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Compression Ratio's

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Posted by: ochster

All else being the same, and the motor is ported and cammed to be a mid to top performer, could you gain some bottom to mid simply by putting a higher compression piston in? like going from 10.5:1 to 12.5:1.

Do you automatically lose revability with the higher compression piston?

Does higher compression, alway's mean higher cylinder and head temps?

I understand thier are many other variables, was just curious if you could use the piston alone as a tuning tool. My thought being you could'nt easily change porting, valve diameter, unless of course you had a spare head.

[This message has been edited by ochster (edited 03-25-2001).]


Posted by: Ol'89r

Ochster. IMOI you would be taking a step backwards by increasing your compression.
Compression makes horsepower, to a point.
That point being the fuel that you use.

If you go that high on your compression, you will have to run race fuel. If you don't run a high octane fuel, you will get pre detonation, especially at low RPM's. Higher compression Does mean higher combustion chamber temps, although, you should not lose any revability.

After going back and reading your other posts, I feel your problem is with the combination that you have put together.

In my experience, I have found that you can build a four stroke engine to work best in two areas, bottom to mid range or mid range to top end. If you want your engine to work in the bottom to mid range you need to use a torque type grind on your cam, large pocket type ports, a short intake track and not so much compression. The stroker crank should work well with this combination. You should get good bottom and mid range power, but, the power will drop off early.

If you want the engine to work best in the mid to top range, I would go with the stock stroke, the high rev cam, high compression, straighter ports with a long intake track and let the thing rev. The Honda RFVH engines like to rev.

You appear to have the combination of the two in your engine with the stroker crank and the high rev cam.

Hope this helps a little.

Ol'89r


Posted by: David Trustrum

To add to above which I totally agree apart from the implication that 4 strokes are useful, (cringe, wait for abuse) but another thing to watch for with higher compression pistons is that they may be heavier & or they may create a longer flame path or more disrupted combustion chamber shape. This was more of a problem in the bad old days of 2V heads. I'd really want to get a recommendation from a decent tuner for a particular piston.

Rather than just installing the piston, measure the current volume & squish clearance. Decide on what gas you can afford to run EVERY time. There are suitable charts around to determine the compression required.
May turn out you can raise the comp by reducing the squish (watch for valve clearance).



Posted by: ochster

From my experience and others I have talked to, it appears anytime you go up in piston size, you give up RPM? Is this just because your adding mass? or friction? both?

David, you mean cc's of the chamber? Are correct squish angles determined by fuel? piston crown shape? RPM's desired?

When you say, "reducing squish", you mean milling the head or cylinder? or infact doing some welding and remilling of the chamber and squish band?

When going up in piston size from 73mm to 78mm, I have wondered about the dynamics of the wider, flatter squish band created around the periphery. I had originally just put a slight radius on the otherwise "crisp" transition, from where the old bore diameter was. Sorry for all of the ?'s I appreciate the input. I feel dumber every time you guy's pose a new angle.



[This message has been edited by ochster (edited 03-27-2001).]


Posted by: SFO

Trial and Error
Two words, one concept, and in the face of statistics, I should have less of an error ratio by now.
Alas, my quest is endless...
Most engine designs seem to respond favorably to certain things, but they are usually different aspects.
The only similarities I have found, have led me, to create formulas, that end up biting me in the behind.
The only thing that I know for sure after 20 years of machining, is how to sharpen a drill.
The deductions that I have gleaned from building cylinder heads are all questionable.
Even proven flow figures do not create the qualities I may be searching for.
In the end, I only know, if it(das motor) is/performs, better or worse, and this might not be a result of what I did, but an assembly anomolie.
Using your motor as a test mule, you can step by step, see what modifications produced, what desireable results.
The system of an internal combustion engine has a high degree of compatability issues.
Trying to focus on one element creates blind spots.
Think system, one step at a time...
Boot camp taught me to pay attention to detail.
Combat taught me that the end is certain, just perhaps not when I might be anticipating it.
Women taught me that,....Aww, shucks...
I have to stop this and get my bike hauler ready for our three day gourmet enduro...
Your humble servant, Bill


[This message has been edited by SFO (edited 03-27-2001).]

[This message has been edited by SFO (edited 03-27-2001).]


Posted by: KLeeneX300

I have a similar question. There are two different big bore kits for my bike (KLX300).One is a 84mm (340cc) piston with 10.5 to 1 compression and the other is a 83mm (331cc) piston with 12 to 1 compression. What would be the difference in the type of power that these kits would make if any? All other things being equal.

------------------
KLeeneX300
** -To spodely go where no spode has gone before


Posted by: Ol'89r

Ochster. IMO, you will give up a little RPM when you go up in size due to the reasons you mentioned. Although, the difference in RPM would be insignificant.

