Is this cam what it is supposed to be?

[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.

[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]

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]

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]

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.

[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.

[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.

[digger]

Digger -

What was the valve lash when you degreed the cams?

EDIT: Just looked at your thread and noticed that you said that you set the rockers to zero clearance. Is that the same as saying zero lash?

yes, it was zero. I did do both on one of the standard cams to see what the difference is. If i find it i will post it up.

[structured]

it is a variable rocker ratio so you might get some info about max lift but it is pretty approximate and you won't learn anything about the LSA or centreline of the lobes etc

Digger -

What was the valve lash when you degreed the cams?

EDIT: Just looked at your thread and noticed that you said that you set the rockers to zero clearance. Is that the same as saying zero lash?

[digger]

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.

it is a variable rocker ratio so you might get some info about max lift but it is pretty approximate and you won't learn anything about the LSA or centreline of the lobes etc

[structured]

what pressure are you talking about? you don't want pressure in the cylinder just before the intake valve opens.

it is obvious by his cams he wanted to limit limit overlap but wanted more duration and did this by using wide centerline for the intake and a relatively small exhaust duration but with faster ramps to get more area under the curve.

the shorter rod will dwell less at TDC not more but it is a pretty small difference and only a concern if chasing every last hp but then it won't neccesaily be the longer the rod the better either. The durability would be more of a concern but we aren't interetsed in OEM life from our engines.

I don't have the 288 anymore, i wouldn't bother measuring the cam anyway its what the valve does that matters. yours is a schrick and there should be a code/Part number on the end of it.

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.

[digger]

Like I said, I may have misunderstood. I am only relaying information which I don't totally understand (something which is almost never the right thing to do).

Here's a thought. Isn't it true that because of the unfavorable rod/crank ratio of a lot of M20 strokers, the piston will have a tendency to "dwell" at TDC? In the same way, isn't quite a bit of energy wasted trying to start the piston moving back down again to start the intake stroke? If you were to open the intake valve a bit later and also close the exhaust valve after TDC but before all of the pressure has escaped, then that pressure would assist in starting the piston moving down. The obvious drawback there would be that the flow of air into the chamber would be delayed slightly. You could mitigate that by opening the intake valve very quickly, having a very broad centerline, and a very high lift. Now, I don't really know if that was what Jim Rowe was thinking, but that's what I thought.


Anyway, back on topic. I disassembled my head today :hitler: in order to measure the cam lobes. I found that they are all uniform and have a lift of approximately 7.2mm ( calculated about 7.24). I believe that Schrick specifies a lobe lift of 7.3mm. It is close enough that I don't think that it is cause for alarm. Unfortunately, I still don't know it the cam was reground.

Digger, could you measure the lobes of your 288 and let me know what the max and min dimensions are? If they are different than mine, then I would know that this cam has been reground.

My cams dimensions are 34.9mm x 27.7mm.

what pressure are you talking about? you don't want pressure in the cylinder just before the intake valve opens.

it is obvious by his cams he wanted to limit limit overlap but wanted more duration and did this by using wide centerline for the intake and a relatively small exhaust duration but with faster ramps to get more area under the curve.

the shorter rod will dwell less at TDC not more but it is a pretty small difference and only a concern if chasing every last hp but then it won't neccesaily be the longer the rod the better either. The durability would be more of a concern but we aren't interetsed in OEM life from our engines.

I don't have the 288 anymore, i wouldn't bother measuring the cam anyway its what the valve does that matters. yours is a schrick and there should be a code/Part number on the end of it.

[structured]

None of that makes any sense . That’s sounds like the backpressure myth.

With the exhaust you need to open the exhaust valve early enough to allow enough time for the gases to blow down and not so early as to lose some of the heat energy out the exhaust. The more you get out the of the cylinder the lower the pressure in the cylinder and the less the piston has to pump through the exhaust valve as it approaches TDC towards the enough of the exhaust cycle this pumping is a “pumping loss”. You also don’t want any residual gas left over so the last part of the chamber volume at TDC can get trapped inside.

why would you want pressure in the chamber at TDC? That will lower the pressure differential that drives the intake flow. People go to extended lengths to tune the exhausts and cam timing so that the exhaust gases are removed by scavenging (as well as normal blowdown and pumping) this minimizes the residual gas which occupies a lot of volume because it is hot. This makes room for the fresh intake charge and the exhaust system can provide a low pressure signal to get the flow in the inlet moving quite fast without having to rely on the piston motion which is quite slow around TDC.

