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?
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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