Compression test, where to go from here?

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Put a small amount of Marvel Mystery oil down #3 let it sit for a day or so and rotate the engine (ignition off). Rings,valves stick with carbon. I have done this many times thru the years with "junk" engines and lots have increased in compression. May as well do it on all 8 cylinders, if cost nothing. Make sure you only
rotate the engine slightly with plugs out a few times, the crank it over good to get the MMO out before you install the plugs.
 
billsdartgt said:
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 -
Click to expand...

Yes another project for 5k! That would not cause my wife to divorce me. :elmer:<-- would be my fate lol. I'm actually trying to get some valve seals, valve locks and spring retainers right now. A guy has those listed with pistons, piston rings, bearings etc for a full rebuild but those are for a 360 and I couldn't use them.

If he won't split it I guess someone here will be getting a killer deal on 360 parts haha.
 
66jim said:
Put a small amount of Marvel Mystery oil down #3 let it sit for a day or so and rotate the engine (ignition off). Rings,valves stick with carbon. I have done this many times thru the years with "junk" engines and lots have increased in compression. May as well do it on all 8 cylinders, if cost nothing. Make sure you only
rotate the engine slightly with plugs out a few times, the crank it over good to get the MMO out before you install the plugs.
Click to expand...

I'll try this. I have to mess with the balancer anyways to put on my timing tape.
 
I would be careful of what you do buy before you actually open the engine up, seems like resale of things you don't need equals less than half of what you paid, or you go past a return window and lose even more
 
My stock 68 340's compression came out to 9.4:1 All stock parts and gaskets. If the cam your putting in is bugger(of course it is) with that low compression, you might lose some more bottom end. if you do have a 3000 stall it won't matter much.
That lower compression cyl wouldn't bother me much after i had done a leak down test to know its not a burnt valve........That's only going to burn more and lose more compression.

How many miles are on this eng????????
 
Cudafever said:
My stock 68 340's compression came out to 9.4:1 All stock parts and gaskets. If the cam your putting in is bugger(of course it is) with that low compression, you might lose some more bottom end. if you do have a 3000 stall it won't matter much.
That lower compression cyl wouldn't bother me much after i had done a leak down test to know its not a burnt valve........That's only going to burn more and lose more compression.

How many miles are on this eng????????
Click to expand...

My odometer shows 041612, but the engine isnt the stock engine, the stock engine is a /6

I also seem to think my odometer doesn't work properly because there is no way I've driven 3k miles this year unless the white part is not a complete mile and is a tenth of a mile.
 
The odometer has 99999.9 miles it can display then it starts over to 00000.0

Mine has gone around 3 times. Right now it shows 17567.3 but it has 317567.3 miles on the car

Yes the white number is 1/10 of a mile.

As long as your speedo is fairly accurate your odometer will be correct.
 
Dana67Dart said:
The odometer has 99999.9 miles it can display then it starts over to 00000.0

Mine has gone around 3 times. Right now it shows 17567.3 but it has 317567.3 miles on the car

Yes the white number is 1/10 of a mile.

As long as your speedo is fairly accurate your odometer will be correct.
Click to expand...

Perfect!

So to answer the question my car has 4161.2 miles on it :thumbsup: lowest mileage A-body on this site? Should command a hefty penny if I were to sell it :rofl:
 
Cudafever said:
My stock 68 340's compression came out to 9.4:1 All stock parts and gaskets. If the cam your putting in is bugger(of course it is) with that low compression, you might lose some more bottom end. if you do have a 3000 stall it won't matter much.
That lower compression cyl wouldn't bother me much after i had done a leak down test to know its not a burnt valve........That's only going to burn more and lose more compression.

How many miles are on this eng????????
Click to expand...

Exactly! That's "about" where all the early 340s came in. The later ones were in the 8:1 range.
 
1970 "Rapid Transit System"......Duster 340



RTS.JPG
DSC_0192.JPG
340 Compression (1).JPG
 
The 'marketing' data in post #37 is what we are talking about.... CR is over-rated by about 3/4 of a point. GM and Ford and overseas mfr's were doing the same in that era.

OP, do the leak down test. It is not an onerous amount of work and you may get some useful data. I'd kinda suspecting a valve not seating quite right. A soft lifter, or a pushrod bent and not opening things like they ought to be, or a cam lobe juuuust starting to go, could also make that kind of minor difference in cranking compression.

