Compression

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Model T Ford Forum: Forum 2015: Compression
Top of pagePrevious messageNext messageBottom of page Link to this message  By Terry Horlick in Penn Valley, CA on Wednesday, December 16, 2015 - 09:35 pm:

Rusty hasn't been running well so today I pulled the plugs and inspected. #1 is oily, with oil leaking out the nearby head bolt.

I suspect this is due to the Horlick oiling system I employ... I literaly deliver about a gallon of oil up to #1 bearing every minute I run the engine. I never spin off rods #1 and #2 anymore but oil fouling is not uncommon. I'm thinking about adding an oil flow restricting valve in the line!

Now the reason for this post...I did a compression test. I'm running a Z head and aluminum pistons. I got the following:



I'm wondering what is a credible compression reading? I'm thinking that #1-#3 may need rings and a valve lap job.

TH


Top of pagePrevious messageNext messageBottom of page Link to this message  By paul iverson freeport ill. on Wednesday, December 16, 2015 - 10:27 pm:

I have found comp. tests to change after a run if it runs good I would leave it alone. a little carbon under a valve maybe? my dad told me when he worked at a ford dealer in the 30s on flathead v8s they fixed a lot of valve problems by a whack with a ballpien hammer on valve to seat them differint style valve setup than T. I would never try that it might make a valve fail later


Top of pagePrevious messageNext messageBottom of page Link to this message  By Terry Horlick in Penn Valley, CA on Wednesday, December 16, 2015 - 11:24 pm:

I guess you are suggesting valve lapping is next... I rebuilt the vaporizer carb a couple of weeks ago. The coils were done about 2 yrs. ago. The Anderson style timer can't need anything.

What pressure is ideal in a Z head engine?

TH

(Message edited by thorlick on December 16, 2015)


Top of pagePrevious messageNext messageBottom of page Link to this message  By Tony Bowker, Ramona, CA on Wednesday, December 16, 2015 - 11:40 pm:

With any cylinder head, the formula for maximum pressure is:
Pressure = (CR -1) * 14.7 psi
With the standard Ford head the maximum pressure is {(3.9-1)*14.7} 42.6 psi
With the claimed CR of 6:1 for the Z head, the value is 73.5 psi
Note that skimming the head or block will tend to increase the CR and losses due to poor valve sealing and bad rings will decrease the maximum pressure.


Top of pagePrevious messageNext messageBottom of page Link to this message  By Roger Karlsson, southern Sweden on Thursday, December 17, 2015 - 06:01 am:

True CR for the Z head (and the Prus) is about 5:1. Measured compression in a good engine is usually about 45-50 psi with a standard head and about 90-100 with the Z or Prus, so it's likely you have some leak. Try squirt some oil into the cylinders and test again - if it changes, it's the rings that doesn't seal as they should - and possibly the valves, if it dosn't change it's only the valves.


Top of pagePrevious messageNext messageBottom of page Link to this message  By Terry Horlick in Penn Valley, CA on Thursday, December 17, 2015 - 01:06 pm:

Roger and Tony, thanks for the pointers. It's been gradually getting worse for maybe a couple of years. These things creep up on you with nothing really shaking you into action.

I have decked the block and head (head at least twice) to the point where I have had piston strikes on the head. I then had to relieve the head where interference occurred. So I am sure that with everything correct this should be up there, probably North of 5:1, maybe better'n 6:1.

I have adjustable lifters but with a vaporizer carburetor and a single piece valve cover I just NEVER get to adjusting them due to the hassle involved. I'm betting that I have terrible valve adjustment... and maybe carbon on the valves and leaky rings... possibly worn valve guides also. It is time to off the carburetor and head and get serious about fresh adjustment. Just to be safe, I will do the wet compression test this morning.

In the past I have done valve adjustments sometimes to a specified lash and other times to piston travel method. Which method do most folks prefer?

TH

Terry


Top of pagePrevious messageNext messageBottom of page Link to this message  By Tony Bowker, Ramona, CA on Thursday, December 17, 2015 - 03:52 pm:

If you have an original camshaft with lift close to 0.250" then the piston travel method will work. However most original camshafts are down at 0.230 or less so the piston travel will yield incorrect results, so use the clearance method. If you have a reground camshaft, then clearance is the only way to go.
By the way, if the Z head has a 5:1 compression ratio, then the pressure will be about 58 psi..... If someone measures 100 psi, it will require a CR of about 8:1... At that, I fear for the splash lubricated Babbitt bearings


Top of pagePrevious messageNext messageBottom of page Link to this message  By Terry Horlick in Penn Valley, CA on Thursday, December 17, 2015 - 04:21 pm:

Test results:

Cylinder #1 #2 #3 #4
Dry psi 50 50 48 60
with oil psi 62 60 58 58


So it looks like rings on #1,2,&3 and valve adjustment. I will also have to pull the valves to check them for lapping or grinding.

Working backwards form Roger's formula the 62 psi reading suggests a 5.2:1 CR. Since the head was hand ground to provide piston clearance the CR may not be exactly the same on all cylinders. Also, it may be a little higher because I am not at STP being at about 1,600' MSL, and about 8ºC. I'm not sure about that since the compression tester read psig not psi!

Well now the only decision left seems to be: engine work in the car or out on the engine stand!

TH


Top of pagePrevious messageNext messageBottom of page Link to this message  By Kevin Weeds on Thursday, December 17, 2015 - 04:46 pm:

I have a Z on one of my T's.. The other two T's have standard heads & compression's vary from 42 to 50 psi but the Z head is 89 - 91 psi.


Top of pagePrevious messageNext messageBottom of page Link to this message  By Tony Bowker, Ramona, CA on Saturday, December 19, 2015 - 08:30 pm:

As Hal indicates in another thread, my formula for the compression test results is for Sea Level!
I am at 1500 ft elevation which has an air pressure of 13.9, (from http://www.engineeringtoolbox.com/air-altitude-pressure-d_462.html), so my readings of a standard Model T will be 13.9*(3.9-1) which is 40 psi. This explains why my readings are always a little lower than my friends nearer the coast.


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