[DPelletier]

Shinyribs;

Good post. I think there have been some clues that the MC mod doesn't do much for upper rpm power for the reasons you mention. I also remain unconvinced that it is necessarily beneficial on a Gen1 (which has different exhaust cams) due to conflicting reports.

As an aside, My 440-6 was built for Pure Stock Muscle Car drag series and used stock lift but longer duration......the very steep ramps helped avoid undue overlap to meet the rule of a minimum of 18" of vacuum at idle....but the engine was never built to last for 100,000 miles either.

Cheers,
Dave

 

[Damocles]

And no need to be gentle. lol. I think the biggest reason I love drag racing was actually just trying different engine combinations. I learned that what is considered too small, old fashioned, too heavy was generally the same parts we could get the most power from. I love the theories on stuff like this. It's one of the few things I can sometimes wrap my head around ( diagnosed with learning disabilities at a young age), so I kinda thrive on it. Always open to be told I'm wrong ( often am) and learn new things.

Can't say you're WRONG about anything!

I might challenge the notion about overlap being more unavoidable than contrived.

Overlap, in my understanding, is useful for SCAVENGING flow at high rpm; that is, the fluid dynamics support the intake and exhaust of mixture, allowing the engine to process a higher volume of fuel and produce more power. Effective at high-velocity fluid flow; at idle, not so much, as witnessed by the "lope" you mention.

As to higher compression test results with the MC mod; hard to grasp the operational principle, for me. Advancing the exhaust cam also advances the KACR "bump" opening the exhaust valve; the consequences of the altered cycle-point of compression-release exhaust valve cracking should be taken into consideration (HEY, the sentence doesn't make much sense to ME, either!).

Flow rate, or volume-per-unit-time, past the valves remains a function of both LIFT and DURATION, seems to me; not, for example, lift alone. Higher lift, certainly, facilitates flow, up to a point.

All this said, your explanation provides the most lucid and plausible rationale I've ever encountered for the tremendous power increases available from advancing the exhaust cam one tooth!

Now, about the "free" additional power generated by the PVC Valve Mod . . .

Seriously; thanks for your pertinent technical analysis; enlightening and thought-provoking.

 

[pdwestman]

Informative posting guys.

The KACR on the Exhaust camshaft only effects the compression at starting RPM's, or < about 700rpm. One can hear it come back into play if we attempt to idle to slowly.

With near maximum valve tappet clearances and a Non-functioning KACR, I don't believe there would be any difference in Cold Cranking Compression between standard cam timing and MC mod cam timing. Why?
Because Intake Valve Closing point is what normally effects CCC in an internal combustion engine, not exhaust opening nor closing.

 

[Damocles]

I assumed a FUNCTIONING KACR; silly me!

 

[shinyribs]

Informative posting guys.

The KACR on the Exhaust camshaft only effects the compression at starting RPM's, or < about 700rpm. One can hear it come back into play if we attempt to idle to slowly.

With near maximum valve tappet clearances and a Non-functioning KACR, I don't believe there would be any difference in Cold Cranking Compression between standard cam timing and MC mod cam timing. Why?
Because Intake Valve Closing point is what normally effects CCC in an internal combustion engine, not exhaust opening nor closing.

I believe the effect could be the overlap issue. There's typically a certain amount of time before the exhaust valve closes fully that the intake valve is already starting to open- the overlap. If the exhaust cam is advanced then the exhaust closing will also happen earlier ( being advanced) in the cycle, which would have the exhaust valve closed, or closer to being closed, before the intake valve starts to open. That's why I can see the logic in all of this, but I've also not looked at any specific valve timing events to see what's up.

I'm tempted to try this.

Any show of hands of who all on this forum has given it a go? And what they thought about it?

Oh, and Dave, regarding: "I also remain unconvinced that it is necessarily beneficial on a Gen1 (which has different exhaust cams) due to conflicting reports. ". The initial test was done with a Gen 2,wasn't it? So are there conflicting reports on Gen 1 vs Gen 2 on all this?

 

[Damocles]

So are there conflicting reports on Gen 1 vs Gen 2 on all this?

Yes.

Some Generation 2 modifiers claim noticeable performance increases, while some Generation 1 riders say, "Not so much," even restoring stock valve timing in some instances.

Then, again; some Generation 1 riders perceive improved performance.

The mod plays to mixed reviews, one might say . . .

----------------------

BTW, the cracked valve might close sooner on cranking with an operational KACR and the exhaust cam advanced, but . . . it would OPEN sooner also, a factor perhaps affecting compression test PSI.

 

[DPelletier]

Oh, and Dave, regarding: "I also remain unconvinced that it is necessarily beneficial on a Gen1 (which has different exhaust cams) due to conflicting reports. ". The initial test was done with a Gen 2,wasn't it? So are there conflicting reports on Gen 1 vs Gen 2 on all this?

Yes the initial testing was done on a Gen2 and yes, as Damocles has said; there have been mixed reviews on the Gen1......the mega thread on .net started by Eaglemike has a ton of back and forth on it. Basically some with Gen1's perceived an increase, some did not (which makes one skeptical as people WANT to perceive a difference most of the time) there were also periodic reports of hard starting.

