Timing curve for mild cams - Chevelle Tech
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  #1  
Old Nov 8th, 10, 10:19 PM
Autoengineer Autoengineer is offline
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Default Timing curve for mild cams

Anyone seen a chart like this showing that for mild cams, lower intial timing is better? I've been playing around with my hei and I increased my timing from 15 intial (36 total) to 18 intial (36 total) MAybe I went the wrong direction? According to this chart, I should be running 10-12 initial. I'm not getting any pinging with the 18 init/36 total timing curve, but I didn't see any improvement either. Might of actually lost a little responsiveness. Any comments on this chart? I pulled this off the internet awhile ago. I've seen similar charts on Barry Grants website as well.

Cam duration ...................Advance
270 ...............................10-12
280................................12-14
290................................14-16
300................................16-18
310+..............................18-20
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  #2  
Old Nov 9th, 10, 3:40 AM
Schurkey Schurkey is online now
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Default Re: Timing curve for mild cams

That's similar to how I set up distributors; except I get nervous any time the initial goes over 16--17 degrees.

Needing more than about 15 degrees initial timing is a sign of screwed-up idle circuits in the carb. Fix the fuel curve, you probably won't need excessive initial timing.

But, yeah, the concept is similar: Stock cams get near-stock initial timing; mild cams get a bit more, as the cam gets more duration, the initial timing goes up...to a point.
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  #3  
Old Nov 9th, 10, 7:07 AM
69-CHVL 69-CHVL is online now
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Default Re: Timing curve for mild cams

We'll, my 280ish HR cam like 40* initial along 15:1 AFR for max vacuum. This is with EFI, so no idle circuits that I know of.
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  #4  
Old Nov 9th, 10, 2:22 PM
SWHEATON SWHEATON is offline
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Default Re: Timing curve for mild cams

Quote:
Originally Posted by Autoengineer View Post
Anyone seen a chart like this showing that for mild cams, lower intial timing is better? I've been playing around with my hei and I increased my timing from 15 intial (36 total) to 18 intial (36 total) MAybe I went the wrong direction? According to this chart, I should be running 10-12 initial. I'm not getting any pinging with the 18 init/36 total timing curve, but I didn't see any improvement either. Might of actually lost a little responsiveness. Any comments on this chart? I pulled this off the internet awhile ago. I've seen similar charts on Barry Grants website as well.

Cam duration ...................Advance
270 ...............................10-12
280................................12-14
290................................14-16
300................................16-18
310+..............................18-20
================================================== ===================

That adv cam dur vs base timing chart is conservative and i find i need considerably more base timing with perf cams in the ranges shown by approx 4-8 deg depending on cam that's is being shown in your list.

With base timing being retarded by approx 4-8 deg or so that will make for a lazzier motor in all rpms below where total timing is fully in.

I find from my many yrs of dialing in timing/carbs for street perf motors that the timing rec i listed below is a very good place to start with an aftermart perf cam(not stock gm cam) for a street sbc or bbc motor that should only require minimal timing tweak/s from this starting point to find the sweet spot.

There's nothing to get worried about when going over 15-16 deg base timing with an aftermarket perf cam requiring the additonal base timing along with running proper octane lvl of fuel required for your setup/comp . So run 93 pump fuel with the street perf timing curve i suggest and if no detonation/ping then try 91 oct fuel,if no ping with 91 pump fuel that then try 89.

But getting the timing curve right often requires the mech adv in the dist to be recurved to reduce timing from mech adv and for it to also come in faster too along with the vac adv being reduced/limited with a restrictor/limiter plate to max of 10-12 deg too.

Its a pia to do but once you get this right for your setup you will have a good running motor with crisper thorottle responce & more power (esp comming off idle and at lower rpm below where total timing is fully in )/better fuel mileage, etc

Here goe's:

* 270-280 deg adv dur approx 16-18 deg base + 18-20 deg mech in dist all in by approx 2500-2600rpm for 36-38 total + vac adv limited to 10-12 deg max to roughly 50 deg + - a couple deg timing when a low load part throttle cruise.

