I was going to tag this on to Marcelino's thread, but I thought it would be better for that thread to cover the mechanical installation issues instead of a discussion about the potential effects of cam timing.

After M's great coverage of this cam timing mod for the KLX, I've been mulling over the possible issues before jumping into this myself. I've had some experience with advanced cam timing on my 4X4. What does a stroked 350 (383) Chevy V8 have in common with a KLX motor?...quite a bit in many of the critical elements common in most 4-stroke engines. My 383 has an adjustable camshaft timing gear which allows 4 degrees of advance. That along with an aftermarket cam that has more advance contributes to increased performance. I also have an ignition system that allows for manipulating the ignition timing while driving. With this setup I can run unleaded regular without any damage.

Now, I'm no engineer or professionally trained engine tech. I'm like most here who were just more interested than most when it comes to learning and working on our machines. It even got me a couple of very decent part time jobs for many years working at a motorcycle shop, a car dealership, and even a bicycle shop while pursuing my full time career in an unrelated field. So if anyone has any corrections or input on my comments here...jump in.

Advancing the cam timing on an engine usually produces more power because the cylinder fills with a larger volume of intake charge (air & fuel) and then compresses a little longer creating more pressure. Now, this is the ideal situation in the ideal engine I'm referring to as I understand it. Advancing the cam timing causes the intake valve to open sooner which would start the intake process sooner when drawing in the fuel/air mix. The intake valve is going to close sooner which allows the piston to compress the fuel/air for a little longer. The exhaust process is similar in that the exhaust valve opens sooner to hasten the expulsion of the combustion gases. Now, this can change a bit with some engines and designs, but it's generally why you see power increases with advanced cam timing...at least as I understand it.

Now, the potential problem here can come from the fact that compressing the intake fuel/air mix for a little longer actually can increase compression which can increase the risk of detonation/pinging in many engines. This was the case with my Chevy, and I was able to control this with an adjustable in-cab ignition timing control device hooked to my MSD ignition box. My 4X4 has a much broader range of operating use than the KLX. I tow a travel trailer, do off roading, and just tool around in it occasionally. This dramatically changes the demands on the engine. I can run a lot more ignition advance with the vehicle empty than when towing. This compares a bit to the KLX in that we wring out our motors on a more regular basis, so timing, fuel octane requirements, and compression all come into play.

If advancing the cam timing on the KLX is creating a bit more compression, there is the chance that the engine's fuel octane requirements could change too. Now, if one has an aftermarket piston that has also increased compression, it may contribute to this issue. None of these elements stand alone to help or hurt engine detonation. I don't know if the KLX will be prone to detonation with this mod, but some have asked the question...and rightfully so. Can all detonation problems be addressed with just richer jetting...not really. Timing might have to be addressed in some cases, but that usually causes a power loss at some point. Besides, the KLX's timing isn't easily manipulated.

I'm just throwing this out for discussion. I notice Marcelino has an FI engine if I read correctly. That will help some of these issues. His engine is also using a stock piston and compression. Those with big bore kits with higher compression should just be aware of the possibility of detonation. My OEM KLX300 piston and cylinder has the same compression as the KLX250S engine, so perhaps it won't be a problem. I'm not saying that this will be an issue with a higher compression KLX engine, but it's better to be aware.

 

TNC, those were exactly my thoughts as well.
As far ad cam timing advance, on your stroker "cheby" motor, you can only adjust the centerline of the cam not the overlap. I THINK what is happening here is the same thing because the 2 cams are being advanced equally????? Or is the overlap being adjusted because the cams are being split 10* each... Confusing,,,,,Im not a tuner by far but i agree with cam advance,, some timing changes will "help" to make the whole package. Due to the limitations of that, as lonk as i don't get spark knock on premium fuel, i can live with the gains. I'd like to be able to address all of it though,, as i am sure you would too!!!!

 

Very nice explanation of the events as I understand them. Either you're absolutely correct, or, we're both wrong.

One thing I'd like to discuss: it seems logical to me that the higher the rpm, the more advance can be utilized, partly due to the intertia of the intake fuel/air mixture. By that, I mean: in a non-boosted intake system, atmospheric pressure is the only thing PUSHING the fuel/air toward the intake valve. The intake valve opening allows the piston to aspirate this fuel/air mixture, which is being pushed by ambient air pressure. At higher and higher rpm's, it's harder and harder to suck in enough fuel/air without resorting to opening the intake valve sooner and sooner, and timing of the spark is also a contributor to this faster and faster rpm state, as the spark can be fired earlier and earlier in the piston's upstroke without causing valve knock (which isn't the same as pre-detonation) since burning isn't 100% instantaneous: the burn takes time to occur. With less burn-time available at higher rpms, it's more efficient to fire the spark earlier.

