10.2 : 1 compression 360 at sea level. Options?

-
nm9stheham said:
The 170 numbers sound about right. I suspect some of your lifters have leaked down and are making the intake valves close a lot sooner on those high compression number cylinders, and that is building up those high cranking compression numbers. So run first it as YR sez....

The good news is that is none of them are low.
Click to expand...
So after warming it up the rear 3 cylinders on each side show 155 PSI. Cylinders 1 and 2 both show 175 PSI, but have the same compression ratio as the other cylinders, so I'm not sure what's going on there.
 
155?
You'e good, case closed.

No.1 and no.2 are 175?

20 psi diff.

2 small chambers or 6 holes with leak down.
Can't know until you measure it all.
 
Or it wasn't cranked thru enough compression events for the numbers to peak.
Some guys think 5 cycles is enough, some go eight.
Some go until you get two or three cycles with the same or similar numbers; that would be me. But I also keep track of how many cycles it takes to get there. A tired cylinder might take 8 or ten cycles,compared to another better cylinder peaking at 6cycles.....in the same engine.If one cylinder takes more than 2 or 3 additional cycles to peak, there is a problem, in that hole.
In any case; MoparOfficial has it right; it's time to find out where it's going

Or the cam is dropping lobes
 
Last edited:
AJ/FormS said:
This is what Mr Wallace Says
Static compression ratio of 10.2:1.
Effective stroke is 2.74 inches.
Your dynamic compression ratio is 8.04:1 .
Your dynamic cranking pressure is 160.92 PSI.
V/P (Volume to Pressure Index) is 135................................................135


This is what your compression tester is saying
Static compression ratio of 12:1.
Effective stroke is 2.74 inches.
Your dynamic compression ratio is 9.42:1 .
Your dynamic cranking pressure is 197.35 PSI.
V/P (Volume to Pressure Index) is 166.............................................166

If after repeating the test with a warmed up,pumped up engine,and the numbers haven't changed much, you will need a second opinion

160 should be fine on pumpgas with that 650, and a solid cooling system.
200 is guaranteed trouble.......with iron heads.

To get a 10.2 with a 365 engine would require a swept of 748, and a total chamber volume of 81.3cc.
With your parts, say 65cc heads, an 8.9 gasket and 5 in the eyebrows, will require 2.4cc in the decks or; pistons down .011, so that sounds about right. I wouldn't call your gauge a liar,....yet

Some guys have reported success with 200psi,aluminum heads,tight Q, and pumpgas. You got the right cam, to make that a funtastic street engine. A VP of 166 is pretty awesome. With a manual trans VP becomes very important.Your 3.23 may be able to carry you all the way to 60mph

for comparison, here's a 69 440 with a 268/284/113/.450 cam, ICA of 67*
Static compression ratio of 10:1.
Effective stroke is 2.83 inches.
Your dynamic compression ratio is 7.79:1 .
Your dynamic cranking pressure is 154.44 PSI.
V/P (Volume to Pressure Index) is 154................................................ 154
As you can see 166 is way more than you need, but it sure is fun


Read about V/P here .
V/P Index Calculation
Click to expand...
I was able to find my notes from measuring compression ratio a year ago.

Drivers side head chambers are all around 61cc (they've been milled and have bigger valves).
Deck Height .041 in. piston below deck
Pistons +5cc Dish
Gasket .039 compressed (8.77 cc volume based on website)

Per Wallace Racing-
Dynamic compression ratio- 7.87:1
Dynamic Cranking pressure- 156.51 psi (very close to warm cranking compression test)
V/P- 131

Passenger's side cylinders 4 and 2 are slightly over 10:1.
Cylinder 4- 10.11:1 or 7.97:1 dyn comp, 159 psi dyn cranking
Cylinder 2- 10.29:1 or 8.11:1 dyn comp, 162.74 dyn cranking

Luckily it's on the passenger side, so I think I'll just pull the head and smooth down the chambers a bit more to raise the CCs to achieve 10:1 for those two.

