Compression ratio on (premium) pump gas

[///Mangler]

The biggest difference is direct injection that allows a CR increase of about 1 full point straight off the bat. Also combustion modelling and understanding has come along way since late 90s

This. And also it matters whether or not the engine is knock limited. Some are not, the power falls off before knock is induced.

T

[EthanolTurbo]

Actually, I think your right. I didn't think of that. Sorry.

Your best bet is E85 if it's available in your country. You could run 14:1-15:1 compression easily. Also, the higher your compression ratio, the more fuel efficient E85 becomes. I'm thinking about going to 14:1 compression eventually. Not sure how big of a difference it'll make but #racecar.

[Altaran]

The new, very high compression ratio NA engines are using the spark plugs as ionic knock sensors to nearly instantly pull/push timing.

Without that ability (which the MSS54(HP) does not have), I wouldn't want to exceed 11.5:1 on 91 octane or 12:1 on 93 octane.


... and, yes-- 98 RON (euro) is the ~same as US 93 octane.

Maybe I should get a different Ecu with that capability. I haven't done any research if any of the ones available have that capability (Halltech, Motec, Bosch Motorsport, ...)

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[BBRTuning]

Maybe I should get a different Ecu with that capability. I haven't done any research if any of the ones available have that capability (Halltech, Motec, Bosch Motorsport, ...)

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I've tuned several S54s with 12.5:1 compression and 288/280 cams on 91 octane (95 RON) with no issue. I find they are just about as knock limited as a stock 11.5:1 engine with stock cams. In both cases, with the engine in good health and a solid tune, we will reach MBT at almost the exact point that knock occurs, or just before (typically MBT is found 1-2* before knock occurring with either of these setups), which is a sign that the compression ratio is very nicely suited to the octane level. Because of this, going to a higher octane on these setups has only little benefit other than for safety and consistency. E85 is a different situation though, because for the injected volume it has more total energy and also very slightly raises the effective CR, so you will always see a small HP gain on E85 when tuned for it, even with a relatively mild compression ratio.

This is all on standalone ECUs. The S54 knock sensors are very good and any decent standalone ECU will have no problem using them for knock feedback and can get you a safe tune that is also riding right on the edge of MBT/knock. This cannot be said for all types of engines or ECUs. I find there are few people actually doing proper tuning on the factory DME, and it seems overly sensitive to knock noise if knock base levels are not adjusted properly, so you must take any sort of tuning related information done with factory DMEs with a grain of salt.

For your original question, your theoretical 13.44:1 is likely still going to be too high for any sort of pump gas. If you can assemble that engine with those specs without valve-valve or valve-piston contact, and with full Vanos range, it will be a fantastic E85 (or 105++ RON) engine.