* 290-300+ deg adv dur approx 18-20 deg base + 18 deg mech adv in dist in by approx 2500-2600rpm + vac adv lim,ited to 10-12 deg max for roughly 50 deg + - a couple deg when at low load part throttle cruise.

Keep in mind you may need to tweak timing a few deg up/dn to find sweet spot along with running 93 fuel to hopefully avoid detonation too with street perf ign timing cruve.

Also ,Very large cams on street in approx range of 295-300+ deg adv dur(maybe 250-260 deg + dur @ .05 deg dur) may do better running timing locked out lets say 30 deg and run a vac adv to ported vacuum to add 18-20 deg timing at low load part throttler cruise.

Thats becasue the larger cams duration makes for low idle vac that can cause unstable idle issues with vac adv hooked to full int vac all the time but when vac adv is hooked to ported vac when at cruise theres plenty of vac to fully activate the vac adv for better fuel miles /better throttle respnce/cooler running motor/and a little more power too.

I run the follwoing timing curve in my mild 396 now 402 bbc with a mild 268 deg hyd ft perf cam with 222/226 deg dur @.05 on a 112 deg LSA with approx 9.7-9.8 comp & stock type 063 closed chamber lrg oval port heads,19-20 deg base + 18 deg mech for approx 38 deg total all in by 2500-2600rpm + vac adv limited to 10-12 deg for approx with 93 fuel.

Doenst ping under all normal driving cond other then when at WOT and i ran some real lead booster to stop it.

But this past summer i got the motor temps reduced on hot days from 190-195 on avg to 175-180 max which should help it not detonate when at WOT without any octane booster due to the motor running 15-20 deg cooler at times.

But i have not had a chance to try it out at WOT with 93 fuel by itself in hot 85-90deg + temps because the cooler fall temps moved in by the time i ran the fuel low enough to require refilling so timew will tell on that one.

Even if it still pings at WOT on a hot day with motor running 15-20 deg cooler i may be able to run less real lead booster which would still be a help.

I tried retarding timing enough to stop the detonation @ WOT but doing so hurt perf /throttle responce enough on my already cubically challanged 402bbc that i was not willing to put up with and opting to run full timing with the real lead booster .

But the real lead boosters seem to be slowly going away so when /if that happens thats when i will be forced to back off the timing for good vs doing motor mods like installing a larger cam that i dont want,install flatop pistons or diffewrent heads to reduce comp etc to reduce compression from the 9.7-9.8 wheres it currently at closer to a more pump friendly 9.0-9.2 comp on 93 fuel with 10% ethenol that may go up to 15% ethenol in future froim what i have been hearing/reading on it.

One more thing,some vortec heads require less base/total timing due to thier design so watch out for that to avoid detonation if your running vortec heads.

Scott
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1969 CHEVELLE SS396,ORIGINAL #'S MATCH,GOT IN 1978,(In 2001 rblt/bored original 396 .030 to 402)/M20/12BOLT/3:31'S
2006 YAMAHA 1700 ROADSTAR SILVERADO (1 owner ,GOT in 2009,4,700 miles/WK end fun)
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Last edited by SWHEATON; Nov 9th, 10 at 2:56 PM.
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  #5  
Old Nov 10th, 10, 10:17 PM
Autoengineer Autoengineer is offline
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Default Re: Timing curve for mild cams

Quote:
Originally Posted by Autoengineer View Post
Anyone seen a chart like this showing that for mild cams, lower intial timing is better? I've been playing around with my hei and I increased my timing from 15 intial (36 total) to 18 intial (36 total) MAybe I went the wrong direction? According to this chart, I should be running 10-12 initial. I'm not getting any pinging with the 18 init/36 total timing curve, but I didn't see any improvement either. Might of actually lost a little responsiveness. Any comments on this chart? I pulled this off the internet awhile ago. I've seen similar charts on Barry Grants website as well.

