Cam sprockets revised with pictures
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Author:  fainya [ Tue Jul 07, 2015 3:07 pm ]
Post subject:  Cam sprockets revised with pictures

I need to modify the intake cam sprocket for my DOHC engine. I want 4 degrees advance on the intake valves.
My question is are the SOHC 16 valve and the DOHC 16 valve use the same cam sprocket?
The DOHC cam gears are round cut, the SOHC cam gear is square cut.
So I did this mod to my DOHC intake cam gear.
The rotory table allowed me to use the factory cuts as Zero for the new positions that will be at 4 and 6 degrees advanced.
On the cam gear it will actually be 2 degrees and 3 degrees.
I had to order a 5 mm carbide end mill to do the new cuts.
3631 shows centering the cam gear on the rotory table then centering the rotory table to the spindle.

IMG_3631.JPG [ 589.58 KIB | Viewed 1820 times ]

3635 is showing the the alignment tube that helped center the gear. The tube is in a collet in the rotory table.
I used a tube instead of a solid piece. The tube is a sacrificial part that will be cut into when making the new slots.

IMG_3635.JPG [ 499.62 KIB | Viewed 1820 times ]

3638 shows the 4 degree slot cut. The rotory table makes this easy. Once the original slot cuts are zeroed,
I can rotate the rotory table by degrees, minutes, and seconds to the new slot location with the proper 2 degree off set,
lock the table down and make the cut.

IMG_3638.JPG [ 463.14 KIB | Viewed 1820 times ]

3639 is showing the cam gear cut for both 4 and 6 degrees. the shiney spots on the gear are where I stamped the cam
gear with numbers of degrees advance.

IMG_3639.JPG [ 426.54 KIB | Viewed 1820 times ]

A quick road test with the intake gear set at 4 degrees shows signs of improvement in the lower RPM ranges.
Why pay $100 for something I can make in less then 30 minutes?

I would like to do a set at 8 and 10 degrees, but I need another set of DOHC cam gears.
If you have an extra set that you are willing to part with, I will cut you a set of cam gears with 2 offsets of
your choice for free in exchange for the DOHC gears I need.

Author:  TheINCRide [ Tue Jul 07, 2015 3:12 pm ]
Post subject:  Re: Cam sprockets

Can't say for certain but I do seem to recall that the SOHC timing belt has square teeth versus the rounded ones on a GT..

Author:  fainya [ Tue Jul 07, 2015 3:21 pm ]
Post subject:  Re: Cam sprockets

Ok I will pull the DOHC intake sprocket and modify it for the 4 degrees I need.
I will do some pictures of the mod.

Author:  t3 ragtop [ Tue Jul 07, 2015 3:58 pm ]
Post subject:  Re: Cam sprockets

you can pick up a set of adjustable cam gears on ebay for $100.

in red or blue anodizing. :wink: ... rs&_sop=15

Author:  suprf1y [ Tue Jul 07, 2015 7:51 pm ]
Post subject:  Re: Cam sprockets

Why do you want to do that?

Do you know where they are phased in the stock position? Do some research. All the info is here and it will save you some time.

Author:  suprf1y [ Wed Jul 08, 2015 7:33 am ]
Post subject:  Re: Cam sprockets revised with pictures

I guess you didn't bother to read up on it before you did it.

The exh setting is 4 degrees advanced compared to the intake setting (so you didn't have to do anything to get your +4) but keep in mind that the intake centerline is 106 degrees. 102 is not typically where you want to set a cam with 192 degrees duration at .050"
The SOHC 16V sprocket is the same.

How many actual degrees did you machine the sprocket off the intake setting to get +4?

Author:  fainya [ Wed Jul 08, 2015 11:54 pm ]
Post subject:  Re: Cam sprockets revised with pictures

Being that 4 degrees is crankshaft timing, I only moved the new cut 2 degrees counterclockwise (looking at the gear on the engine)on the intake cam gear. This should delay the intake valves opening. 2nd cut was done at 3 degrees on the cam gear.
When I degreed my stock cams I had 4 degrees retarded on the intake and 4 degrees advanced on the exhaust.
The way I learned this is delaying the valves opening is advanced. It is just the opposite of distributor timing.
I went to the crane school for 2 weeks along time ago. I still have all of the books from that school.
So I modified the intake cam gear to give me the 4 degrees back. I don't have a dyno to tune with, but my butt meter
says I have lowered the torque curve. The exhaust sound crisp now.
In the old days they included cam shaft degree numbers for both the intake and the exhaust.
As of 98 the geo manuals got really convoluted with a lot of simple stuff just not included in the manuals.
For example the manual says to torque engine pan bolts to 97 inch pounds. It should read 9.7 inch pounds.
I have had to remove quite a few broken 5 mm bolts from engines and transmission.
So when I want cam degree numbers I get out my degree wheel, a dial indicator and base and plot the numbers.
At least this give me a starting point. And yes I agree with 0.050" lift.
For finding TDC I also use the 0.050" method by moving the piston down 0.050" BTDC then turn the crank till the indicator
read 0.050" PTDC. I split the difference in degrees and that to me is TDC.

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