Hi Guys

a Little puzzled here so hoping you guys can give me a hand.

The 500 has a compression ratio of 9.8:1 and boosts a bar or 14.5 psi. That gives you a final compression of around 20.

According to charts, anything above 12.4 final compression will cause detonation on pump gas so how do these motors run then?

Need the info for a build as we need to lower compression to run higher boost.

Thank you

ChrisPRSA said:

anything above 12.4 final compression will cause detonation on pump gas

Click to expand...

That's what we thought in the seventies when turbocharged engines still had compression ratios of about 7:1.

Thanks for the response

So does this mean modern engines on 95 octane can run higher effective compression or does this mean fiat thought this through and built it to handle roughly 20 effective compression.

Lets say you tried pumping 20 psi into the motor, we would need to drop to 8.5:1 to maintain 20 effective compression pr do i have this whole thing backwards

Surly the turbo won't come into as while the air is compressed by the turbo
The fuel itself is only being compressed by the engine ?

chris3234 said:

Surly the turbo won't come into as while the air is compressed by the turbo
The fuel itself is only being compressed by the engine ?

Click to expand...

sorry i dont quite understand what youre saying?

ChrisPRSA said:

sorry i dont quite understand what youre saying?

Click to expand...

The turbo compress air only so it shouldn't matter how much it compresses it as it's the fuels level of compression that will dictate with it explodes/detonates Or not

At least that how I understand it?

Agreed, a very good tune and quality fuel can reduce the chances of detonation however as you add significant amounts of boost to static compression it adds up which means your effective compression rises as well that increases the temperature of the air in the chamber and so fuel with air can ignite spontaneously without spark.

I have always dropped CR on a turbo conversion to about 8.5 on Hondas and Alfas but they were all N/A motors before, however the Abarth 500 is already charged and sitting at 9.8:1 from factory.

We want to run 20psi, more air means more fuel means more power. Yes we will sacrifice ponies when we drop CR but just trying to figure out how much to drop to

This table may or may not help ?
https://www.bp.com/content/dam/bp-country/en_au/media/fuel-news/fuel-octane-power.pdf

Thank you for the table.

We are on BP premium 95 this side and states 9.5 as highest. The stock abarth ratio is already .3 higher without boost.

This is where my confusion comes in. I mean i could always just try replace the pistons with the same CR, just forged and throw in the 20 pounds and try amd tune it right but I want to make sure the groundwork is done properly

Does anybody here have a 500 that pushes out over 250 horses that swapped conrods and pistons? Perhaps share the details of the aftermarket parts used?

If you wants to run 20PSI of boost, you'll need to look at a Static compression ratio of 8.25:1 if you want to keep the Boost compression ratio the same as the stock 19.8:1

You could probably take a nibble out of the stock BCR since that's designed for pump fuel and you might be able to find some 99-octane or similar, so 8.5 might be a goer...

Bear in mind too that 8.25 would be the SCR at sea level and your neck of the woods has a lot of altitude, which works in your favour, if your motor goes for a spirited drive up in the hills. At 1000ft altitude your SCR can be 8.35 for the same BCR.

I wouldn't change the pistons and rods just yet. You could do worse than just boost the beast up as it is, until it starts to complain. Then you'll have a definitive figure for how much boost it can take with the stock SCR.

I dunno what you plan to do next.. I guess it depends on how much BHP you get from the maximum-boosted stock setup up.. but if you intend to tweak the cam-timing too, then that will have an impact on the compression ratio you can use, which will also affect how much power you ultimately make.

If you get a setup that you're happy with, or just needs a bit more or less SCR, then that would be the time to introduce new pistons, once you know what the definitive spec' is going to be. New/beefier rods is always a good idea .. but again, once you know how many BHP you're going to end up with, that would be the time to go out and buy some.



Ralf S.

Hi Ralf

Thanks for the awesome response.

We are aiming for Atleast 300hp to start with but will attempt to keep pushing, the motor will be on the floor with gearbox seperated to do the clutch so changing our pistons and conrods will be within reach.

I have read from previous tuners that stock rods are the weak point at 250hp. I was also leaning towards 8.5 ish from my previous builds but am unsure. I will not be running stock turbo, nor fuel injectors or pump. We are planning for this to be a quick tjet.