Compression test, where to go from here?

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DentalDart

DentalDart

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Did a warm cranking compression test here are the results. I havent done a wet test as I've been busy doing other things...
Engine 1968 340, X Heads, hooker comp headers, 2.5 exhaust, AVS2 650 cfm carb, vacuum? (Dunno have had a hard time getting it lol), anything else you wish to know?

Cyl 1: 155 Cyl 2: 150
Cyl 3: 135 Cyl 4: 145
Cyl 5: 157 Cyl 6: 150
Cyl 7: 155 Cyl 8: 145

Cylinder 3 is my odd ball. Seems like the 135 would indicate something in cylinder 3 isnt in as good of shape as the others. I dont know the internals of the engine and cant see any #s on the piston tops through the spark plugs nor can I see any markings, they just look like flat piece of metal in a dark metal hole. :poke:

I dont have blue smoke coming from the exhaust as I drive, I dont see any misting oil coming from the engine while it idles, anything else I should look for?

Since I already have lifters, springs, timing chain, and camshaft sitting on the side to go in should I get anything else to help correct the problem?

Thanks!
 
Leak down check will show you where you are loosing compression.
 
# 3 is a tad lower than the others, but well within spec IMO, may have a sticky valve or something similar, but I wouldn't worry too much, seems pretty healthy.
 
DentalDart said:
Would I do it on that cylinder only or all of them?
Click to expand...
Do it on the best cylinder and the weak one.
Normally the leak one will show up three places
1 out the exhaust
2 out the intake
3 out the dipstick tube.

1 leaky exhaust valve
2 leaky intake valve
3 dipstick tube rings
 
Marcohotrod said:
are u taking the heads off? a good idea
Click to expand...

I didnt want to... it's not like I know how yet anyways :poke::lol:

I just am trying to make sure when it starts getting the cam, timing chain, lifters and springs I have any other parts that are "needed"
 
sireland67 said:
Do it on the best cylinder and the weak one.
Normally the leak one will show up three places
1 out the exhaust
2 out the intake
3 out the dipstick tube.

1 leaky exhaust valve
2 leaky intake valve
3 dipstick tube rings
Click to expand...

How do you know where it leaked into?
 
brian6pac said:
YEP, put the plugs back in it and run it.
Click to expand...

Oh I've been doing that :D

It just gets put back on stands every couple days for different projects. Today's

20200316_190247.jpg
 
Las Vegas is at 2000ft elevation?
I'm guessing your engine has the original 340 cam.
Any cam you put into it, with a later closing intake, will allow the pressure to drop yet further. At 2000ft you cannot afford to lose much more pressure.
Low pressure makes for soft take off, and the 340 was never known for a powerful take-off . My 68 FSM says to expect up to 175 psi, I'm guessing at sea-level; it doesn't say.
IMO, you need to get to the bottom of this BEFORE you start pouring money into it.I too would recommend to do a leakdown test.
But I would make it easy on myself by backing off the rocker gear, and allowing the tester to blow the piston down to the bottom. If there was still a problem, and this engine was sitting for an unspecified long period of time. I would bop the valve stems a few times to knock the carbon/rust off the seats. You will notice the sound change and the pressure come up. With the pistons at the bottom, you cannot bend the valves during the bopping. ow yur ready for a new compression test.
That's what I would do.

Unless the car is licensed and otherwise ready to drive; then I would take it out for a blast; followed by a new compression test. Then, if the numbers are still down, then it's time for the above. Make sure the oil level is up

Edit:
The Wallace calculator predicts 159psi at 10.5Scr@2000 ft; So it seems either your engine, or the calculator, or both, are pretty much right on. Well, at least three cylinders are,lol.
 
