Quench vs Compression ratio vs Detonation

[Charlie Brown]

I know there is a lot of information on this topic. There seems to be as may recipes for this as there is for your Mom's apple pie - but here goes.
I pulled a 413 engine out of a car a few years back that was running well. I decided to take it apart and see what it might need. Engine stats are as follows -
413 - .030 over (4.22 bore)
3.75 stroke
closed chamber iron heads - factory cc'd at 73.5
flat top piston, down .060 in the bore
aftermarket cam - intake lift .466, lobe center 109, advertised duration 298, exhaust lift .488, lobe center 119, duration 303, overlap 71
factory rated compression is rated at 10.0 to 1
calculators put this combo at around 10.1 to 10.2
above calculations based on factory .022 compressed steel head gasket
So by my calculation, I have around .080 quench
This engine did ping under certain conditions - when under full load and when outdoor temps were high.
Fuel used was 91 octane and ethonal free. I tuned the engine timing as best I could to handle the pinging.
Upon tear down, most of the exhaust guides need replacing. I brought the heads out to the machine shop and he sugessted to mill the heads to reduce the quench to .060 or less.
So after all this, my question is should I mill the heads or not, assuming they are flat. If I mill the heads .020, that will raise the compresion ratio by about a half point, but drop the quench to .060. I'm not liking that much compression on a fairly stock all steel engine. On the other hand, reducing quench seems like a good idea.
I did have carbon build up in the combustion chambers and on top of the pistons, which I'm sure didn't help the detonation issue. Upon re-assembly these issues will be taken care of. So many variables - head chamer cc, head gasket thickness, milling of heads and possibly other areas.

I'd be really interested in some feedback in this regard.

 

[Kern Dog]

Years back when I had some detonation problems, among the many suggestions that were made, I found the following hard to believe:
"Increase the compression ratio to increase quench and this will reduce detonation."
It sounds completely contradictory. It is like you're running on empty so you drive faster to get to the gas station.
I still don't like the sound of it. I tried thicker head gaskets and it stopped the knocking. ANY iron head engine over 9.5 to 1 on even pump premium seems to be at risk of detonation from what I understand.
Now, higher compression with quench using aluminum heads and dished pistons is another thing:

In 2022 I pulled the 440/493 and went through it, using these pistons that contributed to a 9.8 to 1 ratio that runs strong and can even do so on 89 octane!

 

[1 Wild R/T]

Milling closed chamber heads will raise the compression ratio but it won't effect quench.... Either deck the block or new pistons with a higher C/H will effect quench.... At .080 you have no effective quench.... Ideal quench would be around .040, by around .055 quench is pretty much non existent..

The pistons Greg shows are the ideal solution... Doubt anyone makes them for a 413...

 

[R/T Boy 67]

The factory rated combustion chamber size is inaccurate. Those heads are probably closer to 80cc.

 

[1 Wild R/T]

BTW that's not carbon, there is a lot of oil deposits in the chamber, oil in the chamber makes detonation occur easier....

 

[65CopCar]

The pistons being 0.060" in the hole is a big problem for good quench.

 

[autoxcuda]

aftermarket cam - intake lift .466, lobe center 109, advertised duration 298, exhaust lift .488, lobe center 119, duration 303, overlap 71

What cam is that? What is the rest of your combination: intake?, carb?, headers?

Are those heads 2.08 intake and 1.74 exhaust valve size? Most closed chamber 413 heads are small valve?

 

[BSB67]

Milling closed chamber heads…..won't effect quench.... Either deck the block or new pistons.

This, and

The factory rated combustion chamber size is inaccurate. Those heads are probably closer to 80cc.

This.

 

[PRHeads]

Since there is already no effective quench, I’d just use thicker gaskets and lower the compression.

 

[BSB67]

We don’t know you budget or goals.

Probably your best performance boost on a budget would be 75 cc Eddy heads and a better cam.

 

[Dodge Bros]

That cam appears to be a Summit 6401 cam or similar white box cam grind. The seat timing numbers listed SAE numbers and the exhaust side is a misprint in a catelog at some point. It is 308 not 303 on the exhaust, SAE is rated .006” valve lift duration out the valve, it is effected by the rocker ratio.


There will be no benifit to trying to add quench with that small cam with that combination. The compression will be to high. Better off lowering the compression with thicker head gaskets if keeping that cam.

