Rich, 250F questions

Doug - The stock pipe with the stock cam is really pretty damn good. Even Akrapovic can only get about 0.5 horsepower out of a full race pipe, so spending $800 for the good stuff doesn't make a whole lot of sense.

There seems to be more to be had with a cam. I would try a Stage 1 Hot Cam or a Stage 2 if he rides longer tracks and lives really high in the rpm range along with AP mods and sharp jetting. They are cheap and easy to pop in and test. Those wing things are a waste of time and money.

It's possible that some of the aftermarket pipes MAY work better with a different cam combination than they do with the OEM setup, but without a lot of testing it's tough to say. Chances are good he'll never get value for his money with a pipe on this bike.

The ugly reality is, he needs to to top ends more often and the kid should have a practice bike. At 140 hours that crank is getting ready to hatchet the cases open.

Reality check time. These engines start losing ring seal in the 5-8 hour range. Valve springs last about as long as a piston (or less) and when they wear they don't control the valves as well and the power starts to fall. For the average guy it's not a huge deal, but if you are looking for power for fast guys maintenance is the best value for the money.

Cams and pipes don't do **** if the ring seal sucks and the valves are bouncing on the seats or floating before the redline.

If it were MY fast rider, I'd build a good cylinder head with Kibblewhite or Xcelydyne Ti valves, good valve seats and valve guides (like Kibblewhite's Ampco 45 pieces) and Kibblewhite race springs. I'd change rings at 10-15 hours, and pistons with every other ring change. That would keep it sealing fairly consistently. Once that was covered I would do the AP mods to try and get the throttle response the rider wants (for some guys it doesn't matter) and then I'd try some cams but NOT tell the rider which one was Stage 1 and Stage 2. I'd change it let him ride it and see if he liked it. The way the power is delivered has such a huge impact on a rider's confidence that assuming one cam is better than another based on the specs makes no sense.

Good suspension and fresh tires tailored to the terrain will probably make more difference than anything. ;)

Chili said:

Any thoughts on the harder to start/kick with the Hot Cam setup? Is this the nature of the beast?

Also without race fuel any benefit to a hi comp piston?

Click to expand...

It sounds to me like someone screwed something up that resulted in the hard starting.

The high compression piston has a value if you change the cam and it runs a later Intake Valve Close timing.

The point of Intake Valve Close will determine the dynamic or trapped compression ratio of a four-stroke engine.

As an example if you took a YZF 444 with a chamber volume of 34.40 CC, a 13.91:1 mechanical compression ratio and closed the intake valve at 50 degrees ABDC you'd have a trapped compression ratio of 12.16 : 1. If you take that same engine and use a cam with an intake valve that closes 20 degrees later at 70 degrees ABDC the trapped compression ratio would only be 10.50. The difference is about 70 psi of pressure (absolute) before combustion.

Compression ratio and intake valve close are intimately related and need to be considered as you look to modify an engine.

It's pretty simple once the terms are clear. Mechanical or Static compression ratio is the volume difference between the piston at BDC and the piston at TDC . Let's say for example we have a 450cc cylinder with a 36 cc combustion chamber. At BDC the volume is 450+36 or 486cc, and at TDC the volume is only 36cc . If we put it in a formula it looks like this (Cylinder Volume at BDC + combustion chamber volume at TDC) / combustion chamber volume at TDC = Static compression ratio

with numbers it looks like this:

(450 + 36) / 36 = 13.50 static compression ratio

This is the number that the piston manufacturers will list in their catalog because they really have no idea what the cam spec is this is the only number they can list. Plus this number will always be bigger and bigger sells

Trapped or Dynamic compression ratio is the volume difference between the piston at Intake Valve Close and the piston at TDC . Let's take that same 450cc cylinder with a 36 cc combustion chamber. If we close the intake valve at 70 degrees ABDC the cylinder volume is only is 334+36 or 370cc when we finally close the intake . If we plug those numbers in our modified formula (Trapped Cylinder Volume at IVC + combustion chamber volume at TDC) / combustion chamber volume at TDC = trapped compression ratio

with numbers it looks like this:

(334+ 36) / 36 = 10.27 trapped (aka dynamic) compression ratio

The later we close the intake valve the smaller the trapped volume and the lower the trapped compression ratio. Lower trapped compression ratios are less likely to detonate and have a lower octane requirement. So that same 13.50 compression piston in the Wiseco catalog will have very different behavior depending on what type of cam is in the engine.

The opposite is also true. If you put a high compression piston in an engine that closes the intake valve very early the trapped ratio will be very high and need really high octane fuel to keep from detonating. The older XRs were like this.

Some people might be wondering at this point why you would want to close the intake early or late.

Well by leaving the intake valve open longer you can fill the cylinder more completely. The obvious trade off is the fact that at low speeds this late IVC will cause major reversion problems in the intake. Basically with the intake valve open too long the mixture that we are trying to get into the cylinder can back up into the intake port and cause all kinds of problems. I believe this is part of the reason that Yamaha used such small valves in the YZF engines. By keeping the intake velocity high with a small valve and small port, they can minimize the negative effects of late IVC while retaining the high rpm, and lower octane requirement advantages. If this seems confusing, think of it this way. The combination of IVC timing, intake port/valve diameter, and the intake tract length will combine together to determine the rpm range that the engine is most efficient at, and volumetric efficiency at various engine speeds. The inertia generated by the incoming fuel/air mix is the major factor that is used to determine these relationships. Higher port velocities from the smaller valves and ports help to increase the overall cylinder filling up to a point. Once port velocities get above about 300fps, there is a trade off between charge density, filling, and pumping loses.