I had previous smoothed down all the sharp edges in the chamber and on the piston, so hopefully I'll be fine with 10:1 static/ 7.87:1 dynamic with 93 octane.
 
Last edited:
Hang on what? STOP!
You listed the deck height as -.041..That's Negative, so that would be a pop-up of 8.6cc on a 4.03 bore, to subtract from the total chamber volume. That maths out to a total chamber volume of just 66.2cc...... and that makes a 12.3SCr engine.
But if that little minus sign is a slip up and the pistons are actually down in the hole .041, then the total chamber volume becomes 83.4, making the compression ratio 9.96.And so then we get

Static compression ratio of 9.96:1.
ICA of 66* with a 280/110 cam in at +4
Effective stroke is 2.74 inches.
Your dynamic compression ratio is 7.86:1 .
Your dynamic cranking pressure is 156.25 PSI.
V/P (Volume to Pressure Index) is 131.............................................131

So,if that is correct, then 155 on your gauge is dead-nuts on. The high cylinders are probably telling you that the ICA has changed, and that only happens when a cam starts dropping lobes, or the lifter is collapsed.
 
Last edited:
MOPAROFFICIAL said:
155?
You'e good, case closed.

No.1 and no.2 are 175?

20 psi diff.

2 small chambers or 6 holes with leak down.
Can't know until you measure it all.
Click to expand...
It's pretty confusing because No.1 has one of the lowest measured static compression ratios- 9.89:1. It also showed 175 psi on the cold cranking test.

Per Wallace racing calculator- I should be right at 155 psi with this cam and compression ratio, so I'm thinking it's got something to do with the lifters.
 
AJ/FormS said:
Hang on what?
You listed the deck height as -.041..That's Negative, so that would be a pop-up of 8.6cc on a 4.03 bore, to subtract from the total chamber volume. That maths out to a total chamber volume of just 66.2cc...... and that makes a 12.3SCr engine.
Click to expand...
Sorry, just a dash there. My mistake.
Pistons are .041 below the deck at tdc.
 
AJ/FormS said:
Post 55 finished

Pop the valve cover and check the lifter preloads
Click to expand...
Checked out the valvetrain today and the preloads are fine. Even added preload on the intake valve of the high compression cylinder and it didn't change anything. Next I'll try a running compression test and then get a dial indicator/ degree wheel and check the cam timing.
 
Keep in mind: the cylinder pressures can vary simply due to cylinder distortion when the head bolts are installed and torqued, unless you had the cylinders done with torque plates in place. I had one 4 cylinder block have the #4 cylinder go .0035" out of round just by installing the 2 top bell housing bolts and torquing them to 25 psi....

And decks can slope front to rear....
 
nm9stheham said:
Keep in mind: the cylinder pressures can vary simply due to cylinder distortion when the head bolts are installed and torqued, unless you had the cylinders done with torque plates in place. I had one 4 cylinder block have the #4 cylinder go .0035" out of round just by installing the 2 top bell housing bolts and torquing them to 25 psi....

And decks can slope front to rear....
Click to expand...
The only thing I'm really concerned about is the cam lobes starting to go flat on the high PSI cylinders (1 and 2) and reducing duration and increasing dynamic compression which could lead to detonation (I think).

I did take the deck slope into account, and I've recorded different static compression ratio's for each cylinder. For example the deck height for cylinder 7 is .04 and cylinder 2 is .023. After calculating each cylinder's compression ratio, I found that only cylinders 2 and 4 are over 10:1 (cylinder 2 is 10.23:1 and 4 is 10.12:1), so my plan is to reinstall the stock 2.02 intake valves (current valves are Manley) in those chambers which increases the chamber volume by 4cc. That should put me right around 9.9:1 and 9.8:1 in those cylinders. I have adjustable rockers from a 273 (I believe) so if the valves are slightly different lengths it shouldn't matter.

Just trying to ensure this thing lives on 93 octane if possible.
 
By any chance,did you oil up the cylinders prior to the compression test?
If yes,then you'll have to spin the engine over long enough to blow the excess back out, and the redo the compression test .
 
SBM4theW!n said:
The only thing I'm really concerned about is the cam lobes starting to go flat on the high PSI cylinders (1 and 2) and reducing duration and increasing dynamic compression which could lead to detonation (I think).
Click to expand...
FWIW from the (fortunately) few shot cam lobes I have seen, they have all gone at the toe, or peak, of the lobe. No damage was done on the far flanks where of the advertised duration is set duration is set.

I am going to guess that you have some soft lifters there; you're hardly getting the pressure built to pump pump up the lifters. Or the cylinders sealing super-good for some odd reason. 20 psi is like another point in compression ratio....

If you do the timing, check it on #1 and #3, or #2 and #4, to see if they are different by exactly some multiple of 90 degrees.
 
AJ/FormS said:
By any chance,did you oil up the cylinders prior to the compression test?
If yes,then you'll have to spin the engine over long enough to blow the excess back out, and the redo the compression test .
Click to expand...
I did squirt some oil in a couple of cylinders to see if the numbers would change (they didn't), and then ran the engine to get it warm which would have taken care of the excess oil in there.
 
nm9stheham said:
FWIW from the (fortunately) few shot cam lobes I have seen, they have all gone at the toe, or peak, of the lobe. No damage was done on the far flanks where of the advertised duration is set duration is set.

I am going to guess that you have some soft lifters there; you're hardly getting the pressure built to pump pump up the lifters. Or the cylinders sealing super-good for some odd reason. 20 psi is like another point in compression ratio....

If you do the timing, check it on #1 and #3, or #2 and #4, to see if they are different by exactly some multiple of 90 degrees.
Click to expand...
That does make sense with peak lift deteriorating vs duration. I'm leaning towards the lifters bleeding down. I'm hoping that by doing a running compression test the oil pressure would be high enough to compensate for soft lifters.
 
I just did a running compression test and all the cylinders were consistent (130PSI). The two high cylinders in the front of the engine were most likely the result of soft lifters as the running oil PSI leveled them all out.
 
should run on 87 moose piss with those numbers.
 
Hang on, a running compression test?
You don't think that raw gas in the chamber is gonna affect the numbers?
Ima thinking they're all low, cuz the gas washed the oil off the cylinder walls.
Ima thinking you better not run 87, but if you do, you better not stand on it!
From post #49, an ICA of 66* in a 10.2 360, should make 160ish psi, at sealevel. And with iron heads premium gas may be needed at WOT. It will idle on 87 just fine. And cruise just fine on 87. But getting from idle to cruise ,on 87, might be tricky.
I wouldn't change a thing. I run over a half a point higher Dcr with aluminum heads on 87E10 with full timing; thats 185psi. You should be fine on 87/89 for all conditions except perhaps WOT,and loaded. Like second gear.
Here's the thing about hi-compression; it makes a more powerful off-idle to midrange, and it ups steady-state fuel-economy. I would strive to keep it.You said no quench so I'm assuming greater than .080, and that will be fine.
The entire first page of this thread is filled with success stories.
 
Run race gas. End of story. Problem solved.
 
SBM4theW!n said:
I just did a running compression test and all the cylinders were consistent (130PSI). The two high cylinders in the front of the engine were most likely the result of soft lifters as the running oil PSI leveled them all out.
Click to expand...
If you are doing this the way I think you are, the numbers will be low due to the closed throttle and high vacuum.... I would expect very incomplete cylinder filling results at actual running speeds versus cranking speeds, plus not-so-great cylinder clearing due to the mixture not being burned and the internal pressures being low on the exhaust stroke. So the absolute numbers in this test are pretty much meaningless compared to what we normally use (cranking pressures, taken at a very low engine RPM); the comparative numbers between cylinders are what the OP is after in this test.

