Compression ?

[bgraley]

This may just show my lack of knowledge for motors, but I read somewhere that using a lower compression piston set up will give more bottom end power where a higher compression piston will give you more top end. is this true? I would rather risk being in another of Bobs text-to-movie films and know the truth than go on what some dumb guy said on line. thanks for your help.

[dubious01]

higher compression increases torque, it will bve very noticable on bottom rpm, with no noticable change in top rpm power.
Increasing compression is the easiest way to make more power from the cr500. Milling .030 off the head of a stock engine is the cheapest most effective way to increase power on the 500 IMO.
This will net approx 8.5:1 compression, still safe for premium pump fuel

stock is only 6.9:1

[bgraley]

ok cool. I thought what i read was backwards. I am running a 250 engine right now. but the concept is still the same.

[dubious01]

ok cool. I thought what i read was backwards. I am running a 250 engine right now. but the concept is still the same.

I took .030 off an other wise stock , but fresh 01 cr250 with a pipe, and it revved so fast it was unreal.
Made my 04 YZ450 feel LAME!
I think it was faster than some stock healthy 500's I rode. Just hit HARD when the powervalve opened, in 4th, the front tire would come about 18 " off the ground and it would carry it out to a screaming top end. Unlike a stock 500 that dies after the mid hit, this thing screamed up top.

It would have been a great second bike, but he wouldn't sell it to me.

[bgraley]

and you said it is still cool on pump gas? I don't really want to do motor work that will cause me to pay for race gas when i don't race. I don't have money to throw at my hobbies like that. Can't the same thing be accomplished with a thicker base gasket? or does that take it too far?

[AlisoBob]

Thicker base gasket pushes the head AWAY from the piston and also "delays" the exhaust port closing.

Both kill low end power.

[dubious01]

and you said it is still cool on pump gas? I don't really want to do motor work that will cause me to pay for race gas when i don't race. I don't have money to throw at my hobbies like that. Can't the same thing be accomplished with a thicker base gasket? or does that take it too far?

You will have to ensure the tune of your engine is stock.
On a 250 mill .020 off the head.

I have done .030 on the 250 and 500, but I don't want to steer you wrong.

[Tharrell]

Damn, I was impressed with my '01 250 after a fresh stock top end.
I love that engine.

[100hp honda]

how much power you think you gain from shaving .030 off a head ? enough difference to even measure on a modern dyno ?? 1ftlb maaayyyyybbbbeee ? change in air temp could yeild the same results . roostius did all kinds of cylinder/case/head work to get +7ftlbs. so i ask you again, how much power you really think youll gain from simply hacking off .030 ??

[2T500]

On motors that rev high like the 250 then revs is the trick, they usually have little if any you can shave off a head, the thermal coupling is so important that you need to beware when the head gasket is like .010' steel, same for 500 depending on the head used...

the head shave without altering port timing is a bmep trick, add pressure to bump the plan formula but revs are usually so much easier, on a 2 stoke the thing i wonder about is the engine pump over atmospheric effect on any compression increase, its always more significant than you think unless your engine builder is thinking this out for you so my advise is ensure the piston to head clearance means no drama and then run the right gas and use a good engine builder before you change away from stock, reduce comp until you know better if your mixing engine parts.

[dubious01]

how much power you think you gain from shaving .030 off a head ? enough difference to even measure on a modern dyno ?? 1ftlb maaayyyyybbbbeee ? change in air temp could yeild the same results . roostius did all kinds of cylinder/case/head work to get +7ftlbs. so i ask you again, how much power you really think youll gain from simply hacking off .030 ??

It was way stronger than stock, in fact it was near dangerous for jumping if you treated it the same, and did a throttle chop, it would endo like a 4 stroke, blip it hard and it would flip over backwards.
The kid was scared of it afterwards!


I rode the fuckin thing and it felt 20% faster than stock, and it was pullling my 450, wheras before milling the head it was sucking hind on my 450 tail pipe, by a fair margin.

squish was .038, safe enough for a healthy 250 , but I wouldn't run it tighter than that.
We had a spare head, so it was a matter of installing that if it would have pinged on pump

Clyde, you read it all doesn't mean you know it all.

Im not regurgitating something someone else wrote on the internet.
(and i am not refuting roosty's work, otherwise i wouldn't have a 3rd motor of mine going to him before the new year)

I am speaking about my own real world experiences, riding a real bike!
Your going to come on the board and refute the findings and experiences of others? really?

thats fucking ignorant, and by that I don't mean rude.

[87CR500Rider]

I'd chance to say you have other issues if that 500 was sucking tit to a 450. My 87' with a cracked piston and stop sign shaped cylinder would outpull a 450 anywhere on the track. That's also a real life experience.

[hoofarted]

I'd chance to say you have other issues if that 500 was sucking tit to a 450. My 87' with a cracked piston and stop sign shaped cylinder would outpull a 450 anywhere on the track. That's also a real life experience.

He's referring to a 250.

[nmdesertrider]

That's my criteria- if you get pulled by a 450 it's time for a rebuild.

[100hp honda]

how much power you think you gain from shaving .030 off a head ? enough difference to even measure on a modern dyno ?? 1ftlb maaayyyyybbbbeee ? change in air temp could yeild the same results . roostius did all kinds of cylinder/case/head work to get +7ftlbs. so i ask you again, how much power you really think youll gain from simply hacking off .030 ??

