1973 340 detuned?

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jongdoe

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Hello, I'm new here. I have a stock 1973 340, I've read they were detuned and wanted to know what chrysler did to them. The only thing I've read was they choked the exhaust and changed the pistons. Was that it ?
 
smaller intake valve...2.02 down to 1.88

compression ratio....from an advertised 10.5 to 1 to an 8.5 to 1 ...done by dropping the piston down about .100 of an inch from the high compression engine..changed the compression height on piston from 1.84 to 1.74....and they added a cast crank which has no effect on the performance....

the 73 A bodies were heavier too....new heavier bumpers and door beams...
 
So if I was to put 1971 340 pistons in my 73 engine that would raise the compression to about 10.3 to 1 with my J heads with the 1.88 valves
 
jongdoe said:
So if I was to put 1971 340 pistons in my 73 engine that would raise the compression to about 10.3 to 1 with my J heads with the 1.88 valves
Click to expand...

you would raise your compression...Yes..but it all depends on what the head cc are...head gasket thickness...and again...where the piston is in the bore....

best thing is fine a good set of pistons that are lighter then the stock thugs....

but it certainly be a step in the right direction....
 
I think the heads are 65cc's from what I found online. The engine was rebuilt at 50k miles, I've put about 10k on it since I've had it. I was thinking of freshening it up and get some more power out of it since I had it parked for over 5 years
 
Find good aftermarket pistons to get the cr up to about 10 to 1, valve job with 2.02 valves pocket port them if you can, slightly bigger than stock cam will help.
 
The engine right now is stock with a edlebrock performer intake, holley 650 spread bore and headers. I was thinking if changed the pistons to earlier 340 pistons and put a bigger cam in, I would be gaining more power. The engine is in a plymouth scamp,904 trans 2500 stall converter,8.75 rear 323 gears
 
jongdoe said:
So if I was to put 1971 340 pistons in my 73 engine that would raise the compression to about 10.3 to 1 with my J heads with the 1.88 valves
Click to expand...


No, the heads won't make a difference in your compression, the difference is in the pistons...
 
Not sure if your going to find actual 71 and earlier 340 pistons but check KB, speed pro, etc for the +.030 over replacement.
 
Running a stock distributor.

Unless you have emission testing to deal with, one way to make it run better is tune your engine with the initial timing it actually wants. The book numbers suck for performance and idle.

If a stock 73 340 doesn't ahve at least 14* on it, it's likely not in the range it should be. Timing it this way takes work because you have to fix the distributor mechanical advance as well.
 
Don't believe the specs you read online. Stock J heads are 65 cc's-ya right. I've been messing with Mopar's for over 30 yrs. and if there's one thing I've found it's you can't go by advertised specs.

If you do change pistons you better weigh what's in there and find pistons that weight the same (or really really close) or you'll need to have the rotating assembly balanced
 
most engine kits i've seen are for 68 to 71,so i figured if i'm going to freshen up my engine just go for the earlier pistons to boost up the compression. the engine runs good right now the way it is,but like i said earlier it's been parked for atleast 5 years,which i did start it and let it run while parked every 6 months or so.
the distributor is electronic stock,if i remember right the timing was set with the running at i think 3000 rpm to full advance. a friend of mine set everything up,he got into details while doing it,right down to changing airbleeds in the carb. he spent alot of time setting everything up years ago
 
keep in mind,,NOT ALLL but a good percentage of 1973 340s were cast crank,,externally balanced,,requiring the correct torque convertor,,or fly wheel if you have a stick shift,,,this will effect what rods and pistons you change,, a 73 340 with a good intake,,,cam and exhaust will run great in an A body,,,and wont require high octane gas,,,they run great,,
 
I'm doing rebuild on my 72 340 now. I did cc of my head chambers and they went 70-72 cc. Easy to check by setting a CD disc on head w grease to seal edges. Then inject alcohol in it w syringe. I picked a set of speed pro forged pistons L2316F with cd height of 1.840. For my block, heads, gasket should yield 9.5 CR.
 
you guys all know alot more about this then me. i appreciate all the help. if the only things i change in my 73 340 engine is the pistons and cam to the 1968 specs would i be improving the performance? any idea about what hp and cr i would have while using my stock 70-72 cc heads?
 
jongdoe said:
you guys all know alot more about this then me. i appreciate all the help. if the only things i change in my 73 340 engine is the pistons and cam to the 1968 specs would i be improving the performance? any idea about what hp and cr i would have while using my stock 70-72 cc heads?
Click to expand...

the older piston will raise the compression....the 340 cam never changed 73 cam was the same as the 68 auto trans cam...the 68 4 speeds had a little more cam...but both are mild compare to what is available today.

sit down..put a game plan together....
 
I wouldn't use the 68 stick cam its mild, go with a modern grind that has some lift to it!
 
Ah the total timing monster.

Put a light on it at idle.
 
jongdoe said:
I think the heads are 65cc's from what I found online...
Click to expand...

Stock J heads need to be milled .040 to get down to the NHRA min 64.7 cc. Add about .040/.0048 = 8.3 cc for milling and that gives you about 73.0 cc for the largest chamber with stock valves. You will make more power with the stock 340 intake than the Performer, same goes for the Carb.
 
jongdoe said:
you guys all know alot more about this then me. i appreciate all the help. if the only things i change in my 73 340 engine is the pistons and cam to the 1968 specs would i be improving the performance? any idea about what hp and cr i would have while using my stock 70-72 cc heads?
Click to expand...

If you use a set of these with stock J Heads you'll end up with somewhere in the ballpark of 10:1 with a 72cc chamber and a .039 gasket.

http://www.summitracing.com/parts/slp-l2316f30/applications

Of course if I had the chance to do it again(which I will) I'd sell that 340 to someone that thinks they need a 340 and sink the bucks into a 360.
 
You are on the right track with going to the earlier piston - that should help a lot. As you are replacing pistons anyway, just go with a higher compression model piston. Someone has to make one. Check with Mancini or one of the other Mopar specialist shops. As far as the heads go, be aware that if you cut them too much, you will have manifold alignment issues. There is most assuredly horsepower gains in the heads, even just matching the ports to the gasket(s) - along with the intake ports will make a difference. Unshouding the valves and a decent "bowl" cleanup gives you even more! Since you have it apart anyway, check to make certain your block has a nice flat deck - and maybe give it a little .010" cleanup (that isn't a lot, but it all adds up). Cams. Wow. We have So much to choose from! Spend a little time talking with one of the guys from virtually Any of the cam makers out there, explain what you want to do with your car and above all, Listen! In the 60's I had a 273 Commando Barracuda that I recammed - I wanted that drive in idle, but the guys at Crower talked me into a much more suitable cam. Didn't sound bad azz, but it sure did work good! As far as timing goes, 32 to 34 degrees total seems to work well.
Just my ten cents worth. (inflation, ya know!)
Have Fun! Art (R/T)
65 S 331LA, 904/998 & 65 318(soon to be 340) 4speed Barracudas
 
You worked hard doing all that typing to try and help the OP, but this thread was from 2014, and the OP hasn't been here since 2016. I'll bet he's been working hard saving up money for pistons! :D
 
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