Cam duration ...................Advance
270 ...............................10-12
280................................12-14
290................................14-16
300................................16-18
310+..............................18-20
Well I recurved my MSD (ready to run Billet) and went the other direction . I went with the silver bushing giving me around 11 degrees intial and 36 total. That loss in responsiveness at 18 intial was real as I noticed it is definitely more responsive off-idle with the lower intial timing. I looked up the advertised duration on my cam and GM specifies it as 264/273 @.006 That puts me really at the bottom end of these timing curves so that probably explains why 18 initial was too much. I'll have to do some more test driving, but I'm thinking ideal is either the silver or blue bushing for sure with intial somewhere between 10 and 14 for me. Just based on my couple of tries, I would definitely say there's merit to the chart above at least in principle.
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  #6  
Old Nov 10th, 10, 10:23 PM
Schurkey Schurkey is online now
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Default Re: Timing curve for mild cams

Quote:
Originally Posted by Autoengineer View Post
18 initial was too much.
That's been my experience. It's REAL EASY to get TOO DAMN MUCH initial timing.

Too many people use excessive initial timing as a crutch for idle circuit problems. The excessive timing moderates the troubles caused by lean idle fuel mixture, and they think they've "solved" their idle problems.
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  #7  
Old Nov 10th, 10, 10:55 PM
Buzzbomb Buzzbomb is offline
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Default Re: Timing curve for mild cams

Quote:
Originally Posted by Schurkey View Post
That's been my experience. It's REAL EASY to get TOO DAMN MUCH initial timing.

Too many people use excessive initial timing as a crutch for idle circuit problems. The excessive timing moderates the troubles caused by lean idle fuel mixture, and they think they've "solved" their idle problems.
You know that's not a real popular viewpoint when it comes to timing, right?

There is a chart that substantiates the above claim, AND the chart that is in the original post...

http://www.barrygrant.com/demon/default.aspx?page=5
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  #8  
Old Nov 11th, 10, 7:14 AM
dreis454 dreis454 is offline
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Default Re: Timing curve for mild cams

Now It makes sense that my GMPP catalog shows 12* initial timing for Big Blocks that are using the HR ZZ502 cam....I though they were being too conservative.
I'm gonna try 14* in my 454 that is NOW @ 20* and has a ZZ502 HR cam in it.
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  #9  
Old Nov 11th, 10, 8:10 AM
mr 4 speed mr 4 speed is offline
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Default Re: Timing curve for mild cams

I run 25* base and 38* total with my 223/231 @.050 cam
Throttle response is instant and my timing is all in by 2800
..and I have no idle circuit problems.
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  #10  
Old Nov 11th, 10, 11:15 PM
SWHEATON SWHEATON is offline
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Default Re: Timing curve for mild cams

YOU CAN LEAD A HORSE TO WATER BUT YOU SURE CAN'T MAKE IT DRINK/LOL!!!!!

There is something wrong when a motor that truely has a non GM aftermarket perf cam has better throttle responce & runs better in general with only 10deg or so base timing vs 16-18 deg base timing that's for sure.

But if you dont know what your doing installing an aftermarket perf cam needing 16-18 deg base timing and dont recurve /reduce timing given by mech adv in the dist down from stock 25-30 deg to approx 18 deg + - a couple deg the motor will have too much much timing if also running a vac adv thats not also been limited to 10-12 deg max vs stock 18-20 deg timing .

So in a case like that which is common the timing ends up in the 58-60 deg range when all is said & done when the dist & vac adv aren't seup properly to handle the additonal base timing an aftermarket perf cam requires for street operation.

But a race or mostly race appliacation where motor see little to no idle time or not much lower rpm use that's a dif story when it comes to base timing not being near as important with an aftermarket perf cam.

On the street thats when the motor will not run well when you dont reduce/limit mech adv in the dist and also dont reduce/limite vac adv when running the addtional base timing an aftermarket perf cam needs.

Some guys like for example Chris run plenty of base timing @ 25 deg with a fairly mild perf cam for a 454 and the additonal 13 deg to get 38deg total comes from mech adv in the dist with no vac adv and his motor run's just fine with that setup.

I also run plenty of base timing @ 18-19 deg + 18 deg mech in dist in by 2600 ish rpm + vac adv reduced/limited to 10-12 deg for roughly 48-50 deg timing with all timing in when at part throttle low load cruise in my mild 396/now 402 with mild aftermarket perf cam.

