Camshaft Help

[BRAAP]

Just a thought... am I gonna run into piston to valve clearanc problems with Isky's .530 lift cam? Will I realize the potential of the cam with an unported head? Should I just stick with their .495 cam?

 

As for piston to valve clearance, you probably will be fine, especially if you are using a P-79 or P-90 head, though don’t take my word for it. The clearance should actually be checked. To know for sure, you really need to mock up one cylinder of the engine long block with the cam to be used and soft springs on the valves of the #1 cylinder and valve lash set on the tight side. This means the crank, piston and rod, no rings, preferably a used head gasket that is the same as you plan to run, and with a small blob of Silly Putty on top of the piston where the valve would contact the piston if such as thing would happen, (Yes, the same silly putty you played with as kid, every good engine builder has an egg or two of Silly Putty in his tool box), “carefully” rotate the mocked up cylinder through 2 complete crank revolutions. If you feel any resistance, don’t force it. That could be the piston and valves crashing into each other and you don’t’ want to bend the valves. If you were able to rotate the engine through 2 full revolutions, then you can remove the head and you will see 2 valve head imprints in the Silly Putty left from the valves as the piston passed near TDC. You can then use a steel rule or calipers to measure just how thick the Silly Putty is at the thinnest point. That will be your actual piston to valve clearance.

 

As for ported vs non ported heads. A bone stock L-series running a stock cam, stock induction and stock exhaust might realize a couple/few HP running a ported head vs a non ported head. On a full tilt road race engine with triple Webbers, huge headers, .600” lift cam, the difference between a ported and non ported head could easily be 30 HP or considerably more depending on several variables. Just remember, the more radical the power plant, the more power a ported head will unleash vs its non ported variant.

[Collectindust240Z]

I know that I should check the clearance with putty, I'm just trying to get away from taking the head back off my motor. When I grabbed the cam out of my old motor that I blew up, I noticed that one of the lobes was scored fairly badly, but that's a different story. I need to get a new cam for this motor and since I wasn't thrilled with the last cam... I'd just hate to spend the loot on a new cam and run into interferance problems. I REALLY don't want to have to pull this thing apart to have the pistons cut for valve reliefs and put it back together again. While I'm waiting for my new Dellortos, I need to figure out this cam thing. I want a big nasty cam, but I don't want to spend a lot of time and money on machine work. If there is any question, I'll just have to go with a smaller cam. Also, I know that cam timing has an effect on clearance.

[TimZ]

John' date='

Glad you enjoy my posts.

As for valve lash and HP. Hmmm.. Valve events are a VERY intricate and highly sophisticated part of engine design and development. [/quote']

Very true.

 

As for optimizing that cam wipe pattern for performance, we tend to cheat the wipe pattern as far toward the back of the rocker as possible, (towards the rocker pivot/adjuster). This is typically referenced as “cheating the rocker ratio†and essentially cheats the rocker ratio from say 1.5:1 to 1.55:1.

To back up a little here, ideally you want all of the cylinders to breathe equally so all the cylinders are producing the exact same HP. Hopefully the intake tract and exhaust tract is allowing this to happen, (in our L-series, yeah right), and with this theoretical equal cylinder filling happening, you would want the cam timing “at the valve†to be equal for all cylinders as well. We typically take for granted that all the lobes on a cam that we purchase are ground “identicalâ€, but in reality, there are slight differences in the lobes themselves. If you have an accurate micrometer and are able to get repeatable results, (i.e. you have used a mic regularly), you can measure the lift of all the lobes on a cam vs their respective base circles and you will see what I mean. I’ve seen as much as .010†difference between lobes on the same cam. (this does not take into account valve duration as that would make this already lengthy post an extremely long novel, suffice to say, that any differences in duration from one cam lobe to the next will follow suit with the lift.) In an effort to help equalize the valve events, we can cheat the wiping pattern of individual lobes to help accommodate this equal cylinder filling. Also, not all rockers are created equal and due to manufacturing tolerances, can cause the rocker ratios to be off from one rocker to the next.

 

This brings up another point - when you move the wipe pattern around on the rocker pad, you also end up moving the valve event timing with it. If I recall correctly, moving the wipe pattern to the "cheating" side of the rocker advances the valve event timing. This can be easily compensated for if you move them all to the same place, but if you start moving the wipe patterns around on a per-valve basis to try to equalize the lift, you could inadvertently scatter the valve event timing as well.

[JMortensen]

This brings up another point - when you move the wipe pattern around on the rocker pad, you also end up moving the valve event timing with it. If I recall correctly, moving the wipe pattern to the "cheating" side of the rocker advances the valve event timing. This can be easily compensated for if you move them all to the same place, but if you start moving the wipe patterns around on a per-valve basis to try to equalize the lift, you could inadvertently scatter the valve event timing as well.

