maximum compression ratio with 98 octane
 

Hi,

Choosing parts and strategy on new engine build based on 7R small spigot case.
I have to find save compression ratio for the fuel quality in the netherlands.
We have on all pumps 95 octane and on many even 98 octane!

What would be the maximum CR if I will limit myself to a 2.5 liter engine?
Will be on webers IDA40 to start and maybe later on MFI in combination with mod S cams.

I am interested in max CR with or without twinplug.
I have been searching in the threads but difficult to find info as much is related to US fuel quality

Thanks,
Bart

link to a specific thread would be helpfull already!

Octane ratings are different in Europe compared to the US. You know, "the right way the wrong way, and the US government's way".

If a fuel is 95 RON and 87 MON then it will be 91 PON -> 91 PUMP

So according to what I see here, https://www.etuners.gr/fuel/

your 95 is the same as our 91.

I think you would be safe with 9.5 up to 9.8 - 1 CR, with single plugs.

Hi Bart,

I'm not an engineer or an engine builder, but maximum compression ratio depends on a lot of things, like piston dome design, squish/quench, camshaft design etc...
You might want to get in touch with somebody like Steve Weiner, Henry at Supertec, or closer to you, Dirk Scheffer from Edelweiss.

10.3:1 is safe with your cams. Watch your ignition.

Just have in mind that the 10.3:1 from the Carrera 3.2 might not be the same as true 10.3:1, if that is what you are inferring.

Also a mild camd engine like the 3.2 with 95mm bore might not be the same as a small bore 2.5 engine. Carbs or MFI are not as good as the Motoronic to control the fuel and spark.

Lots of variables....

If E85 is available go with that and increase the compression.

There's a lot of factors to this. Maximum dynamic compression with pump gas is a question of volumetric efficiency at various RPM, not just piston dome size and whether twin plug or not.

If one runs 10.3:1 static compression in a motor with small 34mm intake ports and CIS injection, standard CIS cams, one could likely run single plug and high octane pump fuel as the dynamic efficiency would never reach the static ratio. A standard NA engine only reaches maybe 80-85% of its static ratio due to breathing restrictions.

On the other hand, if one ran the same pistons, but with 39mm ports, individual throttle bodies, Mod-S cams, free flowing exhaust, then twin plugs would be a requirement as the dynamic operating ratio would most certainly approach or exceed 100% of the static ratio.