Not unless you need new pistons and or boring to bigger size
Swap pistons for quench?
- Thread starter Spadman
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AJ/FormS
68 Formua-S fastback clone 367/A833/GVod/3.55s
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I'm not gonna tell you what to do.
I'm not even gonna hint.
but first I want to review how you got to 9.5 Scr?
Your swept volume at 4.03 bore and 3.58 stroke is 748.3cc
Your total combustion volume is ;
59cc heads/8.6cc @.039 gasket/10.45cc [email protected]/and assumed 5cc eyebrows =83.05
and so the Scr is (83.05+748.3)/83.05 = 10.01
IDK how you got to 9.5.
but ok, the Q is gonna be .050 +.039+ a bunch= greater than .089; so very safe as to detonation
In fact so safe that, lets swap out the 039 for the 028 and the Scr goes to,
(81.25 + 748.3)/ 81.25 =10.2, and the Q is still way over .078, so is safe.
And this makes the Wallace Calculator very happy, as the pressure is predicted to be under 160psi, with an Ica of 68*; an easy pumpgas number. If she runs clean there, I might advance the cam two more degrees, to Ica of 66*.
So IDK
That big cam is still gonna be a lil soft on the bottom, but she'll suffer 3.23s with a decent convertor.
The point of this story is to go back to the math; I checked mine several times.
Edit, Feb 3
With alloy heads, I once ran the pressure in my 367 to as high a 200 psi, on 87E10 at full power-timing with the Q at .028. I did this with a 223 cam, a manual trans, and 3.55 gear (mostly) in a 68 Barracuda.
Be aware, I tried almost every gear combo you can think of, and;
That combo pulled 2.76 gears reasonably well, and was very happy with 3.23s
With another overdrive combo it pulled 75mph @ 1800 rpm, giving me back over 30 mpgUS in steady-state cruising.
By far, that combo has always been my favorite;
Gobs of torque, plenty of power, instant throttle response,
floor it any time, pulls any gear, drive it anywhere anytime, it was my DD.
The Wallace Calculator puts the V/P of this combo at 172, which is beyond BB Territory. I was very sad when it suddenly dropped cam-lobes.
Even sadder when I upsized the cam just one size.
Those Edelbrock heads, with the small hi-lift cam, driven as a DD, payed for themselves pretty quick. I think I once worked it out to, in two years, running that skunk-pee that everybody loves to condemn.
I'll never again run open-chamber Iron heads, at sub-optimal pressures.
However, I would be willing to try closed-chamber, Iron heads, at high pressure with a really tight Q. and
one thing I've always wanted to try, is exactly what you're contemplating, namely; filling the open Iron-chambers with a tight Q-pad, calculating the Scr, and then selecting whatever ICA will get the pressure up to as high as can be safely run at WOT, with pumpgas, without adding anti-knock; and then translating that Ica to whatever cam it comes on.
If you decide to try this I would be eager to see it.
I have 20 years experience with those Eddies on my 367, with three different cams, four different Scrs, and five different Quench Heights. The car has run four different manual transmissions, two overdrives, and nearly every rear end from 2.76 to 5.38 But no autos, behind the 367. The tires progressed from 245s to 325s., then settled at 295s. On the one and only successful run at the track, it went 93 in the Eighth @7.92, with a 2.4 second 60 ft. It was reported to me by a track official that she was still hazing the tires thru the traps. (yes, crappy old airstrip)
Anyway, the wife is calling, time to bath the dogs.
I'm not even gonna hint.
but first I want to review how you got to 9.5 Scr?
Your swept volume at 4.03 bore and 3.58 stroke is 748.3cc
Your total combustion volume is ;
59cc heads/8.6cc @.039 gasket/10.45cc [email protected]/and assumed 5cc eyebrows =83.05
and so the Scr is (83.05+748.3)/83.05 = 10.01
IDK how you got to 9.5.
but ok, the Q is gonna be .050 +.039+ a bunch= greater than .089; so very safe as to detonation
In fact so safe that, lets swap out the 039 for the 028 and the Scr goes to,
(81.25 + 748.3)/ 81.25 =10.2, and the Q is still way over .078, so is safe.
And this makes the Wallace Calculator very happy, as the pressure is predicted to be under 160psi, with an Ica of 68*; an easy pumpgas number. If she runs clean there, I might advance the cam two more degrees, to Ica of 66*.
So IDK
That big cam is still gonna be a lil soft on the bottom, but she'll suffer 3.23s with a decent convertor.
The point of this story is to go back to the math; I checked mine several times.
Edit, Feb 3
With alloy heads, I once ran the pressure in my 367 to as high a 200 psi, on 87E10 at full power-timing with the Q at .028. I did this with a 223 cam, a manual trans, and 3.55 gear (mostly) in a 68 Barracuda.
