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CRF250x with 13.5 piston
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Posted by: mideastrider---------------------

I've got a 13.5 wiseco piston for my son's 250x. I haven't installed it yet. Does anyone know if it will still run fine on 93 pump gas. Not really wanting to run race gas just because it's not easy to get in my area.(not to mention the $$)

Posted by: Rich Rohrich---------------------

You might run into issues with the stock cam timing especially on hot summer days. If you run the R cam it will be much more fuel tolerant. The intake closes 10 degrees later on the R cam which drops the trapped compression ratio more than enough to be safe.

The other option is to run the stock cam with an adjustable cam sprocket and retard the timing about 4 degrees. Any more than about 4 degrees and you start running into other potential issues. If you do run the the stock cam retarded you'll have to verify the the exhaust valve to piston clearance is sufficient on either side of TDC. The piston chases the exhaust valve closed and retarding the cam timing on a SOHC engine retards both intake and exhaust, so you end up with the exhaust valve closer than normal near TDC, and you have a different piston crown shape, so it's something worth checking. It's likely to be fine, but it's not something you'll want to guess at.

Posted by: mideastrider---------------------

I am running the stage 1 hotcam

Posted by: Rich Rohrich---------------------

If memory serves correct the Stage 1 Hot Cam for the CRF250 closes the intake valve about 6-8 degrees later than the stock X cam if you time it to their recommended LCA. With those specs you should be good to go. Just pay attention on those 100 degree NC days and you should be fine.

Posted by: mideastrider---------------------

Hopefully it will be awhile before we reach the 100degree mark but it will. I think I'll go ahead and install it and see what happens. Thanks

Posted by: a454elk---------------------

That's the setup I have in my 250X and it runs great. You'll like the kick you get with the bottom end as well as up top, good luck! Take a look at disconncting the pink wire which is under the left side plate near the air filter. It lets it rev to the R specs I believe.
Elk

Posted by: mideastrider---------------------

Quote:

Originally Posted by a454elk

That's the setup I have in my 250X and it runs great. You'll like the kick you get with the bottom end as well as up top, good luck! Take a look at disconncting the pink wire which is under the left side plate near the air filter. It lets it rev to the R specs I believe.
Elk

I've read about disconnecting the pink wire but have not did that yet. May go ahead and give it a try also.So you can tell that it helps? What type of fuel are you using?

Posted by: a454elk---------------------

I'm able to run super with no problem so far. I'm keeping an eye on it but it's been fine. Yes, it seems to rev up more and give more of a top end. I'm taking it out this weekend again and will do some more testing.

Posted by: Rich Rohrich---------------------

Quote:

Originally Posted by a454elk

Elky - Do you remember the LCA that Terry set the HC cam to?

Posted by: _JOE_---------------------

I have that piston in my '"r" and it runs fine on 93. All else internally is stock.

Posted by: Ol'89r---------------------

Quote:

Originally Posted by Rich Rohrich

Elky - Do you remember the LCA that Terry set the HC cam to?

Rich.

In. O @ 23 BTDC
In. C @ 49 ABDC
In. Lobe center 103

Ex. O @ 49 BBDC
Ex. C @ 28 ATDC
Ex. Lobe center 100.5

All figures checked @ .020"

Posted by: a454elk---------------------

Quote:

Originally Posted by Rich Rohrich

Elky - Do you remember the LCA that Terry set the HC cam to?

144 degrees west by 135 degrees north with a easterly wind of 56 knots from the north.

Posted by: Rich Rohrich---------------------

Thanks Terry.

Elk, don't you have a lawn to mow, or some poop to scoop?

Posted by: Bodge---------------------

Rich, what does the hot weather change that could hurt this set up? Should he be looking out for detonation or what?

Posted by: a454elk---------------------

Quiet Rich, you invited me into this technical conversation and I gave you my gift of knowledge!

Posted by: mideastrider---------------------

Well I got the piston installed and the bike back together yesterday. Took it to the race today and "Mickey Likes it". He won the race and said it was a noticable inprovement from the bottom to the top.

Posted by: Rich Rohrich---------------------

Quote:

Originally Posted by Bodge

Rich, what does the hot weather change that could hurt this set up? Should he be looking out for detonation or what?

Exactly. All else being equal, higher inlet temperatures increase the chance of knock.

That's the short answer, the specifics are a lot more involved, and probably boring to most, but that's never stopped me before.

So strap in kids, this might be a bumpy ride.

So we know the basic definition of knock or detonation is :

When an air/fuel mixture is brought to a sufficiently high temperature or pressure it will auto ignite without the aid of an external source of ignition.

This auto-ignition temperature is one of the critical factors in the makeup of a fuel. The interesting thing is that tests have shown that even when an air/fuel mixture is compressed and the temperature is kept well above the auto-ignition point a period of time passes before the mixture ignites. This time period is commonly called the delay period, and it has a significant effect on engine knock. The octane rating of a fuel directly addresses the auto-ignition point. Higher-octane fuels tend to have higher auto-ignition temperatures. As a result they are more tolerant of increased cylinder pressure and higher combustion chamber temps.
What lots of people (many big name tuners included) fail to understand is the way the high octane components are distributed within the fuel, and how the chemistry and fuel makeup produce the octane rating can be more important than the octane number itself. In simple terms two 99 MON octane fuels could easily have very different knock characteristics in the same engine.

As we start tuning for maximum horsepower and get closer to the edge, understanding the way various tuning parameters effect the delay period will prove to be vitally important.
If the delay period is long enough the last parts of the charge can be burned before detonation occurs, even if the auto-ignition temperature of the fuel has been exceeded. That’s an important point that the good tuners know and exploit to their advantage through tuning or fuel chemistry.

