In a generic example, what would the practical effect of the following be on performance- Twin cam motor, the head is milled and the block is decked, which in effect lengthens the timing belt and therefore retards cam timing. Assume there is no allowance for degreeing the cams. I understand how much is machined is an important part of the discussion, but what would the tendencies be? Thanks in advance, TomB
It would depend a lot on what kind of engine and which way the belt goes in relation to the crankshaft. It's possible that in V-type engine one bank would be retarded slightly and the other advanced. This is purely academic since when the head/s are reinstalled and the timing belt/s are fitted you would at least be lining up the reference marks to make it run. This would get you "in the ballpark" and might in fact put cam timing just as it was before. No real change. The big change as I see it would be compression ratio and on some engines the intake manifold might not fit anymore.
A Miata motor to be exact- the tensioner side of the belt is on the intake side. The question is if for example if enough was cut off the block and head to retard the cams a half a tooth, how much effect might this be on performance. I can't tell you how many degrees off that might be so I know it's a nebulous question. TomB
You should do it. Maybe 2hp, but you actually would feel the improvement from A to B with the same compression/squish.
Hi Tom, If you are really concerned about the change to your cam timing, look at the links below. http://www.junauto.co.jp/products/cylinderhead-part/cam-sprocket/index.en.html or http://www.gomiata.com/fipemicage.html Then you can degree them in, wherever you want. Just for fun, you might try advancing the intake approx 5-10º from stock and set the exhaust cam as per stock. Might not help in a sniffer test, but could improve the perf a bit (Old Lotus 907 trick). Regards, Bill
An offset key is on my list of things to look into. I was curious in very general terms what would happen to a mid-90's emission compliant motor if the cam timing was retarded say 5 degrees- maybe it's an unrealistic question. It's an area about which I know next to nothing. I do appreciate the help, TomB
This is why the aftermarket has built means to fix that problem. Just buy the pulleys that are adjustible, degree the cams, and get on with life. Alternately, you could take a bunch of measurements and then have a new keyway machined into the existing pulleys. If you take enough measurements, or have several keyways machined, you culd end up with adjustments in 1 degree by simply moving one tooth on the pulley. (1 degree is close enough unless you are running radical cams).
Read posts #6 & #7 You could probably make your own key with little more than a file and a bench vise.
Retarding the camshaft(s) moves the torque curve higher, and if the rest of the motor is up to snuff will increase peak HP. This effect is most pronounced on small-displacement high-revving motors; for example a few degrees on a Fiat SOHC (1.3L, 7000 RPM) will make a HUGE difference. Of course if the rest of the induction and exhaust system aren't up to moving an increased volume of air, you won't gain much of anything at the high end, while the loss of low-end torque will hurt driveability and acceleration. Fortunately, you're also increasing compression, which generally improves torque across the board, so you may find that that balances any loss of low-end torque from the cam timing. There's no substitute for dyno testing if you want data rather than opinion.
If you have enough belt adjustment you shouldn't effect engine output. Anything under 3 degrees makes no difference on the street or track.
