Compression killing the top end?

[smwood79]

I have two different people telling me something different. One says the high compression dome in my 250r kills the top end (rpm)
The other ( the guy that built the motor ) says 250r's love the compression and it should not make to much of a difference, he said to replace the ESR trx6 pipe that is currently on it and I will be happier with the top end. Any thoughts?

[Dave83]

This an ongoing debate.my personal experience has been on my 250R,nephews Pred90,and acouple other dirtbikes,that uping the compression helped out all thru the powerband.It may have cost a few rpm's but they wind out quicker and the top speed actually increased on the smaller bikes.I think the R is as fast as before,hard to tell on a mx track.I do know that a guy here with a 370 ESR motor called ESR and they recomended a smaller dome for dragging his R.He has it in and it picked up a few 10ths thru 300 ft.

Again here, compression has nothing to do with your RPM... the pipe shape (convergence cone lengh, angle and diameter) (Belly of the pipe and its length and diameter ) determines EVERYTHING in a 2 stroke engines.

Right now on my 500 engine i am running a 250 pipe, its smaller so pressure waves return much faster so i get MUCH Higher RPMs than if i had on a 500 size pipe, but it also means im always in the Powerband, so its a crazy monster to ride, from low throttle to WOT it simply goes nuts with the HP.

Whoever is telling you compression changes your rpm is nutz.
Its simply not how 2 stroke works.

[TRX370R]

I've always thought the higher the compression the more power your motor will be making.....doesn't have any effect on RPM. Only downfall is having to run race gas and the higher the compression is only gonna wear out your bottom end faster......but you gotta pay to play I've always been told haha. Just had a though while typing this.....what kinda compression is a nascar motor running??? I'm guessing pretty high to make that much HP and those things are turning 9500+ RPM

[wilkin250r]

First, a basic definition. Over-rev is when your engine is still making decent power once you pass the peak HP rev point. For example, on a 250r you might have a peak HP of 42 at 8200 rpm, but still make 35hp at 9000rpm, and 25 at 9300rpm. Over-rev.

People much smarter than me, with more experience, have said the phenomenon is true, that high compression kills top-end revs. I think it's in the over-rev area, past your peak HP. Higher compression will boost mid-range HP a lot, and peak HP a little, but kills the over-rev.

If we threw it into the above example, higher compression might boost you to 46hp at 8200rpm, but only 25hp at 9000rpm, and 15hp at 9300rpm.

You can push the peak HP into higher rpms with a different pipe design, as Dierwolf is talking about, but you still run into the over-rev problem.

[rebelbanshee]

excessive compression can hurt top end. Look at duncans site about one of the benefits of a PV..it decreases the compression to help overrev

[Dave83]

Im an idiot on this stuff but I dont see how a PV setup would decrease compression.If im looking at this right,compression should be built after the piston passes the exhaust port where the head and more specificaly the dome determine compression.Lower head smaller dome more compression.The PVas I see it ,eases you into the the "hit" by varying the exhaust port.Interested in what some of you more experienced guys think.this is the kind of stuff I like learning.

[Prey]

i just had my motor done by tony kellner

he made my motor run right around 220lbs for mx cause he said that would allow the motor to rev out, he builds his gncc oriented 250rs with more, this reduces the amount the engine will rev out and make ponies but makes more torque for woods riding.

[250R Baja Rider]

Prey and Wilkin250r are right on the money.

High compression will hurt your top end.

If you ride woods and MX you want lots of High Compression which will give you a ton of torque.

For desert riding, where top end is ongoing, you must run lower compression ratios to avoid losing top end.

[machwon]

Ditch that 6 pipe if you want it to rev.

As for the powervalve and compression it would have the same full stroke compression ratio as a motor without a powervalve. Other ways to calculate a compression ratio is called the effective ratio, which accounts for the height of the exhaust port. This would be a different value for a motor with a powervalve assuming the valve was open. If you took two motors, one with the exhaust port raised and one stock, the stock port motor would show a higher static psi on a pressure guage.

[wilkin250r]

To say that a powervalve "reduces" compression is rather silly.

Machwon is right in one respect. In a 2-stroke, you don't actually start building compression until the exhaust port closes. Yes, if you have two nearly identical engines, but one engine has a higher exhaust port, the higher exhaust port would show a lower static compression.

However, this has almost no bearing at all on a powervalve. When the powervalve is fully open, your running 6000 plus RPM, and then you have the pipe effect and returning pressure waves to stuff the cylinder and help build compression. Static compression doesn't mean jack at full RPM while you're on the pipe.

