Timing to get crisper throttle rev off idle? - Team Camaro Tech
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post #1 of 9 (permalink) Old Apr 16th, 08, 02:54 PM Thread Starter
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Timing to get crisper throttle rev off idle?

Seems to be lots of engine timing questions going around lately, so here's one more.

When the engine is idling, if I stab the throttle, it revs pretty good, but I'm wondering what effect timing will have on revving the engine. Will lots of timing give a crisper rev?

I have Accel DFI, with complete computer control over the timing. There is absolutely no hesitation if I punch the pedal while driving, so I'm not trying to cure any real problem, just trying to maybe get a crisper rev. I run 18* at idle currently.

thanks

Dave F. in Rhode Island
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post #2 of 9 (permalink) Old Apr 16th, 08, 03:07 PM
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Re: Timing to get crisper throttle rev off idle?

The word "timing" is so fitting because having the right amount of timing for your engine combo will result in the best performance. If you add too much advance the spark will ignite the fuel before the piston is in the right place for it's next trip down the cyl bore and not enough advance and the piston will have already started down the bore.

Every engine combo is going to like a slightly different advance curve. Testing at the track or on a dyno is the only real way to dial a combo in.

...Dennis

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post #3 of 9 (permalink) Old Apr 16th, 08, 04:33 PM
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Re: Timing to get crisper throttle rev off idle?

Dont change the timing if the top end is ok
Often a slightly steeper curve helps the bottom end
try lighter springs on the counter weights
But not too light..

My Spelling is not incorrect...it is creative

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post #4 of 9 (permalink) Old Apr 16th, 08, 05:52 PM
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Re: Timing to get crisper throttle rev off idle?

I posted this little spreadsheet a weekor so ago...
It may help.

I don't think 18 degrees is enough at idle, particularly with the control you have with efi.

You can set your own varibles in the spreadsheet, including the rpm and MAP scales to match what's on your ECU, the left box in the initial timing line, and the individual mechanical and vacuum (rpm and load) advance curve.

What is in there now is a sample. Change any number in a bordered box and it will recalculate the entire table.

Interestingly, while my carbed engine ran exactly as the sample spreadsheet is, my efi motor would not even run with those numbers! Spitting and popping all over, and my mixtures went dead rich everywhere.

Here's the calculator: https://www.camaros.net/forums/showthread.php?t=131528





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post #5 of 9 (permalink) Old Apr 16th, 08, 08:55 PM
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Re: Timing to get crisper throttle rev off idle?

You're wanting a free revving roller motor with a loose converter. I know the first time I heard a roller motor I made up my mind I needed that. Not wanted, but needed. Now I got some.

What converter and accessories do you have? Just for fun take all your belts off and start the car and rev it. Best have a bungee cord to catch your jaw when it drops.

You won't hurt it to run without the water pump a few minutes as long as there's water in it. I leave my pump off until I get over 160.

Not debating anyones contributions on timing, just saying latent load plays a huge part in an engines free/sluggish characteristics.

My engines think 36ish at idle is pretty neat.

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post #6 of 9 (permalink) Old Apr 17th, 08, 06:25 AM Thread Starter
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Re: Timing to get crisper throttle rev off idle?

JimM, nice spreadsheet. My timing looks pretty similar, except I have only half that amount at idle. I will try as you suggest, with 36 degrees at idle. It's nice being able to tweak the timing on a laptop!

thanks all

-

Dave F. in Rhode Island
'68 Camaro SS Accel DFI 454 (SOLD - I'm a boring Corvette owner now)
Bought my first big-block Chevy in 1970
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post #7 of 9 (permalink) Old Apr 17th, 08, 07:38 AM
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Re: Timing to get crisper throttle rev off idle?

Dave, don't take my "sample input" on the spreadsheet as "gospel"

I'm still very much kneedeep in tuning this EFI, and not at all happy with it right now.

My EFI engine flat out would not run with those numbers. I'm currently running with 27 degrees at idle, and that seems to work pretty well, tho I think these should be more than 10" of vac at idle with the cam I have...

Last night I ran new numbers in the spreadsheet, pulling the initial back to 14, pulling the max vac advance back to 14, and slowing the vacuum curve so I had full vacuum advance at cruise, but somewhat less than full at idle. I'm gonna put that in the ECU tonight and see how it runs.





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post #8 of 9 (permalink) Old Apr 17th, 08, 07:57 AM Thread Starter
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Re: Timing to get crisper throttle rev off idle?

Jim, thanks for the warning, lol. Yeah, I realize it's basically all trial and error. But after reading the 2 nice pieces on vacuum advance, both of which called for loads of part-throttle timing advance, I have adjusted my timing table to add in 51 degrees at light load. I am gonna try 34* at no-load idle (backing off to 18 under load) and see what happens.

I'll post a pic of my timing table after I drive it around and get it fairly nice.

thanks

Dave F. in Rhode Island
'68 Camaro SS Accel DFI 454 (SOLD - I'm a boring Corvette owner now)
Bought my first big-block Chevy in 1970
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post #9 of 9 (permalink) Old Apr 17th, 08, 11:12 AM Thread Starter
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Re: Timing to get crisper throttle rev off idle?

Well, here's my newest timing setup. I drove it around today and it was perfectly smooth and it was also a little better throttle response. The idle sped up quite a bit, so I had to adjust the idle screw down. My idle vacuum also increased from about 12" to about 14".

Heck, the only thing I kinda don't like is that it idles smoother now. Doesn't have as much rumpety-rump sound to it, lol.



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Bought my first big-block Chevy in 1970
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