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MG MGA - Adjustable Timing Gear-Romac?
|The biggest concern I have in this whole engine rebuild process is retiming the cam. |
Having said that I'm not terribly excited about shelling out $250 for a vernier adjustable timing gear.
I've found the Romac Rollermaster system that has 7 keyways for adjustment. $89
What wisdom do you all have to lend in this regard?
|T, unless you need the precise calibrations for the high performance cams, why not just go with a standard timing gear? There a dime a dozen and work quite well!
I've decided to go with a mild performance cam. They require something like 4 degrees of advance. It will drive me crazy if I'm not confident that I have it fairly well dialed in. However the trial error method of offset keys and skipped teeth makes me nervous.
I think that if you can get the 4 degrees from the Romac, you are better off. With a keyed system, you don't have to worry about later shifting of the timing.
|T, 4 degrees can easily be aquired from the knured knob on the stock distributer. You will find that dialing in timing is based upon how the individual motor is performing at the time. That degree of calibration s easly aquired with a standard street set up. It is the high performing racing cams that require the more percise set up.|
The Romac keyway and skipped tooth system sounds simple enough, although I have never seen one nor used one.
I assume it is used the same as the way as the mga twin cam sprocket hole/sprocket tooth timing works for the camshafts.
Divide the number of keyways into 360, and then divide the number of sprocket teeth into 360.
The first answer is the number of degrees advance for one keyway movement forward, then subtract the second answer for the retarding of the camshaft by moving back one sprocket tooth.
This vernier system allows you to find the advance you need. For this to work the number of keyways must be different from the number of teeth, on any engine.
A word of warning. This is camshaft degrees. Is your required 4 degrees for the camshaft or the crankshaft?
|M F Anderson|
Looking at the Romac website it appears that you don't have to do any calculations. They tell you how to simply get 4 degrees.
The vernier method could still be used with their unit but it appears to be unecessary.
|M F Anderson|
I have attached the explanation from the Romac website.
|M F Anderson|
|The marks on the vernier sprocket are for camshaft degrees. 4d at the cam is 8d at the crank. I believe this is what the cam manufacturer is after.|
I have used one of these vernier sprockets, procured from Moss Motors several years ago. The Moss sprocket has only 5 keyways, adjustable from +4d to -4d in 2d increments. It wouldn't get to 6d when I needed it, because it didn't have enough extra keyways. Additionally, if you have an offset key for 1d change of timing you can fine tune these adjustments in 1d increments.
The Crane cam is ground 1d retarded and wants 5d advance for installation, requiring 6d change at the cam. I get there by skipping the chain over 1 tooth for 9d advance, then using a 3d offset key to bring it back to 6d. This can be done with a cheap offset key using the original sprocket.
To get 4d change with a standard sprocket would require a 4d offset key. This is why offset keys are available with 1d, 2d, 3d, 4d offsets. Skip the chain over 1 tooth for 9d and use a 4d key the other way around, and you get 5d net change. In this manner you can walk it around the clock in 1d increments.
If you know the final timing you want, then offset keys are cheap. If you want to fiddle and maybe change the cam timing a few times, then the vernier sprocket makes easy changes in 2d increments, but you still need an offset key to get smaller steps. If you need to make a lot of cam timing changes, the adjustable cam sprocket can change without dismounting the chain or sprockets, and can be set to any odd angle.
|Thanks guys. Its a little more demystified now....I no longer feel compelled to sacrifice a virgin prior to undertaking this part of the project.|
Bill, thanks for the support...I think you confused my question between spark advance (set by the distributor) and cam advance. The cam advance allows you to time the lift of the valves in conjunction with the max compression. Spark advance allows you to set the time of the spark in the compression cycle.
This thread was discussed between 25/08/2008 and 26/08/2008
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