[Dave83]

So Im sorta right in my thinking that compression is built after the piston passes the exhaust port?Im not sure on others,butPv's like ESR,work by"shrinking " the exhaust port from the bottom then opening the port as rpms increase kinda easing you onto the pipe instead of the hard hit.How does the PV help the bottom end power if its closing the port up?Seems to me you would lose a little power having to open the valve.

[MR.BIG]

For mx you want more compression because it will give you more low end grunt where as short track it is better to have lower compression so it will rev better.

[wilkin250r]

Originally posted by Dave83
So Im sorta right in my thinking that compression is built after the piston passes the exhaust port?Im not sure on others,butPv's like ESR,work by"shrinking " the exhaust port from the bottom then opening the port as rpms increase kinda easing you onto the pipe instead of the hard hit.How does the PV help the bottom end power if its closing the port up?Seems to me you would lose a little power having to open the valve.

What?!? The ESR powervalve works by raising and lowering the bottom of the exhaust port, not the top? That can't be right, are you sure about that?

[JTRtrx250r]

hi compression does kill over-rev, and puts a greater load on the rod bearings and everything else, alot of flattrack motors dont have hi-compression as opposed to a motor set up for mx/woods(also depends on the builder/specs/rider), of course not every motor is not the same

[Dave83]

No Wilk,Im not sure on the ESR stuff.Ive only seen one and I was thinking that about the PV.I see how they would work raising and lowering the top of the port so Im probaly dreaming about it being the other way.Anyway,how much power do you suppose is lost by opening the valve?Ive asked ESR about this and they did say that the non PV kits did put out a little more than the PV.

[wilkin250r]

I've heard the other way around, that the PV engine put out slightly more.

Much of it depends upon setup. We know that two identical displacement engines can have radically different porting specs, and thus very different horsepower curves. Also, hig-rev engines have more peak horsepower, at the expense of low-end power. The high-rev characteristic is due largely to raising the exhaust port.

So it would make sense that you could port a powervalve engine for more TOP end power, and the powervalve allows you to keep the low-end.

However, I imagine that the powervalve also restricts some of your porting options, you can't cut your ports into the powervalve assembly.

[baker250r]

so then wut are the advantages of a powervalve motor? do they kick butt on MX tracks?

[Dave83]

Wilkin,the guys at ESR told me straight up that the non PV motors they have do put out a little more than the PV.But the PV was responsive at much wider rpm range.The PV is suppose to let you come off the corners in a higher gear without the bogging.Similar to a 4stroke.the fastest guy at our track used to have a PV265 and said he hated it.I think it was a Duncan.Said he just never got used to the PV and felt like he was always waiting on the power.He said you could lug it and jump off the corners,but it felt flat on the straights.IDK,just one guys opinion,I guess.Personally,I ride an r because i like the "hit".I should be getting my ESR 310 kit this week and I went with the non PV.Couldnt cough up the extra 600for something Im not sure about.Besides,with the bottom end port work,the hit is over as soon as you nail it.

[Prey]

^^^ i think you are going to like it alot, i know that i love mine, made a huge difference over the 265 kit i had in it before, i think i got the trx7 port (it was the bottom/mid port job at any rate)

good luck...hope you like it

[Prey]

http://home.comcast.net/~kstanka/atv/image/Rside1.gif

[wilkin250r]

The main advantage of a powervalve is low-end power and a smooth powerband. That "hit" that we all love so much can break traction on the rear wheels and cause them to spin, losing acceleration out of corners.

Dave, I know exactly what that other guy is talking about. When I first got my 340PV, it didn't "feel" as fast, because it didn't have that hard hit. But when I raced another quad, I found out mine was WAY faster than my stock motor. I used to be dead-even with the other quad up through 5th gear. With the 340PV, I could pull ahead even at 3/4 throttle. During one race, I actually looked over at the other rider and asked "that's it?" because I was only 1/2 to 3/4 throttle to stay neck and neck, the other rider was WFO. I hit full throttle and blew away.

[baker250r]

so.... if u got a powervalve cylinder, and ported it for mid-top end, ur sayin the powervalve would pick up wut ur lackin on bottom end?

