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Detonation: Timing Plugs Compression Heads Etc

Just wondering ,have you verified TDC with a piston stop to make sure that everything is what it says it is?. I'm about to go through the same crap on a ot sbc 383 of unknown parts,compression ect. With unknown parts it's like a ball of string that you need to find the two ends ,sucks.
 
Ok so I slowed the advance curve as much as possible. This was using the largest springs in the MSD kit. That curve again is the flattest curve I could tune to. The MSD curve in the book shows all in by 5500 RPM. But I am running a 14 degree advance bushing. I did not change my initial timing and left it at 20 degrees. So I am limiting all in timing to 14 I see all in by about 3600 RPM. This makes sense I am shortening the max all in advance by limiting the added mechanical advance to only 14. I am not detecting any more detonation.

With an engine thats power band is up in the 4K-5K range I am lugging it around town. Hell its like at idle when I am going 35MPH so I desperately need a better rear gear as that 271 just isn’t right for this application. I knew this going in. So I still need to figure out what rear end I have to see what my choices are. But I need at least a 355 if not taller....probably taller since this is not a highway cruiser.
I've run 3.91's & 3.55's with 26-1/2" tall tires 2.76's with 28" tires and I'd say 3.55 is the shortest gear for any highway driving at all and 3.91's or 4.10's are "around town only" or "tow to the track" gears if that helps any (all 8-3/4" gear ratios). 2.76's are a dog off the line
 
I've run 3.91's & 3.55's with 26-1/2" tall tires 2.76's with 28" tires and I'd say 3.55 is the shortest gear for any highway driving at all and 3.91's or 4.10's are "around town only" or "tow to the track" gears if that helps any (all 8-3/4" gear ratios). 2.76's are a dog off the line
Thanks PB those recommendations are for both automatic and manual? I have an automatic and the 271 pegleg is a real dog. They also make a 373 I believe.
 
Thanks PB those recommendations are for both automatic and manual? I have an automatic and the 271 pegleg is a real dog. They also make a 373 I believe.
Either automatic or manual. The 3.73 is aftermarket only, fyi. Seriously, don't go over 3.55's if you ever want to drive it on the highway. On the other hand, if you can deal with a top speed of 60mph, then go for 4.56's or 4.88's (though I wouldn't suggest those unless it's a track-only car)
 
Either automatic or manual. The 3.73 is aftermarket only, fyi. Seriously, don't go over 3.55's if you ever want to drive it on the highway. On the other hand, if you can deal with a top speed of 60mph, then go for 4.56's or 4.88's (though I wouldn't suggest those unless it's a track-only car)
These are the type of blanket statements that are entirely misleading without context. I drove a 4.10 geared 70 RR back and forth from Peoria, IL to Rolla, MO for years. (275 miles one way). Running 75 on the interstate. I’ve done Dragweek for multiple years prior to having an overdrive with a 3.73 gear on a 4.500 stroke 572 Hemi running 75-80 a good portion of the time. A lot of these decisions need to made based on use of the car, age, tolerance level for noise and rpm, etc... the more gear advantage you can give an engine, the less sensitive it may be to detonation.
 
I am currently running 4:10’s in my daily driver. Its a 5 speed though. Its fine for highway but not sure I would want to cross country in it. Also with that set up first gear is kinda useless your climbing a 70% grade lol.
 
These are the type of blanket statements that are entirely misleading without context. I drove a 4.10 geared 70 RR back and forth from Peoria, IL to Rolla, MO for years. (275 miles one way). Running 75 on the interstate. I’ve done Dragweek for multiple years prior to having an overdrive with a 3.73 gear on a 4.500 stroke 572 Hemi running 75-80 a good portion of the time. A lot of these decisions need to made based on use of the car, age, tolerance level for noise and rpm, etc... the more gear advantage you can give an engine, the less sensitive it may be to detonation.
OK, here's the context (from my previous post) I've run 3.91's & 3.55's with 26-1/2" tall tires 2.76's with 28" tires and I'd say 3.55 is the shortest gear for any highway driving at all and 3.91's or 4.10's are "around town only" or "tow to the track" gears if that helps any (all 8-3/4" gear ratios). 2.76's are a dog off the line

