Highest Compression ratio, without Sac. Reliability

[400MXer]

I was thinking about going with 13:1 comp ratio....but i don't really want to sacrafice my reliability any....should i go with 12.5:1 or 12:1, or just stay with 11:1.


I had a 12.5:1 on order but something got messed up...now i am temped to order a 13:1. I have no problem with running Race fuel...infact, it's almost more availible than reg fuel for me.

Thanx

[Predator36]

The compression that you can get away with depends on the fuel BTU (how hot it burns) and this is a hard number to get. CamII has a lower BTU than turbo blue. Running a lower BTU fuel allows your cooler running engine to utilize a higher compression ratio. Fuel octane also plays a very important part in this. The octane number indicates its resistance to detonation. Another big issue is how you ride the engine. A XC engine must run lower compression than a MX engine due to less cooling caused by engine load verses engine speed.

What it all boils down to is engine temp. In testing I set the compression higher than I think is safe and than monitor cylinder head temp in an actual race. I then machine the piston top and rerun till I see a engine temp that is acceptable.

[killed300ex]

oi would stay with 11.1 just cause if you go any higher you wont be able to run pump gass you will have to run race gas so id stay with 11.1 or if you want higher and wanna run race gas then i thinik reliability would cut off somewhere around 12.5:1 i think thatswhat iv heard
killed

[YZROOSTINYA]

since race fuel is not a issue, then 11.5:1 would prob be my guess, that would be a true comp not a advertised amount. i have no personal experiance with this but 12:1 and higher is alot more than the stock 9.1:1

[Bean]

13:1 is really tough on the stock rod and crank

[trx400ex]

ye man, i was going to run a 13;1 for mx too, then i decided to play a little safer with a 12.5:1...after my banshee blowing up this weekend i figured out what i DONT want to do with my 400, i would run 12;1 at most unless you get a stronger rod anyway thats a huge difrence over stock and ask X-rider about his 13:1 piston
Also what bore are you on?

[jskance]

stock:cool:

[Castor-426ex]

sack reliability??

well...as long as you dont go in her dry you should be just fine:D :D :D :devil

[400MXer]

With a 12.5:1 or 12:1 compression piston and a full race cam, will my valves have enough clearance over the piston. or will i need to do some machining.

[ieatglue]

i ran a 12.5 piston in my ex 2 years ago for mx racing and had to replace my rod twice in 6 months so i sold it.

[ChadEXer]

TC says a 12.5:1 is the absolute highest you would want to go. But if you were running a stock rod I would say go with 12:1!