I suspect you are correct on the lifters. It's nice that a real measurement verified your work to get the cylinders to even compression.
 
Nm9
I've never done a test like this
But I'm thinking it would be like a bicycle pump airing up a bicycle tire , except on steroids.
I'm thinking each little 20% or 30% shot is gonna eventually fill up the tire. Just like when you do a closed-throttle compression test; You always get a number, it just sometimes takes twice as many compression cycles.
But I still think a lot of that pressure flew right on by the rings, as the oil washed out.
I mean look at what happens on an old tired 318 during a compression test. You run the test dry, and a couple are in the basement. So you squirt some oil in there and rerun the test, and these worst ones are suddenly the best ones.You can't believe it and so you rerun the test,not adding oil,and low and behold she drops 10 psi. That can't be right you say, and so you do it again still not adding oil, and now they're back in the basement.
To my way of thinking, this is akin to the running test with the fuel washing the cylinders dry.
And I think that thinking is borne out by the fact that the pressure in All of the cylinders dropped, even the ones that formerly had 155.
Your thoughts?
 
Imathinkin' that the idling engine just ain't gonna fill the cylinders the same; the cylinder filling with a closed throttle is always just a tiny fraction of at WOT. And things are happening at a lot faster rate than when cranking; the cylinder filling just has to be less with the shorter time periods involved..... the mixture has to be accelerated from a dead stop each time, and the shorter the actual valve open time, the less will get in.

Bottom line is that we do the cranking pressures at one speed (and WOT) and this is is at a speed 3-4 times higher with a closed throttle, so for the pressures to be different should not be a surprise.

I really don't see the fuel washing as a problem as the fuel should stay pretty well suspended.

But, hey, that is just the way the 'visible v-8' model in my wee brain works LOL
 
nm9
I'm good with your thinking.
I still like my bicycle-pump analogy,tho. Everytime I have ever done a compression test either with closed throttle as at idle position, or with WOT-position, and at cranking speed, the numbers always ended up the close to or the same,providing enough compression events were allowed for the closed throttle test. In my thinking,the psi just represents the best the chamber can do at cranking speed. And that's the measure we have.
I understand that at WOT, as the rpm increases, cylinder filling can approach or even exceed the maximum capacity of the measured environment, you'll not get an argument out of me, on that point.
But In my thinking idle speed and cranking speed are close enough, both with closed throttles,to not change the pressure reading significantly.
I also agree with the OP's diagnoses that the hi cylinder pressures could have been due to collapsed lifters. I mean from the running results, we now are pretty sure the lobes are ok, so really, there's no other option left.
So the very first compression test was done IIRC with a closed throttle, and so was the idling test. So where did the pressure go for all the 155ers? Were all their lifters also collapsed, just to a lessor degree? That seems rather conspicuous to me; 5 or 6 all the samely collapsed?
I get that every time you shut a V8 off, there will be at least two lifters being under duress, so that probably explains the two high readings. But they should hold their preload long enough to do a warm compression test,else I would throw them away.
I find these thought-exercises fascinating.

A little further
it's been reported that at idle, not all the gasoline entering the chamber is oxidized, right. That's why the idle mixture has to be enrichened; to stabilize the idle,to ensure that all the oxygen finds fuel to react with. So, normally this unburned fuel passes out with the exhaust; but does it all pass thru?
And the condition is aggravated with say....a 280 cam, and 10.2 Scr. So I hear ya that you believe that the gas in the non-firing chamber will stay suspended and pass harmlessly thru.
But think back to olden days,lol, when a vehicle came into your shop with a dead cylinder. During the diagnoses, how many times didn't you pull the dipstick, and find the oil diluted with gas in the crankcase?
Your thoughts?
 
nm9stheham said:
If you are doing this the way I think you are, the numbers will be low due to the closed throttle and high vacuum.... I would expect very incomplete cylinder filling results at actual running speeds versus cranking speeds, plus not-so-great cylinder clearing due to the mixture not being burned and the internal pressures being low on the exhaust stroke. So the absolute numbers in this test are pretty much meaningless compared to what we normally use (cranking pressures, taken at a very low engine RPM); the comparative numbers between cylinders are what the OP is after in this test.

