What compression is "safe"....

[GPracer2500]

...for a 250r? Is 235psi going to be reliable for a rec duner?

I've got an 85 250r that pumps 235psi cold at 1300 ft. That's with a CR head gasket. Base gasket is uknown. It's had work by the previous owner. That includes porting but I don't know who did it or exactly what was done.

I'm still working out what fuel it needs. A 50/50 mix of VP110 and AZ 91 pump seems marginal in the dunes. I need to do some more testing to see what's what. I'm thinking if I run it fat enough the 50/50 mix could be acceptable. It's fine on top, the midrange is where it either needs more fuel or more octane.

Comments?

[clutch22]

seems high for a R.... they come stock with 165psi... my 250R runs at 175psi and i like it there cause for what i do, just riding around out back and down at the dunes, thats fine for me because at 180psi your supposed to be using Avgas wich is 100octane or other racing fuel.
oh.. and I run a 32:1 ratio.

[GPracer2500]

Yeah, my buddies 86 pumps 160psi on the same guage. I purposely checked his because I wasn't sure my guage was reading right. I was getting 235psi in mine and thought that seemed high. Made me wonder about my guage but I do believe it is accurate.

Stock comp on the 85 ATC250R is 170.7 - 199.1 psi.

[trailwart]

i would run 110 octane or higher

[250r4life]

if youre going to mix 110 and 91, i wouldnt be running 235... that is a whole lot of compression... i would try and drop it down to around the 200 range...

[250r4life]

what two stroke oil do you use GP?

[kiesta00]

235 seems extremely high for a 50/50 mix...I'd do like 250r4life said and get it down to 200.

[GPracer2500]

I've been using Maxima Super-M at 38 or 40 to one. I mostly run it because I know I can find it everywhere.

I was hoping not to have to run straight race. I know my other ride doesn't need it. It sucks having to mix up different blends of fuel--I want to run the same stuff across the board. May not be able to though...

I don't want to turn it down any if I can help it. It's strong enough to compete and surprize a lot of newer machines--that's fun. The only changes I want to make are ones that will help it get faster.

[250r4life]

when i went bigger on my dome to lessen the compression, i did not notice going any slower up the hill... if i understand correctly, higher compression mostly helps with the low to mid range anyhow...

i dunno... i didnt like the idea of all that stress that that much compression puts on everything...

when i was running 215, i was mixing vp and av half and half...

[C-LEIGH RACING]

You can run that much compression on the engine but I would only use race gas & very little timming addvance. Keeping it rich like you said is a very good idea when trying to keep the piston cool from high compression.
I've ran some big bores with over 240 compression & they lasted for a good while.
Neil

[ChvyCruzen]

.. if i understand correctly, higher compression mostly helps with the low to mid range anyhow...

i dunno... i didnt like the idea of all that stress that that much compression puts on everything..

I thought so too, that if you raise your compression you only improve mostly low to mid range

200 is too much compression imo

[machwon]

R's like more compression (ratio) and increasing it will increase power from low to high. Compression (psi) can vary from engine to engine depending on the porting. 235 psi is definitely one that should use race fuel.

[GPracer2500]

I'm guessing this engine will see 25hours of duning a year. Maybe less?. I don't mind putting a piston in every year. AFAIK, the ignition timing is stock.

I think I'm trying to talk myself into keeping the comp and just running straight race. :p

[400EX QUADER]

It's fine... Run straight racegas with torco gp-7 40:1 and you should be fine.

[86 Quad R]

deffinately get away from mixing fuels. either lower the compression to run high OCT gas or run straight race fuel in the setup you have now.

[C-LEIGH RACING]

Matt,
200 in compressions not bad at all, I try to shoot for 200, 215 in just about every one of my builds.
You get over 240, 250, now I'm getting a little skidish about that wrist pin bearing, specially if I'm using a old style piston with a lot of side load.
200 is as high as I will go on a top end output engine build.
You put a lot of compression in a top end rpm R engine & boy those two things fight each other.
Well I say fight, it will run like that but the whole time its running I'm looking for the rod to stick out from somewhere its not suppost to.
That has happened before to & let me tell you first hand, it is a very sickening feeling looking at that engine & the oil & coolant all over the ground from holes everywhere.
Its a feeling most every one of the 4 stroke guys will encounter one time or another, & you notice & didnt say IF,,,just a matter of time.

Neil

[86 Quad R]

me en neil are on the same page when it comes to running that high of a compression on a engine that see's high revs for an extended period of time. the 220 - 250 psi is ok for short bursts like 300' dirt or sand drags but, for hill shooting...... it makes me gring :eek2:

i too try to keep all my engines between 185 - 205 that see alot of revs for long periods of time. been using this manner for years with no troubles. some setups are on cast pistons even.

[C-LEIGH RACING]

86,,,I know some of those guys are looking & saying,,whatttt a cast piston.
Yep thats right, a cast piston will last, its all about whos boreing that cylinder & fitting the piston that makes the difference.
:eek: :eek:
Neil