It's as simple as that. ;)

cool rich!


can i offer a translation chili?

get the suspension done,
and have dad pony up and do maintenence more often..
and more COMPLETELY (valve springs/seats/ valvws piston rings etc)

its money better spent

and likely far more HP per $$.


IMO- putting in the cam is fine (stage 1 or 2 ) and the kid might notice a bump in a throotle area that seems all good to him..

but if not done in accordence with a stepped up maintence program,
its a bump that wont
be much ,, or wont last long..

AJ Waggoner said:

cool rich!


can i offer a translation chili?

Click to expand...

Well said AJ :cool:

High Lord Gomer said:

, or does the momentum of the mixture flowing continue to fill the chamber even though the piston has started back up?

Click to expand...

Exactly! That's where the size of the valve the size of the port, the bore size, rod length to stroke ratio and rpm comes into play. A big valve and a big port can flow a lot of air, but they will do it at a lower velocity given the same rpm. The higher the port velocity the longer you can keep the intake valve open and the more completely you can fill the chamber before it starts backing up and causing problems.

The Intake Valve Close timing point is the single most critical timing number on the cam. It's a balancing act figuring out how much is too much or too little.

In simple terms it looks like this

Big ports and/or big valves = lower velocity
Small ports and/or valves = higher velocity
Higher piston speeds = higher velocity (this can come from RPM or stroke length)
Larger piston area = higher velocity (this is the REAL secret to the success of big bore engines)

With that in mind you can see that a big valve or port would need higher rpm to reach the same port velocity. Put another way, a cam that keeps the intake valve open extra long will have more issues at lower rpm because the port velocity is low and the intake charge can back up easier (this is called reversion).

You can see it's a juggling act full of compromises and trade offs. My philosophy when it comes to MX engines has always been to keep the port velocity as high as possible even if it means trading off some high rpm power past the torque peak.

Bikes accelerate with the torque distributed under the peak, and corner to corner acceleration tends to make most riders seem to go faster even if they give up a little on the brief time spent at max rpm on a straight.

Other people who build engines have different ideas in this regard. Neither is necessarily right, just different ways of looking at the same problem.

There is lots more too it than this. Things like airflow through the ports determine how much cam timing is required, but this covers some of the basics.

Does any of this make sense? :whoa:

Maybe this will help. As strange as it seems air has mass and it takes some time to accelerate it. Once you accelerate the air to it's maximum speed it will try and stay moving (body in motion and all that foof). So intake valve closing is a matter of figuring out the point when the air will have slowed down enough to reverse.

For a given intake valve close (IVC) timing this closing point will only be correct for a band of about 1000-2000 rpm, everywhere else it will be closing to late or too early for the rpm and the port velocity. Higher velocity ports tend to be less sensitive to the IVC being "out of time" for the rpm, but the trade off is they tend to flow less air.


When the ports get bigger they get lazier and it takes more time to get the air up to speed so reversals at lower rpm are more common. So big ports will tend to move more total air and show bigger numbers on a flow bench and possibly the dyno, they also tend to be more sensitive to getting the IVC perfect and only get it right in a much more narrow rpm range. They also show dips in the torque curve in places where the cam timing (and by extension the intake and exhaust lengths) are "out of time"

In other words they get peaky. MotoGP engineers have been battling this phenomena since they switched to four-strokes. It's become more complicated since they dropped the displacement limit to 800cc.

See four-strokes are more interesting than you guys realized. ;)

A bigger bore will increase the depression in the cylinder, which increases the "gulp" that the engine takes as the piston descends from TDC. This gets the air mass in the ports moving sooner, and gets it to max speed at a lower rpm. This can carry over as the piston changes direction at BDC and can in some cases allow you to use a later IVC timig.

....

Chili said:

The bottom line is there is no way a practice bike is possible within their budget especially with the costs of University on top of his racing. It sure would make it a bunch easier though as part of the reason more frequent maintenance on the top end wasn't done is simply lack of time for downtime on the bike. Cramming a 16 race Provincial championship over the span of 20 weekends doesn't leave a lot of time for extensive bike tear downs when it becomes at least a couple of evening project in the best case scenario.

Click to expand...

With that in mind I would suggest the following to get maximum value from dollars and time.

Leave the stock pipe on, it works and it's paid for. For the price of a good pipe you can rebuild the cylinder head with Kibblehwhite stainless steel valves, their race springs, and get a good valve job done. A complete cylinder head rebuild done in this fashion should cost about $600 US and will make good power with excellent reliability. You might even get basic porting in that price if you deal with the right folks.;)

For about $150 you can get a Hot Cams Intake cam and get a reliable increase in power. Take a look at the dyno charts on their site to see which cam Stage 1 or 2 best fits the rpm range that he rides in or wants an increase.

From there, change the rings and piston as often as the budget and time allows, and keep a close eye on the crank. Big hours and 250F cranks aren't a good mix.

Using a small o-ring is the best way to deal with the AP pump issue. Kits like the JD Jetting pieces come with the proper o-rings and very good instructions.

It's worth noting that AP pump mods are not a fix for bad jetting, even though people try to use them that way.

VintageDirt said:

I'm not bad, just jetted that way.

Click to expand...

Too rich???? :whoa: :nener: :rotfl:

Never mind. Back to your regularly scheduled programing. :laugh: