Quench vs. Compression on a daily driver.

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Ben A...

Ben A...

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Hey there,
I'm trying to decide on a headgasket thickness for my LA360. It will be a daily driver in Canada, except winter. 91 octane is fine, but I don't want to go higher due to availability. And I'm concerned about detonation.

I can't figure out if quench should take priority over my compression ratios.

I know a head gasket is just for fine tuning an already well designed system, but I'm wondering if all other factors in the engine build were fixed...
Stock stroke LA360 w/ Aluminum heads, bored 4.03
63cc chamber
5cc dished pistons
.017in deck clearance
Cam IVC ABDC: 67deg

Option 1: .026in gasket would be ~10.7:1 static compression, DCR: ~8.37:1. Quench: .043in
Option 2: .039in gasket would be ~10.39:1 static compression, DCR: ~8.13:1. Quench: .056in
Option 3: .056in gasket would be ~9.99:1 static compression, DCR: ~7.83:1. Quench: .073in

I imagine increasing the chamber volume and keeping a .043in quench is ideal, but if I can't do that.. Which is the best way to go here?

I'm not really looking for max performance right now, just something reliable.
A pro engine builder told me that option 2 is the best way to go, and to go easy in timing. But I wonder if I should just go with option 3 because the internet tells me that above 10:1 static and 8:1 dynamic I'm running close to trouble.

What would you do?
Thank you for any help.
Ben

EDIT: Poll--
Option 1: 7 Votes
Option 2: 1 Vote
Option 3: 1 Vote
 
Last edited:
Ben Ainsworth said:
... Oh no.. What have I done?
Click to expand...

It's usually a can of worms. IMO with aluminum heads, you should be able to go with the thinnest gasket and still run pump gas. As far as power output, it's really splittin hairs with any of the three. Now watch the mayhem ensue.
 
RustyRatRod said:
It's usually a can of worms. IMO with aluminum heads, you should be able to go with the thinnest gasket and still run pump gas. As far as power output, it's really splittin hairs with any of the three. Now watch the mayhem ensue.
Click to expand...
Thanks RustyRatRod -- I appreciate your input.
I worried this might be a matter of debate and experiment.
I hear a lot about the importance of proper quench, but not much about significantly lowering my compression from the 'danger zone' and worrying less about quench.
 
Compression makes power! Aluminum heads you can get away with less than perfect quench. You can always adjust the timing, so if it starts to ping take a degree or two out of it. IMO 65'
 
Combo 2 with the limited info. Carb rear gear weight. My car runs great at 11.? 3100 lbs. 4.30 gear quench 180 - 200 degrees-stock cooling system
92 octane pump 10% ethanol.
 
My contribution to the mayhem LOL:

For option 3, at .073", conventional thinking is that you really don't have any quench effect any more. Option 2 is some limited quench effect. Option 1 is getting some real effect.

Just a few checks:
  • What pistons?
  • What cam?
And you do know that you can retard the cam and lower your DCR? I would do that and keep to option 1 or 2.

And increase that 5 cc in the pistons to 6 cc to add in the volume down around the top of the pistons down to the top rings.

With that, and 2 degrees of added retard on the cam timing, you are now at 7.9 to 8.0 with option 2.

I've run 8.3 DCR on the highway on 93 premium with quench.... IIRC in the .050" range. I had to limit ignition timing a bit; can't tell you the exact numbers anymore.. that was 40+ years ago LOL.
 
Marcohotrod said:
Combo 2 with the limited info. Carb rear gear weight. My car runs great at 11.? 3100 lbs. 4.30 gear quench 180 - 200 degrees-stock cooling system
92 octane pump 10% ethanol.
Click to expand...

DP650 carb, 2.76 gear, not sure on weight tbh. Thank you for thoughts here.

EDIT: That said I'll hopefully be changing the gear to shorter
 
Last edited:
nm9stheham said:
My contribution to the mayhem LOL:

For option 3, at .073", conventional thinking is that you really don't have any quench effect any more. Option 2 is some limited quench effect. Option 1 is getting some real effect.

Just a few checks:
  • What pistons?
  • What cam?
And you do know that you can retard the cam and lower your DCR? I would do that and keep to option 1 or 2.

And increase that 5 cc in the pistons to 6 cc to add in the volume down around the top of the pistons down to the top rings.

With that, and 2 degrees of added retard on the cam timing, you are now at 7.9 to 8.0 with option 2.

I've run 8.3 DCR on the highway on 93 premium with quench.... IIRC in the .050" range. I had to limit ignition timing a bit; can't tell you the exact numbers anymore.. that was 40+ years ago LOL.
Click to expand...

Thank you for the thoughts!

- Speed Pro H116CP pistons
- Mutha Thumpr, which is at 102 intake centerline, really not sure about retarding it...

I was wondering best option without increasing volume of the pistons or chamber, but I suppose machining the pistons might be the best choice if I had to go that way... ? But, still hoping to just fine the best head gasket option and see how that goes.
 
To me your choices are 1 and 3

2 loses quench and still pretty much the same cr as 1.

I go with 1, and if it don’t work go with 3.
 
option 1 is the only option. imo
Though I like to see quench at a maximum of .035.
 
