High compression ratio on US pump gasoline?

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Guy Croft
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High compression ratio on US pump gasoline?

Post by Guy Croft »

Can anyone Stateside guide me here?

An 8V TC engine on twin carbs on the highest available octane US pump gasoline (is it 91MON?) - what would be the highest CR you could run?

One assumes inducted air at ambient, engine temp under load 75 deg C max, the right ignition curve, plugs and fuelling etc etc.

The question is due to the the fact that we use RON here and the US uses MON which values are NOT related by formula and are derived by testing by different load/speed and settings methods.

FWIW I can run an engine like that at 190bhp or more at 11/1CR quite happily on UK 97 Oct RON.

Please only answer if you have direct personal experience of this.


G
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Re: High compression ratio on US pump gasoline?

Post by robert kenney »

Hi Guy,
I'm quite conservative on compression really.
As you might remember the 2050 Fiat 8V TC I built for my family member was 10-1, 34-35 total timing and GIIIa cams. Pump premium 91 octane is no issue at sea level at all. Remember it has lots of quench band. Pretty sure I could have gone to 10.5-1.

I have used 190 PSI as the max cranking pressure for our 91 octane.

Robert
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Re: High compression ratio on US pump gasoline?

Post by engineerted »

Guy, Just to correct you a little bit, the US uses the average of the RON & MON ratings at its fuel pumps and the highest availabe rating at the pump is 94. The US Sunoco stations carry the 94 rated gas but not in all states.

Ted
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Guy Croft
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Re: High compression ratio on US pump gasoline?

Post by Guy Croft »

Thanks Rob 'n Ted

you can see how little I know about this.

Hopefully Csaba Vandor at Autoricambi will spot this and decide what he wants in this case for a new build,

many thanks!

GC
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Re: High compression ratio on US pump gasoline?

Post by robert kenney »

The octane conversion between availible sources here in the US has been of particular importants to me. After years of scouring aviation documantation to try and dertimine the suitability of 100LL AV gas (100/130 low lead) I have come to the conclusion that RON to MON is factored by .95 and will be close. Your 98 works out to 93. I regularly build engines that require racing fuel or a minumum of 100-110 AKI with is similar to our pump fuel rating and 100LL crosses over very closley from a timing/detonation resistance stand point to C12 racing fuel.
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Guy Croft
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Re: High compression ratio on US pump gasoline?

Post by Guy Croft »

Sorry Rob I don't understand your info. Please put in simpler terms for me would you?

Thanks,

G
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Re: High compression ratio on US pump gasoline?

Post by robert kenney »

It wasn't you but my rather rambling post.

To convert between RON and MON factor by .95 so your 98 is equal to about 93 octane here in the USA.
The other info was a poor attempt at correlating the calculation to my experiences with AV gas and racing fuel.
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Re: High compression ratio on US pump gasoline?

Post by WhizzMan »

I was told that the readily available MON93 would be equivalent to our RON97 or RON98. I'd check with your customer to see what he can actually get at the pump there. I'm fairly certain that the last time I was there, 93 was available at every station I saw in New Jersey and Washington (state). Be aware that there are quite a few people complaining about high ethanol percentages in pump fuel there. This may be relevant for tuning, choice of fuel lines etc.
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Re: High compression ratio on US pump gasoline?

Post by vandor »

Hello,

Here in Texas 93 octane (average of RON and MON) is widely available. I have not seen 94. All gasoline here has 10% ethanol content (I now use FI rated fuel hose even on carbed cars, as the cheaper ones will degrade).

I'd rather be conservative, as it's pretty hard to reduce CR once the engine is built. There are thicker headgaskets available, but they would mess up the squish. I think 10.2:1 is as high as I want to go.

Csaba
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Guy Croft
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Re: High compression ratio on US pump gasoline?

Post by Guy Croft »

10.2, roger that.

G
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