Identifying pistons?

[Loa]

Hello,

I finally managed to find and read the vin number on my 1973. I knew it wasn't a matching numbers so I never really needed to check, but I was curious. The car is from the US (hamtrack plant) but the engine is from Canada (windsor plant) and, more surprisingly, seems to be a 1970. Vin is 0RXXXXXX.

Now I'm curious to see what kind of pistons I have in it, and wether they're original 1970 (high compression) or if they've been changed to the 1973 lower compression. I've read a few posts here but I'm just not sure.

Here they are:





Also, does the faint "30" on the piston mean that it's been bored .030 over?



Thanks

[edl94]

Yes the 30 indicates it is bored 30 over. With those valve reliefs I think those are the higher compression pistons. I think the lower compression pistons did not need valve reliefs.Check the piston bore depth at TDC

[Chryco Psycho]

I tend to agree , check piston height at TDC  if it is above deck it is the 10+ comp pistons , definatly bored .030

[Loa]

Damn... I should have checked for this before putting it all back together. These pics come from when we rebuilt it in april-may. Now it's closed up, and I'm not sure I wanna take it apart again for this. NEver even thought I could have a 70 engine in there...

Any other way I could test it? Could the car's performance (let's say 0-60) give any indication?

Loa

[Chryco Psycho]

compression test warm should be 150 psi or better

[Loa]

Ok, will try to do that this week.

Thanks for the info.

Loa

[Chryco Psycho]

Comp test is not really a great way to tell but if it is below 150 I doubt it will be 10:1 pistons , cam can bleed off a lot of the pressure though .

[Loa]

I was planning on doing one anyway because I have no idea in what shape the pistons, cylinders and rings are. Which will add to my problem if I get low compression.

I just tested my 0-60 time for fun: I got between 7.8 to 8.1 seconds (over 3 attempts) with 727 tranny, 3.73 diff, 4bbl vacuum Holley. Note: I was letting the transmission decide when to shift, as my tach's dead. It shifted into third at about 50-55.

Loa

[Loa]

Hello,

Tested my pistons yesterday, and they're all between 160 and 170 lbs, except one at 155 lbs.

Loa

[Chryco Psycho]

sounds like 10:1 to me

[73EStroker]

0-60 time should be way better than that with 3.73 gears! Might need setting up some more.

[Loa]

Well the timing was set using regular gas, as I thought it was a 73 motor back then. I'm using premium now, and will redo. Carb will also be retuned when this is done, but other than that, I don't know...

Also, probably that a better driver with a better timing method would get better results...  :-)

Loa

[jimynick]

I'd've thought the car would do nearly 50 in first with the pedal to the metal and shifting into third at 50-55 is short shifting it big time. If you were driving along at 60 and floored it, it'd kick back down into second to pass, so you're sure not over revving it. Just my 

[Loa]

Hello,

Thing is: I've never had a working tach on this car, so 1) I don't know where the redline is and 2) I can't estimate RPM by ear.

Using the RPM calculator, if I were to go at 50mph on the first speed, the motor would be turning at 5500 rpm. Is that alright on the '70 340?

(Adding a temporary tach until I can repair the one I have is on my to-do list.)

Finally, while the transmission is a 727 with the same gear ratios as the 1970 would have had, the one I had came from a 1976 dodge/plymouth truck (PK3898866). I know nothing about transmissions, but if the shift points are built into it, maybe they're different from the 1970 challenger 727...

Loa

[Chryco Psycho]

5500 rpm for a 340 is fine . You should be able to feel the power nose over & the car will not pull as hard as you go past the rpm where the engine is making the best power