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K model cam alterations

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Post Wed Mar 25, 2009 6:33 pm

Posts: 140
Location: USA
I need some help checking my math, i'm wondering if I use the large diameter rollers of an EVO which is .9375 vs .8550 of a stock roller, which gives me a diffrence of .0725, and divide that in half which equals .03625 and if i add that increased roller diameter to the lift of a KH cam (.344) would I end up with a lift of .38025? I have several KH cams and thought that if I used the larger diameter rollers of the EVO's that I could make myself a poor man's KHK cam (.375). Any help would be appreciated. This may be an example of having sat around and having ate to much wonderbread, wondering and wondering. :D

Post Wed Mar 25, 2009 6:37 pm
Pa Site Admin

Posts: 5805
Location: Ohio USA

You may gain lift but you also lose drop. Pa

Post Wed Mar 25, 2009 8:31 pm

Posts: 693
Location: somerset, oh usa
Lift is determined strictly by the lobe providing said lift; plus rocker arm ratio, if applicable. So I guess the lift and fall gains and losses are telling you that net gain is "0" Pa?!?, which it will be.
The benefit of using a larger roller will only be in duration.

Post Wed Mar 25, 2009 8:56 pm
Pa Site Admin

Posts: 5805
Location: Ohio USA

The diameter of the roller will determine lift and drop.

Post Wed Mar 25, 2009 9:10 pm

Posts: 693
Location: somerset, oh usa
Only if you mount it eccentrically.

Post Thu Mar 26, 2009 5:37 am

Posts: 140
Location: USA
Okay guys, I guess it's back to the drawing board. I guess it doesn't look like there would be any benefit to running the larger rollers. If anyone has ever done this let me know. Thanks so much. :D :D

Post Thu Mar 26, 2009 7:22 am
Pa Site Admin

Posts: 5805
Location: Ohio USA

Are we talking tappet rollers ?????

Post Thu Mar 26, 2009 7:53 am
panic

1. using a larger diameter roller will not change lift
2. using a larger diameter roller will increase nominal (zero lash) duration very slightly, and will affect every other point in the lift curve (more lift at the same point of rotation, and longer duration at any lift except 100%), but not in the same proportion
3. the effect is not regular (i.e., not similar to using a high-ratio rocker)
4. the effect is most noticeable in the first few degrees of both the opening and closing ramps, where the lift per degree of rotation rises (the math looks complicated)
5. the effect decays as the roller approaches the nose (where the vector is more vertical with less side-thrust), and almost vanishes near the max lift point (where it is zero)
6. the effect repeats in mirror image on the closing side
7. overlap is entirely composed of opening and closing events, and becomes effectively larger
8. exhaust opening will have slightly faster blow-down, and a stronger wave return
9. intake closing will slightly reduce the cranking compression, but add some inertia fill ABDC (under ideal conditions)
10. more spring tension is needed for the same lobe profile and RPM
11. side-thrust in the block goes up, but should still be reasonable
On balance, probably an improvement over a stock or very mild cam. May be a step backwards on a hot cam (i.e., changes are not all beneficial)

Post Thu Mar 26, 2009 8:12 am

Posts: 140
Location: USA
Panic,Thanks for your time and consideration in providing an indepth evaluation regarding the use different sized tappet rollers.It appears that I am just going to have to change camshafts.Again Thanks to all for your help. :D

Post Thu Mar 26, 2009 10:59 am
Pa Site Admin

Posts: 5805
Location: Ohio USA

Image

This is what was pictured in my mind when lift and drop was spoken of. I see now, if there is enough tappet adjustment available, the length doesn't change a bit. Thanks Panic....for knocking the rust out from between my ears. :wink:

Post Thu Mar 26, 2009 1:15 pm
panic

Soitinly! as Curley used to say.

It's a potentially useful tuning tool, since you can do only intakes or only exhaust (intake is more important). If it helps, I would guess the engine can use more cam. If the power doesn't impress you and there are symptoms (jetting, flat spots) either the cam is pretty close now, or more overlap isn't the right direction, etc.
Most stock motors with stock cams will be slightly faster, but may need minor mixture tweaks.
I'm not sure how important this is - but too much lash is going to do more damage than with the stock roller, so watch it carefully until it stops moving around.


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