MOST knock occurs well after the spark is fired, but it's still a matter of exceeding the auto-ignition (heat and pressure) threshold of the fuel. The root cause of why the threshold has been exceeded isn't always so obvious.
Knock/Detonation variables and their relationship to the delay period are really worth understanding if you want to tune for real horsepower.

• Inlet pressure
• Ram effects from intake or exhaust tuning, higher atmospheric pressure, etc
• Increases flame travel speed (denser mixtures burn faster) but the higher overall pressures decrease the delay period.
• Higher inlet temperatures
• Higher inlet temps lower the charge density, which slows the combustion flame speed and decreases the delay period.

• Compression ratio
• Raising the compression ratio will increase the charge density and improve the flame speed but the higher pressures will decrease the delay period.

• Ignition timing
• Advancing the ignition timing decreases the delay period

• Air/Fuel ratio
• The air/fuel ratio that produces the best power tends to have the fastest flame speed and the highest pressures, but it also tends to have the shortest delay period, which tends be the deciding factor. Sad news is the best power air/fuel ratio also has the best chance of running into detonation.
• Going richer than best power will tend slow combustion and decrease delay time, but the extra fuel tends to lower combustion chamber temperatures and cylinder pressure so the auto-ignition temperature is less likely to be reached before combustion is complete. This is especially true for two-strokes.
• Going leaner than best power will tend slow combustion and decrease delay time. But you quickly reach a point where you’ll get intermittent misfires (that go largely undetected by the rider) which tends to lower combustion chamber temperatures and cylinder pressure so the auto-ignition temperature is less likely to be reached. On two-strokes the lack of piston crown cooling from excess fuel tends to negate this because the chamber temps increase.

• Engine Speed
• Higher engine speeds reduce the time available for pre-flame reactions to occur which tends to increase the delay period
• Cylinder size
• Increased bore diameter provides a longer distance for the flame to travel increases the time available for pre-flame reactions and reduces the delay period

• Combustion chamber design
• Small tight combustion chambers provide the longest delay periods

That’s just the short list above, but it’s obvious many of these variables are contradictory.

One of the reasons tuners love lead (TEL) in fuel (whether they realize it or not) is it has a very positive effect on increasing the delay period as it interrupts some critical chemical reactions that lead to detonation. As a general rule, even in fuels with equal octane levels and distillation curves, a leaded fuel will provide a detonation safety margin that isn’t present in the comparable unleaded fuel.

That’s enough rambling for now; hopefully it made some sense and provided a little fuel for thought.

If you want to find out more about the delay period and the rest of this madness, check out the following references.

References –

The Internal-Combustion Engine in Theory and Practice: Vol 1 Thermodynamics, Fluid Flow, Performance & The Internal-Combustion Engine in Theory and Practice: Vol 2 Combustion, Fuels, Materials, Design - Charles Fayette Taylor
- Published by MIT press Vol 1 ISBN: 0262700263, Vol 2 ISBN: 0262700271
This pair of volumes sell for about $63 and amount to an entry-level college course in engine design, and function. These books helped drag me out of the "just another dumb-ass mechanic" club. If you don't feel comfortable spending hundreds of dollars on books, buy these two, study them and you'll see a whole new world open before your very eyes.

Elements of Internal-Combustion Engines - A.R Rogowski
- published by McGraw Hill
It's an OLD book (1953) that has a Library of Congress catalog Card Number: 52-12361 instead of an ISBN number. This book along with “The High-Speed Internal-Combustion Engine” by Harry R. Ricardo are out of print but they are classic texts that are worth tracking down. Any good college engineering library will have copies.

Automotive Fuels Reference Book - Keith Owen & Trevor Coley
- Published by SAE ISBN 1-56091-589-7
This is the bible of all things automotive fuel related. 963 pages of WFO info. It's a humbling experience reading this book; it makes you realize how little you really know.

Lean Combustion in Spark-Ignited Internal Combustion Engines - A Review
Germane, Wood, Hess - SAE paper 831694 . Cheap at $14.00 direct from the SAE www.sae.org

Quote:

Originally Posted by a454elk

Quiet Rich, you invited me into this technical conversation and I gave you my gift of knowledge!

Anything you want to add Elky?

Posted by: a454elk---------------------

I'd hate to burst your bubble but my explanation was a bit longer. Based on the technical difficulty and the time zone difference from London to Bejing, I concur with your explanation of early detination. Also, seeing a doctor can help with early detination.

Posted by: Rich Rohrich---------------------

Quote:

Originally Posted by a454elk

I'd hate to burst your bubble but my explanation was a bit longer. Based on the technical difficulty and the time zone difference from London to Bejing

The time zones always confuse me.

Posted by: Ol'89r---------------------

Quote:

Originally Posted by a454elk

Also, seeing a doctor can help with early detination.

Don't they have a pill for that?????

Posted by: Rich Rohrich---------------------

Quote:

Originally Posted by Ol'89r

Don't they have a pill for that?????

Yeah but it's made by VP.

Posted by: a454elk---------------------

Hold the presses, 89r is up before noon! I see HE is confused about time zones as well. I guess it's not age discriminatory.

Posted by: Ol'89r---------------------

Quote:

Originally Posted by a454elk

Hold the presses, 89r is up before noon! I see HE is confused about time zones as well. I guess it's not age discriminatory.

Shut the hell up copper before I come over there and give you a drive-by-slappin'!!

Posted by: a454elk---------------------

Shut your cake hole fool, we all know you're afraid to drive the 91.

Posted by: Ol'89r---------------------

Quote:

Originally Posted by a454elk

we all know you're afraid to drive the 91.

Well yeah!

Isn't everybody?

Posted by: a454elk---------------------

Not me my friend, that is my hunting ground.

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CRF250x with 13.5 piston

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