Cam timing

Reggie · Apr 16, 2010

I've just been checking the valve timing on my new PW3 cam.
To get the 0.145' lift on standard settings, my crankshgaft is 2* ATDC. Obviously if I advance the cam 5*, I achieve 0.145' lift at 3* BTDC. The reality is it probably will not matter which setting I use as I am not racing. Also by advancing the cam I am more likely to have valve to piston contact (haven't got that far yet) I think.

I understood that advancing a cam gave power delivery lower rather than higher in the rev range, but I may have just had a bad dream as I can't remember where I read that?

So does anybody have an educated opinion as to which setting is likely to be beneficial?

comnoz · Apr 16, 2010

For street use I would advance it 5 from where your at. It will run better everywhere except 6000 up. Jim

Reggie · Apr 16, 2010

Jim wrote

For street use I would advance it 5 from where your at. It will run better everywhere except 6000 up. Jim

Thanks for that advice Jim. I am also fitting a Steve Maney stage 2 big valve head (3mm oversize inlet valves & 3mm oversize exhaust valves ) and 34mm carbs. Although as I said I am not racing, with these parts on my engine, is the advice still the same?

comnoz · Apr 16, 2010

Yes, unless you are willing to give up a bit more midrange for a bit more on top. The difference will be small. Jim

Reggie · Apr 16, 2010

Thanks for the advice Jim.

Reggie

bpatton · Apr 16, 2010

Reggie said:
I understood that advancing a cam gave power delivery lower rather than higher in the rev range, but I may have just had a bad dream as I can't remember where I read that?

I had that same bad dream. In mine someone said that the single most influential event as far as cam timing goes is intake valve closing. High rpm likes it later, lower rpm develops more torque with earlier closing relative to piston position.

swooshdave · Apr 16, 2010

bpatton said:
Reggie said:

I understood that advancing a cam gave power delivery lower rather than higher in the rev range, but I may have just had a bad dream as I can't remember where I read that?

Click to expand...

I had that same bad dream. In mine someone said that the single most influential event as far as cam timing goes is intake valve closing. High rpm likes it later, lower rpm develops more torque with earlier closing relative to piston position.

Which is (surprisingly not involving a hijack, for once) why modern engines employ variable valve timing.

rustynuts · Apr 16, 2010

swooshdave said:
Which is (surprisingly not involving a hijack, for once) why modern engines employ variable valve timing.

With that sloppy old timing chain, Commandos have variable valve timing too!

MexicoMike · Apr 16, 2010

Yep, new vehicles have variable cam timing and variable intake volume as well. The things that make for good top end power (big valves/lots of cam lift/overlap) kills low/midrange power. In the old days before variable cam/intake/computer controlled ign timing- you set up the engine for what you wanted - good low/mid or good top end. The only way to get both at the same time was to increase displacement or compression. But compression alone is not worth all that much - about 3% Increase per full point increase. But increasing the compression was a way to help keep some low end while increasing the top end.

Nowadays, your new vehicle can have both...but our Norton Commandos cannot!

RoadScholar · Apr 16, 2010

MexicoMike said:
Nowadays, your new vehicle can have both...but our Norton Commandos cannot!

I would be surprised if Jim Comstock isn't fiddling around with variable cam timing; most have seen his beautifully machined geared cam drive, how much trouble would it to plumb oil pressure to one of those gears? Volvo, amongst others, has been doing this for years and the actuators, located inside the cam drive pulleys are very small and quite simple.

RS

comnoz · Apr 16, 2010

It's been on my mind for some time now. Jim

bpatton · Apr 16, 2010

A nice tricloidal oilpump running off the timing chest would open up a lot of possibilities. Including hydraulic valve adjusters ala Porsche, pressurized cam bearings, and a cure male pattern baldness.

swooshdave · Apr 16, 2010

comnoz said:
It's been on my mind for some time now. Jim

Don't give him any ideas, aren't we still waiting for his gear drive? :mrgreen:

comnoz · Apr 16, 2010

The gear drive is hopefully in it's last design. There ended up being so many changes to make it happy on a street bike and still be easy to install that I felt I needed to hold on to it until it has been through a bunch of miles and a variety of applications. As you know a failure in the cam drive can be catastophic. Jim

swooshdave · Apr 16, 2010

comnoz said:
The gear drive is hopefully in it's last design. There ended up being so many changes to make it happy on a street bike and still be easy to install that I felt I needed to hold on to it until it has been through a bunch of miles and a variety of applications. As you know a failure in the cam drive can be catastophic. Jim

Jim, that poor motor has been around for 60+ years, if you need a few more to get it right, take it. :mrgreen:

comnoz · Apr 16, 2010

Yeah, pretty wild that 60 years later there are still people crazy enough to be doing developement on them. Jim

Keith1069 · Apr 19, 2010

Have 600 miles on my PW3 now and had to advance 5 deg to achieve the TDC lift, it came out at .152" and the torque (for a 750 with big ports and 32mm carbs) is good compared to the std cam. No signs of valve-piston conflict.

Cam timing

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