Scott
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1969 CHEVELLE SS396,ORIGINAL #'S MATCH,GOT IN 1978,(In 2001 rblt/bored original 396 .030 to 402)/M20/12BOLT/3:31'S
2006 YAMAHA 1700 ROADSTAR SILVERADO (1 owner ,GOT in 2009,4,700 miles/WK end fun)
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  #11  
Old Nov 11th, 10, 11:39 PM
SWHEATON SWHEATON is offline
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Default Re: Timing curve for mild cams

Quote:
Originally Posted by dreis454 View Post
Now It makes sense that my GMPP catalog shows 12* initial timing for Big Blocks that are using the HR ZZ502 cam....I though they were being too conservative.
I'm gonna try 14* in my 454 that is NOW @ 20* and has a ZZ502 HR cam in it.
Dan,the only thing you will notice is a little less power/less throttle resp when running 14 vs 20 deg base timing .

I find in general that 16-18 deg base timing is where many sbc/bbc street perf motors with aftermarket cam's like to be and thats comming from working on & installing too many to count perf cams or just dialing in the timing & carbs over the past 40 yrs,not just from working on my car or maybe 1-2 other peoples cars so i know for a fact thats the case.

And the aftermarket perf cams i am talking about are in the rough range of 262-290 deg advertised duration,lower dur are ok at 15-16 deg base and larger dur cams 17-18+ to some at 20 deg +.

But if you were to pull the base timing in your motor all the way back to 10 deg base it would be very notceable running like a slug loosing 10 deg base timing comming from 20deg to 10 deg base timing.

But since your motor is running well at 20 deg base why would you want to pull 6 deg base timing out if it esp when you just went thru the process of dialing in the base timing with your new cam a little while back and had already tried less base timing and found it didnt do as well there from what you said at bthat time?

Scott
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SCOTT
1969 CHEVELLE SS396,ORIGINAL #'S MATCH,GOT IN 1978,(In 2001 rblt/bored original 396 .030 to 402)/M20/12BOLT/3:31'S
2006 YAMAHA 1700 ROADSTAR SILVERADO (1 owner ,GOT in 2009,4,700 miles/WK end fun)
1977 KAWASAKI KZ1000 (AM ORIGINAL OWNER ,GOT IN 1977,NOW has 29k miles/WK end fun)
2002 MAXIMA (DAILY DRIVER/1 owner,GOT 3/2013 w-44k miles)
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  #12  
Old Nov 12th, 10, 12:48 AM
Autoengineer Autoengineer is offline
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Default Re: Timing curve for mild cams

I'm wondering how converter stall speed affects or lessens the effect of intial timing. Think about a car like mine with a 2000rpm stall. I give it some gas off the line and its trying to put power to the ground around 2000rpm. The timing at 2000rpm is important. Now take a car that has a 3000rpm stall. Assuming the typical timing curve has all the timing in by 3000rpm, that person could lock out their distributor and its really not going to matter. Correct?
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  #13  
Old Nov 12th, 10, 9:25 PM
SWHEATON SWHEATON is offline
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Default Re: Timing curve for mild cams

Running proper base iming is much more important for mostly street perf motor then a mostly strip setup that has plenty of stall etc and doesnt see much street duty in traffic/idle time.

So yes if you have 3k stall and motor is close to or at 3k rpm most of the time at cruise with mech adv setup to be all in at 3k rpm then base timing isnt as important esp if that motor doesnt see a lot of idle time traffic where if base timing was retarded it woiuld affect off idle power also making off ilde throttle responce more soggy/sluggish too.

Scott
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SCOTT
1969 CHEVELLE SS396,ORIGINAL #'S MATCH,GOT IN 1978,(In 2001 rblt/bored original 396 .030 to 402)/M20/12BOLT/3:31'S
2006 YAMAHA 1700 ROADSTAR SILVERADO (1 owner ,GOT in 2009,4,700 miles/WK end fun)
1977 KAWASAKI KZ1000 (AM ORIGINAL OWNER ,GOT IN 1977,NOW has 29k miles/WK end fun)
2002 MAXIMA (DAILY DRIVER/1 owner,GOT 3/2013 w-44k miles)
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