195 psi too high for pump gas?

[Waternut]

Just as the title says... I know I'm flirting with the line here but I mostly do freestyle and run WOT only for split seconds and then go back to quarter throttle or so. I just got a reading off my new head with 195psi. But keep in mind this is off a Harbor Freight compression tester so I've come up with what I believe my compression ratio is... I used the following equation and numbers for compression ratio:

81mm bore, 42.22mm from the top of the exhaust port to the deck, and I quickly filled the cylinder with mineral spirits at TDC to get 29cc for total volume at TDC
CR = (volume of the cylinder above the exhaust port + filled volume at TDC) / filled volume at TDC

I came up with a CR of 8.5. Is that safe for 93 octane running a total loss at 29° static timing?

 

[WFO Speedracer]

A lot is going to depend on what timing you are running with that.

 

[Waternut]

Well I know for sure that static timing is 29°. I believe I started retarding the timing at 4500rpms and brought it down to 14° at 8000rpms.

 

[restosud]

a lot of deto doesn't happen at higher rpm's.it usually occurs during peak torque at maximum cylinder filling.
your calculated c/r could mislead you as the expansion chamber(depending on rpm) also helps to stuff charge into the cylinder thereby increasing cylinder pressure although c/r seems acceptable.
as a general rule your cranking psi is a little high.try it and do regular inspections and maybe take some timing out.

 

[Freakrider]

you could blend less and less avgas each time with inspections to slowly get to 93...

 

[keefer]

Why not just run 50/50 race gas? A few dollars more for some fuel is a lot cheaper than a deto destroyed engine.

 

[Waternut]

I guess I could try to find somewhere around here that sells race fuel although currently I don't know of any or even how much race fuel actually costs.

 

[keefer]

Look near the little race dirt or asphalt race tracks. We have a pump around here and I notice that there is usually a station that has a pump near some of our local tracks. It runs between 5.50 & 6.00 a gal around here for 110 leaded.

 

[texh20racer]

Just run a mix of pump/Av gas. Its a lot cheaper and some times easier to come by. Plus its consistently high quality.

 

[Mark44]

I am currently running pump gas and 195 compression without issue. No wide open runs for more than a few seconds.

Mark44

 

[Scorn800]

Was this the head you showed pics of with Deto rash?
Now you know why.

 

[Waternut]

I am currently running pump gas and 195 compression without issue. No wide open runs for more than a few seconds.

Mark44

Well I may try 93 gas for a short ride and see what happens. So far the only place I've found that sells race fuel sells 112 octane for $9 a gallon :shocked::shocked:
I know you've gotta pay to play but that's ridiculous!

 

[Freakrider]

have you called around to your local small plane airports ?

 

[Waternut]

have you called around to your local small plane airports ?

There are two airports right around me but isn't av gas a really lightweight fuel with a slow burn? I was under the impression that running av gas in a high compression motor isn't any better than running normal pump gas...almost like running an octane booster. It can help with pre-ignition but that's about it.

 

[ger87410]

There are two airports right around me but isn't av gas a really lightweight fuel with a slow burn?

No

If I remember correctly, all gas weighs the same.

It's just a higher octane gas dyed a certain way to tell how the taxes are paid on it. Jet fuel is special stuff.

 

[restosud]

yes,avgas is lighter as a molecule is missing from the carbon chain.it does work if you mix it with premium however.20 to 50% is usually good.

 

[Freakrider]

according to group K SITE it's lighter than pump gas (to get the plane off the ground easier) ....made w/ heavy and light molecules....still makes decent power w/ high psi...I have run it twice w/ 200 psi and it is arm pulling power.....

 

[Waternut]

Ok if it's not going to noticeably decrease the power, I would definately feel more comfortable running a gallon or two of av gas with 2-3 gallons of premium as opposed to straight premium and hope for no detonation.

 

[ger87410]

A new lesson's learned everyday. :smile:

U might wanna stay away from the 100/110LL (low lead). I've read somewhere that lead's bad on the engine. Dunno if true.

 

[Waternut]

A new lesson's learned everyday. :smile:

U might wanna stay away from the 100/110LL (low lead). I've read somewhere that lead's bad on the engine. Dunno if true.

I know it's bad for catalytic converters but never really heard anything about the engine itself.