Detonation on 408 Stroker

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KevC

KevC

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Hi All

I have a 73 Duster with a 408 Stroker.
Ever since it was built I have been having detonation (ping) and running on when ign is killed.
I have gone through 3 different ignition systems 1) Original Mopar 2) MSD 3) Pertronix
It works much better with the Pertronix Billet Distributor than with the others, and by tuning weights and springs in the dizzy it is almost there.
Timing is 33 degrees all in.
Compression is 10.3:1
Heads are Mopar Performance cast ones.
I use 99 octane fuel always.
I have tried several different heat range of spark plugs
It has a Holley red Top fuel pump
Now I only get ping at WOT, even 3/4 is fine but as soon as it is WOT it pings
The carb is brand new Holley 750 vac sec, and has not been touched, so I'm wondering if this may need different Jets / accelerator pumps etc?
The car also tends to run on of I turn it off in Park, if I shut it down in gear, where the revs are lower it is usually ok
Thoughts / opinions welcome
Thanks
Kev
 
What are the complete camshaft specs? Was compression ratio measured during the build and assembly process? Have you backed off total timing any more? Is it burning oil?
Compression ratio at 10.3 to 1 is getting up there for cast iron heads.
Run on or dieseling at shutoff may or may not be related. What is idle RPM in neutral?
 
Cam CompCams 20-232-4 Hydraulic-Use in 340-360 Street Machine. Dual exhaust, 3.53-3.91 gear, 9:1 compression. Headers & aftermarket intake, 2500 stall. .480 / .480 Lift – Hydraulic 230 / 230 Duration @ .050 280 / 280 advertised
The compression is what the builder gave me on collection so presume it was measured during build time.
It isn't burning oil
I will back the timing off a touch this weekend
In neutral, RPM is about 950
Heads specs:
Heads Mopar Performance Heads 4772576MP Or P4529269 360 hp Head
Valves Mancini Racing Stainless Valve Set SEV-2678SG 1.60” Exhaust / 2.02 Intake
Thanks
Kev
 
Open chamber heads from what I can tell(LA heads, not magnum heads)........ no quench unless you have quench dome pistons.

Iron heads with no quench + 10.3cr = easy ping.

Probably be better off with a factory hp360 type of advance curve and run a vacuum advance........ at least as far as ping prevention goes.
 
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Thanks PRH
Pistons are Diamond Flat top PN: 51026 - not sure if that helps
So what would you suggest? Thicker Head Gaskets?
 
Do a compression check. I cant see detonation running 99 octane unless you are up around 11.5+ and/or your timing is messed up. What does your mechanical advance curve look like ? And are you running vacuum advance ? If so which canister and is it hooked to ported or manifold vacuum ?
 
Hi over there. I have a 408 Magnum with 10.2 to 1, Edelbrock ported aluminum close chamber heads and a .555"/540" lift intake/ exhaust, 246*/252* @ .050 with a 108 lobe separation angle.

It cranks to 200psi when cranking compression checked, never pings or runs- when shut down, all while using 91(R+M/2) octane California fuel. It is remapped factory EFI'd and runs very well. I suspect your 99 octane rating is derived using either the Research or the Motor Octane method. Ours over here is the average of the two.

I too suspect the open chamber iron heads. A buddy of mine hade the same issue with a 470' low desk stroker motor. What is your cranking compression?
 
KevC said:
Cam CompCams 20-232-4 Hydraulic-Use in 340-360 Street Machine. Dual exhaust, 3.53-3.91 gear, 9:1 compression. Headers & aftermarket intake, 2500 stall. .480 / .480 Lift – Hydraulic 230 / 230 Duration @ .050 280 / 280 advertised
The compression is what the builder gave me on collection so presume it was measured during build time.
It isn't burning oil
I will back the timing off a touch this weekend
In neutral, RPM is about 950
Heads specs:
Heads Mopar Performance Heads 4772576MP Or P4529269 360 hp Head
Valves Mancini Racing Stainless Valve Set SEV-2678SG 1.60” Exhaust / 2.02 Intake
Thanks
Kev
Click to expand...

Well there you go. Iron heads lots of compression and a tiny camshaft. You're going to have to physically change something to fix your problem. The DCR is too high.
 
If even close to zero deck and flat top piston with 65-63ish cc chamber you are more like 12:1. My x head with IIRC 65 cc chamber with a flat top piston, .010 in the hole is 12.2 to 1 with 5cc valve reliefs. Numbers are just off the top of my head. No pun intended. Not sure what your stock chamber size is.
 
jbc426 said:
Hi over there. I have a 408 Magnum with 10.2 to 1, Edelbrock ported aluminum close chamber heads and a .555"/540" lift intake/ exhaust, 246*/252* @ .050 with a 108 lobe separation angle.