Also, IMO, reducing squish can be done in several ways, depending on the design of the combustion chamber. Some engines, like your XR, use a flat top piston and the piston comes up to the head and stops. In other engine designs, the piston actually comes up beyond the head gasket surface and into the combustion chamber. The squish area is the flat band area around the piston that you spoke of. The clearance between this band and the cylinder head is the squish area. I have run this as close as .025 -.030 in some engines.

You can change the squish area by milling the head, milling the cylinder or by using different piston designs, all depending on the engine design you are working with.

SFO made a very important point, " The system of an internal combustion engine has many compatability issues." I toally agree with this. Everything in an engine has to work along with everything else.

When designing a modified engine, first, you have to determine what you are going to use the engine for. If it is a drag racer or flattracker that is going to be run wide open most of the time, you can use very radical specs on compression, cams, etc. Since you are not concerned about how the engine will perform at a low RPM, throttle response, etc, you can use very radical specs in an engine like this.

If you are going to use the engine for trail riding, TT or MX, you will have to use much different specs, lower compression, less cam lift, etc, otherwise you will lose dependability and ridability.

You will want this type of engine to pull well from bottom to top, have good throttle response and top end performance will not be as important since that is not the range you will be running in most of the time.

The end result is the type of power to the rear wheel. I have seen and ridden many high horsepower bikes that are completly unridable. It may be the fastest thing on the dyno, but, if you can't make it work on the racetrack, (throttle control, traction, etc.), all of that horsepower doesn't do you much good.

KLeeneX300. There again, what are you going to use it for? IMO, the 84mm piston with the 10.5 compression ratio would be good for trail riding and torque conditions. The 83mm piston with the high compression would be better for top end performance.

Just my humble opinon.

Ol'89r



Posted by: dirt bike dave

KleeneX300 - my buddy had the 331cc 12:1 piston in his KLX300, and it improved the bike dramatically over stock. He also replaced the carb and exhaust system. With the mods, it was a sweet running (and handling) trail machine.

The high compression 331 makes usable power over a very wide range and will rev very high. It makes much better low and low-mid than stock, but not nearly as much down low as an XR400. If you could breed a DRZ400 with a KX125, you would get a KLX331. Never tried a KLX340, but adding some power to the KLX really increases the fun factor.

[This message has been edited by dirt bike dave (edited 03-28-2001).]


Posted by: ochster

Concerning squish volume;

Is it accurate to say, you want as little volume as possible?

Determining factors being, mechanical clearances, parts distortion or flexation, so you don't make componet contact. And also fuel?

Woulld it then be accurate to expect this to be a trial and error process? And not something one could figure out mathematically?

Using my 13:1 piston as an example. If I was to find out I had a squish of say .050, and I wanted to try to improve performance. It seems to me, by milling either the head or cylinder, I would be upping my compression more, plus creating a worse problem with valve to piston clearance. So would I be better off starting with say a 11:1 piston? Then through several tries, mill small amounts, and test. Seems this way I would have more lattitude as far as valve to piston clearance, and not running insane comp. ratios, while figuring squish volume. I also could see this done with different head gasket thicknesses. Then once you achieved optimum results, maybe have a piston made to your desired specs?

Is this a logical way of determing optimum squish volume?

Is this something one should be concerned with? Or does the piston manufacture more than likely, have this down to a science when they R&D thier pistons? (wich I doubt they could, cosidering production tolerances)

I know I'm rambling, but I can't turn my brain off!

[This message has been edited by ochster (edited 03-28-2001).]

[This message has been edited by ochster (edited 03-28-2001).]


Posted by: Ol'89r

Ochster. If your looking for max compression then, Yes, you want as little volume as possible. I don't believe there is a formula for figuring this out, at least not one that I know of. I would use clay to determine the clearance.

The idea is to machine either the head or cylinder inorder to allow the squish band on the piston to come as close to the mating area on the head as possible without touching. There are several factors to consider, rod stretch, mechanical clearance, carbon build-up, and wear, etc. Also, you would want to add a little clearance for overreving,(missed shifts, etc.).

The heighth of the squish band should be the same on all production pistons. The difference between the two high compression pistons would be in the dome configuration. If you were to decrease the clearance in the squish band, then you would have to flycut the valve pockets in the piston dome to compensate for it otherwise the valves may hit the piston.

I suppose you could have a special piston made once you determine the proper squish clearance.

As far as using different head gaskets, you would have to completly eliminate the head gasket to decrease the squish area enough. Since the squish band on the piston normally comes up even with the top of the cylinder, then, you add the thickness of the gasket.

Hope this helps.

Ol'89r

[This message has been edited by Ol'89r (edited 03-29-2001).]


Posted by: ochster

Ol'89r, you stated you had run squish clearances as close as .025-.030. Would you consider this a minimal clearance for my apllication?

