche-working-on-variable-compression-connecting-rod/

I always thought that having a hydraulic piston moving in the combustion chamber (in the head) would be a great idea in varying compression ratio, but it obviously wouldn't be able to change it by 6 compression points like the Nissan engine, at least not without a bunch of work.

[piggdekk]

squish isn't necessary at all revs and all speeds, but it certainly is when the combustion chamber temperature is high enough to promote self ignition. I would say the last 40 years of engine development have gone in this direction with great results.
Feel free to try how the same engine works with and without an effective squish band!

[K24madness]

Originally Posted by dannobee

For those wondering about reducing compression by adjusting deck height, here's how the manufacturers are using it on the fly. Obviously squish isn't a very high priority.

https://www.roadandtrack.com/new-cars/car-technology/a15942295/heres-exactly-how-infinitis-variable-compression-engine-works-and-why/

And guess who now has a patent on variable length connecting rods? Obviously it doesn't matter as much as we thought.

https://www.enginelabs.com/news/porsche-working-on-variable-compression-connecting-rod/

I always thought that having a hydraulic piston moving in the combustion chamber (in the head) would be a great idea in varying compression ratio, but it obviously wouldn't be able to change it by 6 compression points like the Nissan engine, at least not without a bunch of work.

Comparing a small bore direct injected water cooled power plant is a poor comparison.

I grew up playing with big twin Harley Davison Evo engines. It’s about as close to porsche engine as you can get. Squish played a HUGE role in optimizing engine combinations! We would shim the barrels up to get zero deck (or really close I forget specs). While the compression went up the engine responded by accepting more timing without knock. Opposite of what you’d expect. Exciting the mixture with proper squish in a large bore engine helps promote proper flame travel. Too big of a gap in the squish band also creates a detonation pocket for early ignition.

There’s a great book by William Denish that explains lots of science that applies well to our air cooled Porsche application. https://www.amazon.com/Twin-High-Performance-Guide-Vol/dp/0964011522

My local Harley mechanic was a drag racer and worked in nuclear power plants on compressors. He was REALLY into the science of it all and I watched lots of examples of his theory prove out in the real world.

When building my 993 turbo conversion I took all that Harley squish experience and applied it to my build. Looking at stock 3.6 993NA pistons I noticed they had raised mini domes at the edges to push (squish) the mixture to the center of the combustion chamber. On top of squish effect it made the combustion chamber a better shape by shortening flame travel. We set proper deck height by milling the barrels to achieve ideal squish. I added some flame rings and custom cams that I spec’d and the whole combination works great. I works with 5-7 psi on 93 pump fuel and 9psi on 100 oct with better timing. Been going strong for 9 years like that. Last AWD dyno was on wastegate springs only. 8psi peak tapering to 6psi. Now the EBC holds 9psi to redline. I was shooting for 12psi (maths says it’s realistic) but stopped at 9psi since it feels so good?

[Mixed76]

Originally Posted by K24madness

What was your compression ratio?

Sent from my Nokia 7.1 using Tapatalk

[K24madness]

I believe it ended up at 10.6:1

It was so long ago I’d have to dig up my notes. It’s definitely lower than the advertised 11.2:1

I am closing the intake valve a lot later so dynamic compression is down quite a bit from stock too. When compared to 8:1 993tt with small cams the dynamic compression ratio delta is not nearly that bad.