It was explained to me that this was the reason that all the intake and all the exhaust valves should be set in the head at the same height. Assuming the head is not warped then the same size lash pad is used and the same wipe pattern obtained from all the intakes or exhaust valves. That's the way it was described and that's the way it worked out on my head anyway.

[TimZ]

It was explained to me that this was the reason that all the intake and all the exhaust valves should be set in the head at the same height. Assuming the head is not warped then the same size lash pad is used and the same wipe pattern obtained from all the intakes or exhaust valves. That's the way it was described and that's the way it worked out on my head anyway.

Yes, that's right - if the the valves are not installed to the same heights, then all bets are off. I think you might be able to compensate with different size lash pads to get the wipe patterns back in line in this case, but I'm not sure what this does to the rocker ratios.

[jgkurz]

In short, the answer to your first question John is…. Maybe….

 

Thanks Paul. I hoped for just a short answer and got so much more....

 

I'm off to study my "How to Modify Your Nissan/Datsun OHC Engine" and "How To Hotrod & Race Your Datsun"

 

John

[BRAAP]

I know that I should check the clearance with putty, I'm just trying to get away from taking the head back off my motor. ............ Also, I know that cam timing has an effect on clearance.

 

True indeed. Altering the cam timing will alter the relationship between the piston and valve as they chase each other.

 

 

 

 

Very true.

 

 

 

This brings up another point - when you move the wipe pattern around on the rocker pad' date=' you also end up moving the valve event timing with it. If I recall correctly, moving the wipe pattern to the "cheating" side of the rocker advances the valve event timing. This can be easily compensated for if you move them all to the same place, but if you start moving the wipe patterns around on a per-valve basis to try to equalize the lift, you could inadvertently scatter the valve event timing as well.[/quote']

 

 

Yessirree… Good point. Being as cheating the cam wiping pattern is merely angling the rocker in relation to the cam lobe itself, and as such, this alters the point in time at which the cam lobe starts to push down on the rocker itself, (point in time in relation the crank/piston position), in effect, altering the valve timing of that particular cam lobe…..

 

 

 

It was explained to me that this was the reason that all the intake and all the exhaust valves should be set in the head at the same height. Assuming the head is not warped then the same size lash pad is used and the same wipe pattern obtained from all the intakes or exhaust valves. That's the way it was described and that's the way it worked out on my head anyway.

 

Yes' date=' that's right - if the the valves are not installed to the same heights, then all bets are off. I think you might be able to compensate with different size lash pads to get the wipe patterns back in line in this case, but I'm not sure what this does to the rocker ratios.[/quote'']

 

Even if the head was severely warped and then shaved flat on both top and bottom, the valve stem tips of the middle cylinders would be closer the cam lobes as compared to the outer valve stems, (stem heights would shorter in the center vs the ends). By using thicker lash pads on the outer cylinders, the rockers will remain at the same angle in relation to each other, and it is that angle of the rockers that will alter the valve event timing from one valve to the next. If the same thickness lash pads were used on all valves in this scenario, then there would be an issue of differing vale event timing as the rockers would be at differing angles and the wiping patterns could even be off the rocker pad, which is very bad. By varying the thickness of the lash pads bringing the wiping pattern to the same location on the all the rockers, the valve event timing would not change from one valve to the other. Another approach as mentioned previously is to tip the valve stems which would achieve the same results as using varying thickness lash pads. The end result would be consistent valve event timing. With this same warped head, there is a very minor issue of the outer valve stems being angled, not perpendicular to the bores anymore. It takes more warpage than we typically see with these heads for that to actually become an issue in regards to valve stem to rocker/cam geometry……

 

 

 

 

I’d like to stay and chat with you guys more on this topic, but the rest of this week I need to devote to our customers. .

 

Good night guys…

[BRAAP]

Paul- I've wondered about this in the far back of my mind but never crunched the numbers to see how it works out...on a fairly radical reground cam (like the one you pictured) where the base circle is about where the core shaft itself is, used on a P79/P90 head...how would N42/N47 valves (.10" longer) work out, and if the length is too much then shim the towers to compensate? Let's say the stock lash pads are .160", do you end up with lash pads as thick as .250" when you set these cams up? If so, then why not just start out with the longer valves (which might allow more valve spring choices/flexibility)? DAW

 

 

 

Hmmm…. Never really though about that. I promise that by the end of this weekend I will actually try that scenario out on one of the P-90’s or P-79s that are currently in process of customization.

Yes, this would allow more valve choices for sure..

 

I’ll report back by the beginning of next week….