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I spent some time today disassembling a stock cylinder head assembly and noted some interesting findings. I looked at the cams, the ports, lash adjusters, rocker arms, intake runners and plenum very closely and this is what I discovered: The intake ports on the head are considerably smaller in inside diameter than the runners. The intake ports have a machined bevel cut on the flange area where the runners meet the ports. My guess is that maybe this was done to help promote air velocity through the intake ports. Otherwise, why wouldn't the ports have just been cast larger in I.D. and closer to matching the I.D. of the runners :?:
By studying the cams very closely, I noted some interesting findings. Both cams appear to have been made from the same billet shafts, with the exception of the distributor drive gear cast on the intake cam and the dowel pin locations for the sprockets. Here's the real interesting comparison: Both cams have what I call a "dual profile", whereby the opening slope of the cam lobe is different from the closing slope of the lobe. The intake cam is ground in such a way as to basically open the intake valve rapidly and then close the intake valve more gradually (pop the valve open, keep it open, and then gradually close it). The exhaust cam is the same way, only backward from the intake cam (gradually open the exhaust valve, keep it open, and then slam it shut).
The rocker arms have a machined curved surface (not flat like the 7M bucket and shim). This basically means that the rocker arms have a variable ratio rather than a fixed ratio. In laymens terms, the valve opening speed is steadily increased and the closing speed is steadily increased as the cam lobes open and close the valves, basically accelerating the profile of both cams. As the rocker arms are pushed down by the cam lobes, the ramped surface of the rocker arm increases in height, thus accelerating the speed at which the valves open. The valves start to close rapidly and then slow down before they reach the seat. This is pretty darn ingenious if you ask me!! 8)
When you combine all of these different characteristics of the induction system and valvetrain, I can't help but think that perhaps the clever engineers that designed the 5MGE had several things in mind: Intake port airflow velocity, exhaust gas scavenging, intake plenum volume, runner length, yada-yada!
If the 6MGE has the exact same cam grinds and port configuration, then I suspect that I may know why the 6M is so much more efficient than the 5M. Perhaps a much better VE! :wink:
I know this is a lot of info., but I thought this would make for some interesting insight and fuel for thought! Please correct me on anything that I may have wrong or have otherwise misinterpreted on this post!
Wheww! Sorry for the lengthy post! :oops:
 

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Dave,

You are scaring me. Are you SURE you aren't a rocket scientist?

When I talked to Barry at Shadbolt cams he mentioned that the asymetrical cam profile was due to the cam contacting on the follower which is mounted on the side. On a pushrod engine or an overhead cam engine with direct acting cam-on-valve, the pattern would be symmetrical.

He suggested that the assymetry was to equalize the rise and fall rates which would be different because of the changing rocker ratio. That is so cool that you noticed it. He had to point it out to me.

I also suspect that you are AGAIN right on the money with the variable rocker ratio. When I was measuring the stock cam height I measured a rocker ratio of about 1.73:1. On the TRD cam with increased lift, the rocker ratio at maximum lift was about 1.79:1. I thought that was too big a difference to be a measurement error. I assume that as the taller lobe sweeps to the right, the leverage of the system is increased. Thank you for pointing that out. I was feeling very guilty for poor measurement technique.
 

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Anytime you move the peak lobe lift in relation to the lobe centerline you will slightly adjust the rocker/follower ratio in in an OHC engine that uses them. This and increasing ramp speeds are part of the "cheater cam" methodology. Also why they are refferered to as "cheater" cams.
(For those that aren't familiar with the term,... In racing classes where the stock lift and duration MUST be maintained, these techniques are used to squeeze that litttle extra HP/torque out an engine while maintaining the stock cam "profile".)
Too bad the 7M doesn't have a rocker at all. The disc riding on the cam has to spin to prevent excessive wear, so the lobes are ground on a slight slant, and the disc MUST be flat and offset slightly fore and aft to the centerline of the lobe. I belive it was VW that pioneered/perfected this arrangement.
Jamie
 

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Discussion Starter #4
The same type of asymmetric profile is also sometimes used when building supercharged engines. The results are phenomenal! 8)
 
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