It cranks to 200psi when cranking compression checked, never pings or runs- when shut down, all while using 91(R+M/2) octane California fuel. It is remapped factory EFI'd and runs very well. I suspect your 99 octane rating is derived using either the Research or the Motor Octane method. Ours over here is the average of the two.

I too suspect the open chamber iron heads. A buddy of mine hade the same issue with a 470' low desk stroker motor. What is your cranking compression?
Click to expand...

You have two big things he doesn't. Quench and more camshaft. That's why you're gettin it done.
 
mbaird said:
Do a compression check. I cant see detonation running 99 octane unless you are up around 11.5+ and/or your timing is messed up. What does your mechanical advance curve look like ? And are you running vacuum advance ? If so which canister and is it hooked to ported or manifold vacuum ?
Click to expand...

Why? With iron heads and that small cam? You're not gonna run 11.5 with iron heads without a LOT of science and work. And he sure ain't gonna do it with that cam he has.
 
Thanks for all your inputs guys, I really appreciate it.
To answer one question asked, the billet Pertronix dizzy has no vacuum advance
Based on your assessments I need to either change the heads, lower the compression ratio or what else?
 
KevC said:
Thanks for all your inputs guys, I really appreciate it.
To answer one question asked, the billet Pertronix dizzy has no vacuum advance
Based on your assessments I need to either change the heads, lower the compression ratio or what else?
Click to expand...

First, let's get this out of the way. Are you CERTAIN the static compression is a MEASURED compression, or is that just what you are repeating? If we make a recommendation, it needs to be one that works for you and if all the info isn't correct, it't won't. So the more info you can give, the better. I think a camshaft change will do it, but we need to be SURE first. ........AND make SURE everything ELSE is working right first. Distributor, ignition....everything. The last thing we want is for you to make a big change and it not help you.
 
Hi, I haven't measured the compression, it's just on the stats the engine builder gave me, so no, I can't be certain.
Here is what he gave me:
4.030 Bore
Piston to Deck .024” (11.5cc) Head Gasket 9cc
Head 69cc
Total 89.5cc
Compression Ratio 10.3:1
 
KevC said:
Hi, I haven't measured the compression, it's just on the stats the engine builder gave me, so no, I can't be certain.
Here is what he gave me:
4.030 Bore
Piston to Deck .024” (11.5cc) Head Gasket 9cc
Head 69cc
Total 89.5cc
Compression Ratio 10.3:1
Click to expand...

Well, those seem to be measured specs.......they are all we have to go on so.......
 
He's in Europe so his 99 octane is approximately equivalent to the US 93 octane premium. Not race gas but not regular swill either.
 
What length rod? Stock? I assume 6.123? Also if you know it, deck clearance? If not, I can play around with all the other numbers and ge tthe compression right.
 
gzig5 said:
He's in Europe so his 99 octane is approximately equivalent to the US 93 octane premium. Not race gas but not regular swill either.
Click to expand...

Thanks for that! I hadn't noticed.
 
Childs & Albert Con Rods I-Beams
6.123 Rod Length
Piston to Deck .024” (11.5cc) Head Gasket 9cc
Thanks
 
gzig5 said:
He's in Europe so his 99 octane is approximately equivalent to the US 93 octane premium. Not race gas but not regular swill either.
Click to expand...
Like I said I have a similar combination. Maybe not that light of a cam but still. This is great information here as when he said 99 octane I just assumed it was 99 octane LOL. I recently tried running mine on 92 non ethanol and it wouldn't shut off so I have to toss a gallon or two of 110 race fuel so it will turn off....
 
Yeah, I get 8.2 DCR using the cam you have now which is probably conservative. Take a look at this one.

COMP Cams Specialty Camshafts 20-324-4

I cannot get into the Comp site right now to get the full specs. I don't know if it's me or them. But rest assured, it will have a late enough intake valve closing event to fix your problem. I like the fact that it is a low lift cam too. Less stress on everything. It's almost like a cheater cam. Way nastier than what you have now. Maybe @yellow rose can come up with the IVC and give you the DCR numbers. I cannot get into the comp site to see those specs. I like that cam though.
 
KevC said:
I think you may be mistaken regarding the fuel. Shell V-Power is 99 RON high grade fuel?
Thanks
Kev
Click to expand...

Step out and run a compression test and post up the numbers. That right there will tell the story. I would want you to do that anyway before you change anything.
 
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