Compression Ratio

[SuperLooper]

Is it true that compression helps top end? It was my understanding, from Eric Gorr and a few others, that compression hurt top end. Now I'm told, from a trustworthy local race builder, that increased compression in 2 and 4 strokes makes power everywhere. He also stated compression hurting the top end was a myth. What's the deal?

If you take the head from a 1988 CR500 and mill .010-.020 from the surface, will it increase top end? What compression ratio will I end up with? Can someone explain the pros and cons of compression? Thanks for all the help!

 

[bclapham]

Originally posted by SuperLooper
Is it true that compression helps top end? It was my understanding, from Eric Gorr and a few others, that compression hurt top end.

that is what i found. be carefull what you get into, it can get quite complicated if they take off too much. i might air on the side of caution with an 88 bike, espeically if its a 500 with loads of power anyway.

good luck

 

[Rider 007]

That's the way I understand it also, BC. It seems if you up the compression then you increase the pressure the piston has to overcome as it nears TDC, thereby increasing the resistance against it. It seems to me this would hinder it's ability to rev.
I know in my old pickup I would advance the ignition timing via the distributor and it would gain a lot of response off the bottom. When I retarded it, it lost the punch but would rev much higher. I attribute that to the fact that it was having to fight the combustion pressure when it ignited earlier in the stroke, due to the advanced timing, hindering it's ability to rev.

 

[flyinzuki]

I thought it was due to the MSV increasing because of the tighter squish.
I guess it would be true if the original setup was allready optimized, but we all know about factory tolerances.
Or the rumor may have come from people that have raised the compression by using a thinner base gasket, or turning down the base of the cyl. effectively ruducing port duration.

 

[MikeeeP]

More compression in most cases = less top end.

More compressoin in most cases = quicker reving / more torque / hp at that given RPM (easy explanation).

The only bikes that you will see rev higher with more compression, are bikes that are under a load and underpowered. What I mean is that a PW50 can't pull top end because it doesn't have enough power. Increase the compression a little and under a load (under way) it will rev higher as long as the compression isn't too high and start to hinder that. Some street legal enduro bikes had that problem. When trying to overcome rolling resistance, rider weight, wind drag, the smaller bikes would top out on the tach ad XXXX RPM, but if you milled the head a bit raising compression they would pull past that number because they had the hp/torque to do it.

I don't think I'd mill the head on a CR500. Maybe install a BP8ES plug or BP7ES plug and see how you like that. Eric got me doing that way back in the early 90s, late 80s??? Probably early 90s. Now, I sell probably 5 boxes per month of those through my shop to local riders. They love them.

MP

 

[Glitch]

What does a projected tip plug, I think thats what the 'P' stands for, do to the performance of a bike MikeeeP?

 

[crankin355]

By shaving the head you increase compression ratio. Doing so will increase combustion pressure and subsequently torque output of the engine regardless of r.p.m. When combustion pressure is increased, ignition advance may need to be decreased to avoid detonation. Other things that can be done to avoid detonation include enrichening of air fuel mixture and/or running a colder spark plug.
By changing the gasket under the jug to a thinner one you are altering compression ratio and port timing. By dropping the ports, you are increasing compression ratio, retarding port opening, advancing port closure and decreasing port duration. Doing so will give you an increase in low/midrange power and even more of the 500's violent hit.
If you are looking for more top end, get a larger carb (39.5 mm Keihin from KX500 or aftermarket) and a Pro Circuit pipe. You could also mill the head and put a thicker gasket (same thickness as removed from head) under the jug. This will advance port opening, retard port closure and increase port duration without changing compression ratio. Or, you could take your engine to your trustworthy local engine builder and have him do more extensive mods that could give you significantly more power than stock, when combined with the larger carb and pipe.

 

[SuperLooper]

It's hard to tell if my cylinder & head have been turned down, without having specs to compare with stock. The bore is now 1mm over, intake & exhuast ported, PC pipe/silencer. This bike has a violent mid range and no top end. How can I take away the mid range explosion and increase top end. I would like the bike to run 100 mph for short distances yet remain stable on technical hill climbs and tight woods? Is this asking too much from the 500? Don't get me wrong, I'm addicted to the power the bike has but would like to have a little more control when negotiating obstacles.

 

[crankin355]

You are asking alot from an 88. The 85-88 are notorious for hitting hard. The larger carb will take a little away from the bottom but probably wont help with the midrange hit, although it will give you more top end. Have you tried flywheel weights? They smooth things out a little, make it harder to stall and easier to start. Check out http://www.steahlyoffroad.com

 

[SuperLooper]

I looked into flywheel weights a few months ago and was suprised to find they aren't recommended for the 88 500. The 88 , I am told, has the heavist crank of all CR500's. The bike does great down low through second gear, third gear is a bit silly though. It would be nice to harness that power and put it on top.