Issues timing a new rebuilt 408

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Mopar204

Mopar204

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not the smartest when it comes to engine building and adjusting but I put together a 408 stroker myself last year and can’t seem to get rid of a timing knock and an underpowered engine. 408 with scat stroker kit forged flat top pistons. Ported 202 heads. Roller lifter 525 lift. M1 mopar high rise intake. 650 Holley. Set my idle timing at around 14ish and at high rpm my timing is going past 40 with mechanical advance. Not sure if that’s normal or not so I’ve come to the mopar experts here lol. Compression is around the 10.5 area
 
Limit your mechanical advance.
You'll want max. 34-36° of advance, depending on cr-ratio and Dynamic cr-ratio.
 
This requires removing the D, taking it apart, welding the slots a bit shorter, evening them up, and slapping it all back together again.

I highly recommend a compression test to see where you are at.
 
Are you using a vacuum advance and if so is it on ported or manifold vacuum? I would think you need to limit mechanical advance to about 30 degrees and hook the vacuum advance to manifold vacuum and have it limited to about 24-26 degrees. So at start up and idle you have the vacuum advance pull the advance all the way to 24-26 degrees and at cruise you have a fair amount of advance but at wide open throttle the vacuum advance drops off and the mechanical takes over and give you full advance. You will have to play with the spring tension on the mechanical weights. Good luck on your mission.
 
get an fbo limiter plate, always adjustable and easier to install than welding slots, id start around 20 initial myself and unhook vac advance for now
 
I'm with AJ...a spot of weld is pretty easy.
the limiter plate,springs,shipping and exchange add up for us guys north of the border
 
Turns out my distributor had an adjustment plate on it with screws and slotted holes. Fixed the issue with too much timing. As for compression. I should’ve figured out my compression ratio before throwing everything together but I was in such a hurry on getting it on the road. Compression test is high, like over 200psi which doesn’t seem right to me
 
I used Quiksteel putty on the advance slots of a distributor on an old motorcycle (with points ignition) and then just used a round file to make the slots give the advance I wanted. That stuff is still in place, works fine...and was cheap.
 
Mopar204 said:
Turns out my distributor had an adjustment plate on it with screws and slotted holes. Fixed the issue with too much timing. As for compression. I should’ve figured out my compression ratio before throwing everything together but I was in such a hurry on getting it on the road. Compression test is high, like over 200psi which doesn’t seem right to me
Click to expand...
Yeah, I can't see that much pressure either with a 10.5Scr-408 with a .525 roller cam.Something doesn't seem to be computing.
However, to get a 408 cuber,DOWN to 10.5Scr requires a TOTAL chamber volume of 88cc. So depending on where your heads came in at, this might require a good bit of deck clearance.
I wonder if your ICA is big enough to drop the Dcr into pumpgas compatibility with iron heads, and full power-timing...
 
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AJ/FormS said:
Easier sure, but you can mod the D and be back on the road in an hour........ or so,lol.
Click to expand...

I did that 'dance' once about 5 or 6 times in a day, to try and really dial in the distributor.
Having the R&R these cumbersome Mopar distributors get kinda old real fast, especially dealing with hot engine and parts.
It's the main reason I have a digital ignition now. :)
 
In order to get close to 10:1 with my x head 408 I had over a 20cc dished piston. I’m a little over 12:1 with the same combination except a 5cc flat top. They are still down in the hole .010, chamber is about 63cc .027 head gasket. You may be high in static compression. You will pick up a bunch going to bigger carb. I ran a mighty demon 825 on mine for awhile. Now run 800 avs eddy.
 
BigBlockMopar said:
I did that 'dance' once about 5 or 6 times in a day, to try and really dial in the distributor.
Having the R&R these cumbersome Mopar distributors get kinda old real fast, especially dealing with hot engine and parts.
It's the main reason I have a digital ignition now. :)
Click to expand...
I hear you.
I modded my D with a screw on top to do away with that ignorant clip. Then I modded several cams, And several pairs of weights.
Then I got me a dial-back-from the cab ignition module, and a G-techSS.
After that it was just a few minutes per change
After that I got a lot of local tuning business,lol.
 
With all the research I have been doing for a stroker, the flat top pistons put you way up there close to 12:1. I think for any reasonable build you have to have the dish. You don't say what heads you have.
 
J heads. Could I just get a thicker head gasket to lower my compression or is that a ghetto way of doing that? I knew I should’ve done more research and asked around before throwing everything together
 
Mopar204 said:
J heads. Could I just get a thicker head gasket to lower my compression or is that a ghetto way of doing that? I knew I should’ve done more research and asked around before throwing everything together
Click to expand...
I would get a second witness as to the cylinder pressure.
If your pistons were dished and you didn't deck the block, you might be in pretty good shape.

But I see there is 14cc difference in their 10.8 kits to their 9.3 kits, with 70cc heads. It would take an extremely small cam to make 200psi tho with the 10.8@70cc kit.So I would tend not to believe that compression tester. The more typical 268 cam might make 175psi. Still too much for iron heads, but you get the idea.
 
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AJ/FormS said:
I hear you.
I modded my D with a screw on top to do away with that ignorant clip. Then I modded several cams, And several pairs of weights.
Then I got me a dial-back-from the cab ignition module, and a G-techSS.
After that it was just a few minutes per change
After that I got a lot of local tuning business,lol.
Click to expand...

This might help you out.
dist info on curves.png
 
So if I want decent compression I should pull out the flat tops and put in dished pistons?
 
Well i appreciate all of the help guys. From the sounds of it the pistons need to go if I don’t want headaches and reasonable compression ratio. Maybe this time I’ll get the heads ccd and find out my bore and get everything calculated properly.
 
Fix your 40°+ timing first before tearing into the engine and pour in money.
New pistons also means rebalancing the crank assembly.

Installing thicker headgaskets might not mean you will be 'ping'-free.
If your engine might has a decent quench-height now, a thicker headgasket might make that less optimal and make the engine even more prone to ping.
 
I fixed the pinging. I adjusted the limit on the mechanical advance, spark knock is gone and timing is around 35 with no ping
 
If it’s not pinging then you may not have an issue. They could be so far in the hole that your compression is pump gas friendly. So is it pulling better?
 
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