Doubt regards high compression pistons

[EdderSpisder]

Hi there,
Im Edder from Mexico and i contact you because im restoring a Fiat Spider 74 with 1800cc. Well, recently i bought high compresion pistons because the Fiat store did not have on stock for regular flat pistons. My machinist said that will increase a lot compression but my concern: if is there another thing that i have to consider or modify in the head of motor or the cams?
Really, this is my first restoration and im nervious to fail in the engine to over heat/ warm it.
Items that i bhave are the following:
Oil pump
Water pump
engine gasket kit
High compression kit
Main and rod bearings
Valve kits
My block and head are from original equipment with single weber carb.
Also is a true that all 1800cc engines in Europe uses hih compression pistons?
Thanks in advance
Edder

[131]

Measure combustion chambers and pistons, then work out what compression will be. Plenty of calculators online. What is the octane rating of the fuel you have access to? I've recently put an 1800 head and longer duration cams on my 2 litre with domed pistons, static compression is around 11:1, with the cams, dynamic compression is about 9.6:1, I have to run it on 95 or 98 octane fuel, it pings too much on 91. Keep in mind that our method of calculating fuel octane is different to the US.

[focodave]

Edder,
I am running a stock(except for the pistons), newly rebuilt 2 litre with stock fuel injection and exhaust.
The pistons are 10.2-to-1 static compression units and the bore is .8mm over-bored.
I am running at 7 degrees initial advance, and I use spark plugs that are 2 steps colder than stock.
I have absolutely no ping using 91 octane fuel --- none at all.
The difference in torque, with the new pistons, is simply amazing compared to the stock units.
I am at 4K to 10K elevation, though, so this effectively lowers the compression ratio to about 9-to-1.
Detonation is what will kill your engine.
As long as you can avoid it pinging, you are OK.
With a high-compression engine, you may have issues if you are close to sea level elevation and you can't get high-octane fuel.
Just some more thoughts to make you lose sleep over.....
Dave

[EdderSpisder]

Hi Mick and team
I can use the 93 gasoline and maybe an aditive to increase around of 5 octanes. I will find how to measure the combustion chamber and pistons because im really new. If you could help me with a link as a visual aid would be better
Thank you again

[vandor]

How high are the domes on the pistons? 4mm - you are ok. 8mm - you may have problems.
You really need to have your machinist measure all the volumes and calculate what the actual compression ratio will be.

[EdderSpisder]

I guess those pistons are 8mm . what is the risk to regrind or bore 1 or 2mm the flatness of dome?
Maybe a crack with the detonations becase the thickness of thet are will be reduced?
So complicated... because return the parts to the dealer and get back to mexico should be 200 usd wasted in shipments plus the customs rates

[narfire]

Perhaps get hold of Mark at his shop and enquire about a thicker head gasket. I'm thinking that might help knocking a few points off the CR ?

Saying that, I believe several might be running the taller domed pistons, I believe Jim (fiat218) is or will be as I sold him a set I had in the cupboard and were not going to use them any time soon. Beek is doing the engine build.
Chris

[vandor]

>I guess those pistons are 8mm . what is the risk to regrind or bore 1 or 2mm the flatness of dome?

1-2mm is not going to reduce the CR enough. You need to take off ~4mm, but then the top of the piston may not be strong enough.

[EdderSpisder]

I have this note from the dealer:
10.2:1 STATIC Compression on a 2.0 ltr and 1800 will increase about .2:1 more compression

Since the point of view that my spider is 1800 which is the recommended static CR?
Thanks

[vandor]

8mm domes do not result in 10.2 compression ratio, not in any of the engines where I calculated the CR. As I said, you need to have your machinist measure all the different volumes and calculate the actual compression ratio. It's foolish to build an engine and not know what the actual compression ratio is!! It's hard to change it once the engine is already built.

I have an actual 9.8:1 CR in my engine and every now and then it still pings on 93 octane gasoline.

[So Cal Mark]

this is one of the reasons we had .080 mls gaskets made and they are in stock

[narfire]

I'm a bit confused about the CR and pinging and octane . I believe the higher octane number can alleviate pinging in higher CR (does it increase the flash-point?) engines but then I saw the Mazda's "Skyactive" engines are stated to be 13:1 and run on regular and my VW Passat is 9.5:1 and is recommended to use 91 or better.
What gives? Is it the timing?

Sorry for the off topic but perhaps others can get educated as well.
Thanks
Chris

[BEEK]

it has to do with combustion efficiency, cam timing and modern electronics, which include a knock sensor, if the computer detect 1 count of knock, it will add fuel to the mixture and / or retard the timing to stop the detonation. prehistoric carburetors and early efi cannot do this

[wizard124]

I'm a bit confused about the CR and pinging and octane . I believe the higher octane number can alleviate pinging in higher CR (does it increase the flash-point?) engines but then I saw the Mazda's "Skyactive" engines are stated to be 13:1 and run on regular and my VW Passat is 9.5:1 and is recommended to use 91 or better.
What gives? Is it the timing?

Sorry for the off topic but perhaps others can get educated as well.
Thanks
Chris

Direct injection maybe? Seems like if the fuel is injected at exactly the right moment, there would be no pre-detonation. I too would like to know the real answer.

[darksider415]

I'm a bit confused about the CR and pinging and octane . I believe the higher octane number can alleviate pinging in higher CR (does it increase the flash-point?) engines but then I saw the Mazda's "Skyactive" engines are stated to be 13:1 and run on regular and my VW Passat is 9.5:1 and is recommended to use 91 or better.
What gives? Is it the timing?

Sorry for the off topic but perhaps others can get educated as well.
Thanks
Chris

Direct injection maybe? Seems like if the fuel is injected at exactly the right moment, there would be no pre-detonation. I too would like to know the real answer.

I can speak for the Mazda Skyactiv engine, since I daily-drive a Mazda3 with it... It's direct injection, versus port injection. That's the big difference.