I did a compression test on my 390 this weekend. I was wondering what everyone thinks. I do not get any smoke and the engine runs great, I was just attempting to assess the condition of the engine.

Cylinders Pressure
1 137
2 130
3 130
4 115
5 130
6 130
7 130
8 115

Do you think there is a problem with cylinders 4 and 8? Are these reading close to what they should be (close to that of the original readings?)?

Thanks for any opinions!

 

Nothing at all to worry about. As long as the cylinders are within 75% of eachother you are fine. The pressures are pretty normal for a low compression FE. You should be happy. How many miles?

 

You may want to consider a leak-down test to see the condition
of your engine...

 

I was considering a leak-down test, but I have not been successful at finding a plan to build a guage or a commercial guage for sale.

The odometer shows about 16000 miles. Your guess is a good as mine on how many actual miles it has. What is the compression ratio in this engine?

 

About 7.8:1 give or take.

 

Either J C Whitney or Harbor Freight has a leakdown tester for less than $50. I bought it and have yet to use it... Has two pressure gauges, and the air supply connector and the adjustment valve. Also got compression gauge with multiple plug adapters for $30 or so.
You did have the throttle wide open when you did the test, didn't you?
If you want, I could go dig up the plans & explanation how to build a leakdown tester, but the price was too good to take the time to chase down all the components and do the assembly.
tom

 

What is a normal compression number for a 390? I am guessing 110?

I have not found it listed anywhere using the search feature on-line and can not find it in the repair manual (CD-ROM). My Chlitons manual says to use the values listed in Chapter Two ... they do not have any charts for compression in Chapter Two ;D


I got
90
90
90
68
82
72
78
75

About normal for a 100K engine?

The lowest 68 came up to 80 with oil and the other cyls. I tried with oil did not make a 2 psi difference. With a 65 motor and 65 heads I am guessing at least a valve job with harden seats is in my future. Except for excessive blow-by (?) the motor feels strong.

 

Sorry, clicked post twice.

 

So I'm new to this forum thing, and to FE engines for that matter. My question is not about my truck but rather my 65' galaxie, but this is the best thread I could find anywhere on the topic. I have had some issues with the 390 lately and I'm wondering if these compression numbers point to anything specific, such as a bent valve.
#1- 113 psi
#2- 100 psi
#3- 124 psi
#4- 99 psi
#5- 131 psi
#6- 130 psi
#7- 130 psi
#8- 119 psi

A little back story for it: Ran one night and then wouldn't fire next day. Broken linkage on the leaking 4100 carb, rebuilt carb, still wouldn't fire. Changed plugs, wires, cap and motor since I didn't do it when I bought the car. Still no fire. Checked the timing chain and sure enough that was it. I was able to take the chain off the sprockets by hand with no tools and without removing sprockets. I'm 97% sure I got the new ones lined up properly and the ignition timing is spot on. Then fuel pump went out and replaced with an electric. Now fires up but back fires through tail pipe and intake both. Plus it doesn't have the power to idle in gear, even at 2200 rpm it dies if I put it in gear (automatic). Wondering if I bent a couple valves when chain slipped. Its a 64' engine (c4ae) all original and has 116,000 on the clock. This has been 2 weeks of hell and any and all advice would be greatly appreciated.

P.S. As best I can figure, it is all original and stock and cam from the factory with 300hp 427ft# and 10.1:1 compression. Head casting number c4ae 6090g. Car manufactured 12/64, hence 64' parts on 65' car.

 

I've never heard this 75% stuff ever. I have seen no more than 15% between the top number and low number. This is published in a number of factory service manuals.

 

Ok, so I went ahead and re did the compression check after putting a little oil in the cylinders to seal the rings. It has always burned oil bad since I bought it so I knew the rings where somewhat bad. Here are the new results.
#1 135
#2 135
#3 180
#4 185
#5 205
#6 210
#7 195
#8 210

Needless to say, even with 1 and 2 being a little lower than the rest, I would imagine the valves are no longer the prime suspect. I guess I need to re evaluate the timing chain and figure out if I might have set it off a tooth or something. Still, any input would be greatly appreciated.

 

If you had the timing chain off a tooth it would run like crap.

 

Ya, it runs real rough and doesn't have the power to stay running in gear, not to mention the bad backfire from both intake and exhaust. I guess my chore for tomorrow is pulling everything back of the front of the engine and looking at the timing chain. I suppose since this was the first time I have done a timing chain I may have got it wrong just from inexperience. Thank you for the input Bear 45/70.

 

Ok, so I finally got around to pulling the front cover off and checking the timing chain today, despite the 9 inches of snow it dumped on us last night. The little dimples are lined up perfectly. My new question would be is there any way you could get the cam gear of by 180 degrees? I have heard it mentioned but it doesn't make any sense to me because once the crank goes around once, wouldn't it be lined back up? Second question: is there any way that the dimple could be off by a tooth? This whole thing is really throwing me for a loop, I would post pictures if I knew how to upload them into here but unfortunately I'm better with wrenches than key boards.

 

Here's how to post pictures: one way that is.....
1. Log in to FTE.
2.Click User CP (In the Top Left Hand Corner)
3.(On the Left, There will be a list, The 5th Option Down, will say "Pictures & Albums) Click That.
4.Then, Click Add Album, Enter a title, and description if ya want to.
5. Click Upload Pictures, Then click Browse, Locate the Files of your truck. (You can add 3 files at a time. There is a size limit to the pics you upload. I think it's 800x600 pixels. If you need to resize an image on your computer, you can right click it and open in paint, then click resize and type in the pixels, save and you're ready to upload)
6. Click Upload.
7. Add a description to the individual pictures if you want, then click save.
8. Add more files if ya want.
9. Click The Picture you want to post, it will give you two codes at the bottom, Right Click The Second one (It say's BB Code, Then Say's [IMG]http:blahblahblah...[/IMG])
Then, Once that is highlighted, Right Click, Then Copy, Then Paste it into your post.
You can add up to 30 Images in one FTE Post.

It is possible it could be off a tooth, use some kind of straight edge to make sure they line up.