250 Gte Valve timing

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chris
Posts: 219
Joined: Sat Mar 02, 2013 7:52 am

250 Gte Valve timing

Post by chris »

Dear All, dear Tom,

I have to set the valve timing on my Gte. I know that
the factory marks are only of use for pre-mounting
and that the right timing has to be checked with
a degree wheel and pointer using opening times.
Thats clear.

I do have mounted both camshafts with the according marks aligned. For the right head the marks align and number 6 cam lobes are on the up at same height.

For the left side there is a difference between the aligned marks and the position of the cam when the cylinder number 12 cam lobes are on the up with both cams at same height. The difference is about
15 degrees.

I am just wondering what is the better position to start with before doing the exact setting of valve timing? The position with aligned marks or the one with both lobes at the same height?

What do you normally do when remounting an engine for pre-assembly - start with the factory marks or cam lobe position (up and equal)?

Thanks a lot
Christopher
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tyang
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Re: 250 Gte Valve timing

Post by tyang »

Hi Christopher,

I usually start with the factory marks as a starting point. Be very careful! If you feel something interfering, stop and check.

Jumping 7 teeth on the chain sprocket moves the cam by 1 degree. Which way? I always have to do it to remember which direction the cam turns.

Tom
'63 330 America #5053
chris
Posts: 219
Joined: Sat Mar 02, 2013 7:52 am

Re: 250 Gte Valve timing

Post by chris »

Hi Tom,

Thanks for your answer and support. Appreciated!

BR
Christopher
DWR46
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Re: 250 Gte Valve timing

Post by DWR46 »

Tom: I apologize for being "picky", but the actual minimum amount by which you can change the cam timing is 2 degrees on the camshaft which translates to 4 degrees on the crank. For every 7 teeth you move the cam chain sprocket, the flywheel moves 148 degrees 10 minutes, and for every bolt hole you move the cam sprocket, the flywheel moves 144 degrees, therefore the smallest amount you can move the cam timing is 4 degrees 10 minutes of crankshaft rotation. so any time you can get the cams to be within plus or minus 2 crankshaft degrees of the "theoretical" specs, you are as good as you can get. Also due to errors in the cam grinding equipment of the era, and 50 years of normal wear on the cam lobes, you will never get all the opening and closing numbers to match the cam specs. That is why we generally set up the cams on "centerlines" ( the center of the opening and closing of the Intake Cam).
chris
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Joined: Sat Mar 02, 2013 7:52 am

Re: 250 Gte Valve timing

Post by chris »

Hello Dyke,

thanks for your very detailed input. If you speak of centerline of inlet, would that mean you dial in the cams to 19 degree after TDC for the inlet in regard to the tip of the lobe of the inlet valve (inlet opens 27 before, closes 65 after per handbook/ 27+65=92/2=46-27=19). If yes I guess you measure the tip with a dial on top of the inlet?

Thanks
Christopher
DWR46
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Re: 250 Gte Valve timing

Post by DWR46 »

Chris: Your cams have a 109 degree centerline. 27 BTDC, 180 as the piston goes down, and 65 as it starts back up. Total duration is 272 degrees, Divide by 2 to get 136, and subtract the 27 BTDC, gets you a centerline (max lift) at 109 degrees ATDC. Set the flywheel at 109 degrees ATDC and move your cam to max lift at that point.
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tyang
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Re: 250 Gte Valve timing

Post by tyang »

DWR46 wrote: Sat Apr 25, 2020 10:03 am Tom: I apologize for being "picky", but the actual minimum amount by which you can change the cam timing is 2 degrees on the camshaft which translates to 4 degrees on the crank. For every 7 teeth you move the cam chain sprocket, the flywheel moves 148 degrees 10 minutes, and for every bolt hole you move the cam sprocket, the flywheel moves 144 degrees, therefore the smallest amount you can move the cam timing is 4 degrees 10 minutes of crankshaft rotation. so any time you can get the cams to be within plus or minus 2 crankshaft degrees of the "theoretical" specs, you are as good as you can get. Also due to errors in the cam grinding equipment of the era, and 50 years of normal wear on the cam lobes, you will never get all the opening and closing numbers to match the cam specs. That is why we generally set up the cams on "centerlines" ( the center of the opening and closing of the Intake Cam).
Hi Dyke,

I have to admit, my training is from Francois that never did the math on the procedure. Thanks for explaining the math for more accuracy!

Tom
'63 330 America #5053
chris
Posts: 219
Joined: Sat Mar 02, 2013 7:52 am

Re: 250 Gte Valve timing

Post by chris »

Hi Tom,

absolutely right. I left out the 180 degree in the calculation. Autsch. Thanks for correcting me.
Absolutely clear.

Christopher
chris
Posts: 219
Joined: Sat Mar 02, 2013 7:52 am

Re: 250 Gte Valve timing

Post by chris »

Just realised what confused me. English Handbook mentions inlet Cam times as following:
Opening start 27 before T.D.C
Closing end 65 after T.D.C
But closing is after bottom dead center.
I guess the translation used T.D.C for every punto morto instead of differentiating between top and bottom D.C.

No excuse for me not directly realising it in above calculation.

Thanks
Chris
chris
Posts: 219
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Re: 250 Gte Valve timing

Post by chris »

Here the handbook details I relate too above
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