I've run all the combinations listed above & have tested 3 of these combinations within the last 60 days and I'm a drag racer too. 26-1/2" tires w/3.91's put me just over 4000 rpm at about 70mph. In my opinion, I don't think it's a good idea to hold a motor for several hours at 4000 rpm....not if you want it to last for 100,000 miles. Running 3.55's with 26-1/2" tires put me somewhere around 3500rpm (or so) & I drove on the highway like that, but I didn't drive cross-country. It still seems a little revved up to me, but tolerable. In my opinion, I would not run shorter gears than 3.55's for a street/highway car. Just in general.....an engine turned at 2000rpm will last twice as long as one turned at 4000rpm.
 
I am driving a 4,10:1 ratio diff with 28" tires.
It's doing around 3700 rpm @ 70mph, which is not pleasant at at.
Would like to convert soon to 3,23:1 ratio for just this reason.
It's not running at a high load but revving like that for prolonged time, as PurpleBeeper mentioned, is an unwanted situation.
Nice ratio for the track, but not for a street car.
 
I went from 3,500 RPMs at 70 in 4th w/a 3.54 to 2,750 at 70 in FIFTH w/a 4.10!
BEST of both worlds!!
:bananadance:
 
I only have a 3 speed lol. Being the OP this info is very helpful. I have confirmed that I have an 8-1/4 axle so it looks possible to get that 2.7:1 dog leg out and get a 3.55:1 in there with that hot 340. Should be a lot more spirited to drive. I am probably longer term going to de-cam it a bit as the guy who built didn’t know what he was doing or had it set up for a 4 speed race car. I am running a bit retarded for not but with 3:55:1’s it will run a heck of a lot better too! All your inputs are valuable and not wasted on me!
Thanks for keeping this discussion alive and spirited!
 
My 3.54 behind a warmed up 440 6bbl and the "infinitely variable stall speed" 3rd pedal was a very good gear for me if I would have stayed with a 4 speed. 3,500 RPMs not that bad on the highway at around 73 mph...
BUT the difference between 3,500 and 2,750 seems greater than the 750 RPMs would lead you to believe "math wise".
A LOT less noise, much better gas mileage (not a big deal) and peace of mind.
BUT you have a small block, so 3.55 even better for you.
 
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THREAD REVIVAL!!

Kern, my motor is a low deck 400 B block 451 mild stroker (using 400 rods) with automatic, 3.55 gears, 28" tall rear tires, aluminum Eddy RPM 84cc closed chamber heads and flat top pistons with cylinders .030" over. Calculated compression ratio is 10.2:1. When John and I put my new heads on, I knew we were going to bump up the compression to around 10.0: 1 or so. Going for optimum quench volume, it was recommended to use a .030" thick Cometic gasket, putting it at 10.2:1 which the Eddy heads should make tolerable on 91 octane. They didn't have those in stock so we went with .027" thick Cometics, the respected mopar head machine shop guru telling me .003" won't make much difference in compression ratio. Here's the thing, current timing was done on a dyno with carb jetting and my initial/total timing is now set at 17/32. Any more than that and it will ping. I had pinging with the old 906 heads too (but without carb jetting) and hoped the aluminum heads would solve things. It did help quite a bit, but trouble-free only for a short time. I had no pinging at first. At the Sacramento track 2 years ago, night time 90 degree outside temp, full throttle ping unless I splashed it with a few gallons of 110 octane mixed with the crappy California 91 (too much ethanol in that fuel). I ran it again at Willow Springs the year after and in the morning cool air the engine was so happy. By later in the day, even with 5 gallons of 110 mixed with 10 of 91, the motor pinged at WOT. Some have told me it could be due to carbon build up increasing compression ratio (already? with only a few thousand miles on the motor?). I don't drive the car that often, but would like to get this sorted.