Be aware, I tried almost every gear combo you can think of, and;
That combo pulled 2.76 gears reasonably well, and was very happy with 3.23s
With another overdrive combo it pulled 75mph @ 1800 rpm, giving me back over 30 mpgUS in steady-state cruising.
By far, that combo has always been my favorite;
Gobs of torque, plenty of power, instant throttle response,
floor it any time, pulls any gear, drive it anywhere anytime, it was my DD.
The Wallace Calculator puts the V/P of this combo at 172, which is beyond BB Territory. I was very sad when it suddenly dropped cam-lobes.
Even sadder when I upsized the cam just one size.
Those Edelbrock heads, with the small hi-lift cam, driven as a DD, payed for themselves pretty quick. I think I once worked it out to, in two years, running that skunk-pee that everybody loves to condemn.
I'll never again run open-chamber Iron heads, at sub-optimal pressures.
However, I would be willing to try closed-chamber, Iron heads, at high pressure with a really tight Q. and
one thing I've always wanted to try, is exactly what you're contemplating, namely; filling the open Iron-chambers with a tight Q-pad, calculating the Scr, and then selecting whatever ICA will get the pressure up to as high as can be safely run at WOT, with pumpgas, without adding anti-knock; and then translating that Ica to whatever cam it comes on.
If you decide to try this I would be eager to see it.
I have 20 years experience with those Eddies on my 367, with three different cams, four different Scrs, and five different Quench Heights. The car has run four different manual transmissions, two overdrives, and nearly every rear end from 2.76 to 5.38 But no autos, behind the 367. The tires progressed from 245s to 325s., then settled at 295s. On the one and only successful run at the track, it went 93 in the Eighth @7.92, with a 2.4 second 60 ft. It was reported to me by a track official that she was still hazing the tires thru the traps. (yes, crappy old airstrip)
Anyway, the wife is calling, time to bath the dogs.
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Thanks for checking my numbers. Your error is using 5 cc’s for eyebrows. These pistons have 4 and are advertised as adding 10cc. I used the Summit calculator btw. Easier than crunching the numbers myself. The manufacturer recommended installed centerline is 4 degrees advanced, so 66 degrees ABDC. The Wallace calculator shows why Old Man Mopar said if I cam it I need compression. Even 4 advanced the dynamic compression is only 7.5 and estimated cranking pressure of 145. I am aware I can play with the installed centerline to tune in dynamic compression.
My heads are iron open chamber J’s that have been milled years ago and I don’t remember what I requested then but checking the remaining area where the quench pad would go reveals the two are not the same. ( On a side note, the machine shop milled the heads with the valves installed and clipped the intake valves. I bought new valves and installed back then) Therefore, I’m going to go over the heads carefully before I finalize my piston selection. From what I have read, it would require a quench of 30 to 50 to be meaningful. Also I could stand a point of additional dynamic compression ratio. One piston that might work for me is the KB step UEMKB373 that has an .085 quench dome with a thick gasket. I’ll post the final numbers in my build post when I get there.
I’m an engineer by training so I can do the numbers. It’s the real world experience that is valuable to me. Thanks.
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Thanks Old Man Mopar. You’ve got me digging in on the effect of cam timing on dynamic compression. I don’t want to leave anything on the table. I’m in it for performance, not sound.
AJ/FormS
68 Formua-S fastback clone 367/A833/GVod/3.55s
- Joined
- Jan 19, 2014
- Messages
- 25,845
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If you use the Wallace Dynamic compression calculator, it has the advantage of spitting out a V/P index, by which you can compare sub 3000 to 3500 rpm power. What it attempts to do is deliver number by which you can compare not just before and after changes, but compare your engine to any other sized engine. Thus it gets eye-opening in a hurry.
For instance, I have, in my head the V/P numbers for most of the Mopar small blocks. a 5.2 is 124, a 360 is also about 124, a 318LA is 118, a 225 slanty is 87 and a 273 is I forget around 98 I think. a 68 440 magnum IIRC is 145. So if your engine comes in under 100 well that is 273 territory, and you don't want your 360 to run down there, cuz below say 3200, it's gonna feel like a 273. but if you can coax your 360 up to 135, well it's getting spunky. By 145 it's feeling like that 440 Magnum...... below 3200.
Numbers above 150 are for frying tires.
Mine has run up to a tic over 170, but is currently at ~156
The V/P numbers are just numbers, until your engine pegs the tach for 7 or 8 seconds in the Eighth.
You said you were planning another engine for the future. Put this engine together as is with the pistons you have and stick in a smaller cam. Save the bigger cam for the next engine and you can build the compression and quench to match.
That’s a valid idea. Thanks for the input. We’ll see. Right now I’m taking time out to read Vizard’s books before my next step on the engine. I’ve got plenty to do otherwise. Just finished buttoning up my four speed and I’ve got the heads to finish.
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