[Prey]

Originally posted by baker250r
so.... if u got a powervalve cylinder, and ported it for mid-top end, ur sayin the powervalve would pick up wut ur lackin on bottom end?

here i go. i just hope i get this right, and keep in mind there are several kinds of PV's and i dont know how similar each is

with that said, when porting on non-PV cylinder, i think the lower your exhaust port the more low end you have, the higher it goes the more mid and ultimately top end you have

you will notice that there are no port options with a PV cylinder, this is beacause...........

in a PV engine, the exhaust port is much larger than a non-PV. what the PV is essentially is kind is like a flood gate with a top and a bottom, when the motor is at low RPMs the flood gate is all the way shut (ie, the top of the flood gate is down essnetially lowering your exhaist port ) as you increase RPMs this "gate" rises, raising your exhaust port to make more power in the mid range and then fully raised which is making your power on top end.

with that said, i agree with the ppl who said the PV does not make more HP, it is not inteded to do that but it is to make your power band much wider

[baker250r]

So, u preaty much dont port a powervalve cylinder? i guess that explains why i never hear about people porting their dirtbikes..... would u port ur intake and polish ur exhaust tho? and how about the transfer ports? would u port them.. lol, Im askin this cuz i am thinkin of upgrading to a PV, and want to know all about them b4 i drop the chunk of change and not know anything about it.

[powerslider]

A PV cylinder can be ported like any other to whatever you want. Baker250R call LRD and talk to Arlan, he will answer any questions you have. There is much mis-information here about this. I am very happy with mine and if I were building another engine it would definitely have a PV.

[Prey]

there was talk of ESR, ESR does not offer porting options with their PV cylinders, sure there is porting to be done like giving the overall exhaust port size, porting on a PV is no where near as caveated as with a non-PV

no one is really knocking it that i saw, some just said they prefer the arm ripping power vice the smoother but equal power of the PV

[machwon]

Each style of motor has its advantages. Powervalve motors are very nice to have in most all forms of racing. Drag racing and dune riding are two places I would considering saving the extra cash as both areas are typically riden with the motor pushing wide open all the time. Also, the bigger the bore on a 250 the more torque it has from the piston size and the need for that powervalve goes away. PV or not, either can be made fast. As for the porting on the powervalves, most come rather mild from CT, ESR, etc., if your into dune or drag racing. They width on the exhaust ports are also limited due to the powervalve size, which again gives the total topend advantage to the non PV.

[wilkin250r]

Originally posted by Prey
you will notice that there are no port options with a PV cylinder, this is beacause...........

in a PV engine, the exhaust port is much larger than a non-PV.

You're half right.

One thing you need to understand is that there is MORE to porting than just the exhaust port. The transfer port size, height, and angle will also affect the powerband, as well as boost ports and basic port configuration.

You are half right because the powervalve does limit porting options. With the powervalve assembly in there, you are limited with the size and shape you can cut the exhaust ports, which in turn limits your options on the transfer ports to avoid a porting mis-match between intake and exhaust.

Also, powervalve exhaust ports can come in different widths for different port time area, which will change the power characteristics. You may not be able to alter it with conventional porting and grinding tools, but you can certainly design a powervalve with certain port characteristics from the start.

[Prey]

^^^ wooo hooo, i will take 1/2 right.......

if you notice on my 1st post, i said "i hope i get this right"

i was 1/2 way there :blah:

[baker250r]

hmmmm...... i guess i kinda stole the thread here..... lol, sorry :devil:

[smorris]

I've owned both and the pv is def. the easier one to ride!
The curtis sparks 350 pv cyl with a sparks big bore tt pipe
had PLENTY of power bottom and top...And very smooth.
And for anyone thinking it wont hit , just get one and stab the throttle!!The pv cyl (I had two of em)You could come off a corner and roll on the throttle easy in a higher gear and it would pull it
unlike a non-pv.Or you could run it in a lower gear and nail it and it would rip off your arms!!Tons of torque.And NO LOSS of top end power.Of course it had a little more on top with the 250 tt pipe
than with the big bore tt pipe.But at a huge loss of bottom and mid.Also ran an esr pipe trx-6 I think. Again,more on top than with the big sparks pipe, but no bottom end and it hit REALLY hard when it did come on.By far the 330-hi rev tt pipe was much easier to ride.And I seriously doubt anyone riding that bike would notice a 1/2 hp loss on top!If its jetted right and running like it should there are very few engines that will beat it in a heads up drag race cc for cc.Even those ole yamaha banshees ( if they are a true 350 cc)not one of those 535 strokers making 100+hp.
my two cent worth...........