At this point I'm pretty sure I'm going back to the stock crank with a 105mm Carillo rod, 80-81mm 11:1 J&E piston. I will, as I have alway's, be running good VP fuel. My thoughts are going with the 11:1 piston gives me some "room", for valve to piston clearance while milling for squish adjstument. Seems that by going to higher comp. piston, I limit myself to be able to tune the squish, without running into valve to piston problems. Understandably I may still have V-P clearance problems, but should be less likely with the lower comp. piston. I could be talking about a non issue, I have never even measured the stock squish. Does this sound logical?

Mike at Kibblewhite, figured he would be getting back with me Friday, on flow figures of the head. I know that will dictate alot.

Ol'89r, I Thank You! for your patience.




Posted by: sKiNNyWhEaT

WOW! im looking for more ways to get more power outta my xr430. it seems ive stumbled on a good topic! as i was currently thnking about milling my headfor more compression. in currently riding a 96 xr400 with the xrsonly 430 10.5:1 compression pistion. i take it carillo makes coustom rods from this topic, i was wondering if anyone stroked the xr400?. but now in thnking mill the squish to minimal specs and grab as mutch compression as possible, its sems to me that upping my compression gives gobs of torque. but adds to the nasty stalling at low RPM's {i thnk adding flywhell wieght will counter that effect tho?}. the piston that came in the 430 kits has sizable valve releifs in it already. does anyone have spec for this type of mod for the xr400? i have machine shop access and can do all my own machine work. im shure xrsonly has done this and know the specs i should follow, but they probly would want me to buy "their" modified head with core. im in search of that torque god. any help is greatly appreciated! im mainly climbing vertical hills and bottomless mudholes. o yea.. after rebuilding my xr engine i have this nasty stalling problem, ONLY when the crankcase LOWer brather tube enters water..? it seems as tho when the fram touches water the bike wants to stall , and i have to work! the trottle and clutch to keep it alive??? andyone have any ideas on this? im thinking: to mutch crankcase presshure and the UPper tube alone cant flow enough air to releive the increased presshures in the crankcase due to the 430 kit..?
srry for disrupting the squish band topic.

skinny


Posted by: Ol'89r

ochster. The engine that I was referring to with the squish figures was a 750cc triple road racer engine. It ran a much higher RPM than your XR. Although, the piston size is similar to the XR, I think I would give the XR a little more clearance to start with. Perhaps, .040 / .050. And yes, determining this clearance is pretty much trial and error.

The compression ratio of the piston should not matter in regard to the depth of the valve pockets. The valve pockets should be the same in either piston. Only the heighth of the dome should be different.

If it were my engine, I think I would go with the lower compression piston. If you go too high on compression, it will make the bike harder to ride on trail. If you are going for max HP, like a drag racer, then go for the high comp piston.

Skinny. In my experience, I have found that a good torque grind cam and a Mikuni pumper carb would probably compliment your XR with the 430cc kit. By going higher on compression, you would only add to your stalling problem. A heavier flywheel would help.

Also, in regard to the breather tube, this is a big problem with the new YZ and WR Yamaha four strokes when you get in deep water. Have not heard of this happening to the XR's though. Re-routing the breather tube to the airbox has cured the problem with the Yamahas, so, you might want to try that.

Thanks for disrupting the squish band topic, my head is starting to hurt. http://dirtrider.net/ubb2/wink.gif


Posted by: Ol'89r

ochster. One topic that we didn't touch on was valve overlap. You will notice that when you degree your cam in, the exhaust valve opens before the intake valve closes. As you probably know, this is the overlap.

Normally, the more radical the cam, the more overlap. On a high RPM engine you can get away with a lot of overlap. On a torque type engine, you will lose bottom end performance if you have too much overlap.
At lower RPM's, too much overlap takes away from the vacuum effect of the intake track, making it very hard to jet an engine like this. There again, there is a happy medium where everything works well with everything else.

I feel there is a lot of room for improving the modern four stroke engine. By using modern materials and electronics, I believe we are just on the cutting edge of some exciting technology. I think we will see, mechanically, electronically or hydraulically advance and retard systems for camshafts. Power valve type systems for intake and exhaust tracks and advancements in electronics, maping, etc. This will give us the best of both worlds in an engine that will perform from the very bottom, to the very top and every where in between.

They already have a lot of these system in testing and in use in other forms of racing such as Formula 1 racing and it is only a matter of time that the manufactures start using them on motorcycles.

Stick a fork in me, I'm done.

Ol'89r


Posted by: motopuffs

To take a step backwards here, what about compression ratio versus heat (part of the original question)? I'm not sure if it is logical for a higher compression engine to produce more heat.

The same volume of fuel and air are being compressed. BTUs available are equal.