It was way stronger than stock, in fact it was near dangerous for jumping if you treated it the same, and did a throttle chop, it would endo like a 4 stroke, blip it hard and it would flip over backwards.
The kid was scared of it afterwards!


I rode the fuckin thing and it felt 20% faster than stock, and it was pullling my 450, wheras before milling the head it was sucking hind on my 450 tail pipe, by a fair margin.

squish was .038, safe enough for a healthy 250 , but I wouldn't run it tighter than that.
We had a spare head, so it was a matter of installing that if it would have pinged on pump

Clyde, you read it all doesn't mean you know it all.

Im not regurgitating something someone else wrote on the internet.
(and i am not refuting roosty's work, otherwise i wouldn't have a 3rd motor of mine going to him before the new year)

I am speaking about my own real world experiences, riding a real bike!
Your going to come on the board and refute the findings and experiences of others? really?

thats fucking ignorant, and by that I don't mean rude.

if you claim a good size gain in power from simply milling the head then ill take your word for it. i sure didnt see any big gain in power. in my experience i would compare shaving a cr500 head in itself with no other engine mods to that of cleaning cast flaws. people were raving about the new found power in simply cleaning cast flaws from ports, naturally i had to see if there was any truth to it........ several wasted hours of my life ill never get back.

[dubious01]

I never saw anything from cleaning casting flash either, but -.040 off a stock CR500 head gave me more torque , bottom end through compression, than any pipe or carb has ever done. Reeds, PWK , and pipe might be close in performance increase, but thats not all bottom torque, and allot more dough than milling the head.

I thought I would jump back on this thread casue I found out the guys are having issues with the 02+ RC valve CR250's, jetting the mikuni, and also those engines are reported to have .080 piston to head clearance, maching it down to .040 is helping the jetting problems, but best results have been PWK 36mm. Still screams on top, but pulls on bottom and mid like the old HPP 2001 motors.

I read this on several threads in Thumpertalk forums.

I happen to have one of these motors in my next conversion bike, and I will be sure to check it out for myself, as well as compare the clutch assemblies.

I really want a barnett basket with stainless inserts.
The hinson one I have looks pretty worn. Not much better than stock IMO.

[Roostius_Maximus]

Still screams on top, but pulls on bottom and mid like the old HPP 2001 motors.

when we were running a 2000 and 2002 cr250s there was no question that the 2002 had what felt like twice the torque, and the 2000 would rpm higher, but didnt pull to that rpm as hard as the 2002

[rsss396]

I believe shaving .030 off a .068 squish gap would make a noticeable difference
The UCCR would go up about a point and the squish velocity would go up allot since you have just about cut the squish gap in half.

A higher squish velocity increases the fuel burn rate in essence acting allot like advancing the timing.

[dubious01]

I believe shaving .030 off a .068 squish gap would make a noticeable difference
The UCCR would go up about a point and the squish velocity would go up allot since you have just about cut the squish gap in half.

A higher squish velocity increases the fuel burn rate in essence acting allot like advancing the timing.

Thank you, my point exactly was the increase in compression.
1 point UCCR= to about 10% more power, as long as detonation, and cyliner head temps are maintained, without detuning through timing or jetting.

In fact, it felt more like about 20% increase, with 1 size larger on the main, no detonation, on pump gas, -.040" on the stock 500 head.
I will refer to my notes and provide cc change in the head tomorrow.

[100hp honda]

who cares . stock head is crap regardless what you shave off. with 19mm wide band how much fuel you think never gets burned under there ? they designed it to run on old piss watered down chevron gas. and it did a mighty fine job of serving that purpose

[AlisoBob]

.... they designed it to run on old piss watered down chevron gas.

I prefer ARCO

[Slomo]

I try to stay out of these threads, but I cant stand it no more...........
How many of you power hungry dudes can ride a STOCK CR500 to it's potential???? If you can, post up some video of yourself being badass gnarly so the entire web-o-sphere can see how truely sick you are.
Clyde- meet us on the way down to Hoonfest and jump in. There are about 8 of us this year driving 1250 miles one way to ride Hoonfest and we would love your company. Bring your sick, crazy, gnarly, badass whatever in the hell you have bike and let us all witness it first hand....................................................................................................................................................................................................................................................... I will personally help fund your trip so you have no excuse not to go.

[AlisoBob]

I will personally help fund your trip so you have no excuse not to go.

Thats a "Tea-Baggin' " SMACKDOWN if I ever heard one!

Hop in the truck Clyde!!

[100hp honda]

no need to get all cranked up . call pete tomorow he will explain the head deal

[dubious01]

Clyde, you have nothing positive to contribute in this thread. You are a post whore, thread jackoff!

the stock ML3R head is 66 cc, 30 of which is bowl, and 36 in the squish band chamber, and below the bowl with a 2mm deep squish clearance at the outer edge of the chamber.
.080 is the biggest reason any hydrocarbons aren't burning... MSV sucks with that clearance...


Cutting .040" or 1mm off that equates to 12cc removed.
66cc-12cc= 54cc

stock advertised corrected compression is 6.9:1 at 66cc, which works out to 8.4 :1 at 54cc
My engines have all sustained this mod on pump fuel.
all 7 bikes, with numerous rebuilds over the past 24 years I have ridden CR500's.

Until you have spent the $10 on a head gasket, and $20 to mill a stock head, and actually tried this mod, I invite you to shut the fuck up,....
.... actually I challenge you to.