 

[PRHeads]

It’s also a good idea to cc a chamber, so you really know what CR you’re actually working with.

 

[autoxcuda]

Seems like a small step bigger cam than stock?

Camshaft Manufacturers Description:
Fair idle, good midrange power camshaft. Requires 9.5:1 and higher compression, 2,000+ stall, and gearing. Good mild bracket racing cam.
Basic Operating RPM Range:
2,000-5,500
Intake Duration at 050 inch Lift:
224
Exhaust Duration at 050 inch Lift:
234
Duration at 050 inch Lift:
224 int./234 exh.
Advertised Intake Duration:
298
Advertised Exhaust Duration:
303
Advertised Duration:
298 int./303 exh.
Intake Valve Lift with Factory Rocker Arm Ratio:
0.466 in.
Exhaust Valve Lift with Factory Rocker Arm Ratio:
0.488 in.
Valve Lift with Factory Rocker Arm Ratio:
0.466 int./0.488 exh.
Lobe Separation (degrees):
114

 

[66 Sat]

BTW that's not carbon, there is a lot of oil deposits in the chamber, oil in the chamber makes detonation occur easier....

I've had detonation issues on my poly for about for about 4 years since the stroker rebuild - 10.8:1 compression, small-ish cam. I was getting oil into one cylinder from a leaking steel shim intake gasket, one spark plug would get oily and really crusty. The other cylinders may have been seeing smaller amounts of oil too.

Finally re-did the intake with a composite gasket 9 months ago and now the plug is clean. Detonation seems to have gone away too.

Carburation seemed to help in my case as well. I was getting more detonation with an AVS2 650 than with a Holley 650 D/P.

 

[autoxcuda]

I've had detonation issues on my poly for about for about 4 years since the stroker rebuild - 10.8:1 compression, small-ish cam. I was getting oil into one cylinder from a leaking steel shim intake gasket, one spark plug would get oily and really crusty. The other cylinders may have been seeing smaller amounts of oil too.

Finally re-did the intake with a composite gasket 9 months ago and now the plug is clean. Detonation seems to have gone away too.

Carburation seemed to help in my case as well. I was getting more detonation with an AVS2 650 than with a Holley 650 D/P.

You have higher octane gas in Australia?

10.8:1 on US pump 91 Octane is very tough with an old motor with small cam. With California 91 spec, it just won’t happen.

 

[66 Sat]

You have higher octane gas in Australia?

10.8:1 on US pump 91 Octane is very tough with an old motor with small cam. With California 91 spec, it just won’t happen.

We get 98 RON which is the equivalent to 93-94 US.

We also have 91 & 95 but both my cars have to use 98.

 

[lewtot184]

From looking at the pictures there's so much oil, maybe too rich, head gaskets leaking and who knows what else that no modern gas will support. Good quench is impossible with that combo. Try somewhere between 9:1-9.5:1 compression with a very clean burn.

 

[Don Frelier]

Unless you're a stock set-up purist, I would suggest looking for some other heads. Stealth or e-street.
Maybe a set of redone ironl heads if you can find a good deal.

Do a real cc test on the heads and run a calculator before choosing a gasket set.
If you stick with iron I would try and get towards 9:1.

 

[Runcharger]

Not sure what the end goal is. Engine had an oil control problem and will never make quench with those pistons in the hole that far. Fixing oil control will probably repair the detonation if thats all you are looking for. After that, different pistons and heads will get you farther performance wise.
If it was mine I'd either ball hone it and freshen the heads as cheap as I could or set it in the corner and build a 440 based new combo. But it's not my money...
Also, if the heads are flat and in good gasket sealing condition I would not mill them. If you had to mill them I would use a thicker head gasket.

 

[Charlie Brown]

Some very good information provided. Thank you all.The end goal is not to dive deep into this engine. My main goal is to try and reduce / eliminate the detonation working with what I have. Oil control seems to be at the top of the list along with a reduction in compression. The heads are out for reconditioning which should help alot with the oil controll issue. The cylinder bores look excellent, with cross hatch still present and no ridge. I'm thinking I'll use the Fel Pro .039 gaskets instead of the factory steel units which spec out at .022 compressed. This should provide about a .35 reduction in compression. I understand this will increase quench, but it seems to be of less importance compared to the other areas of concern. As an aside, this is the car that has this engine.