The higher the compression, the more violent the combustion. An equal amound of fuel and air at low compression would burn very slowly.

It seems to me that at higher compression ratios, more mechanical energy is released (expansion of gasses) and less heat energy.
The law on conservation of energy states that if the available BTUs are constant, if there is an increase in one type of energy released, there would have to be a decrease somewhere else.

Diesel engines have a very high compression ratio, and they run very cool compared to gasoline engines. Here in Iowa, we see tractors with the radiator completely covered up in the winter.

Also consider heat transfer...the faster and more violent explosion produced by a higher compression ratio will give the cylinder, piston and head less time to absorb the heat energy.

Am I on the right track, or has my train derailed?

[This message has been edited by kookooformotopuffs (edited 03-31-2001).]

[This message has been edited by kookooformotopuffs (edited 04-01-2001).]


Posted by: WWR

kookoo,
I believe you have it backwards with the diesel/heat thing. Diesels tend to run cooler. This is why they need the air block on the front of the trucks in winter, so the engines will warm up/maintain operating temps better. But the diesel/gas comparison has too many differences to actually compare.

In a gas engine, the higher compression ratios heat the air/fuel mixture more and confines the mixture in a smaller area which helps enhance your volumetric effeciency (similiar to dynamite, the tighter it is wrapped, the stronger it can be). The increase in compressin temp and combustion temps from higher compression directly relate to the operating temps increasing. But this is just the tip of the iceburg. An expert could write a very thick book to explain every detail of encancing gasoline 4-stroke performance/operation, and I am no expert.

------------------
Paul '97 YZ250, '86 VFR400R Mechanical "nut"


Posted by: Joe Turner

Quote:
Originally posted by WWR:

In a gas engine, the higher compression ratios heat the air/fuel mixture more and confines the mixture in a smaller area which helps enhance your volumetric effeciency


Volumetric efficiency isn't related to compression ratio, unless there is a big giant dome in the middle and it effects the cylinder head flow.




Posted by: motopuffs

Ooops, thanks for correcting me, that's what I meant to say.

I know there are other variables, but if changing the compression ratio from say, 9:1 to 10:1 has an effect on heat in the engine, wouldn't you think at 20:1 in a diesel we would be seeing more heat?

What is the definition of VOLUMETRIC EFFECIENCY?

An engine is actually a fluid pump (air and fuel are both fluids). I would think that the more mixture we can pump into the chamber, the greater our volumetric efficiency. If you could measure the pressure in the cylinder as the intake valve starts to close, you could get an idea of the engine's efficiency. Of course, the exhaust stroke is another issue.

So, a less efficient engine could run a higher compression ratio without knocking, where a blown or turboed motor would need a much lower compression ratio.

Elevation/air density is also a consideration.

Anyway, what is the official definition for VE?

Sorry if I'm boring you guys, I'm actually not into four strokes but I enjoy engine theory.


Posted by: Joe Turner

Volumetric efficiency (VE) is used to describe the amount of fuel/air in the cylinder in relation to regular atmospheric air. If the cylinder is filled with fuel/air at atmospheric pressure, then the engine is said to have 100% volumetric efficiency. On the other hand, tuned length runners on the intake and exhaust can increase the pressure entering the cylinder, giving the engine a volumetric efficiency greater than 100%. However, if the cylinder is pulling in a vacuum, then the engine has less than 100% volumetric efficiency. Normally aspirated engines typically run anywhere between 80% and 100% VE. So now, when you read that a certain manifold and cam combination tested out to have a 95% VE, you will know that the higher the number, the more power the engine can produce.

Basically, volumetric efficiency is effected by your carb, intake manifold, headers, and cam specs. Compression Ratio effects THERMAL efficiency and pumping losses (mechanical losses)



Posted by: WWR

Joe,
thanks for the simple explanation, it cleared a few things up for me.

THERMAL effeciency. Yea, thats what I meant! http://dirtrider.net/ubb2/smile.gif

------------------
Paul '97 YZ250, '86 VFR400R Mechanical "nut"


Posted by: kevkon

Having experience with engine building for NHRA drag racing, I would say that the issue of higher comp. ratios must be, as mentioned in a previous reply, part of a "total package".
There are many instances with internal combustion engines where mods can negatively effect OVERALL performance even while improving SPECIFIC performance. That is to say, you may gain in peak hp, only to find a loss in performance range,ie; no bottom/mid range power.
In addition, increasing your compression ratio without adjustments elsewere may cause problems such as ; pre-detonation, spark blow-out from inadequate ignition system, overheating, mechanical interference (valves to piston etc), improper timing,hard starting, and others.
I think in general dirt bike tuning is difficult because of the wide range of power called for, it is always easier to add performance when you want it from a specific area.



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