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I would live to have those numbers. 105 to 120 with150,000 miles since last rebuild. (At 5000 feet, cold, dry stock 273)
 
AJ/FormS said:
Las Vegas is at 2000ft elevation?
I'm guessing your engine has the original 340 cam.
Any cam you put into it, with a later closing intake, will allow the pressure to drop yet further. At 2000ft you cannot afford to lose much more pressure.
Low pressure makes for soft take off, and the 340 was never known for a powerful take-off . My 68 FSM says to expect up to 175 psi, I'm guessing at sea-level; it doesn't say.
IMO, you need to get to the bottom of this BEFORE you start pouring money into it.I too would recommend to do a leakdown test.
But I would make it easy on myself by backing off the rocker gear, and allowing the tester to blow the piston down to the bottom. If there was still a problem, and this engine was sitting for an unspecified long period of time. I would bop the valve stems a few times to knock the carbon/rust off the seats. You will notice the sound change and the pressure come up. With the pistons at the bottom, you cannot bend the valves during the bopping. ow yur ready for a new compression test.
That's what I would do.

Unless the car is licensed and otherwise ready to drive; then I would take it out for a blast; followed by a new compression test. Then, if the numbers are still down, then it's time for the above. Make sure the oil level is up

Edit:
The Wallace calculator predicts 159psi at 10.5Scr@2000 ft; So it seems either your engine, or the calculator, or both, are pretty much right on. Well, at least three cylinders are,lol.
Click to expand...

The car is licensed, insured and driven... driven a lot.

It was sitting for a while, just driven sporadically that I know of. The last 7 months it's gotten about 700 miles, when it isnt taken apart for some reason.

I'll try a leakdown test tomorrow after the sway bar is on. I want to test out the radiator, trans cooler, and sway bar as well as adjust my carb some more.
 
To what end? Are you gonna tear it down over a 10% variance? I tell you man, some of you guys are so gullible it hurts to watch. Have fun. I'm out.
 
RustyRatRod said:
To what end? Are you gonna tear it down over a 10% variance? I tell you man, some of you guys are so gullible it hurts to watch. Have fun. I'm out.
Click to expand...

No I'm not, just everyone says leak down test so might as well do it right, just a spoon full of oil in the cylinder after I drive it :poke:?

It's not like I'm doing anything else. Its spring break for school and I'll probably be out of school for a couple more weeks due to the Corona, I mean even my daughter has it :rofl:

20200316_204919.jpg
 
I also need to find a spot to insert my water and oil gauges. Was reading about that last night.
 
dental 23 lbs is obviously not in spec and the leak down my tell you where but worth a rebuild????

do throw some new stem seals on
listen to aj
he'll tell you if you are going backward with a cam
do a cycle of 10 full throttle up to some illegal speed followed by a coast- repeat
have someone watch he exhaust for smoke and swab the pipes with a paper towel
report
retest- that might rebreak in the rings- won't help valves much
all the best
 
For your perusal; inputs of;
4.04x3.315,
factory 268/276/114 cam,
Ica of 64*,
2000 ft elevation

Static compression ratio of .................... 10.5
Effective stroke is 2.57 inches.
Your dynamic compression ratio is 8.37:1 .
Your dynamic cranking pressure is ................. 163
V/P (Volume to Pressure Index) is ................. 129

Static compression ratio of ..................... 9.6
Effective stroke is 2.57 inches.
Your dynamic compression ratio is 7.67:1 .
Your dynamic cranking pressure is................ 144
V/P (Volume to Pressure Index) is, .............. 114

>Notice the pressures reflect what your cylinders "should" be making, to what 2 of yours "are" making. Granted your gauge could be reading low
>Notice the VP has dropped;
from 129,about what a 5.2 Magnum makes at sealevel,
to 114, about what a smoggerteen makes at sealevel.
That is a huge drop.and reflects a C/R change of nearly a full number, or over 9%
Of course with a 3000 stall, you might not notice this, and especially not with performance rear gears.
Nevertheless, after the tires stop spinning,your WOT power will be affected by the Scr reduction, whatever it might average out to. If you don't need the missing power then......... by all means just drive it.

readabout VP here V/P Index Calculation
 
Last edited:
AJ/FormS said:
For your perusal; inputs of;
4.04x3.315,
factory 268/276/114 cam,
Ica of 64*,
2000 ft elevation

Static compression ratio of .................... 10.5
Effective stroke is 2.57 inches.
Your dynamic compression ratio is 8.37:1 .
Your dynamic cranking pressure is ................. 163
V/P (Volume to Pressure Index) is ................. 129

