Relationship between Cam Timing and Ignition Timing

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RRoller123
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Re: Relationship between Cam Timing and Ignition Timing

Postby RRoller123 » Sat Sep 17, 2016 7:52 pm

Thanks, that is what I have been looking for.
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Re: Relationship between Cam Timing and Ignition Timing

Postby v6spider » Thu Sep 29, 2016 1:48 pm

My experience with this is it depends on where the car is going to be driven. Track or street. Large lift cams with heavy overlap seem to do best set at 0° in my v6.. although the engine builds way more power if the ignition timing is advanced say 10-12° btdc. However, low rpm drivability suffers from bucking and jerking on deceleration, coasting, and at take off from a dead stop. I usually adjust timing for the street by listening to the engine idle and adjusting it to the smoothest point of idle while at the same time maintaining as much advance as possible. That way drivability is better for street..

Rob
Last edited by v6spider on Thu Sep 29, 2016 11:32 pm, edited 2 times in total.

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RRoller123
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Re: Relationship between Cam Timing and Ignition Timing

Postby RRoller123 » Thu Sep 29, 2016 5:55 pm

Thanks, that is helpful. I used to time my X19s by vacuum, highest steady reading, worked like a charm.
'80 FI Spider 2000 (rolling "resto")
'74 X1/9 (many years ago)
'79 X1/9 (fewer years ago)
'86 Jaguar XJ6 (2nd Hobby Car)
2011 Chevy Malibu (daily driver)
2010 Chevy Silverado 2500HD Ext Cab 4WD/SB
2015 Keystone Montana High Country 343RL

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v6spider
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Re: Relationship between Cam Timing and Ignition Timing

Postby v6spider » Thu Sep 29, 2016 11:29 pm

RRoller123 wrote:Thanks, that is helpful. I used to time my X19s by vacuum, highest steady reading, worked like a charm.

Yup that works well when you have stock cams. With a cam that has lots of overlap you have to play around with it to find something you are willing live with on the street.. the method I described is something I arrived at while trying to smooth out the bucking was getting at low speeds..my first inclination was to advance the ignition timing. That actually produced more hp and torque but increased the bucking. Retarding the timing down to where the idle smoothest gave me something livable to start with. I had to bump it up slightly because it was popping up through the carb on startup every so often its perfect now.

Rob

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Re: Relationship between Cam Timing and Ignition Timing

Postby RRoller123 » Fri Sep 30, 2016 5:45 pm

Yep, That is what I found as well. Slightly advanced over the point of best smoothness at idle seemed best. Worked for years, stock X19 cam.
'80 FI Spider 2000 (rolling "resto")
'74 X1/9 (many years ago)
'79 X1/9 (fewer years ago)
'86 Jaguar XJ6 (2nd Hobby Car)
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2010 Chevy Silverado 2500HD Ext Cab 4WD/SB
2015 Keystone Montana High Country 343RL

PatTroy
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Re: Relationship between Cam Timing and Ignition Timing

Postby PatTroy » Fri Feb 10, 2017 9:59 pm

Lot of generalizations about burn rate on this post. I can safely say there are factors that I didn't catch in my scan that affect burn rate (and therefore the correct spark timing for optimum torque). Also, the description of in cylinder pressures only really tells part of the story when relating to engine output.

To speak about the OP's question, the overlap between cams affects the blow through or residual trapping behaviour and I think this will have the biggest effect on burn rate (and therefore the position of the optimum spark). Someone mentioned that there are more DOF in a twin cam setup than a single cam and that's all true. Being stuck with one cam is a burden.

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v6spider
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Re: Relationship between Cam Timing and Ignition Timing

Postby v6spider » Wed Apr 12, 2017 11:41 am

PatTroy wrote:.... Being stuck with one cam is a burden.

How is a single cam a burden? There are certain advantages to a single cam pushrod engine. One of which is the ability to use roller lifters that allow for steeper ramps on the cam profile, higher lift figures, and more importantly less friction. Dual overhead cam setups offer none of that.

Rob
Last edited by v6spider on Wed Apr 12, 2017 8:37 pm, edited 1 time in total.

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Re: Relationship between Cam Timing and Ignition Timing

Postby So Cal Mark » Wed Apr 12, 2017 2:31 pm

with a single cam you're limited to the specs the cam was ground to.
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v6spider
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Re: Relationship between Cam Timing and Ignition Timing

Postby v6spider » Wed Apr 12, 2017 2:52 pm

So Cal Mark wrote:with a single cam you're limited to the specs the cam was ground to.

Only lobe separation angle.. you can still advance or retard the cam timing.. it's a 50 to 75hp gain just by going with a roller cam setup and true roller rockers vs flat tappet setup. That's not even including the performance grind. Also you can increase the lift ratio by changing the rocker arms to ones with a higher ratio. Aside benefit of this is it also increases duration as well.

Rob


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