Static vs. Dynamic Comp. on pump gas

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Jan 9, 2013 | 08:17 PM

Fordenthusiasts

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Tuned

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From: Kingman, Ks

Static vs. Dynamic Comp. on pump gas

I'm in need of a little schooling on compression ratios and what I can run pump gas on.

I'm trying to get parts together for my 351w build and I'm trying to figure out if I'm at a safe compression ratio.

So far I have a 351 bored .030 over and decked 5 thousandths because of surface imperfections. I'm going to be getting some GT40 heads that are rebuilt and they were milled 5 thousandths too. I plan on using a .041 compressed head gasket unless I need thicker. I've decided on using the Comp Cams 35-349-8 cam also and I am using these pistons: Speed-Pro Hypereutectic Pistons ZH336CP30 - SummitRacing.com

According to my calculations that is about 9.45:1 static and 7.7 dynamic compression ratio. First of all are my calculations correct and second is that safe to run on 87 octane gas?

Jan 12, 2013 | 06:07 PM

Fordenthusiasts

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From: Kingman, Ks

Wow, I'm a little surprised that no one has chimed in on this. I just can't seem to find a definitive answer as to what compression ratio I can run on 87 octane. I'd always heard to keep it under 9.5:1 static but with a little research it seems like dynamic ratio is more important. I just can't seem to find that magic number that people go by. So any help or guidance here would be great.

Jan 12, 2013 | 09:45 PM

Conanski

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From: Ottawa, Ontario

The reason you don't see any definite answers on what you can run on '87 octane is because it varies wildly for even different copies of the same engine. I don't think you'll have any problems getting this combo to run on '87 it's just going to be a matter of how much ignition advance it'll tolerate under heavy loads.

Jan 13, 2013 | 05:58 PM

baddad457

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From: south louisiana

I'm with Conanski here, whether it'll run on 87 will depend on how much timing you give it. It may want 89 too. I had an Explorer 5.0 with the stock F4TE cam in it in an 89 Ranger. It was fine with 87 in winter, needed 89 in summer to avoid the pinging.

Jan 13, 2013 | 08:41 PM

Fordenthusiasts

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Tuned

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From: Kingman, Ks

Alright, thanks guys thats what I needed to know. So it sounds like I'm on the edge. I would run 89 octane but all the 89 in my area has the ethanol blend in it and I don't like running that stuff, and there is no 91 or higher octane for 35 miles from where I live.

Well I guess I will just have to see what it does and adjust timing from there. Thanks again guys I didn't realize that there were so many variables even for the same engine combo.

Jan 14, 2013 | 06:43 AM

baddad457

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From: south louisiana

Strange that the station you're getting the conventional 87 from doesn't have premium. I haul gas, and the bulk plant I work for sells only conventional gas, but even it too has some ethanol in it. We can only get 87 or 91 octane in conventional, to raise the 87's octane, you only have to add more premium to it. That's how the refinery raises the octane, either by blending ethanol or adding more premium to it. Our 87 is a blend of 84 octane and 91 octane fuel. My 77 Comet's 331 prefers 93 octane fuel, so I blend 91 conventional and 93 octane Chevron ethanol blend. So far it has yet to affect the carbs on it.

Jan 14, 2013 | 08:23 PM

Fordenthusiasts

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From: Kingman, Ks

Yah, I work in a small town that has only 1 public gas station, and 2 stations at which you have to have their card to buy gas. The one public one carries 87 and then 89 with 10% ethanol in it. I have a card at one of the other stations (its run by the local co-op) and all they offer is 87 with no ethanol. We used to have a station that carried 91 octane but it closed a couple months ago. So there aren't many options unless I was to drive 35 miles out of my way to fill up.