TOO: "World According to..." request

Posted by Leadfoot on April 29, 1998 at 10:51:03:

I don't know if anybody has already asked, but I have a few ?'s about using adj. cam gears to change cam timing on a n/a DOHC motor. Does changing the intake cam timing affect the compression ratio of the motor? Why? Will overlap remain the same if both in/ex cams are advanced or retarded the same amount? I realize the major benefit of having adjustable gears is being able to move the powerband of your motor, but can the gears extract "hidden power?"

Re: TOO: "World According to..." request

Posted by eg2 frank ( on April 29, 1998 at 13:09:11:
In Reply to: TOO: "World According to..." request posted by body on April 29, 1998 at 10:51:03:

Changing the intake timing will affect the effective compression ratio. You are asking the intake valves to open/close at different crank angles (different piston velocities) so the amount of air that goes in changes. If you advance or retard the in/ex cams by the same amount the overlap will remain the same. In the V8 world where the overlap (lobe seperation angle) is deterine by the cam grinder the end user can only choose to advance/retard both cams at once. On DOHC motors we can easily experiment with different combinations. In general if you advance both cams, you will shift the curve down, have more low end torque, and broader powerband but less peak. Retarding both gives the opposite.

Sure it can extract hidden power... The best way to tune cam gears is on a dyno with repeatable results. The dyno operator should try to minimize environmental factors...

Re: TOO: "World According to..." request

Posted by T.O.O. on April 29, 1998 at 19:25:19:
In Reply to: TOO: "World According to..." request posted by body on April 29, 1998 at 10:51:03:

eg2 frank answered this pretty well, however, you can indeed affect the engines compression ratio. The intake valve opens slightly during the end of the exhaust stroke, and this is known as the "overlap" period because both intake and exhaust valves are open slightly at the same time, with the exhaust closing, and the intake opening. If you advance the cam (especially the intake) it will open the valve sooner, and close it sooner, since all you've done is "move" the fixed intake event. The intake valve doesn't close at the bottom of the intake stroke, it remains open after BDC to attempt to get some additional mixture in the cylinder using inertial ram. Since the piston is traveling up on the compression stroke while the intake valve is closing, the mixture takes the path of least resistance, and so the piston shoves some of the mixture back up the intake port before the valve closes.
If you have less mixture in the cylinder to compress, your dynamic compression ratio will be less, and if you close the intake valve earlier on the compression stroke, the rising piston will have more mixture to compress since less was pushed back up the intake port. The intake valve duration , especially when the valve closes can have tremendous effects on dynamic compression. Now understand that dynamic compression has almost nothing to do with static compression ratios. Static compression ratio is simply the ratio of head and cylinder volume when the piston is at BDC. vs TDC.
An adjustable gear an a SOHC engine will provide the ability to shift the power band slightly to suit you. With twin cams and dual adjustable gears, you can obtain greater results, but keep in mind that the emissions may suffer. My best advice would be to make notes and mark the original cam position, and do the same with the "improved" cam position, then you can easily return timing to stock when necessary, then back where you like it when you're ready....and you won't need to pay some "tuner" to do it for you...................................T.O.O. .....................................