The reason for smaller exhaust duration or more accurately a earlier exhaust valve closing angle (less overlap) is that at low rpm there is not enough velocity in the exhaust flow to stop the exhaust gas in the exhaust primary pipe being drawn back into the cylinder during overlap because the piston is moving down with both valves open drawing both new intake charge and exhaust gas. There is also a situation at overlap where a low pressure region in the exhaust a certain rpm can draw intake in through the intake valve and straight out the exhaust valve.

Like I said, I may have misunderstood. I am only relaying information which I don't totally understand (something which is almost never the right thing to do).

Here's a thought. Isn't it true that because of the unfavorable rod/crank ratio of a lot of M20 strokers, the piston will have a tendency to "dwell" at TDC? In the same way, isn't quite a bit of energy wasted trying to start the piston moving back down again to start the intake stroke? If you were to open the intake valve a bit later and also close the exhaust valve after TDC but before all of the pressure has escaped, then that pressure would assist in starting the piston moving down. The obvious drawback there would be that the flow of air into the chamber would be delayed slightly. You could mitigate that by opening the intake valve very quickly, having a very broad centerline, and a very high lift. Now, I don't really know if that was what Jim Rowe was thinking, but that's what I thought.


Anyway, back on topic. I disassembled my head today :hitler: in order to measure the cam lobes. I found that they are all uniform and have a lift of approximately 7.2mm ( calculated about 7.24). I believe that Schrick specifies a lobe lift of 7.3mm. It is close enough that I don't think that it is cause for alarm. Unfortunately, I still don't know it the cam was reground.

Digger, could you measure the lobes of your 288 and let me know what the max and min dimensions are? If they are different than mine, then I would know that this cam has been reground.

My cams dimensions are 34.9mm x 27.7mm.

[digger]

I don't think he was referring to the dynamic compression (i.e. closing the intake valve sooner). Instead, the idea is to keep some pressure in the chamber while the exhaust closes in order to "push" the piston over and down from TDC. Note that in the MM cam there is much less overlap between the intake and exhaust lobes in order to take advantage of that pressure. The Schrick 288 has a ton.

None of that makes any sense . That’s sounds like the backpressure myth.

With the exhaust you need to open the exhaust valve early enough to allow enough time for the gases to blow down and not so early as to lose some of the heat energy out the exhaust. The more you get out the of the cylinder the lower the pressure in the cylinder and the less the piston has to pump through the exhaust valve as it approaches TDC towards the enough of the exhaust cycle this pumping is a “pumping loss”. You also don’t want any residual gas left over so the last part of the chamber volume at TDC can get trapped inside.

why would you want pressure in the chamber at TDC? That will lower the pressure differential that drives the intake flow. People go to extended lengths to tune the exhausts and cam timing so that the exhaust gases are removed by scavenging (as well as normal blowdown and pumping) this minimizes the residual gas which occupies a lot of volume because it is hot. This makes room for the fresh intake charge and the exhaust system can provide a low pressure signal to get the flow in the inlet moving quite fast without having to rely on the piston motion which is quite slow around TDC.

The reason for smaller exhaust duration or more accurately a earlier exhaust valve closing angle (less overlap) is that at low rpm there is not enough velocity in the exhaust flow to stop the exhaust gas in the exhaust primary pipe being drawn back into the cylinder during overlap because the piston is moving down with both valves open drawing both new intake charge and exhaust gas. There is also a situation at overlap where a low pressure region in the exhaust a certain rpm can draw intake in through the intake valve and straight out the exhaust valve.

[structured]

I don't think he was referring to the dynamic compression (i.e. closing the intake valve sooner). Instead, the idea is to keep some pressure in the chamber while the exhaust closes in order to "push" the piston over and down from TDC. Note that in the MM cam there is much less overlap between the intake and exhaust lobes in order to take advantage of that pressure. The Schrick 288 has a ton.