offset key

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bobc
Posts: 37
Joined: February 9th, 2008, 10:50 am

offset key

Post by bobc »

Which degree of offset key is best? How do you check that you are correct after putting the motor together?
Dietmar
Site Admin
Posts: 650
Joined: June 24th, 2006, 11:56 am

Re: offset key

Post by Dietmar »

bobc:

Can't be specific as to which is the best as : "one size does not fit all". The key is used to retard the cam timing.

Depending on your application, a 3 degree key works well and is probably most popular, but here again, it depends on the application.

To see if it is correct, you need to measure the cam timing by inserting a pushrod into the #1 intake hole, have a degree wheel installed with a known TDC ( and be sure of this location as some degree wheels are off a bit). Afix a dial indicator to the pushrod, rotate the engine pulley and see where the intake closes at .040" of lift. Stock is 35 degrees, 30 minutes so add the 3 degrees and you should get 38'30" . Word of warning- a 3 degree key does NOT always give you three degrees retard ( and be sure to install it correctly) as the cam itself might be off, so be sure to measure - called trial and error.
If you have more questions, feel free to e-mail or call.

Hope this helps
Dietmar
http://www.quixoteracing.com
SR Racing
Posts: 1205
Joined: June 24th, 2006, 1:58 pm

Re: offset key

Post by SR Racing »

How do you check that you are correct after putting the motor together?
Unless someone has used a cam doctor on the cam and gear, don't even bother with the degree wheel pulleys. They do not have enough resolution and and can be inaccurate. You need a degree wheel that is at least 12 or more inches in diameter. (ours is 18") This is bolted to the crank and then the cam is timed to the crank. And hopefully you do this before the engine is fully assembled. :)
brian
Posts: 1348
Joined: June 26th, 2006, 12:31 pm

Re: offset key

Post by brian »

As you can tell, degreeing a cam is no easy task and, as it's been said, just because you are using a known offset key, it may not result in achieving the exact timing change you expect. Manufacturing variances have a way of stacking up and changing the actual results. Here's a link with a detailed description on cam timing:

http://www.stockcarracing.com/techartic ... index.html

The 3 degrees is the most common retard run in vees but may not be the most advantagous number for your particular motor. Head design, internal manufacturing variances, camshaft accuracy, exhaust system and last but not least, the length of tracks you plan to run will effect your choice of cam timing. Simply said, the more retard you run, the higher the power band will peak, but a the cost of low end power. If you are going to run Road America with long straightaways and high speeds, you may want to run more retard. If you are going to run a shorter, tighter track, less retard will be needed. I try to customize cam timing to the customer's needs.

The GCR was changed a few years ago to allow adjustable cams. While the market doesn't really support our needs, SR may have them. The advantage of the adjustable cam is that you can go through the oil pump opening and change the timing slightly without splitting the cases.
The above post is for reference only and your results may vary. This post is not intended to reflect the views or opinions of SCCA and should not be considered an analysis or opinion of the rules written in the GCR.
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