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 Posted: 05-31-2008 09:56 pm
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Tim Knowlton
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can anyone share with me their experience with changing the timing of the cam gears. What is the advantages of timing at  intake 110 exhaust 110 or intake 115 exhaust 115. What is the mager difference and what can i expect if with the different settings? I have a 5 speed all stock except the dellorto 40's.  thanks for the feed back.

Tim knowlton

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 Posted: 06-01-2008 04:54 am
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Jensen Healey
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I have never tried it but the 115/115 timing should move your torque and HP peaks to a lower rpm. What is your goal?

Kurt

http://gglotus.org/ggtech/907camdial/907dialcam.htm

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 Posted: 06-02-2008 08:02 pm
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Judson Manning
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The Elgin article is quite good and does go into some depth regarding practical limitations.  Paragraph 6 under the section Camshafts - "Part 2 choices" is of great interest to a 'stock' JH engine as the example is for a 8.1 c/r where the JH c/r is 8.4.

If we use the 110 timing the stock "C" camshaft with 272^ duration, the intake valve closes at 66^ABDC.  With the 115 timing we're very close to that magic 75^ number the article mentions = very poor low end performance.

As we look at the 907/910/912/920 progression from 1970-1995 we see ever decreasing values of duration (272^ to 252^) and MOP timing (115^ to 104^) with increases in c/r (8.4 to 10.9).  One could only hope the 1995 variant (920) performed better than the original!

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 Posted: 06-03-2008 01:14 am
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Tim Knowlton
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Judson,

Thanks for the reply. So please give me in laymens terms what the 110 timing does for me as apposed to the 115.

Thanks

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 Posted: 06-03-2008 02:49 pm
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Judson Manning
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Tim,

One issue we didn't discuss was the total valve overlap.  You get more overlap when using the 110/110 timing which should equate to more top-end power at the expense of low-end torque.

However, switching to 115/115 forces the exhaust to open earlier and the intake to close later.  Both of these conspire to reduce low-end torque as well.  Kind of a damned if you do, damned if you don't delima.  The 110/110 is really the lesser of the two evils.

We could discuss theory until the cows came home or just try it and see.  With stock pistons I honestly don't think you'll see much difference either way.  Without a dynamometer, g-meter, etc. it would be difficult to quantify any real changes.  If it ain't broke, don't fix it!

Next time you shim your valves, my advice would be to install a 107 cam from the club store on the exhaust side and run 110/110 timing.  If budget permits, buy a second 107 for the intake side.  Drivability and low-end performance will increase with each upgrade.

If you ever do replace the pistons as part of a full rebuild, also invest in a set of later Lotus HTD sprockets with 104/104 timing (i.e. 'green dot' version).  The higher c/r pistons will be able to take advantage of the increased overlap for a boost in top-end performance. 

Judson

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