Timing Curve (two step vs straight)

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Timing Curve (two step vs straight)

Post  eoverhage on December 31st 2013, 7:48 pm

Trying to get the best curve for my engine delivering power and fuel efficiency.
Engine is a 429 bored 30 over, pistons are 9.1 to 1, comp cam XE274H, D0VE C heads with mild porting, Scorpion Roller Rockers, Mallory Unilite (YH adv) dizzy (does have VA but that is a separate issue to address), RPM intake, Holley 750 Street Avenger, C6 with 2600 stall, 3.00 locker 9", car weighs 4100lbs.
Timing is currently set at 12 degrees and does not reach full advance of 36 degrees until 3400 rpm (VA disconnected of course)
Engine is running about 2600 rpm at 60 mph so not reaching full potential.

Want to re-curve dizzy to achieve full advance at around 2600 rpm but not sure if best to go with two step or straight to achieve best performance.
For a performance street car, which config is best

Thanks

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Re: Timing Curve (two step vs straight)

Post  LivermoreDave on January 1st 2014, 4:59 pm

With the information you provide, both mechanical and vacuum advance options should benefit your application. The Mallory unit is supplied with an advance key for adjustment. I'm not sure if the distributor's vacuum canister has an adjustment inside the vacuum hose port. If so more adjustment available there. It will be a trial and error fix, no standard procedure, tune it until the application likes it!

Dave.

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Re: Timing Curve (two step vs straight)

Post  LivermoreDave on January 3rd 2014, 7:51 am

Advance Curve Change Kit. . . . . Part No. 29014
Required to change the amount and rate of
advance within a range of 14° to 28°. Includes
degree keys, springs, and instructions to plot
advance curves.

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Re: Timing Curve (two step vs straight)

Post  eoverhage on January 3rd 2014, 10:11 am

Thanks Dave,
I do already have the kit and did limit the dizzy to 22 degrees as the engine likes 12 degrees initial and wanted to cap the total advance to 34.

I have also used two different step curves but not completely happy with performance. Even using the lightest (pink spring) I still did not have full advance until 2900 rpm and cruising rpm is still considerably lower than that. I know the two steps on most cars is to limit advance in mid range RPM to avoid detonation but not sure if it is needed for my set up.
Was planning on installing springs to get straight curve this weekend and see how it performs as it looks to be the only way I can get to full advance at the 2500 or so RPM level.

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Re: Timing Curve (two step vs straight)

Post  LivermoreDave on January 3rd 2014, 11:10 am

Is it possible to use one spring instead of two?

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Re: Timing Curve (two step vs straight)

Post  eoverhage on January 3rd 2014, 11:41 am

You could but not recommended.

To create a two step curve, you must use one specific heavier spring and the initial rate of advance controlled by the lighter spring.
This allows a quick ramp up of say 14 degrees of advance starting around 1000 rpm and holds that level until about 2000, the remaining 8 degrees of advance come in more gradually from 2100 through 3000.

To achieve a straight curve, there are paired spring recommendations that achieve on paper achieve my top line goal.
You do not have that hold on advance and the curve is a continuous ramp up of advance pretty relational to increasing RPM.

Guess I will just try the single over the weekend and see if I am getting any detonation and how the manners are from stoplights, etc...

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