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
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
* 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.