Cam Changes for a De stroked Motor

[thundair]

I want to build a 500cc twin with a 75mm piston and 55.5mm stroke.
because of limited cams, My other options are to bias cam follower profile and rocker ratios. Can I do anything with either/or both of these?
The motor is vertical twin with 6" rod and 11.5 CR for a Land Speed Record
The original stroke was 88.9mm
The cam is @.04 .436 lift 312 D 106 LC
Int 50 BTC Ex 82 ABC
Int 82 BBC Ex 50 ATC
Rocker ratio stock is 1.13
I posted this on Speed Talk with no response So I may be in left field on this one.

Cheers
Thanks in advance

I don't know anything but the people that do.

 

[MikeHalloran]

I'd have to lay it out to be sure, but in my head, I think you might have to 'destroke' the cam or the rockers, because you need to reduce the clearance volume to maintain or increase the CR on a destroked engine.

I.e., piston to valve clearance might become a problem.



Mike Halloran
Pembroke Pines, FL, USA

 

[thundair]

I do have a .359" lift with a 284* duration

Cheers



I don't know anything but the people that do.

 

[dicer]

There is more to proper cam design than you think.
You are changing many parameters with the stroke change, that is if it was tuned ie designed for optimum before the changes. Someone with a good simulation program could get you close to what you need.

 

[thundair]

The cam follower is rectangular (A65 BSA) and I am taking .o70" off the leading portion of the shoe. I think it will retard the opening enough for my budget.
Oh well its not an engineering question anymore so I will stop.

Dicer
I have to play with the cams that I have, so I am not trying to redesign the cam but trick the valves into thinking there is a compatible cam driving them..

Cheers

I don't know anything but the people that do.

 

[patprimmer]

Changing the follower approach angle to the cam is effectively redesigning the can and will impact on valve acceleration rate.

Is it a twin cam. Playing with lobe centre seperation on a twin cam is safe enough so long as you have enough piston to valve clearance.

Regards
Pat
See FAQ731-376 for tips on use of eng-tips by professional engineers &

http://eng-tips.com/market.cfm

for site rules

 

[BigVlad]

Mr. Thundair: This is actually a very interesting question. Assuming that the contact surface of the follower is flat. Grinding the whole surface of the follower so that it is slightly angled will retard, or by turning it around and angling it the other way, advance the timing. The amount of retard/advance will be (in cam degrees) the same as the new angle - or double the angle in crankshaft degrees. The lift will also be increased (and, thus, as Pat said, the acceleration as well). However as you are probably only aiming at about 4 camshaft degrees of retard, the effect on lift/acceleration etc. will hardly be noticeable. 4 degrees would multiply the lift etc. by a factor of about 1.002 - about 1 thou.
These effects come about because the line of action of the follower becomes slightly offset from thr line of action of the cam. The offset essentially causes the circular motion of the lobe's flank and nose to become involved in the lift. The effect is usually described in textbooks etc. using a roller follower as an example - but it is much the same principle.
No matter what you do you cannot change the overall duration of the cam profile by manipulating the followers/rockers etc. You can change the lift by changing the rocker ratio but probably there is no need to (and expensive).
The 500cc engine will "see" the original cam as a bit "fiercer" than the bigger 800cc engine. I would have thought that this is what you want for a top-speed-only engine. I personally would use (or at least try) the cam and followers as they are. I also think the CR could go up quite a bit.
As other people have pointed out, make sure the valves/pistons etc. don't clash.
Good luck and try not to fall off at 200MPH.

 

[larrylcoyle]

The last thing you would want to do is to change the rocker arm angle. The only effective thing you can do with rocker arms is to change the ratio which also changes the effective duration as well. If you want to change the valve timing, then you need to change the camshaft. Do it right the first time and you will have less trouble in the future. Why do you think you need to change the cam anyway?

Also, from the specs you list, are you measuring the cam at .040" lift? Are you saying the measured duration at .040" lift is 312°? Usually the accepted method of comparing camshafts is to measure them at .050" lift.

Larry

 

[thundair]

They are limited profiles for this bike and I do get stuck trying to use what I have. Megacycle cams gave the numbers @ .04" I agree that most cam profiles are @ .050"

I will try to find out what a high performance cam profile would be so I know what I'm shooting for, and see how close I can get with all the trick I have available.

I probably could get a cam designer to profile a cam just for this motor but for some reason this has become interesting for me.

I think Bigvlad was talking about the push rod angle changing as I lower the head 2 1/4".

That brings up another trick in my bag (I don't know how it applies now) and that is to raise or lower the rocker pivot. I am going to remake the rockers anyways to change the ratio.





I don't know anything but the people that do.

 

[BigVlad]

Thundair: BigVlad was referring to the angle on the face of the followers (you mentioned on 22 May that you were taking 70 thou' off the front of the follower to retard the cam timing).
Also, are you sure about lowering the head 2 1/4" (57mm)? I would have thought 17mm would be about what you need.
I still think you should try the original cam(s) and not alter the rockers etc.

 

[thundair]

Your correct it would normally be a delta of the stroke 33.5mm/2=16.75
The rest is a change from a 150mm rod length to 100mm and I just remembered the compression height was shortened 15mm
with the new J&E pistons.
I have a 750cc bike that I took 18mm off the barrel and made new carbon fiber push rods not for weight savings but stiffness as I used the 32 valve Ford beehive springs.
I guess what I am saying I am not in new territory in the machining side of the project it just that when I make a mistake at work I write an ADCN and get it changed, but at home when I cut metal I can't afford a do-over.



I don't know anything but the people that do.

 

[larrylcoyle]

Okay guys, I guess I have to throw my years of experience in internal combustion engines and camshafts out the window here. Why do you think that by grinding the end of the follower at an angle you will retard or advance the cam and change the lift? Don't think this is physically possible as this is fixed. The only thing you change by grinding on the follower is the rocker arm geometry.

Larry

 

[thundair]

Larry
I wasn't into the angle but as my cam followers are rectangular I was hoping to take the leading edge completely away hopefully change the ramp engagement angle to my lifter.

I have followed your post and read them with great respect it keeps me honest or at least pragmatic in my wonderland.

I don't know anything but the people that do.

 

[BigVlad]

larrylcoyle: I am not entirely sure what thundair means about grinding the front of the follower. What I was referring to was that changing the angle of the whole contact slope of a flat-surfaced tappet will either advance or retard (depending on the direction of the slope) the cam timing slightly. If you look at USPTO 6357406 (on the USPTO website) you will see that this is the principle they are using. (The patent is from BorgWarner). Less clear is the fact that tilting the contact slope either way will increase the lift slightly - BW don't mention this as an effect.

Thundair: A four-inch rod? - I've seen longer rods on lawnmowers.