Taking from your experience, but keeping in mind I want to stay in the optimum quench zone, should I try a thicker head gasket? Say .040"? How did you determine your .075" thick head gasket thickness? That sounds pretty thick.

The initial timing number will have no effect on detonation but the total timing will.
For one thing, the weight of the car and the LONG 2.71 gearing do not help at all. A 3.21 or 3.55 gear (From a 8 1/4" axle in a Durango or Dakota) would improve acceleration and improve mechanical leverage...which does reduce the risk of detonation.
As stated, a delayed or s-l-o-w-e-r advance curve will help. The "All in by 2500" schmere belongs with the other age old terms like 3/4 race cam. Distributors with mechanical advance use 2 springs. One trick is to use one light and one heavy spring. This makes for a two step advance rate where part of the advance comes on quickly while the rest of it is not "all in" until something like 3000, 3500 rpms, give or take.
I dealt with detonation with my 440/493 for years and stumbled on a few winning "fixes". I posted here and maybe 3 other places with responses totalling well over 5000 in number collectively. This was back between 2011 and 2013. I went with thicker head gaskets, a fix that was equally bashed as it was supported. It worked for me after several other fixes did not. 110 leaded gas was the only fuel that allowed me to hit full throttle without any knocking. I tried delayed spark, colder plugs, leaner jetting, richer jetting, open exhaust, different camshafts, different amounts of clearance with rocker arms to valves, lower temp thermostats.....
I went from approx 10.98 to 1 to 10.07 to 1 using a .075 head gasket from Cometic. There have only been a couple of incidents of knocking since then, if I recall right, they were when I drove the car after an extended period of sitting...The gas had surely lost some of it's punch.
 
One thing to know when speaking of gearing: A "Tall ratio" refers to one with numerically low numbers. A 2.71, 2.76, 2.94 is a TALL gear. Many also refer to them as "Highway gears". Taller may be rooted in the theory that a tall person can travel greater distance with fewer steps due to the longer stride. A car with taller gears can travel greater distance with fewer engine revolutions due to its lower numbered gear ratio.
A 3.91. 4.10, 4.56 is a LOW gear, a short gear.
Tall gears are called tall because the pinion gear diameter is physically larger ("taller"). Also, you can remember it because "tall" gears allow a higher top speed for the same RPM. The number is a RATIO (i.e. 3.55:1).

Here's a great explanation:
https://www.randysworldwide.com/resource-center/size-matters/
When ever the gear ratio in a differential is changed pinion diameter changes. There are two things that must change in order for the gear ratio to change. First, the gear ratio is determined by the tooth combination. The number of teeth on the ring gear divided by the number of teeth on the pinion (eg: 41/11 = 3.73) equals the gear ratio. Second, in order for the teeth to have the proper mesh and contact, the relative size of the two must be changed to match the ratio. If the gear ratio is 3.00 to 1, the distance from the pinion shaft centerline to center of the pinion teeth must be 1/3 the distance of the center of the ring gear teeth to the ring gear center line. This means that the pinion shaft is roughly 1/3 the diameter of the ring gear for a 3.00 to 1 gear set. This also means that the pinion size for a gear set that is 5.00 to 1 must be only about 1/5 the size of the ring gear. The ring gear diameter is generally constant for any particular differential design regardless of the ratio. This means that the pinion size changes with the ratio and for lower gears (higher numerically) the pinion teeth are smaller.
 
Back on topic on my WOT sometimes detonation in warm weather issue -
Summit provides a pretty succinct recommendation for quench:
https://help.summitracing.com/app/answers/detail/a_id/4941/~/engine-quench
How is it measured?
Quench is a calculated dimension. It uses the following formula:

Quench = Deck Clearance + Compressed Head Gasket Thickness

How does it affect performance?
The right amount of quench can promote fast, complete burn of the air/fuel mixture. It creates turbulence, which forces the air/fuel mixture toward the spark plug. This reduces the possibility of Detonation. It also leads to cleaner emissions.

What should it be?
Quench should be as tight as possible, without the piston contacting the head. Recommended quench depends on your max rpm and the type of connecting rods you use.

