Is this cam what it is supposed to be?

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So I have this Schrick cam which has a 288 serial number on the end. I bought it second hand and while I was always a bit suspicious of it, I never had a chance to verify what it was until now. I assembled my engine and measured the cam today. Here is what I found:



Now, I hope that I made a mistake somewhere, but the duration does not look anything like 288 degrees. Am I missing something?

Ignore the lift values. I adjusted them to match the 288 cam spec because I was unable to locate the dial indicator's pointer directly on the valve retainer. The actual measured values were a bit lower than what you see. I am going to measure the lobes to check the actual lift this week.

[nando]

is that chart in cam degrees? if it is you need to double it for crank degrees which is what cams are spec'd at.

also - the quoted peak lift value from schrick is without valve lash. typical valve lash is .025mm or .010". I believe the advertised duration is measured without lash and right at valve open/close. Industry standard is .050" open/close and with valve lash. if you're measuring this on a head, you've got valve lash there that would add some duration. if you wanted to get a better measurement you'd set the eccentric to 0. the rocker arm also multiplies how the valve opens/closes (it's abuot a 1.25:1 ratio, but it varies with opening time).

got a higher res pic of that chart? or a pic of the cam? is it stamped "schrick" between cyl 1 and 2?

I think if you wanted to measure it and find if it matched schrick's specs, you'd have to measure on the cam lobe itself and not the valve.

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- I uploaded a better image. Don't have a bigger one available. I can upload a better one later if you still can't see.
- Yes, it has the Schrick logo cast into it.
- I am measuring the cam while the engine is fully assembled and out of the car.
- The location on the chart is the crank location in degrees, not the cam location.
- 0 degrees on the chart is TDC.
- Right now, I calculated the exhaust peak timing to be at about 119 degrees (9 crank degrees advanced from spec). That is only because I haven't adjusted the cam gear yet.

While measuring lift, I had the dial indicator pointer sitting on either the eccentric nut or on the eccentric. I know that this will give a false reading but I was unable to place it directly on the valve retainers. Instead, I multiplied each lift value by the ratio of the (spec lift/measured peak lift). Once I measure the cam lobes, I will adjust that ratio to be the (actual valve lift/measured peak lift).

As for the rocker arm ratio, I measured at the valve (not the lobe) so it isn't relevant just yet. I will need it to convert from the lobe lift to valve lift.

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Pics of cam:

[digger]

what does it say on the end of the cam?

It doesn't plot out like a 288 when you look at the overlap

I measured a 288 a few weeks ago at the valve and plotted the result

http://www.e30tech.com/forum/showthread.php?t=108802

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Digger -

That's exactly what I am looking for.

Also, I notice that you mentioned a reason for the MM exhaust duration being much smaller than the intake duration. Jim Rowe once told me that one reason why he did that was to maintain some compression in the cylinder. Apparently it helps to keep some of the tourque and low-end power which is often lost with bigger cams. He made up for the shorter duration by using an extremely high lift, something which stock rockers and springs aren't capable of supporting. His solution was then to create progressive rate springs which have a lower peak pressure even at such high lift. Of course, I may have totally misunderstood him.

[digger]

Digger -

That's exactly what I am looking for.

Also, I notice that you mentioned a reason for the MM exhaust duration being much smaller than the intake duration. Jim Rowe once told me that one reason why he did that was to maintain some compression in the cylinder. Apparently it helps to keep some of the tourque and low-end power which is often lost with bigger cams. He made up for the shorter duration by using an extremely high lift, something which stock rockers and springs aren't capable of supporting. His solution was then to create progressive rate springs which have a lower peak pressure even at such high lift. Of course, I may have totally misunderstood him.

if he wanted torque in the lowend he shoudn't have done what he did with the inlet side thats where the compression is made, the dynamic compression in that thing means it throws alot of what comes in back out and needs min 12:1 CR IMO, it relies a fair bit on inertia that one....you could go larger on the MM exhaust side without much detriment to torque if you set the ECL appropriately without hurting emmisions and idle quality. It is a pretty quick ramp

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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.

[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.

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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]

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]

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]

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]

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]

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.