How it all plays out in a particular engine design is beyond my pea-sized brain, but, I'm thinking pre-detonation in the KLX engine with an advanced cam-timing will be more prevalent at lower rpm ranges (if it is going to happen), rather than at higher rpm ranges, for the reasons listed above. SO, if your Marcellino's modification doesn't ping below 4000 rpms when you wring the throttle, it won't ping at higher rpms. Am I thinking correctly on this? Assuming carburated bikes in this instance.

 

TNC and Blackheart58, I'm going to have to disagree with your explanations. Advancing the cam LOWERS the rpm power range.

Crane Cams |
Advancing the cam will shift the basic RPM range downward. Four degrees of advance (from the original position) will cause the power range to start approximately 200 RPM sooner. Retarding it this same amount will move the power upward approximately 200 RPM. This can be helpful for tuning the power range to match your situation. If the correct cam has been selected for a particular application, installing it in the normal "straight up" position (per the opening and closing events at .050" lifter rise on the spec card) is the best starting point.

Ride on
Brewster

 

Hey TNC, I just did the mod today in full (with some ACR issues) and posted results in Marcelino's thread. Somewhere around post #160, I think.

I think what we've done with Marcelino's mod is to advance the intake cam timing by 10 or so degrees but retard the exhaust cam by an equivalent amount.

Marcelino's original issue is that the stock cam timing is set for an upper rpm range that is literally not possible with our KLX250's as set up. Therefore he added some valve overlap taken from an unachievable top end and gave us some increase in volumetric efficiency in the lower and mid ranges. Looks like it could be a case of all gain and no pain.

If I'm wrong or misunderstood anyone's posts on it, please sound off. Wouldn't be surprised to see Marcelino weigh in, soon, either.

 

The cam mod on the KLX advances the intake and retards the exhaust .
The small block Chevy has a single cam so if you advance the intake automatically you advance the exhaust , so what you gain on one side you loose it on the other . The only thing you could do with it itīs play JUST a few degrees with the cam to whatever position makes your day . Or you could get an aftermarket cam . Itīs a small block Chevy so you have 1001 choices .
The EFI on the KLX does nothing as far as the detonation is concerned , the ECU does not have a input for a Knock sensor , and the bike does not have one . Itīs mounted only on the bigger bikes ( ZX 6 , ZX10 , Z 750 and probably a few others ) . The way the KLX deals with detonation is by having the ignition advance reduced a lot at the factory . I think many KLX owners do get preignition ( similar to detonation but has different causes ) . Itīs mainly caused by a poor mixture and/or a spark plug thatīs too warm for the engine .

 

Damn ! now thatīs some reading comprehension
Kansas KLX you get A+
The kind of foreplay (camplay) we did with the KLX itīs just not possible with a single cam engine , you wonīt get the engine arroused , you might just get him pissed .

 

Nothing was stated about power or power ranges. I'm talking about making changes in relation to piston movement and valve opening, and how those changes affect getting fuel into the combustion chamber. I'm saying at higher rpms, advancing the valve timing to open sooner would allow more fuel/air to get into the combustion chamber than if you DIDN'T open the intake sooner.

Then, the spark advance is another topic.

 

Yes, I agree with the Crane info you posted. I don't think I said the powerband moves up or even stays in the same place with a 4 degree advance on the cam gear. It definitely moves down in the rpm range. In my Chevy engine the power signs on quicker and signs off quicker...ideal for a big, heavy 4X4 that almost never sees 5000 rpm.

I was only trying to address the possibility of more compression and therefore the possibility of detonation...especially on some piston kits that have already raised the stock compression. Am I missing something?

 

Yes, I get the issue of 1 cam vs. 2 cams and the ability to manipulate the difference between them, but doesn't the possible increase in compression during the intake stroke still occur, or can occur, with the advance in cam timing? With either 1 cam or 2, the advance in the intake valve opens sooner and closes sooner. As I understand it, this gives the compression stroke a longer shot to compress the mixture, hence the possibility of higher compression. Maybe I'm off with my understanding of this, and then some of this may change with a given engine design. But isn't this generally the case?

I would disagree with your take on the FI not being a benefit to combating detonation, even without a knock sensor. The atomization in a properly designed FI is almost always better than a carb IMO. The FI on the KLX is just a throttle body design, isn't it? While this isn't as efficient as a direct injection version, it's still usually better than a carb. Better atomization is almost always a benefit to stemming detonation...proper air/fuel mixture being assumed, of course.