High compression pistons

2 replies to this topic
  • mzanden

Posted July 29, 2003 - 10:10 AM


I have heard it recommended "if using greater than or equal to 11:1 compression ratio pistons to use race fuels or additives". Has this proven to be correct or only under race conditions? How much power gain will I be missing out on if I use a 10.5:1 piston as opposed to a 11:1 or 12:1?

If anyone is interested it will be getting a Carrillo Rod and a Hot Cams stage 1 cam. I'm staying with stock bore. Any other thoughts or ideas would be appreciated.

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  • irondude

Posted July 29, 2003 - 10:37 AM


The trade off on the high compression piston is of course fuel availability vs. performance gains (not reliability). Baja seems to be the biggest issue- as in Johnny Campbell ran the 10.5:1 piston for the Baja 2000, but the hi compression piston for the Baja 1000. He figured he might have to go with local gas in a pinch for the longer 2000. For Supermotard guys its a no brainer- get the hi compression piston.
Horse power gains depend on what else you have on the bike i.e. an opened up airbox (including cut out number plate) and larger headers. Obviously carburation will be affected as well. If you are choked up stock- you can still plan on a couple horses with the high compression piston, if you have the cam, bigger jets, opened intake and exhaust, plus the pipe--well you are gonna gain at least 7-8 horses. Prossibly more like 10. I'm sure most will say these numbers are to low... Johhny claims 67 horses on his race bike-with the high compression piston-but we all know about quoting dyno results.... Short of an intense exercise on a dyno with each component-its hard to say for sure what the high compression piston alone will give you. I would gestimate (conservatively) 3-4 ponnies if everything else is right on vs the stocker. Of course it depends on the dyno, etc, etc. The big bore 680 kit will give you 2-3 (gestimate) on top of that. I've talked to Rob a bit on this subject-I think he will give you similar numbers.
---I have the stock piston with the cam, carb, pipe, airbox mods. For me its a great trade off in versatility vs. performance. Off road i just dont have the ability to use any more engine. On top it would be nice to do 114 mph - with 15/46 gearing-for pavement sections in Baja races, but for me not at the expense of being reliant on race gas...My bike hits ALOT harder with 15/48 gearing than my buddies 14/48 - with standard uncorking done but little else.
Try opening up the airbox with the side cover cut outs-re-jet (or get a Qwiksilver carb), get a bigger header pipe, and enjoy that cam. Ride it hard and then see if you really need the higher compression, and if the trade off is worth it.
Smart choice for the crank. :)
Your bike is gonna smoke!
Give us a ride report when its back together!!

  • JR650

Posted July 29, 2003 - 01:01 PM


General rule of thumb regarding compression ( works pretty close in dyno tests i have seen, between 3 and 5.5%) is 4% per point of compression, within reason, a point at 16:1 is not going to make as big a difference as a point at 6:1 so you might be missing out on 5-10% max by going with 10.5 versus 12, remember if you play with a big cam, you can run more compression. 11:1 sounds kinda stupid to me, but then again i dont know how much advance the BRP is running, could be alot. I was kinda looking into this a couple weeks ago, and honestly, i think if you polished the chamber to eliminate hot spots you would be okay at 11.5 or 12 with no detonation on 92 octane, but this is just my opinion, I have decided to go 11.5 in my bike when i do it. It also depends on how hot your bike runs, the hotter it is the more detonation prone it is. Hope this helps, JR

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