Doing it right -need some advice

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MoparLeo said:
If you want to go bigger in the future, sonic test the block. .040 on a 340 is going pretty thin.
Click to expand...

Nah. I wouldn't go bigger. The rebuild happened 10 years ago, but it has maybe 1000 miles, maybe. I did the motor and drove it for a blip and then resto modded the rest of the car, drive all last summer. Then decided 4 speed with passon OD gearset. So here we are.
 
You don't need to know the compression ratio at this point. Even if you knew it, the Scr by itself would be relatively meaningless.
What you really need to know is the cranking cylinder pressure; it will tell you everything you need to know; save yourself the cost of the gaskets.
Here's my take on pressure for iron heads;
under 125 is just extremely poor.
125 to 135 will make a ho-humm streeter, but your starter gear of 3.09 x 4.10= 12.67 will mitigate that.
A modest minimum target for your gears would be 140 to 150psi
150 to 160psi is good to very good;
over 165psi is probably bad.
Your Second gear ratio is 1.67 x 4.10 =6.84; which is the same as mine (3.55 x 1.92=6.82), so I know it will be fun. The more pressure you have, the more fun it will be, until it gets into detonation. Your overdrive ratio makes 65 ~2440, which should be about right for a 340 with a 4-speed cam..
 
AJ/FormS said:
You don't need to know the compression ratio at this point. Even if you knew it, the Scr by itself would be relatively meaningless.
What you really need to know is the cranking cylinder pressure; it will tell you everything you need to know; save yourself the cost of the gaskets.
Here's my take on pressure for iron heads;
under 125 is just extremely poor.
125 to 135 will make a ho-humm streeter, but your starter gear of 3.09 x 4.10= 12.67 will mitigate that.
A modest minimum target for your gears would be 140 to 150psi
150 to 160psi is good to very good;
over 165psi is probably bad.
Your Second gear ratio is 1.67 x 4.10 =6.84; which is the same as mine (3.55 x 1.92=6.82), so I know it will be fun. The more pressure you have, the more fun it will be, until it gets into detonation. Your overdrive ratio makes 65 ~2440, which should be about right for a 340 with a 4-speed cam..
Click to expand...

Interesting, ok. Well I could bolt up the flywheel and bellhousing and get th cranking compression.

Keep in mind this is Passon OD gear set. Not factory ratios. Here they are.

Screenshot_20220118-201921.png
 
plumkrazee70 said:
Interesting, ok. Well I could bolt up the flywheel and bellhousing and get th cranking compression.

Keep in mind this is Passon OD gear set. Not factory ratios. Here they are.

View attachment 1715855323
Click to expand...

Nice ratios, certainly better than the Mopar-OD box! .The new numbers become;
Starter of 2.66 x 4.10= 10.91 Excellent
Second of 1.59 x 4.10=6.52, a reasonable second for a 360, but only ho-hum for a low-pressure 340.
And 60=4870 with 27s
Cruise gear of .80 x 4.10=3.28s ; So 65=2650 with 27s, a lil high but easier to make adequate cruise-timing with.
The gear ratios are a nice progressive 1-2-3 with a short Fourth, perfect for a 340;
2.66-1.59-1.00-.80od with splits of .598-.629- .80.. The Commando box, which I really like is;
3.09-1.92-1.40-1.00 with splits of .621-.729-.714
I think yur gonna like that Passon box. I'm looking forward to seeing how it works out for you
 
once you get it running like you want, a vacuum advance may be a good thing on the street...
 
AJ/FormS said:
You don't need to know the compression ratio at this point. Even if you knew it, the Scr by itself would be relatively meaningless.
What you really need to know is the cranking cylinder pressure; it will tell you everything you need to know; save yourself the cost of the gaskets.
Here's my take on pressure for iron heads;
under 125 is just extremely poor.
125 to 135 will make a ho-humm streeter, but your starter gear of 3.09 x 4.10= 12.67 will mitigate that.
A modest minimum target for your gears would be 140 to 150psi
150 to 160psi is good to very good;
over 165psi is probably bad.
Your Second gear ratio is 1.67 x 4.10 =6.84; which is the same as mine (3.55 x 1.92=6.82), so I know it will be fun. The more pressure you have, the more fun it will be, until it gets into detonation. Your overdrive ratio makes 65 ~2440, which should be about right for a 340 with a 4-speed cam..
Click to expand...

Well here are the results in of the cranking compression test. I took all the plugs out and had the throttle wide open.
1: 150
3: 150
5: 135
7: 135

2: 140
4: 135
6: 130
8: 135
 
There you have it;
the straight thru average is 138.75
which is sitting on the hi-end of ho-hum, and on the low end of modest.
However, the two 150s seem anomalous, they are 7.3% higher than the average and the 130 is 6.5% lower, for a spread of over 13%
My guess is that you started your test at #1, and ended at #8, and that your battery was slowing down. If true, then the picture may be better than it looks.


For me, having had 177 to 185 since 1999,
this pressure would be unacceptable.
The good news tho is that your engine should be happy with any old gas.
 
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plumkrazee70 said:
What do you think I should do, based on that and my given goals? Should I be concerned with the difference?
Click to expand...

If it was me, I would double check compression with battery charger hooked up on 50amp or better. Then leakdown test to see what is causing the compression difference in the cylinders. Valve seat touch up may be in order depending. I would also be pulling that cam and getting one to work better with your cylinder pressure.
 
First thing I would do is put a charger on the battery and pump it up
The Second thing I would do is REDO the test.
I would squirt a little oil into every cylinder, then bump her over a couple of revolutions, then repeat on the oil, then walk away for an hour.
This oil will fall into the clearance between the piston and the cylinder wall, be stopped by the top ring, then by capillary action it will climb up and all around the top ring, maybe finding the gap and crawling down onto the second ring. No matter.
When you get back, you gotta pump all that oil outta there else it will artificially increase your pressure. This will make a mess, so cover the engine with an old blanket or something.
After the oil is pumped out, you can begin the test; but this time run it in reverse, starting with the LAST cylinder that you previously ended with;
and make sure the cranking rpm stays about the same.
This is what I would do.
Your pressure will be slightly higher, due to that the oil has coated the tops of the chambers, and was Not pumped out. You can't do anything about that. But your engine depends, even while running, on that oil sealing the rings both to the cylinder walls, and to the ringlands. So IMO, you will get better results in the new test. Hopefully the numbers will even up as well.
I guess we'll see.
 
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AJ/FormS said:
There you have it;
the straight thru average is 138.75
which is sitting on the hi-end of ho-hum, and on the low end of modest.
However, the two 150s seem anomalous, they are 7.3% higher than the average and the 130 is 6.5% lower, for a spread of over 13%
My guess is that you started your test at #1, and ended at #8, and that your battery was slowing down. If true, then the picture may be better than it looks.


For me, having had 177 to 185 since 1999,
this pressure would be unacceptable.
The good news tho is that your engine should be happy with any old gas.
Click to expand...

Yes, you're right. I started at #1 and worked my way back. 1,3,5,7,2,4,6,8. In that exact order. I only have a battery tender. I don't have a charger that has like a jump start feature. I put it on the tender when I left last night to be sure it was charged up. Im gonna repeat the test to be sure.
 
I’d say YES! Since it says it is a healthy engine. Your clear to sail on.

AJ nailed it.

What can be done now is to proceed with confidence.
 
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