Bonneville America Forums Forums Technical Forums Performance Mods Fine tuning the cam timing

Has anyone "degreed in" his cams yet? I've not seen it mentioned, but mass production being what it is (inacurate), degreeing in even a stock cam can sometimes have a very beneficial result.

Since the intake and exhaust cams are separate, even greater flexibility can be had over the common old single cam design.

Slightly retarded cam timing helps keep smog levels down, but really kills bottom end power. (Vehicle makers usually retard cams in production.)

Slightly advanced cam timing helps bottom end and midrange power, but hurts smog numbers.

Tighter lobe spread hurts idle a little (lumpy), and drops the very top end power, but is great for bottom end and midrange performance.

One way to set up these cams would be to get the factory spec for lobe spread (also called "lobe center"), and if it seems a little wide, narrow it up some (I'd go for a 106* spread), and then advance both cams so that you get a 4* advance from split overlap. Not all performance cam grinders like to degree in cams using the split overlap method, but ol' Isky "hisself" has always done so. On many (car) engines, this way can be done without a dial indicator rigged onto a valve lifter or rocker arm or valve spring retainer.

Oh, cam timing changes cam be made a few different ways, the most popular being to use a special offset woodruff key. (I made one once using a hand file on a thicker-than-stock key, when I advanced the cam on my wife's old 2.2 L Dodge 4 cyl.)

If these Hinckley twins don't have a keyed-on gear somewhere in the drive mechansism, another method will need to be used.

You don't have to "trust me" that proper cam timing is important to have a good running engine - but it's true none the less.

Hmmm, maybe I should be posting this on jojje's forum.....

You should PapaDean, there actually is a forum post on Jojje's forums that talks about degreeing the cams. I know it was a big deal back when my VW Super Beetle was still on the street. Anyways, Jojje's forums have some good stuff there!

Some discussions on possible cam timing senario's.
cam timing

Cam timing changes should be small, as only a couple degrees can make dramatic performance changes.

My experience with race cars and some HP street cars, has been that the range from 4 degrees advance (over stock) to 2 degrees retarded is about all that should be done.

Overly advanced cam timing kills both top end power and gas mileage (per Competition Cams tech man to me years ago). Retarded cam timing really hurts bottom end power and throttle response.

It's important to know the stock cam timing specs so a considered decision can be made on changes.

When cam timing is checked, those results should be compared to the factory specs, and if the cams are retarded from that stock spec, better overall performance can almost always be had by bringing the specs up to stock. Advancing from there might bring even better performance, but will probably take the engine out of smog spec.

Remember that all of these changes are more complex with a twin cam engine than it would be with something like an old 350 Chevy/360 Mopar/351 Ford single cam engine.

To check stock cam specs, one will need to attach a degree wheel to the crankshaft end, install a pointer for that degree wheel (and justify top dead center), get a dial indicator mounted to contact a valve spring retainer, and then record readings at whatever lift the factory uses for its specs - maybe .020" of lift, or possibly the race industry standard of .050" lift, the so-called 'duration @ .500'.

Yes, the task is somewhat complicated, but the results are worth the hard work is takes both to learn how to do it, and then to actually do it.

No, not another "lecture", just a quick note.

Maybe I should have stated that all this timing adjusting, or "cam degreeing", is really just getting the phasing of the valves' opening and closing to the piston location going up and down the cylinder.

The phasing affects how the cylinder is filled with mixture (and how it gets rid of the burned gasses), especially at lower rpms.

I hope the blokes from Thunderbike are reading this. This is a good reason to depart from the original design and key the timing gears on their cams. Although it would be a bit of a trick since the gears are in the middle of the cams and would require a good size hub.

How right you are, Ed. Some sort of adjustability needs to be available to properly phase the cam to the piston position. As little as 2 degrees off can make a serious difference.

On mild cams, such as typical production grinds, this is not so important, but with longer duration specialty cams, it is very important to get the cam timing "just right".

Just bolting in a new cam and hoping that everything is OK, doesn't get it.

What to do if your cam timing is slightly off and you can't make a change to your cam position:

If your cam is slightly retarded, run your valve lash at the maximum amount suggested by your manufacturer.

If your cam is slightly advanced, run your lash at the minimum amount....blah, blah, blah.

Oh, and after you break in your new set up, recheck your valve lash, as wearing in might change it a little bit.

Nice thing about this new Triumph twin valve train design, is that it seems very stable and doing adjustments is not very common.

I wonder if it is possible to make slight cam timing adjustments by winding the anti-backlash gears the other direction?