Too high compression on 964 89mod 3.8L
 

I am working on and converting my 964 3.6L to 3.8L these days but my compression is estimates to 12.26 which is over what I want.
There is not enough material on the top of my piston (FVD Brombacher) that can be milled by this and spacer rings do not exist for the 3.6 engine so then it is only the top lid left if possible ......
Does anyone have any tips on how others solve this?
http://forums.pelicanparts.com/uploa...1574246145.JPG
http://forums.pelicanparts.com/uploa...1574246145.JPG
http://forums.pelicanparts.com/uploa...1574246145.JPG
http://forums.pelicanparts.com/uploa...1574246145.JPG

lower compression
 

First determine all pistons are equal weight.There is enough material on the top flat of your piston to remove material.Take one piston and mill the top a bit.Check compression.When you reach the desired level of CR JUST machine the other tops until the piston weights are the same as the 1st.I am just having my first coffee but there is a scale of weight removed per gram for CR.Ciao Fred

Then there will be a trip to the machine shop on Tuesday next week

Depending on what cams you are running, I would determine whether material needs to be removed in the valve pockets first. I forget what the number is, maybe 2 grams per cc, but you can find that in a conversion table. Your machinest will need to know what amount you would like removed based on the grams/cc equivalent. He will just take weight measurements, much easier than doing the cc measurements. Will get you close. Bob

Have your machinist either measure or sonic check the thickness of the top of the piston. You can lose a bit of compression by milling the dome off the top.

How did you determine the actual compression ratio? If the engine is for class racing or somewhere else where the compression can be checked, I'd be sure to know exactly how big the dome is by cc'ing the dome in the cylinder, then compare it to the mathematical size of the cylinder with the piston down the same amount. I wouldn't count on the spec from the piston catalog being accurate enough.

Another option is to rebush the piston pin holes on the con rod and offset the hole to bring the piston down in the bore a bit. But it isn't optimal. First choice would be to mill the dome on the piston.

I have a friend who has the formula for my compression.

The cylinder is 102mm
Stroke 76.4
Volume left when piston is filled
with 55.2ml of water

also, ollies make base shim gaskets for 3.6/3.8 jugs

Now I do not know the workshop before but I sit on a theory that the cutter used a
3D milling so that my judgment remains in my stamp after work is done.

Is it possible and buy shim and add under cylinder to
964 3.6L also as it is for older type of engines?

Thicker cylinder base gasket will lower CR. By far easier then grinding down the pistons.

Chris

[QUOTE=r lane;10663705]Depending on what cams you are running, I would determine whether material needs to be removed in the valve pockets first.

The only advice I would consider at this stage.

If you have not measured the piston dome and chamber volumes, do so before you remove any material from anything. Establish the actual static compression, as built.

I think stock heads have approx. 91 cc volume.

Assemble the engine to measure the piston dome volume and while there you will be able to measure the piston deck height. This is a critical number with these engines with these large bores.

You are building an old fashion engine here. Hemispherical chamber, domed pistons and twin spark plugs. So build it for this design.

These engines are knock sensitive due to the large bore diameters. Squish will give you more return than compression ratio. You have to push the gas from center out in both directions with consideration to the swirl happening inside the chamber from pockets, dome shape etc.

Lowering the squish by base shims or machining off the top of the dome will lower the squeeze on the mixture and in most cases will lower the engines ability to run advanced timing. If you are using pump gas, then this is a huge factor.

Depending on what you want the engine to do, choose your path wisely. If its just a grocery getter, then maybe the simple way forward is OK. But if you consider performance to be important, then do your homework.

You want piston to valve clearance to be enough just to be safe, and the pistons want to "just" clean off the carbon from the chamber roof, and the piston deck clearance to be as tight as safely possible. This maximizes the squish and forces the gas to the spark plugs and minimizes the end gasses from igniting under pressure.

You build in safety factors with clearances by compromising performance. But you know what those compromises are before you start the engine.

If it came to it you could certainly remove material around the valve pockets without affecting piston thickness. Might ought to take off the sharp-looking edge there anyway.

Agreed that base gaskets are a lot easier way to do it.

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I think this will be right

At the top of the piston there is room for 1,864 oz of water
Cylinder is 4.02 inches
Stroke 30.08 inches

The engine will be used on long trips not racing and I have experience that it is knock sensitive from a previous car that I worked with so I will expose myself to this again.
To add and add some more information, a 300 hp set from fvd has been purchased, which consists of a new EPROM and a new MAF + a new camshaft that I have no description on now since I am going to bed now .

Usually domes with that deep of valve relief have a rather thick crown area.
Would you happen to know the thickness of the dome?

regards

Are you sure that your measurements are correct? As Neil said, remeasure and recalculate everything.

1mm thick base shim will lower the compression ratio 1.62 points.

Just curious how you use base shims on a 964 as it seals with an o-ring in
a groove in the bottom of the cylinder, different from all the earlier engines.

Did anyone actually read Neil's post?

Only a fool would install thicker base gaskets to reduce compression.

You can easily achieve 300hp with stock pistons and cylinders.
FVDs MAF-kit and "2.1mm camshaft" will get you there with some larger injectors an custom tuning.

Have in mind that those cams will raise your idle hydrocarbon levels in your exhaust... In Sweden they can't pass inspection in a 964... Not sure what your limits your car will have ta pass.

The day did not go as I had hoped because it is NOT possible to machine my pistone top since there is so little aluminum goods on pistone top(8mm) as it comes from factory, so the next step will be and find a solution two increase the distance(+1mm) between cylinder and cylinder head.

I will also check all my numbers again and check cylinder with piston and cylinder head separately so that I can calculate cylinder head 100% correctly ..

Hi Sigwil,
I would have thought 8mm is quite a lot of material. taking 2mm off would still leave 6 mm, wouldn't it? Increasing the distance between head and cylinder should really be the last resource since it'll make the engine more prone to pinging.
Couldn't you shave more material from the valve pockets? I do realize this would be (far) more expensive then a thicker head gasket!
luca

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....

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

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

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

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

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

Buy the correct pistons and sell the old ones.

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...

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

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.

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

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?

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.

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 :(

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.

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!

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?

What was your compression ratio?

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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.