cam degreeing

[ochster]

A recent cam I did, actually showed to be out of spec, by near 6-7 degrees. This difference was actually between the intake opening and closing specs. I merely split the difference, and preceded.

Was just curious, if this is common? I'm figuring it to be either a slight change on the cam manufacturer parts, and not wanting to change all kinds of printed specs. And or, the simple reality, that the final product may not have been made to print.

In this particular application, the cam sprocket was slotted. It was equally machined on both sides of the oem 6mm hole (slot). Even after splitting the difference in lobe spec, my final setting ended up a couple thousand from one side of the slot.

 

[Rich Rohrich]

Ochster - What lift point are you using to determine IVO & IVC? Are you sure it is identical to the cam card spec, in terms of lift point and measurement location (i.e on valve, on rocker , etc) ?

 

[ochster]

I think you mean the .040 lift spec, wich I used. That is what the card indicated. It did not indicate an actual position for the indicator, I used the top edge of the spring retainer.

I have been thinking that with all the componet modifications to set up squish, I have probably retarded the cam orientation, as the slack in the chain is taken up by the adjuster. The head, top and bottom of the cylinder have been cut. As well as running a thinner base gasket.

This is on a RFVC.

 

[Rich Rohrich]

Ochster - So where exactly is the difference showing up, on the open or close point?

 

[DEANSFASTWAY]

I sometimes have situations like that on Honda 4s when I have rebuilt a crank or pressed on a new lower timing sprocket. What was it an XR 600r?Are you saying you cant get the valves to come open at the right time or like the cam was made wrong and wont match the cam card? Just curious DEAN

 

[Rich Rohrich]

Maintaining the 100.5 Intake LCA is easily fixed by splitting it as you suggested. But it doesn't acount for the 6 degrees of duration difference between the card and your measurements. If the 255 duration @ .040" card spec is what you ordered it sure seems like they shipped you the wrong cam (or it isn't ground correctly). Sounds like it's worth getting the grinder to verify which is which given how big a difference you are seeing. It only amounts to .0001 sec difference of total intake open time @ 8000 rpm but it still seems like they should have shipped you what you ordered. Depending on the pipe you are running the overlap difference might prove to be significant.

 

[DEANSFASTWAY]

RICH or others: On a RFVC XR type head when you drop the base circle on the cam lower on a regrind do you think that changes the rocker ratio much ?Weve been trying some drops on stock XR /TRX cams on a Van Norman crank grinder, just dropping the base and trying to simulate some other regrinds as followed by their specs and were wondering how far is really acceptable. We were thinking that .010 at the cam turns out to like .030 at the valve, but if you go deep and it changest ratio might get higher. Adjuster studs still seem centered . Just tinkering.Any Ideas.?

 

[Brianc]

Ochster,

On an overhead valve engine, if you mill the head or use a thinner gasket, you will retard your valve timing. The tensioner removes the slack, but cannot correct the timing. Are you using an old timing chain? It could be stretched. An old chain doesn't need to stretch much to retard your timing 6 degrees.

 

[ochster]

Brianc,

There were some clean up cuts made to the aluminum sleeved cylinder, and the head. All componets are new oem parts, as well as using a slotted cam sprocket. I have a couple of new cams, made by a well known grinder. After near 30-40 set-ups, I have concluded both cams are off of thier respective spec card. One is only a couple of degrees, the other is near six. After two day's of playing and trying to split differences in thier errors, I see you really cannot. It is simply impossible to "split" the specs. I'm thinking, I would like to run some simulations, to see the relevance of my differences in specs. I have a bunch of one-off stuff done to this engine, and I'm sure my engine perameters would dictate something other than what I actually have anyway. I may just head to the dyno with my bag full of tools, and see where I'm at. Wouln't happen to know a "friendly& willing" grinder would you?

 

[Rich Rohrich]

I think the questions I asked earlier might have caused some confusion here.

Changes in the distance from crank center to cam center due (cam centerline height ) to deck height changes, or as Brian pointed out cam chain stretch will change the cam PHASING because of changes to the lobe center point in relationship to the crank. It will not have ANY effect on the total open duration. Cam phasing is nothing more than a realtionship between the cam and the crank position. Total valve open duration is ground into the cam and doesn't change.

Mark - If you measured it 40 times and you are missing 6 degrees of total duration then the cam doesn't match the card spec PERIOD.

 

[ochster]

If all else remained the same, and you increased the size of the intake and exhaust valves, would this not be the equivelant of a higher lift?

Is there any relevance, in thinking that a larger valve face (heat sink), atleast in theory would help chamber cooling? Hurt chamber performance?

Can you effect intake velocities, with a larger seat and valve, all else being the same?

Would a breather tube in the head cover (RFVC), function as an aid in cooling?

 

[Rich Rohrich]

Originally posted by ochster
If all else remained the same, and you increased the size of the intake and exhaust valves, would this not be the equivelant of a higher lift?

I had a similar conversation with cam designer Rick Jones a few years back where I asked about the relationship between lift and port flow and here's what he told me:

When the port area is adequate , you will find by increasing the port flow 10% at critical lift your valve lift can decrease by only the square root of 10% if you're to acheive the same amount of cylinder filling. So by decreasing the port flow 10% and leaving the lift alone you will see a 4.88% decrease in cylinder filling. If you lowered your lift by 10%, an increase in port flow of 21% would have to occur for your cylinder filling to remain the same. If you did not increase the port flow, a loss of 10% in cylinder filling would occur.

Originally posted by ochster

Is there any relevance, in thinking that a larger valve face (heat sink), atleast in theory would help chamber cooling? Hurt chamber performance?

I wouldn't think it would matter one way or the other in terms of cooling.

Originally posted by ochster

Can you effect intake velocities, with a larger seat and valve, all else being the same?

Larger valves will tend to have lower velocities than small ones at the same rpm.

Originally posted by ochster

Would a breather tube in the head cover (RFVC), function as an aid in cooling?

Probably not directly, but if it helped the oil control it might minimize the chance of trace knock which would have a big influence on cylinder head temps.

Air cooled engines can be helped quite a bit by picking the correct fuel and using it as an aid in controlling combustion chamber temperatures.

 

[ochster]

Thank You Rich,

Just some abstract thoughts/questions I had.

I currently have 5 gallon cans of both C12 & C14, for trial.

 

[Rich Rohrich]

Mark - If you are going to run this thing hard then C12 is a poor choice due to it's low end point temp. High specific output air cooled engines need some piston cooling help and C12 won't do it unless you run it way too rich on the main. What makes C12 a great fuel for other engines makes it a poor choice for the XR. C14 would be a better choice but you'll trade off some throttle response. If you are interested I can suggest a mix of VP fuels that would work pretty for your app.

 

[ochster]

Interested...How about I fly to your place, and mow your lawn with scissors?