Max RPM Connecting Rod Type Quench Range
6,000 or less Steel 0.035 - 0.045 in.
6,000 + Steel 0.038 - 0.043 in.
Any Aluminum 0.050 - 0.065 in.
Notes
  • Aluminum rods get longer as they get hotter.
    • Increased quench will prevent the piston from contacting the head.
  • Don't run more than 0.060 in. quench trying to lower Compression Ratio.
    • This will slow the combustion process and could cause Detonation.
So right now my pistons are .015" in the hole and I've got a .027" thick gasket, so I'm at .042" quench. If I pulled my current head gaskets and went from .027" to .030" to get to the max recommended quench of .045" for my setup (only an increase of .003") do you think I would notice a difference in WOT detonation resistance since I likely have some carbon buildup? My original compression ratio calculation of 10.2:1 was assuming a .030" head gasket is used to make .045" quench, but as stated, the aluminum heads currently have .027" gaskets, so the compression ratio is slightly higher with that difference of -.003". Also, is this .045" max quench number for my Steel Connecting rod motor with cam that maxes out at 5800 RPM a hard and fast number? What happens if I go up to .050" total by using a .035" gasket? Do you think chances of detonation could remain since now I'm in the non-optimum quench zone?

For reference, with my old iron heads and head gaskets, the engine builder had measured and calculated my compression ratio to be 9.7:1 and it pinged in warm weather at WOT with total timing at 32.

OK, so I also dug up my original engine info and plugged in all of the values into an online compression ratio calculator (can we trust these calculated ratios as reliable?) Can someone please verify the values shown below are correct for a 451 (400 block) B motor? This is what I have from my notes. Pistons are JE flat tops. The gasket bore diameter I got off of Cometic's info on Summit.

I used this online calculator:
https://uempistons.com/p-27-compression-ratio-calculator.html
.030" over CYLINDER BORE SIZE = 4.370
PISTON STROKE LENGTH = 3.75
HEAD GASKET BORE DIAM = 4.410
COMPRESSED HEAD GASKET THICKNESS = .027"
COMBUSTION CHAMBER VOLUME = 84 cc
PISTON DOME VOLUME (Negative) = 6
PISTON DECK CLEARANCE (Negative) = .015

This is giving me a calculated compression ratio of 10.18:1.
Plugging in a .030" gasket thickness gives a ratio of 10.1:1.

I found the old email from the shop who did these Eddy RPM heads and here is what he originally said:
According to my calculations, assuming the heads are actually 84cc, I get 9.99cr for a .030 gasket, and 9.82 for a .040 gasket.

Why the difference? Not sure how he calculated...


THREAD REVIVAL!!

Kern, my motor is a low deck 400 B block 451 mild stroker (using 400 rods) with automatic, 3.55 gears, 28" tall rear tires, aluminum Eddy RPM 84cc closed chamber heads and flat top pistons with cylinders .030" over. Calculated compression ratio is 10.2:1. When John and I put my new heads on, I knew we were going to bump up the compression to around 10.0: 1 or so. Going for optimum quench volume, it was recommended to use a .030" thick Cometic gasket, putting it at 10.2:1 which the Eddy heads should make tolerable on 91 octane. They didn't have those in stock so we went with .027" thick Cometics, the respected mopar head machine shop guru telling me .003" won't make much difference in compression ratio. Here's the thing, current timing was done on a dyno with carb jetting and my initial/total timing is now set at 17/32. Any more than that and it will ping. I had pinging with the old 906 heads too (but without carb jetting) and hoped the aluminum heads would solve things. It did help quite a bit, but trouble-free only for a short time. I had no pinging at first. At the Sacramento track 2 years ago, night time 90 degree outside temp, full throttle ping unless I splashed it with a few gallons of 110 octane mixed with the crappy California 91 (too much ethanol in that fuel). I ran it again at Willow Springs the year after and in the morning cool air the engine was so happy. By later in the day, even with 5 gallons of 110 mixed with 10 of 91, the motor pinged at WOT. Some have told me it could be due to carbon build up increasing compression ratio (already? with only a few thousand miles on the motor?). I don't drive the car that often, but would like to get this sorted.

