Maximum compresion?

[300rman]

what would be the maximum compression i could run before i have to graduate from pump gas to race gas mix?

so far, i have come up with 190 PSI is about the threshold, and when about 200PSI is reached, race gas mix must be run.

i am running at about 150 PSI now, and am having port work done, and since its apart, i would like to do all other mods i can, without running race fuel, that is.

[SWIGIN]

i wouldnt say it must be run at 200psi....it dependes on the bike.

my 89 q-racer had a turboed crank, national tt poted cyl, reshaped head and all the bolt ons and i normaly ran at 220 psi with no problems and once i ran 240psi and it ran fine till it sucked in the base gasket....lol

p.s. by ran fine i mean no pinging

[BC250R]

"what would be the maximum compression i could run before i have to graduate from pump gas to race gas mix?

so far, i have come up with 190 PSI is about the threshold, and when about 200PSI is reached, race gas mix must be run.

i am running at about 150 PSI now, and am having port work done, and since its apart, i would like to do all other mods i can, without running race fuel, that is. "

I have just spent the best part of the last 18months going through this myself. You will find a few old posts here where i asked the same question but the responses are so varied from other members experiences that you will find that you just have to work it out for yourself with their input as a bit of a guide.
So here is my 2 cents worth.

After a full rebuild with new crank(longrod), bearings, gaskets, pistons etc. national proted jug, 66.50mm piston and oringed head with 20cc dome i ended up with 205psi.

After running in correctly and doing some plug chops i found the bike to have pretty good power but would ping under load and get real hot on 98octane pump gas standard ignition, 39pj, 200 main jet.

Adding tolulene to my fuel to increase octane, ifound that the pinging was reduced but the motor would still get hot and when it did the pinging would come back, motor got hotter etc etc then the orings would melt, riding over for the day.

I never continued to push the motor once the pinging started, so i have been lucky that the orings let go before the piston.

I lowered the compresion down to 185psi by tacking the dome out to 22cc but still retained the oringed head. The same results were had, just the time frame was extended before melt down each time. I found that i had to keep an eye on the temp and rest it alot while everyone else kept riding.

Fed up after the last blow out of the orings and sick of mixing the fuel, i purchased a brand new head(coolhead) and 22cc dome and fitted it with a genuine head gasket instead of the orings. this reduced the compression to a way too low figure of 125psi. (ideal should be about 165psi by the way)

Wanting to go riding, i thought i'll give it a go anyway. To my surprise, on 98octane with no additives, my bike went easily as well as the rest of the 250r's i ride with, keeping with it in drags, dune circuit races and sand oval trak races, AND did not get hot, ping or die! I was stoked.

Since then i have added a PVL ignition and have been able to advance the hell out of my bike as well as lean it out and to this day (about 15 good thrashing rides)(touchwood) the bike has run flawlessly and with the timing advanced it makes more power than the extra compression ever did. All on pump gas.

I dont know how fuel quality relates between the US and OZ but 98 should be 98.

So thats my story, do with it what you will.

Cheers, BC in OZ

[cr480r]

My Lt was cranking 215lbs... so clearly psi is not any indication of octane requirements... I ran it several times on straight pump with no pinging issues... I ran 110 at the dunes for insurance though... I would imagine that an air-cooled engine would be more likely to ping at a given compression ratio than a water pumper though...

[300rman]

hmm. so before trying to bump compression to anything extremely high, i should probably try an aftermarket ignition system, correct?
a Ricky stator would be a good place to start i would imagine, are there any "performance" spark coils for a 84 R?

its running at around 150 i believe, and wouldnt mind bumping it up to around 170, which seems to be the "normal" compression range