Poly Stroker Build Timing Issue

[katmachine62]

I have a 62 Dodge Dart Sedan (in Australia) which I've had a stroker 318 Poly (now 392) built with mild cam ([email protected], .450 lift, running a 2bbbl Holley 500cfm (AFR's: 12.8 - 13 @ Idle and Cruise, into the 10's under load). Everything is new, and engine is balanced, 10:1 compression, running standard 727 with 3.23 rear end. I only run 98RON Octane Premium fuel...
However, from day one, I have only been able to run approx 3/4 degrees of intial timing without suffering detonation (under load at approx 2800/3000 rpm).
I had the original Distributor completely rebuilt and Pertronix 2 module added. Specs as follows - Base timing should be -/+ 14 BTDC, Curve - Mechanical Advance starts at 1000 rpm, has 12 deg @2000 rpm, and 16 deg of Total at 3600 rpm (3600 - 4000rpm is the max my engine will ever pretty much see) Vacuum Advance is 21 deg at 19" of Vac, just for reference.
Interestingly, at 8/10 deg BTDC of initial timing the car starts a lot easier, and runs substantially cooler; at 3 degrees I need to give the engine about 6 full pumps of the throttle to cold start, and the engine runs approx 20% hotter!
Before I spend more money seeking expert shop opinion, dyno testing etc., thought I would run it past the brains trust here for any helpful insights... thanks in advance...

 

[66 Sat]

Unfortunately you have a bad combo of big cubes (for a small block), small cam, high compression and relatively low octane fuel (our 98 is not as good as US high octane)
I have the same cam in my stroker poly (only 354 cubic inch) and with 10.8:1 compression I can only run about 12° initial, otherwise same detonation issue as you.
If it's only at part throttle then it's the vacuum advance unit adding too much timing. I pulled mine and JB Welded a couple of small washers under the arm to limit the amount of advance it can add.
You could also try a bigger cam, or maybe some thicker head gaskets to lower the compression a bit.

 

[turbine68rt]

What spark plugs are you using? Are they the proper heat range for your application?

 

[katmachine62]

What spark plugs are you using? Are they the proper heat range for your application?

Great question... I just thought of this, via searching the FBBO site.
I am currently using the recommended Autolite 85 spec plugs... My ground strap suggests that this plug may be too hot for my application ( I have no timing / colour change until right at/on the base ring)
So, I was potentially going to test some Bosch (which I already have in my stash as recommended, or NGK BP5S )
Your thoughts!?...

 

[katmachine62]

Unfortunately you have a bad combo of big cubes (for a small block), small cam, high compression and relatively low octane fuel (our 98 is not as good as US high octane)
I have the same cam in my stroker poly (only 354 cubic inch) and with 10.8:1 compression I can only run about 12° initial, otherwise same detonation issue as you.
If it's only at part throttle then It's the vacuum advance unit adding too much timing. I pulled mine and JB Welded a couple of small washers under the arm to limit the amount of advance it can add.
You could also try a bigger cam, or maybe some thicker head gaskets to lower the compression a bit.

You have higher comp than me, and can run 12 deg initial... That is a very different scenario to mine... I should be able to run 8/10 deg initial I would have thought...

 

[408Swinger]

Put a vacuum pump on your vacuum advance to see where it is coming in at . If there is a hexagon on the can it is adjustable with a small allen wrench . Assuming you have a Mopar distributor.

 

[katmachine62]

Put a vacuum pump on your vacuum advance to see where it is coming in at . If there is a hexagon on the can it is adjustable with a small allen wrench . Assuming you have a Mopar distributor.

Please read the vac adv specs I provided... No vac adv until 19" (light throttle). I'm not having any issues until I push her significantly...

 

[408Swinger]

Missed that . Wow! How much vacuum does that engine pull?

 

[turbine68rt]

Great question... I just thought of this, via searching the FBBO site.
I am currently using the recommended Autolite 85 spec plugs... My ground strap suggests that this plug may be too hot for my application ( I have no timing / colour change until right at/on the base ring)
So, I was potentially going to test some Bosch (which I already have in my stash as recommended, or NGK BP5S )
Your thoughts!?...

I'm currently using Bosch Platinums in my 440 with excellent results. These are replacements for a number of different heat ranges. They don't miss a beat and zero pinging.

