Highest compression ratio for petrol ?

[bigtoe]

Hi, I've got a honda SL125  running on methanol and I think the compression ratio is 13 -1, is it possible to run it on petrol ? can anyone help, thanks

[Tim754]

Yep but could/should/would run like a one legged dog in a uncleared mine field. Lower the comp and enjoy a romp, or run it on the methanol.
Bigtoe find a manual for the SL125 and compare the standard comp with what you say yours is. Of course the work shop specs would be for Leaded petrol RON as sold at that time.  Better try to find and construct a second engine for petrol usage. 

[oldfart]

Av gas should be ok at 13:1   fresh is best as should have an octane rating of 120 on the rich side
Will  have to alter your timing to suit ...and jet sizes

[Tim754]

Bigtoe  I run avgas  in the 754 on 12 -1,  116 octane . What you going to be using the little SL for?

[Lozza]

BP98 is on the limit of deto at about 12.5:1 geometric which is what you work on in those..............those..............other powerplants.

[Nathan S]

Lozza will probably never talk to me again, but...

We all talk about static compression ratio, but when it comes to the maximum comp ratio for any given fuel, what matters is the dynamic compression ratio.

The maths is beyond my beer-addled brain, but the simple version:
1. The more aggressive cam is, the more static comp ratio you can run.
2. The smaller the bore, the more static comp you can run.
3. The shorter the conrod, the more static comp you can run (but short conrods have their own, bigger draw-backs).
4. The 'cleaner' the combustion chamber is, the more static comp ratio you can run.

In your case, I'd reckon you could get away with 13:1 IF the other stuff was in your favour. But 12.5:1 is probably only a head gasket away, so that's a safer bet.

[Colin Jay]

Bigtoe,
I am running a SR500 yamaha engine that I built to run in the DLRA salt lake speed trials on 105 octane unleaded race fuel, and it is currently cranking a 12.5:1 static CR. The engine was originally built to run on methanol with a 14.5: CR and the compression was lowered by cutting a new cylinder base gasket from thicker gasket paper than the std gasket, from memory the paper was 1.6mm thick. You could try what Tim has done and just ad an extra gasket of two.

CJ

[Tim754]

Oops sorry I accidentally removed one of my posts above. I meant to close one I was writing, got called away came back later and well I am getting older...stuffed up It was a jest with Nathan S, and about the fact that I vary the 754s compression not with head gaskets but with the amount of standard thickness base gaskets (from one to eight) average amount used is five . Tim754

[pancho]

why take it off methanol? a small engine like that should cope with high octane petrol but methanol has other advantages, for example if you run 2 percent lean on petrol which can happen with atmospheric changes, the aluminium piston will become the next fuel source resulting in a big disaster.methanol can cope with up to 30 percent rich [but NOT lean at all.] plus the added power boost gained by the cooling effect on the incoming charge. just remember to remove all methonol from the carby and fuel tank at the end of the days racing as the methanol combines with water & attacks the carby body and makes horrible jelly. cheers wally.

[Freakshow]

That must be that HOT little Dirt track meth SL125 that was for sale on the bay, a whiles back ....DOh   sweet little ride that was, bloody ebay searches missed it tilll it was toooooooo late.  Bigtoe If you cant get it to run, id buy it back for what you paid 

[Viper666]

Correct me if I'm wrong or someone else has already said this but most modern 4 strokes run 13 or 13.5 to 1.
Rejetting will be neccessary but should run fine on 98 octane.
Viper666

[bigtoe]

Thanks everyone for your help, I decided to percervier wiyh the methanol and I think I might have cracked it after many hours of testing, I"ve lost count of how many times I've had the carby apart trying different main jet,needle jet, needle clip position combinations. it runs a 32mm mark 2 amal carb, when I got it had about a 500 main jet and 120 needle jet, it was made to run flat out for dirt track but would'nt run at all at lower revs which you need for a vmx track, so i finally settled on 360 main, 213 needle jet and one clip position up from the bottom, I'm hoping it's not to lean, how do you check that with methanol do you do the it the same as you do with petrol and check your plug ? Yes freakshow it was that bike and I got it fo just $800 dollars!! just to rub salt into the wound.