Head shave timing retarding a myth?

[hinsonracing]

I can no longer find the articles, bit had read somewhere once upon a time that for every .010" shaved on our engines that it retards the cam timing 2* degrees. I am wondering if it was based upon math or realistic data. I am thinking it was only on math and is false and here's why:

First off please note I race Saturns on dirt tracks so this is not street oriented....

After learning this "information" on my latest engine build I have my head shaved .030" and the block was decked .030" to bring aftermarket oversized pistons back to zero deck "actually around .006" protruded. Knowing per the forum community this should DRASTICALLY retard my cam timing, I decided to degree my engine. I fabbed up two solid lifter buckets and set them at .003" lash (smallest feeler gauge in toolbox). When degreeing my engine measuring I got the following at the .050" spec ( .047" to compensate for the lash).

Intake
Opens -11* ATDC
Closes 25* ABDC
Centerline of 108.50*
Spec is 107.25*
So only 1.25* Retarded

Also on a side note when advancing engine timing it was found per factory tooth an average of 18* (+/-) was changed not 9.45* as previously read in prior materials

If that is the case, then all of Saturn "timing knowledge" is incorrect? I am looking for more evidence of the forum math here asap as now I am scratching my head and want to finish this motor up this week.

[SMGordon1259]

Does it look like this?


shaved head.jpg

[hinsonracing]

Does anybody have any input besides the smarta** with a photo of someone's bald head?

[Jon2001sc2]

I’ll start off by saying that I can’t ever remember seeing a specific amount of timing change per amount shaved off the block/head. While there is some change, the build you have descibed sounds a great deal like how we prepped out ITA engines. We would admit to shaving to service limits as allowed by the class and then covering up how far past that it had been taken, for a long time we snuck adjustable cam gears in there to tune it so we weren’t really concerned with what the timing change was on the stock gears. Also we never would correct or deny any stated timing change as it was used as a cover explaining the loping that came from running the custom cut Gude race cams. So the reality is that it was probably a bogus number that was thrown out there and had always been in the best interest of racers not to correct it to allow for covering up more creative rule interpretations.

[SMGordon1259]

old school way to adjust timing is to start at good value and then advance the timing by 1 or 2 degrees then do a test drive. If it starts pinging, back the timing off 1 or 2 degrees until the pinging stops. this should work for all overhead cams


sorry you can't take a joke.

[ProDarwin]

Also on a side note when advancing engine timing it was found per factory tooth an average of 18* (+/-) was changed not 9.45* as previously read in prior materials

I'm no engineer... but wouldn't this be straight math? ( 360 / # of teeth on gear) = change per tooth

360 is not evenly divisible by 9.45

[hinsonracing]

I’ll start off by saying that I can’t ever remember seeing a specific amount of timing change per amount shaved off the block/head. While there is some change, the build you have descibed sounds a great deal like how we prepped out ITA engines. We would admit to shaving to service limits as allowed by the class and then covering up how far past that it had been taken, for a long time we snuck adjustable cam gears in there to tune it so we weren’t really concerned with what the timing change was on the stock gears. Also we never would correct or deny any stated timing change as it was used as a cover explaining the loping that came from running the custom cut Gude race cams. So the reality is that it was probably a bogus number that was thrown out there and had always been in the best interest of racers not to correct it to allow for covering up more creative rule interpretations.

Probably the most honest answer I've gotten yet. I had tried on another forum but all I got that "it is hard mechanical facts that each tooth is a 9* change ... don't exceed .010 -.015 machining or you will have issues...the pcm can only compensate for a little bit". All I have been trying to find out is what I measured agreed upon to be correct math and if the 9* per tooth is as legit as many claim who/where was this proven other than putting a protractor on the sprocket or tell me to measure and times pi..... only reason I even degreed my engine was because of the supposed drastic timing change and was after every lit bit I can get because I'm limited on rules and having to chase v-tecs. Any additional and useful input would he welcomed and appreciated.

[hinsonracing]

So I arrived back home from work and headed straight to the shop, set the degree wheel and dial indicator back and remeasured my open close events for stock timing marks and for one tooth advanced. Here is the following at .050" lift.

Factory marks:

Intake opens -12* ATDC
Intake closes 25.5*

With factory specs being
Intake opens -7.5*
Intake closes 27.5*

Knowing cast cams aren't perfect castings when mass produced the differences I have measured are:
Opening 4.5* retarded
Closing 2.5* retarded
In summary of factory times with an engine with .060" removed from engine and block an average of 3* cam retardation is to be expected not 12*

Advanced 1 tooth at camshafts only:
Intake opens 7* BTDC
Intake closes 8* ABDC

Results with current machine work advancing 1 tooth at the camshaft are :
Opening 14.5* advanced
Closing 19.5* advanced
Average of the two being 16* advanced.

I have not yet measured the factory exhaust cam as at this point I have the opinion the the changes would be similar +/- casting flaws of mass production. I will measure on my own accord and if a large difference is found I will update.

