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Discussion Starter #1
Hello,
New poster. I have read ALOT on here but never posted. I have a 1985 ATC 250R

Question:
In general terms how much compression can you run with a stock bore and stroke before it starts negatively impacting max RPM?
(For purposes of this question assume intake/carb/exhaust/porting/octane are all adequate to achieve desired effect)

I ride exclusively in the sand, and mostly big steep dunes. However, at one of the locations we ride there are miles of pure flats where we drag race/top speed run so being the fastest top speed is great for bragging rights.

Top speed is not so important that I want to mess around with gear ratio’s, so just assume factory front and rear sprockets.

Hence my question how much compression can I run for HP before it slows me down as far as top speed?
 

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Discussion Starter #3
This part of my post kind of got buried in my question, I said

“(For purposes of this question assume intake/carb/exhaust/porting/octane are all adequate to achieve desired effect)”

So with that in mind, any idea?
 

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This part of my post kind of got buried in my question, I said

“(For purposes of this question assume intake/carb/exhaust/porting/octane are all adequate to achieve desired effect)”

So with that in mind, any idea?
Not an accurate answer available, all setups based on intended use, gearing, fuel, IGN, ect...are different, yielding varying answers. In very-very general terms; =>195 lbs

Carlos
 

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Carlos at BDT has helped me out huge in this area and has explained this subject to me once it was years ago but I keep in my head when I’m building my personal motors. Carlos endless supply of knowledge and very helpful so just ask him and most important comprehend what he is saying!
Here is my twist and I am far from right but I only run at Glamis as well. I love conservative approach on this I would rather ride all weekend but have quick bike rather than have fastest bike and break within 2 hrs of riding. So make your choice?
Today I love this combo 88-89 cylinder, OEM 1985 Honda ATC 250R head gasket, VForce 3 reeds, Stock cylinder head do not cut or mill unless head not flat, polish cylinder chamber to mirror, iridium plug I use BR8EIX NGK, and if you can find one on the cheap use a CR 500 38MM carb get cable from someone parting out an 84 Honda atc 250R.
I try to keep compression no more than 195LBS just to avoid unnecessary strain on crank you are riding on sand that’s allot of load on engine. But also pipe is key so buy a good pipe Carlos at BDT with Kenney makes awesome pipes but buy a pipe for your dune application. I run VP 110 with super unleaded at 32:1 with yamalube. My bikes run great and most important they last all weekend no issues.
Again ask Carlos he’s a good man and willing to answer your questions about your application. He knows what he’s doing, good luck as well and don’t be scared one bit!
 

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Discussion Starter #6
Thanks for the replies. I understand and expect only very “general” answers. So assuming a CR of 195, is 110 fuel “generally” required? I have no problem running 110, it’s very accessible to me and I don’t ride often enough to be deterred by the cost. I ask because I understand that too much octane when not necessary to avoid detonation, can have a negative effect.
 

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No not necessarily required!
I personally would say 50/50 mix of race/super unleaded be used, now the race fuel in my opinion needed or I use because it has a slower burn than super unleaded. Take this into mind you are riding in sand your bike has great load spinning tires to keep turning tires now add paddles into mix and cylinder pressures or temps will rise and that is where race fuel comes into play. That slow burn will help to stabilize cylinder temps a little better than super unleaded fuel.
Again I don't know much about how ut works or the chemistry involved but I know how much my engines run and what I put them thru using fuels and oils I use.
 

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Riding at Glamis or sand top mods or parts I would do today in this order of course general maintenance required!!!
1) Make sure engine no air leaks
2) EHS Racing air box lid
3) Roll Pegs
4) 1985 ATC 250R OEM NOS head gasket
5) BDT Pipe
6) VForce 3 Reeds
7) NGK Iridium plug BR8EIX or BR9EIX which ever required
8) Properly dyno tuned ATC, TRX 250R by someone who knows or familiar with machine
 

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Discussion Starter #9
Just as a follow up to those who might read this later (I know I read on here for years before joining and posting). I already have a pretty good parts list. My 85 ATC 250R has been ported and the head was milled bringing the compression back to 185 (I ride at 4200 ft) by a local Salt Lake City builder. I have a BDT intake, BDT pipe, 38 PWK and carbon fiber reeds. I have been running a blend of 110 and 91 for an octane of about 100. The bike runs great.

I posed my question because I have been considering going to a thinner head gasket. If I can increase compression (low mid power) without decreasing my max RPM, I figured why not.

Thanks to Azetecgwynn for his posts subsequent text and information. it is nice to just talk with people enjoy these bikes.

And a big shout out to Carlos. Not only has he responded to way too many of my text messages, but he also called me talked for over 30 minutes explaining compression and squish. He explained to me what to do and how to do it (If I decide to change gaskets). It should be noted (because of some of the talk on here) that Carlos neither bad mouthed my builder, nor did he try and sell me anything. In fact his suggestion was to just ride it and enjoy it.

Anyway, I'm trying to decide if upping my compression (and reducing the squish) is worth the hassle, like will I even notice a difference? I'm stuck between what I should do (which is just ride it) and what I am compelled to do, which is to just tinker with it?

Thoughts?
 
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