From one of the guys that were part of the testing the comment was made that there might be 1hp on the Gen1......so given the fact that several people weren't happy with it, I decided not to bother and went back to focusing on my suspension! ;-)


Dave

 

[shinyribs]

Ok then. That's what I originally thought, too, but somehow I read your post to say something else. My mistake there.

 

[pdwestman]

Yes.

----------------------

BTW, the cracked valve might close sooner on cranking with an operational KACR and the exhaust cam advanced, but . . . it would OPEN sooner also, a factor perhaps affecting compression test PSI.

Cold Cranking Compression with an Active KACR is Primarily affected by the Closing Point of the Exhaust valve off of the KACR during the Up Stroke of the piston on the Compression Stroke of the 4 stroke cycle! Lift of the KACR and Duration of the KACR has very little effect.

Opening point of the KACR has no effect, and neither does Closing point of the Intake valves when dealing with an Active KACR (on a KLR650).

 

[Damocles]

Cold Cranking Compression with an Active KACR is Primarily affected by the Closing Point of the Exhaust valve off of the KACR during the Up Stroke of the piston on the Compression Stroke of the 4 stroke cycle! Lift of the KACR and Duration of the KACR has very little effect.

Opening point of the KACR has no effect, and neither does Closing point of the Intake valves when dealing with an Active KACR (on a KLR650).

If the opening point of the KACR has no effect, then . . . is piston velocity and swept volume vs. crank angle constant?

Didn't think the volume swept by the piston is CONTINUOUS and UNIFORM through the 180 degrees of the compression stroke.

Rather, thought the volume varied with the SINE of the crank angle; i.e., considering BDC as zero degrees, a maximum volume vs. rotation (and maximum piston speed at any given rpm) would be found at 90 degrees, decreasing to a minimum volume (as in zero) at zero and 180 degrees . . . volume swept vs. rotation would be a sinusoidal function . . .

Since I thought the piston velocity varied with crank angle, I thought the volume swept by the piston varied with crank angle also. Thus, the crank angle where the KACR cracks a valve would affect the compression released, and thus the PSI registered in a compression test. The closer to 90 degrees the cracking of the valve, the more compression released.

If the piston velocity is constant throughout the compression stroke, my assumptions appear invalid and I stand corrected.

 

[shinyribs]

That makes sense. Yer a smart cookie. :thumb

 

[Damocles]

That makes sense. Yer a smart cookie. :thumb

Thanks, but . . . my postulations play to DECIDEDLY MIXED REVIEWS!

I thought piston displacement vs. crankshaft rotation varied; minimum at BDC and at TDC; maximum halfway between the two. If that premise is correct, then the crank angle where the KACR cracks the valve would affect the magnitude of the compression released, or so it seemed.

The MC Mod moves the angular point of operation of the KACR by 15 crankshaft degrees; I'd expect some consequence from the change, but . . . maybe not!

 

[Retfird]

Shinyribs, have you done the advance mod...yet? Your point about doing valves while in there is a valid point if they were to tight to begin with this could cause a loss of compression and power. If they were a couple thousands looser than factory specs i cant see how that could make much of a difference in the 7-10% change in power? I noticed the low end torque change the most, dont venture into the KLR zone much on my 09...

Full choke on cold starts with no throttle cured the hard starts usually fires right off, no choke and a slight twist on hot starts usually works to avoid the clack clack depending on where that piston is..... The slight twist is a like starting an Old Norton after you bring it to top dead center and a long serious kick on the lever if the twist is a little off....its kicking time! Having that kick start option on a second gen would be nice.

You guys who have given theory and factual ideas of ICE engines and cam functions is quite interesting! I guess this is one reason why this forum is so good, adults having conversations!

 

[Surfersami]

Here is my take on why I feel more "power" after the MC mod. I ride at relatively low RPMs. It is rare if I rev past 4500. I found that between idle and my normal 4000~~ ish shifts the engine seems to pull harder and faster to those levels. From what I have read in earlier posts this makes sense. Long duration/long overlap is great for high RPM, an area I very rarely use. When I stated I feel more power throughout the rev range, it was my rev range. (see above.) It makes sense that closing the exhaust valve a little early would boost the torque, and that is the difference I am feeling. I don't think this would work to well on a Ninja 250, unless you never spin the engine up to its crazy redline. I can count on 1 hand the number of times I have revved over 5500 RPM in my 73000+ miles with my KLR. Power fell off fast above those RPMs so I never go there. I the question that was posed that led to ten pages of response was how does this make power across the whole rev range. I don't think it does, I think it makes usable power through most of it and the rest is perceived unjustly. Bring on the dyno charts while I pop my popcorn.

 

[terry_g]

Years ago I bought a used 1981 Yamaha XS Eleven Special. It had amazing top end power but was weak at low RPMs and got horrible fuel mileage.
You had to be careful not to rev it past the red line. I was pretty sure someone had installed high performance cams.
I pulled the valve cover to check the valve clearance and found the intake cam was 4 teeth advanced on the sprocket creating a lot of valve overlap.
I timed the cams correctly and put the bike back together.
It was way nicer to drive the low end power was great It still had good top end power just less and the fuel mileage increased by about 50%.