Don't get impatient. Do this BEFORE you tear it down for the new parts. Guys make this mistake all the time.... they fail to make worthwhile tests before tearing things apart. Then they run into a mystery, and realize that they could have had the info needed to figure it out IF they had done the testing first.

Look at the plugs. See if there is anything different about #3.
 
RustyRatRod said:
Exactly! That's "about" where all the early 340s came in. The later ones were in the 8:1 range.
Click to expand...
I have a 72 340 in my duster with aftermarket KB 243 pistons, and stock "x" heads and have 8.98 compression, measured.
But with with careful camshaft selection I make 160 psi cranking, so it is decent on pump gas.
 
DustyEd said:
1970 "Rapid Transit System"......Duster 340



View attachment 1715488823 View attachment 1715488828 View attachment 1715488829
Click to expand...

Nominal 10.5:1 compression ratio was a best case scenario.

If you had the shortest block, with the longest rod, the tallest compression distance and the smallest chamber volume and you'd get that CR.

In the real world, the blocks were only cut until the cleaned up, which most of the time was taller than it could be. Same with the heads.
 
Wyrmrider said:
whatever nhra allowed
don't forget the stroke tolerance , valve faces, piston notches et
Click to expand...
I do believe this was it was based upon. To get a 3/4 point CR increase, you need to get rid of about 6-7-8 cc's of volume for these engine sizes. That will take the typical production head chamber volumes right on down to about the NHRA numbers.
 
nm9stheham said:
The 'marketing' data in post #37 is what we are talking about.... CR is over-rated by about 3/4 of a point. GM and Ford and overseas mfr's were doing the same in that era.

OP, do the leak down test. It is not an onerous amount of work and you may get some useful data. I'd kinda suspecting a valve not seating quite right. A soft lifter, or a pushrod bent and not opening things like they ought to be, or a cam lobe juuuust starting to go, could also make that kind of minor difference in cranking compression.

Don't get impatient. Do this BEFORE you tear it down for the new parts. Guys make this mistake all the time.... they fail to make worthwhile tests before tearing things apart. Then they run into a mystery, and realize that they could have had the info needed to figure it out IF they had done the testing first.

Look at the plugs. See if there is anything different about #3.
Click to expand...

I'm going to do a leak down. I dont know how to tell if it goes in the intake, exhaust, or dipstick though.

My spark plugs all looked good. The 5-8 plugs had a little more oil on them than the rest and it looked like maybe they were a touch lean.

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take a look at the rn12lyc type plug
extended tip even more than the excellent plug style you have
not race plugs no miss no foul plugs great for emmissios
i use them a lot in LA
i can check for current numbers
both 5/8 & 13/16 hex available
 
Wyrmrider said:
take a look at the rn12lyc type plug
extended tip even more than the excellent plug style you have
not race plugs no miss no foul plugs great for emmissios
i use them a lot in LA
i can check for current numbers
both 5/8 & 13/16 hex available
Click to expand...


He can do that 12 heat range with a 5/8 hex if he gets the RC12LYC. The N is the 13/16 hex and I'm not a fan of that 13/16 hex unless that's all you can get.
 
DARTGAME said:
Reason for different spark plugs? Noticed some autolites and some champion.

Ian
Click to expand...

Well that's a funny story... they were all champions when I replaced the plugs, wires and distributor cap. Then when I was installing the headers I broke a couple of them... went back to the store and they had no more champions to match so got the autolites that were supposed to be the "same."

Wyrmrider said:
take a look at the rn12lyc type plug
extended tip even more than the excellent plug style you have
not race plugs no miss no foul plugs great for emmissios
i use them a lot in LA
i can check for current numbers
both 5/8 & 13/16 hex available
Click to expand...

I dont know the difference in the Hex's usually I just or the "heat range." I just go off of past suggestions from FABO on what's good and what's bad for the cars. Usually I do the good, but occasionally I break the rules :p

When the cam, timing chain, lifters and springs go in so will these NGK plugs that I've read good things on here about. I also used to run all NGKs in my quads and dirt bikes so hopefully these work well in the 340. Honestly I wouldn't be able to tell if they didnt unless i have some nasty misfires
 
cross ref hat number yr posted if you wish
working from a small tablet so hard to look up
it was so much easier with the catalogs

THANKS YR
 
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