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Discussion Starter · #1 ·
Let me give you some specs first then I'll explain:
408W stroker
10.8:1 compression
Autolite 3922 plugs
MSD 6AL w/MSD distributor
Compcams XR274R solid roller 274/280 adv. dur, .602/.608 lift
Performer RPM AirGap intake
Edelbrock 800cfm Thunder AVS
93 pump gas

I think my car hates me. It will ping off idle when I'm letting out the clutch,then intermittenly while cruising, which is probably just being drowned out by the exhaust. I can swing the initial timing from 8-16 deg. and it remains the same, although it stumbles like crazy at 8. It seems to quiet down the most around 12 deg., but the ping is still there. Cruising at 2000rpm and mash the throttle and it takes off like a scalded cat, after the wheels stop spinning. I just can't get the darn thing to stop pinging. I've even drained the tank and put in fresh gas. I've installed an adjustable vacuum advance, and installed a vacuum advance limiter that I can adjust as well. I've also tried both ported vacuum and full manifold vacuum sources. Nothing makes a difference. I have a F.A.S.T dual band AFR monitor hooked up and monitor both banks simultaneously. I've got it running pig rich, cruising around 12-12.5:1 so I know it's not a lean condition. The idle is about 14:1, which is just fine maybe a tad rich. I'm currently putting in about 12 on the vacuum canister. It idles great with about 10-12" vacuum. Just pings and runs like crap up to about 2000-2200 rpm. Never had trouble dialing in a car before, really stumped on this one. Generally speaking if you're pinging you pull out some timing then go from there, but it just won't respond.

Any help is appreciated and I'm thinking 10 initial is a good place to start without chance of pinging. I know there are alot of similar setups out there, maybe somebody ran into the same issue.
 

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Discussion Starter · #3 ·
I'll do a compression test tomorrow, but it's a brand new build. The engine has only about 15 miles on it, and that's from driving around trying to tune it and break in time. Brand new AFR 205's.
 

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Discussion Starter · #6 ·
I'll throw some in today, what's burning me is that it doesn't respond to pulling out timing.
 

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how was your deck clearance? how thick is your head gasket? if you quench is to great it will ping like crazy. best quench is .035-.045.
 

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Discussion Starter · #8 ·
The quench is .0355, .0035 above the deck and .039 compressed head gasket thickness. I can try a thicker head gasket.
 

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Discussion Starter · #9 ·
Tomorrow I'll pull the plugs, and read them. See what I'm getting, and maybe purchase the head gaskets to help open up the quench and lower CR slightly.
 

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if you had + .035 deck height and .039 gaskets, yea thats to close. going to be hard to find a gasket .070-.080 thick, unless you go copper.
 

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Discussion Starter · #11 ·
No, no, it's .0035 and .039, that gives me a quench of .0355, which is within the .035-.045 area. I was thinking maybe a .041-.047 gasket which would bring me into the higher end of the quench range. Although I don't want to go too high or that will also cause ping also, not enough quench. Yea, .004 quench would probably put my piston through my head. :nono:
 

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Discussion Starter · #13 ·
58cc chamber with Probe SRS 10674 pistons, which are a -22cc reverse dome. I'm going to play with it some more today. It actually get's worse when you back out the timing. I leaned it out a bit and brought the timing up to 12 initial 10 on the can. And it responded positively. Still has a slight ping just when the clutch is released, but not while cruising. I'm thinking of swapping the mech. adv. bushing out to keep the total down, and bringing the initial up to 14 and see how it responds. I may swap my 800 avs for my old performer 750 and see how it responds too. Oh and the plugs were all looking good, medium tan color.
 

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With the initial, centrifical and vacuum advance all in, at whatever RPM that this occurs, (Should be between 2400 to 3200 RPM, depending on the type of transmission) the total amount of timing advance should not exceed 52*BTC. Without the vacuum advance connected, total advance should be between 36 to 38 BTC.

With a manual trans, all in should be at a higher RPM than with an auto trans, say about 3000-3200 RPM. Is the advance coming in too soon? Have you tried a colder heat range plug? Are your plugs projected tip? Try a set that has a recessed tip. This may help to keep the ground strap from retaining heat and acting like a glow plug.

The compression ratio is a bit on the high side for 93 Octane, but with aluminum heads, you may be able to get by if the timing advance isn't coming in too soon and/or if the plugs aren't retaining too much heat.
 

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Discussion Starter · #16 ·
10 deg. initial, 10 vacuum, 31 total, without vacuum, all in at 3400 rpm. With vacuum, no load, the total is 41. The tranny is a manual. I've trimmed the ground back on the plugs and rounded off the edges to prevent hot spots. I know the 3922 is the coldest Autolite offers. I'll have to do some digging for another brand. I think I'm going to swap out the 800 for the 750 and tune around that. The carb is giving me a lean condition under light acceleration that won't go away until you give more throttle. Sounds like the stepup springs, but it's not responding to spring changes. This carb may have some ghosts, so I may be trying chasing my tail here. I'll do the carb swap tomorrow, and test further. I've also just filled it up and added some octane booster. No help there. It's a light ping, not a continuous condition. It's just annoying and shouldn't be there. I should be able to get that to stop. It's most notable as the clutch if fully released lasting only for 1/2 sec. Then intermittent very light pinging while accelerating lightly. I'm thinking that might be from my lean condition during transition from cruise to power mode. Either that or the ping is causing a false lean reading on the A/F meter. Wish I could quiet the exhaust to hear things better. :)
 

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I think that there is confusion between the vacuum advance and centrifical advance. The base timing and centrifical should total between 36* to 38* BTC all in by 3200 RPM. This is with the vacuum advance disconnected. With the vacuum advance connected at 3200 RPM, no more than 52* BTC.

From what I can tell, you aren't advancing enough.
 

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Discussion Starter · #18 ·
You're correct, I have the timing backed down to try and counter the detonation. I have 10 initial, and with the vacuum disconnected I'm getting 31* total. I'm running on the conservative side to try and prevent the detonation. I'm definitely not running the amount of timing I should be. What I'm going to do today is pick up some NGK racing plugs with a recessed tip and a bit colder than the Autolite 3922's I'm running now. It responds much better with 14* initial and 35* total, but I was turning it back to try and eliminate the pinging. I'm now thinking that it's not due to timing, as it pings the same with 10*/31* and 14*/35*. I'm going to set it with 14* initial, 35* total w/o vacuum and the new colder plugs. See how it feels there. It's supposed to rain all day today, so I don't think I'll get a road test in.
 

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sounds to me like your compression is to high.
Or the octane is too low.

A 10.8:1 compression nominally requires a 98 pump octane gas. It will always ping on 93. You can fudge the octane a little with timing, etc., but not 5 octane numbers. If you could, then no one would really need premium in a new car.

Adding about 40% ethanol to 93 octane gas might be about right. Ethanol by itself is about 115 octane.
 
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