None of the above...just to add to the confusion...1 has not enough quench and 3 has to much compression....plus you will need the 2.76 gears for driving at 150kph on the 401...
 
In Manitoba,with an aluminum headed 367, it is almost impossible to have too much cylinder pressure. My combo has run over 180psi ....... on 87E10. One combo was pushing 200.
Currently she is running 180psi with a Q of .034 .... And I often/usually, having done some oiling mods, spin it up to 7000/7200 . Just because.
And she runs 205* minimum coolant temp.
The Scr is ~11/1, has been as hi as 11.3
With a 4 speed, she has run a starter gear down to 8.59. But currently runs 10.97..
That Thumper is awful hard on gas, especially around town. You can make the same amount of power with a regular cam of similar .050 duration,and get waaay better fuel efficiency, But you would lose the wicked idle-lope.
The 2.76s do NOT have to go, because ; with a regular TorqueFlite automatic,your power peak is gonna be a tic over 5000 rpm/57 mph, and 65 mph will be ~5700. I actually like it. But to get off the line briskly, that engine will like a bit higher than stock stall, say a 2800.
The bonus here is 65mph=2230@zero-slip.
But a 3000 will provide a lil more excitement on the 2-1 kickdown at 32 mph.
IMO, with this combo, 3.23s is the most I would install for a DD
 
Last edited:
Ben Ainsworth said:
Hey there,
I'm trying to decide on a headgasket thickness for my LA360. It will be a daily driver in Canada, except winter. 91 octane is fine, but I don't want to go higher due to availability. And I'm concerned about detonation.

I can't figure out if quench should take priority over my compression ratios.

I know a head gasket is just for fine tuning an already well designed system, but I'm wondering if all other factors in the engine build were fixed...
Stock stroke LA360 w/ Aluminum heads, bored 4.03
63cc chamber
5cc dished pistons
.017in deck clearance
Cam IVC ABDC: 67deg

Option 1: .026in gasket would be ~10.7:1 static compression, DCR: ~8.37:1. Quench: .043in
Option 2: .039in gasket would be ~10.39:1 static compression, DCR: ~8.13:1. Quench: .056in
Option 3: .056in gasket would be ~9.99:1 static compression, DCR: ~7.83:1. Quench: .073in

I imagine increasing the chamber volume and keeping a .043in quench is ideal, but if I can't do that.. Which is the best way to go here?

I'm not really looking for max performance right now, just something reliable.
A pro engine builder told me that option 2 is the best way to go, and to go easy in timing. But I wonder if I should just go with option 3 because the internet tells me that above 10:1 static and 8:1 dynamic I'm running close to trouble.

What would you do?
Thank you for any help.
Ben

EDIT: Poll--
Option 1: 1 Vote
Option 2: 1 Vote
Option 3: 1 Vote
Click to expand...

vote 1
 
Because you asked, Option 1 with aluminum heads. Option 3 if you were using iron heads.
 
option 1
and tune
option 3 is in the zone where you get more detonation no matter what your compression
option 1 you have no burn on the intake side, quench -two cool surfaces and squish
#3 what do you have?
 
Ben Ainsworth said:
Thank you for the thoughts!

- Speed Pro H116CP pistons
- Mutha Thumpr, which is at 102 intake centerline, really not sure about retarding it...

I was wondering best option without increasing volume of the pistons or chamber, but I suppose machining the pistons might be the best choice if I had to go that way... ? But, still hoping to just fine the best head gasket option and see how that goes.
Click to expand...
Have you actually measured the piston to deck height? I ask becasue, in adding up the numbers, piston height is 9.573" in a nominal LA deck height of 9.600". So that is a deck clearance of .027", not .017" as originally posted. Quench gap is now .027 +.028 = .056" with option 1, the thinnest head gasket. This is all based on the nominal deck height so it may vary + or - a bit in the actual block.

That .027", along with another 1 cc in the piston top area (use 6 cc rather than 5 cc, and that 1 cc is something you should always add in), gets you to 10.25 SCR at the thinnest head gasket (option 1), and an SCR of 8.1. Retard the cam 2 degrees and you get to 8.0 on the DCR. (And that ICL of 104 gives you an IVC of 67 degrees, by the way. I am thinking you have the 20-601-4 cam; is that the right PN?)

This new deck height now points to option 1 for me.

Don't have any fear of retarding the cam a couple of degrees. You will have as much low RPM torque as you need, even with this big overlap cam. The cam ICL is set to 102 simply to help make up for the low RPM torque-killing character of this cam type in a stockish piston setup, but you'll have plenty of cylinder pressure with the H116CP's to work around that. Easy to set the cam timing with one of the multi-keyway timing chain sets.
 
And this is why guys get into trouble running pump gas and higher than “normal” compression ratios.

The cam should have been the LAST piece selected based on everything else.

Now the OP is trying to adjust the CR to fit what he has, and that’s always bad.

Ive said it before, but I’ve tested quench. If you can get it and IF the CR isn’t too high getting it, run it. If you can get the CR you want and don't have quench, run it. It’s not a small block Chevrolet. If you can get both the CR you want and quench, do it.

I’m changing out my head gaskets to raise the CR. By doing that, I’m closing the quench down to .042 from .062 and even with the quench in the “danger zone” I can run full timing and have zero detonation issues, but I planned for a CR that high.
 
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