Static compression ratio of ..................... 9.6
Effective stroke is 2.57 inches.
Your dynamic compression ratio is 7.67:1 .
Your dynamic cranking pressure is................ 144
V/P (Volume to Pressure Index) is, .............. 114

>Notice the pressures reflect what your cylinders "should" be making, to what 2 of yours "are" making. Granted your gauge could be reading low
>Notice the VP has dropped;
from 129,about what a 5.2 Magnum makes at sealevel,
to 114, about what a smoggerteen makes at sealevel.
That is a huge drop.and reflects a C/R change of nearly a full number, or over 9%
Of course with a 3000 stall, you might not notice this, and especially not with performance rear gears.
Nevertheless, after the tires stop spinning,your WOT power will be affected by the Scr reduction, whatever it might average out to. If you don't need the missing power then......... by all means just drive it.

readabout VP here V/P Index Calculation
Click to expand...

Very nice that you did this, but this is all like reading spanish, except for the loss of power part...:rofl:

I'm going to... put the parts in that I have, drive the piss out of it then maybe when I need more power or it blows up do a rebuild. I think this is what @RustyRatRod would say, no offense to you @AJ/FormS. I just like driving the car more than keeping it taken apart right now.

I just figured if it was open and there was a part that was definitely needed while the other parts were going in then I'd try to do that as well.
 
Doc it looks like your car is drivable and usable as is since you showed us you can do a burnout, and also it looks like your where most of us have been at some point and some of us are also now. since your car is seemingly functional I would just drive it, keep stockpiling your parts and when you are ready to spend some time and a wad of cash then get started on what ever you want to do but be prepared for whatever you encounter. based on the number of things you described just swapping your cam and valve springs will be a gateway to more discovery, very doubtful that once open the engine you will close it up without wanting to take care of the new discoveries, same for the transmission swap, how do I know? because I just started in late October to replace a few leaking freeze plugs and the strut rod bushings, and now - another new cam, all suspension parts, reworked heads, new parts, more new parts, and I am still not done, this wonderful hobby is a sickness for which the only cure is MORE LOL, this is one of the reasons some of us have several cars, perhaps that may be your best bet, just go get another project LOL -
 
AJ/FormS said:
For your perusal; inputs of;
4.04x3.315,
factory 268/276/114 cam,
Ica of 64*,
2000 ft elevation

Static compression ratio of .................... 10.5
Effective stroke is 2.57 inches.
Your dynamic compression ratio is 8.37:1 .
Your dynamic cranking pressure is ................. 163
V/P (Volume to Pressure Index) is ................. 129

Static compression ratio of ..................... 9.6
Effective stroke is 2.57 inches.
Your dynamic compression ratio is 7.67:1 .
Your dynamic cranking pressure is................ 144
V/P (Volume to Pressure Index) is, .............. 114

>Notice the pressures reflect what your cylinders "should" be making, to what 2 of yours "are" making. Granted your gauge could be reading low
>Notice the VP has dropped;
from 129,about what a 5.2 Magnum makes at sealevel,
to 114, about what a smoggerteen makes at sealevel.
That is a huge drop.and reflects a C/R change of nearly a full number, or over 9%
Of course with a 3000 stall, you might not notice this, and especially not with performance rear gears.
Nevertheless, after the tires stop spinning,your WOT power will be affected by the Scr reduction, whatever it might average out to. If you don't need the missing power then......... by all means just drive it.

readabout VP here V/P Index Calculation
Click to expand...

All of that is WRONG for two reasons. First, NO 340 ever came rated at 10.5:1, they were rated at 10.2:1.

Secondly, every single Chrysler engine made (especially in that era) was horribly over rated on compression, so the 340 that was supposed to be 10.2 was in all likelihood down in the mid 9s.

All of this was due no doubt to what you already know and what we talk about here on a very regular basis. Deck heights being too high and or crooked. Combustion chambers being bigger than the average spec, head gaskets with thicker than blueprint spec and on and on.

So with "that" taken into consideration, his engine is actually more healthy than it may seem.

You give some great advice on here, but it helps if it's correct.
 
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