Taking from your experience, but keeping in mind I want to stay in the optimum quench zone, should I try a thicker head gasket? Say .040"? How did you determine your .075" thick head gasket thickness? That sounds pretty thick.
 
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'd take a few ccs out of the chambers and keep the squeeze, unshroud the valves
going thicker is not going to help
damn ca gass
check the plug threads for hot spots
try all the tuning tricks, cold air, block heat to the manifold first
 
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THREAD REVIVAL!!

Kern, my motor is a low deck 400 B block 451 mild stroker (using 400 rods) with automatic, 3.55 gears, 28" tall rear tires, aluminum Eddy RPM 84cc closed chamber heads and flat top pistons with cylinders .030" over. Calculated compression ratio is 10.2:1. When John and I put my new heads on, I knew we were going to bump up the compression to around 10.0: 1 or so. Going for optimum quench volume, it was recommended to use a .030" thick Cometic gasket, putting it at 10.2:1 which the Eddy heads should make tolerable on 91 octane. They didn't have those in stock so we went with .027" thick Cometics, the respected mopar head machine shop guru telling me .003" won't make much difference in compression ratio. Here's the thing, current timing was done on a dyno with carb jetting and my initial/total timing is now set at 17/32. Any more than that and it will ping. I had pinging with the old 906 heads too (but without carb jetting) and hoped the aluminum heads would solve things. It did help quite a bit, but trouble-free only for a short time. I had no pinging at first. At the Sacramento track 2 years ago, night time 90 degree outside temp, full throttle ping unless I splashed it with a few gallons of 110 octane mixed with the crappy California 91 (too much ethanol in that fuel). I ran it again at Willow Springs the year after and in the morning cool air the engine was so happy. By later in the day, even with 5 gallons of 110 mixed with 10 of 91, the motor pinged at WOT. Some have told me it could be due to carbon build up increasing compression ratio (already? with only a few thousand miles on the motor?). I don't drive the car that often, but would like to get this sorted.

Taking from your experience, but keeping in mind I want to stay in the optimum quench zone, should I try a thicker head gasket? Say .040"? How did you determine your .075" thick head gasket thickness? That sounds pretty thick.
Your SCR "should" (there's that word again!) be okay on premium, even ours, with the tighter quench buuut since it still pings, 1) how fast is your advance curve, 2)what is your cranking psi, and 3) what are your cam specs (intake closing angle, duration, and is it installed advanced/retarded/straight up)? That will tell us a lot..
 
Your SCR "should" (there's that word again!) be okay on premium, even ours, with the tighter quench buuut since it still pings, 1) how fast is your advance curve, 2)what is your cranking psi, and 3) what are your cam specs (intake closing angle, duration, and is it installed advanced/retarded/straight up)? That will tell us a lot..

68 Plymouth Sport Satellite
727 3-speed auto
3.55 gears, Suregrip, 28" tall rear tires (275/60/15)
Edelbrock performer RPM intake (so no heat cross-over)
Quick Fuel SS 735 CFM (750 body with smaller secondaries) - primaries jetted up
Firecore vacuum advance distributor (blue springs installed for medium advance curve. Timing all in by 3000. Tried stiff black springs and motor didn't like it. Current timing post-tune = 15 initial, 32 total (it pinged at 33 total))
Cranking pressure is 165 psi
Cam specs (installed advanced by my builder):
Lunati Voodoo hydraulic flat tappet, Part # 60302,
Lift = .475" intake / .494" exhaust; Duration @ .050" = 220 int / 226 ex; LSA = 112 degrees
Ppst-carb jet and tuned Wheel dyno'd numbers = 391 HP, 523 FT-lb torque
Running 26" Griffin aluminum rad and 180F thermostat. Engine temp =185F.

Again, this is only pinging at WOT when outside temp is above 80F.
Thanks for your input guys!

Also still hoping to hear back from Kern since he's a buddy and also lives with our crap California ethanol laced fuel.
 
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