Overall summary:
During measuring this evening I measured each event 4 consecutive times getting the same measurement each time. I made sure the dial indicator started on 0 completed on zero and recycled to ensure no discrepancies. I zeroed the degree wheel before starting and after ending measurements to ensure my wheel did not move/change. Overall I feel comfortable that I can conclude the 9.45* per cam tooth to be incorrect. I conclude in fact that using a dial indicator that is not super precise from coming from harbor freight (most probable reason for unmirrored results between open/closing) that in fact each cam tooth is approximately 19 to 17.5* to bring an average result of 18.25* of cam timing change per cam tooth.

[hinsonracing]

Okay now it gets weirder...... measuring the factory exhaust cam I get the following:

Exhaust cam set on factory timing marks with the above mentioned machine work:
Opens 21* BBDC
Closes 11* BTDC

Factory spec is
Opens 24.5* BBDC
Closes -12 ATDC

Results:
Opens 3.5* retarded
Closes 23* advanced
????????????

Here's where it gets crazy (I double checked my setup for zeroing out):

Set at one tooth advanced:
Opens 39* BBDC
Closes 31* BTDC

Results:
Opens 15.5* advanced
Closes 43* advanced (WTF inserted here)

Conclusion.... the opening events are consistent with the intake cam with machine work applied. HOWEVER, even after double checking and remeasuring, I am not sure how the closing events are so far off. I measured four times consecutively on factory and advanced teeth and got the same numbers. With no apparent wear, I am under the impression the maybe this cam was possibly a bad casting of what it was intended to be.

[ZombieSatty]

If I'm following correctly, I'm confused as well. Are you positive it's the right cam? 2 intake cams can fit in the head etc.

[hinsonracing]

If I'm following correctly, I'm confused as well. Are you positive it's the right cam? 2 intake cams can fit in the head etc.

I had the twin intake cam installed on the previous build for this head. For this engine I reinstalled the exhaust camshaft that was originally in this head from factory.

[ZombieSatty]

It's been a while, i can't remember, but are the cam gears the same between in/ex?

[hinsonracing]

It's been a while, i can't remember, but are the cam gears the same between in/ex?

Yes sprockets are identical. The pip on the cams is what makes the difference. For twin intake cam mod your new tdc mark is 180* from factory tdc mark then retarded 1 tooth.

[S.Bretz]

I did some math years back that said 1 degree of retard around a .020 deck of the head. I am not sure where you are getting 2 degrees at .010 decking.

My numbers were based on an old vernier caliper and measuring a 200k mile cam gear and guessing where the effective radius was at.

It nice to see someone take the time to mock the system up and do the proper measuring. Thank you for adding knowledge to the community.

[S.Bretz]

I'm no engineer... but wouldn't this be straight math? ( 360 / # of teeth on gear) = change per tooth

360 is not evenly divisible by 9.45

Both numbers are correct (assuming the math is correct on 360/ number of teeth).
~9 degrees in cam degrees, ~18 degrees when referring to the position of the crank. The crankshaft moves 720 degrees per every 360 degrees of the cam movement.

[S.Bretz]

old school way to adjust timing is to start at good value and then advance the timing by 1 or 2 degrees then do a test drive. If it starts pinging, back the timing off 1 or 2 degrees until the pinging stops. this should work for all overhead cams


sorry you can't take a joke.

Cam timing and ignition timing are two separate things. You are referring to the ignition time, the OP is referring to the cam timing.

[S.Bretz]

Now that I think about it, what Jay and I used to to before we had adjustable cam gears was to file down the alignment notch on the cam gear. 0.010 removed might equal to a change of 2 cam degrees when you remove that much material that close to the center of rotation... any change there would multiple the amount of change on the effective circumference of the cam gear.

When looking at the engine, filing the left side of the notch would retard the cam gear, filing the right would allow you to advance the cam gear, but there would also not be anything to hold the cam alignment post so the cam would have the possibility of slowing reverting back to the stock position if the cam gear would slip (not sure if it would actually slip once the bolt it tight but the possibility was there).

[siris]

If your basing all your info on the thread I made years ago about stock cams, I pulled all that information off of the saturnwiki. Which I think is defunct now? Maybe? It's been a while since I have been on the site.

[hinsonracing]

Both numbers are correct (assuming the math is correct on 360/ number of teeth).
~9 degrees in cam degrees, ~18 degrees when referring to the position of the crank. The crankshaft moves 720 degrees per every 360 degrees of the cam movement.

So even though I measure an average of 18* change by moving the cams a tooth and leaving the crank at factory timing mark, I'm still only moving 9* by the cam? If that's the case by degreeing cams would be half of what you measure....not sure if that's right. I'm not denying crank turns 2:1 to the cams, but that would bunk all knowledge of degreeing a cam wouldn't it?

[ProDarwin]

Both numbers are correct (assuming the math is correct on 360/ number of teeth).
~9 degrees in cam degrees, ~18 degrees when referring to the position of the crank. The crankshaft moves 720 degrees per every 360 degrees of the cam movement.

I'm aware of that, but if you move the cam by 1 tooth, the crankshaft has no bearing on it. If you move the cam by 1 tooth it should advance or retard by (360/#of teeth).