 

[66 Sat]

Please read the vac adv specs I provided... No vac adv until 19" (light throttle). I'm not having any issues until I push her significantly...

Does it ping at full throttle (foot to the floor), or only with heavy throttle, say 3/4 push?

 

[katmachine62]

Missed that . Wow! How much vacuum does that engine pull?

Yeah, plenty... Mild cam and all

 

[katmachine62]

Does it ping at full throttle (foot to the floor), or only with heavy throttle, say 3/4 push?

Definitely at 3/4 effort, and also floored, from memory

 

[katmachine62]

I'm currently using Bosch Platinums in my 440 with excellent results. These are replacements for a number of different heat ranges. They don't miss a beat and zero pinging.

Okay I'll definitely try those and report back , hopefully in a few days time

 

[66 Sat]

Definitely at 3/4 effort, and also floored, from memory

That's something you'd want to confirm. If it doesn't do it at full throttle then it's the vacuum unit adding too much and tipping you over the edge. Try disconnecting and plugging it, run 10° initial and see if it pings.
If it doesn't ping then you know it's the vacuum unit and you can start experimenting by limiting the amount it's adding or adjusting the rate if you have an adjustable unit. Both these options cost nothing.

Just looking at your original post, you say you only have 16 degrees all in? Should be around 34. Are you saying 14 initial plus the 16 to give 30?

I wouldn't start buying new spark plugs yet, just determine exactly what's going on before throwing money and parts at it.

I know from my experience that unfortunately these motors seem prone to detonation with high compression and a small cam. Mine will still do it sometimes if accelerating uphill in a high gear. Can depend on the temperature of the day or even a bad batch of fuel. I wish the engine builder had gone for say 9.5:1 and gave up a few horsepower as it's very frustrating trying to sort it out.

 

[turbine68rt]

I wouldn't start buying new spark plugs yet, just determine exactly what's going on before throwing money and parts at it.

He stated that he already has different plugs in stock.

 

[66 Sat]

He stated that he already has different plugs in stock.

I missed that thanks.

 

[Don Frelier]

I hate to give more bad news but this will only get worse as you get some carbon buildup in the cylinders.

 

[66 Sat]

My engine builder put a quench pad in the pistons but I still have problems so I think you are right.
It's a shame because it's a nice torquey engine that can easily pull the high gears at low revs.
Lowering my compression or putting in a bigger cam is on my list for future modifications.

 

[katmachine62]

That's something you'd want to confirm. If it doesn't do it at full throttle then it's the vacuum unit adding too much and tipping you over the edge. Try disconnecting and plugging it, run 10° initial and see if it pings.
If it doesn't ping then you know it's the vacuum unit and you can start experimenting by limiting the amount it's adding or adjusting the rate if you have an adjustable unit. Both these options cost nothing.

Just looking at your original post, you say you only have 16 degrees all in? Should be around 34. Are you saying 14 initial plus the 16 to give 30?

I wouldn't start buying new spark plugs yet, just determine exactly what's going on before throwing money and parts at it.

I know from my experience that unfortunately these motors seem prone to detonation with high compression and a small cam. Mine will still do it sometimes if accelerating uphill in a high gear. Can depend on the temperature of the day or even a bad batch of fuel. I wish the engine builder had gone for say 9.5:1 and gave up a few horsepower as it's very frustrating trying to sort it out.

Thanks for your input!
I have already tried disconnecting the vac advance and running with 10,8,6 deg.if initial and it still detonates unfortunately... And at 3/4 or full throttle I'm thinking I would have nowhere near 19" of vac for the vac advance to be activated
I (should) have approx 12/14 deg initial, and (confirmed) have 16 deg mechanical built in to the dizzy curved, for an overall Total of 30 deg....
I already have the Bosch plugs and am curious to try, albeit not optimistic ... Any other suggestions or insight would be much appreciated...

 

[66 Sat]

Mate I'm out of ideas unless you haven't really got 10:1 compression and it's much higher, or the TDC mark on your balancer isn't correct.
If it's still detonating at only 6 degrees initial with the vac advance plugged then I don't know, but there are many others on here far more knowledgeable than I am - maybe put it on the General Forum and see if you get more input?
Also can you go back to the engine builder and see what he says? Verify what he's given you?