8mm Dome piston success stories

[evm1024]

Hi All,

I built a block years ago and it is sitting waiting for me to put the head on it and get the engine built. It has 8mm dome pistons and I had intended to put one of the 2mm thick gaskets on it to get the CR down to something reasonable. This is a 2 liter motor and doing the calculations I come up with a CR of 10.4 : 1 assuming the gasket does not squish too much. If I shaved 2mm off the pistons I could get down to 10 : 1.

So the question is: Hey your guys with 8mm pistons tell us your story. How did it work for you? What did you do to make it work?

Of course now that 4 mm dome pistons are available I should take it apart and replace the 8mm ones but....

Regards!

[djape1977]

depending on what engine head you plan to use. they don't have the same combustion chamber.

i used domed pistons with 1.8 head and stock head gasket lasted me about 100miles. than i replaced it with one for volumex and it worked fine for next 30k and than it desintegrated too.

[131]

Of course now that 4 mm dome pistons are available I should take it apart and replace the 8mm ones but...

What oversize are they? You could always sell the 8mm pistons to offset the purchase of lower domed ones, someone chasing compression will pick them up.

[131lover]

Hi!
10.4-1 in comp should work fine if you use 98 octane!
My friend runs spider abarth replica with a 2l and has 10.8-1 in comp,he uses it for historic rally and for transport to work in the summers and have done so for the last five years.

cheers
Alf

[131]

The new car I picked up last weekend has domed pistons and according to the previous owner that built the engine, CR is about 10.5:1. I drove it back from Sydney, doing about 1370kms, it ran fine on 95 without the slightest hint of an octane issue.

[djape1977]

maybe it's a stupid question, but is there a co-relation table between cylinder pressure, one you can measure with a compression tester, and compression ratio?

what does, for example, 145psi (10 bar) mean translated into compression ratio?

[vandor]

When I had 8mm dome pistons I measure nearly 11:1 CR. Shaving the domes 2mm hardly made a difference. Of course the dome has the smallest diameter at the top...

> but is there a co-relation table between cylinder pressure, one you can measure with a compression tester, and >compression ratio?

Compression pressure also depends on what cams you are using, how well the rings a sealing, etc, so there are too many variables for it to be a reliable guide.

[djape1977]

i know that it's variable, but taken that everything is new, engine is cold etc.

compression ratio is something you need to mesure and calculate. for starters, you need to know or measure the volume of compression chamber...

[vandor]

On a stock TC with stock cams and 8:1 CR I usually get ~150psi on a freshly rebuilt engine. If forget how much exactly the 10:1 CR engine with cams had - I think ~190psi.
I've seen stock used engines that were running well with no signs of being worn out that had ~120psi of compression pressure.

[sptcoupe]

Csaba - I don't recall how high the domes are on the coupe's motor pistons (3mm?), but I just did a compression test and they were all between 186-188. That's with a calculated 10.1:1 CR.

[djape1977]

ok, that makes sense. i've had couple engines running at 190ish with dome pistons and smaller combustion chambers, and a freshly rebuilt argenta engine last year at 140.
one i was mentioning earlyer was running at 210psi measured on freshly assembled engine prior to first startup.

detonation has been an issue ever since leaded fuel was forbiden even on bog standard engines so i use octane booster aditives all the time. other solution, although not really applicable on 124 spider but rather on sedans, is what most people do here anyway, run their cars on LPG. no self-detonation issues there. high compression engines just love it.

[vandor]

> I don't recall how high the domes are on the coupe's motor pistons

They are 4mm.

[evm1024]

Time Has Flown....

I stalled out working on this engine (and the car, '82 or '83) for years. It has been sitting in the garage as life took me away. But the other night I started to do work on the car a bit and will get back to the engine build. I really do not remember all of the details but have notes from pipetting the head and other such work.

The head gasket that I plan on using is a thick one from Guy Croft so hopefully it will last a while.

The 40-80, 80-40 cam I was planning to put in the car has some rust on a few faces so I ended up buying a new one that should be soon. Time to put the head on and see what clearance I get.

The car was a barn find with rust here and there (from mouse pee!) I have done a few things:

- fixed drivers floor pan with a few rust holes
- replaced break lines
- Red IAP springs
- KYB
- new ball joints
- new tie rods etc
- new rotors, calipers
- repacked bearings
- gutted interior
- Pulled the seats apart and straightened then welded a set of braces inside, reinstalled in car
- homemade carpet set with sound proofing

Lots to do, the engine, transmission and gas tank are out....

[So Cal Mark]

what is "pipetting the head"?

[131]

what is "pipetting the head"?

CCing the combustion chambers.
https://en.wikipedia.org/wiki/Pipette