340 screw-up

[B.Tallent]

finally tore my 69 340 apart, mild surprize,mild shock. the compression ratio spec'ed out to 7.28:1 no wonder my commpression tests where at 60 p.s.i. the bores are
STOCK along with the crank not even any ridge in the bores,COOOOL. i have a question about the old pistons they are 1.75 comp. height so i believe smogger 340 slugs but the casting #'s don't match and also there is a pentastar cast inside on the wall. someone also extensively ground down the skirts and reshaped them the casting # is3671155 i'm going w/kb piston but i'm curious

[rumblefish360]

What are you wondering about?

[B.Tallent]

just wondering what pistons they are from the cast # don't match what mopar said they should be and i don't remember ever seeing a pentastar cast inside a piston before.

[rumblefish360]

Hmmmm, neither do I. I look at my stockers from the 400 later.
The only numbers I have are in the how to hot rod your s/b mopar book.
I do not see them in there.
Perhaps, the maker of those pistons put the pentastar on them for easy ID purposes. Or past MP parts.
The numbers for the 69-340 fall under a group. , 68-71, 3004516-7 @ 720 g. weight w/1.84 comp. distance.
Are these Fordged or cast. That ratio seems very very low.

[Eric Strong]

I've never heard of a 340 with that low a compression ratio, unless it was super charged or turbocharged. But even with that low a c/r you should still get over 100 lb. on a compression test. I think you have more than compression ratio problems if you are getting only 60 lb. Was that for all cylinders or just one?

Quick Claims Adjuster!!!

[B.Tallent]

i know the motor has been into because the engine paint was first painted blue and then it was sprayed over with black, neither color is proper for the year.this engine was in a 31 coupe i bought a few years ago. the pistons are .080 down in the hole with .040 gaskets and 75cc heads it came out to 7.28. it also has a 318 spec. cam this car was built from a guy who builds hot rods. what a mess i have. externally the motor had all the right #'s but it did'nt run like one. the x head are starting to sink the xhst's so i will get seats in it. that compression was all the holes. the pistons are confusing because of the numbers and that mostar cast in the skirt

[Eric Strong]

Sounds like you will be spending a little more than origanally planed. Wasn't 10.5:1 the stock comp ratio on that engine? I guess you'll be investing in a new set of pistons. The cam doesn't sound like your first choice either. Well you have the heads off you might want to polish the combustion chambers, to allow just a little extra in the compression ratio. I plan to change my heads next spring for aluminum, or I would be polishing my 906s and bringing my copmression ratio back up to 10:1. Currently I'm running 9.3:1 and the difference is more than a little noticeable. So I kind of know how you feel, but 7.28:1 is kinda scary. That would be like drivin' a chebby. yuck!!!

Quick Claims Adjuster!!!

[rumblefish360]

Your idea of K/B pistons sounds fine. I would suggest flat tops with the valve reliefs at zero deck height. Polish the head chambers and a Fel-Pro gasket @ .039 thick.
You'll be at 9.5-1 or real dang close.
This should leave you real nice for pump gas with enuff comp. for stout cam selection if needed.
The fuel required for yesteryears comp. ratio is no longer here. So the old 10.25-10.5 ratios of then are a bit much now.
ERIC, your correct.

[dave571]

You'd better double check your bore measurement.

Usually factory over bore pistons have the pentastar on them. Factory oversize pistons are made in .020" increments from stock bore.

There is a very good chance your engine is already .020"over

What are you using to measure the bore with?

ps: I bought a 340 that was .020 over with pentastar marked pistons in it. I was pretty choked when I realized it was .020 over.

[goose69]

So are you trying to build a cruiser motor or what type of outcome are you looking for? I built a 340 for my 'Cuda that was aimed at being a mild street motor and good for cruising. I started with F.M. flat top pistons w/ valve reliefs and a steel crank. Then I used 360 heads ( had to look through quite a few sets to find some that weren't cracked between the valves like they tend to do) ported them slightly ,relieved the bowls ,and match ported them. I then used a Comp Cam 270 camshaft and an Edelbrock performer RPM manifold ( cause it was the closest to the LD340 intake) and then an Edelbrock 600cfm carb ,but then swapped to a holley 750dp when at the track. To keep it simple I ran stock 340 exhaust manifolds and Edelbrock mufflers (which performed alright ,but sounded wierd with the stock tips). But I have to tell you this was the best cruising motor I've built and it suprised a lot of people when I stuffed my foot into it ,especially with 3.91 gears and a factory high stall ,but normally I ran 3.23's. This motor NEVER got hot or loaded up ,but sure got all the jobs done that was asked of it.

[MY340]

If some previous owner ran a blower on the 340 that would explain the low compression pistons. Since it was in a hot rod that might explain it. Just a thought......