I suspect you are correct on the lifters. It's nice that a real measurement verified your work to get the cylinders to even compression.
Click to expand...
Exactly, just for a comparison between cylinders showing 155 hot cranking and 175 hot cranking. Running they all show 130, and cold cranking they all showed 180s. I wish I knew why the front lifters were bleeding down. The engine is highest at the front of the car. But I guess it doesn't matter if everything works fine with running pressure.

The low PSI running is interesting, but I think it's got a lot to due to running vacuum. There wasn't any liquid gas in/on the tester.

Another interesting note- cold/ hot cranking with and without the throttle open had very little effect (5psi), but I guess you also don't see much vacuum just using the starter.

Eventually I'll pull the passenger side head (easy) and get to 10:1 across the board. For now I have to play to cylinder 2 (gotta love uneven decks). 10.29:1 static, and 8.11:1 dynamic compression ratio. I'll just fill her up with 93, add a little booster or race gas and dial back the timing to 31/32 initially and slowly work my way back up to an ideal tune. Want to play conservative with current exhaust (comes out the side) as I'm afraid I won't hear the pinging.

Winter plan- Cold air intake/ hood scoop and better fan and maybe pulling the head, which all together should allow me to run safely on 93, but we'll see!
 
AJ/FormS said:
nm9
I'm good with your thinking.
I still like my bicycle-pump analogy,tho. Everytime I have ever done a compression test either with closed throttle as at idle position, or with WOT-position, and at cranking speed, the numbers always ended up the close to or the same,providing enough compression events were allowed for the closed throttle test. In my thinking,the psi just represents the best the chamber can do at cranking speed. And that's the measure we have.
I understand that at WOT, as the rpm increases, cylinder filling can approach or even exceed the maximum capacity of the measured environment, you'll not get an argument out of me, on that point.
But In my thinking idle speed and cranking speed are close enough, both with closed throttles,to not change the pressure reading significantly.
I also agree with the OP's diagnoses that the hi cylinder pressures could have been due to collapsed lifters. I mean from the running results, we now are pretty sure the lobes are ok, so really, there's no other option left.
So the very first compression test was done IIRC with a closed throttle, and so was the idling test. So where did the pressure go for all the 155ers? Were all their lifters also collapsed, just to a lessor degree? That seems rather conspicuous to me; 5 or 6 all the samely collapsed?
I get that every time you shut a V8 off, there will be at least two lifters being under duress, so that probably explains the two high readings. But they should hold their preload long enough to do a warm compression test,else I would throw them away.
I find these thought-exercises fascinating.

A little further
it's been reported that at idle, not all the gasoline entering the chamber is oxidized, right. That's why the idle mixture has to be enrichened; to stabilize the idle,to ensure that all the oxygen finds fuel to react with. So, normally this unburned fuel passes out with the exhaust; but does it all pass thru?
And the condition is aggravated with say....a 280 cam, and 10.2 Scr. So I hear ya that you believe that the gas in the non-firing chamber will stay suspended and pass harmlessly thru.
But think back to olden days,lol, when a vehicle came into your shop with a dead cylinder. During the diagnoses, how many times didn't you pull the dipstick, and find the oil diluted with gas in the crankcase?
Your thoughts?
Click to expand...
I think the gas in the cylinder would be more an issue with a cold engine. I ran it up to operating temp before doing the running test, so the fuel shouldn't have had any issue vaporizing in the hot intake. While doing these test the exhaust would make your eyes sting a bit, but the compression tester never showed any raw fuel in or on it.
 
You must log in or register to reply here.
-

Back
Top