Is this cam what it is supposed to be?

[Curt Hayes]

When I first got the cam I checked the part number on the end and it matched that for a Schrick 288. I know that the cam was a 288 at some point in its life. My concern is that it may have been reground for some stupid reason, evidenced by the apparent discrepancy in duration. That is why I am measuring the cam, to see if it is the same as another 288. For that purpose, it is a lot more accurate to measure the lobe lift, not the valve lift.

Last time I saw a reground cam I could see the change in material color ("colour" to you, digger. :) ) going from the base circle up the lobe. This was because the regrind had gone through the original hard surface to the soft material underneath. The two are slightly different grays so I could see where that had happened. I'd expect to see something similar on your cam if it's reground.

Digger's right in that measuring lift at the cam won't tell you about valve overlap because of the variation in rocker ratio as the lobe sweeps from one side of the rocker pad to the other.

[structured]

Last time I saw a reground cam I could see the change in material color ("colour" to you, digger. :) ) going from the base circle up the lobe. This was because the regrind had gone through the original hard surface to the soft material underneath. The two are slightly different grays so I could see where that had happened. I'd expect to see something similar on your cam if it's reground.

Digger's right in that measuring lift at the cam won't tell you about valve overlap because of the variation in rocker ratio as the lobe sweeps from one side of the rocker pad to the other.

I understand that. My thought was more that I would be able to tell if my cam was measurably different from someone else's which is known to be to spec.

Regardless, I am going to put the cam back in and degree it once again. This time I will make sure to set the valve lash to zero. It is quite possible that it was very large and made the cam seem so much smaller.

[structured]

So it looks like I was right. The valve lash must have been huge because I was losing like 50 degrees of duration from the cam. I put in some light springs today and degreed the intake side again. Here is what it really looks like (much closer to what was expected):



Unfortunately, despite solving one problem I found a new one. After setting the lash to zero (or very close to it) I noticed that the engine was binding due to the valve just barely hitting the pistion. I had so retard the timing a ton to get it to spin freely. I just hope that it was binding before and I didn't notice! That would blow if I bent a valve (or several). I doubt it though. I would assume that it takes a lot of force to bend them into the plastic range.

Aside from disassembling the block and getting bigger reliefs cut, I don't see any option but to use an oversized headgasket. I am going to plot out the valve clearance relative to the crank position tomorrow. Hopefully that will tell me how much advance I can get if I use a slightly bigger headgasket.

[digger]

You sure that is the intake lobe?

You lose a lot if duration if you measure the closing and opening points but that is because of the ramps. If you compare the duration at say 1mm and higher (more important) it’s a much smaller amount.

Here is one of my earlier attempts, it is a bit rough but gives an idea as to what the lash does. it works out to be approx 0.25mm vertical offset on the graph




It appears the schricks are ground deliberately with some advance into them. Adding in the lash moves the valve farther from piston but if touching with zero lash that is simply too close. Valves bend easily especially if contact is on the edge of valve face which creates some large bending moments (only a 7mm stem). Would want to bend a valve than smash pistons though. You can retard the cam which makes the exhaust more likely to hit, if you advance the intake is more likely to hit since the cam is pretty symmetrical you are snookered. Advancing and retarding for PtoV clearance is not a great idea anyway.
You need a pretty decent spec motor (high CR, different intake manifold etc) to come close to utilising all that cam anyway. I would sell it and get something a bit more appropriate. You’ll end up with something that will be better 95% of the time…….not to mention avoiding a catastrophe

[structured]

Yeah that is probably what the valve lash means when you compare zero lash to what it it supposed to be. Mine must have been much more than that. I had set the valve lash a long time ago, but I must have made a mistake and I never checked it before degreeing the cam for the first time.

I am positive that I degreed the intake lobe.

It was hitting on the intake valve pretty bad. I had to retard it about 2 degrees from the Schrick spec (i.e. exh. peak timing is at 108* instead of 110*).

I know that it is too big for my engine (Super eta w/ +0.05 pistons, "i" head). I probably will never make enough compression. I would love to sell it and get something a bit smaller, but nobody seems to want to buy it...

[digger]

Positive that it is the intake lobe. I had set the valve lash a long time ago, but I must have made a mistake and I never checked it before degreeing the cam for the first time.

It was hitting on the intake valve pretty bad. I had to retard it about 2 degrees from the Schrick spec (i.e. exh. peak timing is at 108* instead of 110*).

I know that it is too big for my engine (Super eta w/ +0.05 pistons, "i" head). I probably will never make enough compression. I would love to sell it and get something a bit smaller, but nobody seems to want to buy it...

the intake valve opens before TDC, your graph shows the exhaust lobe closing just after TDC as it should

[structured]

the intake valve opens before TDC, your graph shows the exhaust lobe closing just after TDC as it should

Haha. I was wondering why the graph seemed counterintuitive. I didnt know the convention so I just plotted it the way it came out. I definitely measured the intake, not the exhaust.