340 - This thing should run better

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finallygotadart

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Just installed a budget rebuilt 72 340 in my 67 dart. First time ever rebuilding an engine. It sounds great and runs pretty smooth, but it does not have the grunt I was expecting. Please help me tweak it. Here are the details of the rebuild -

.030 over 72 block with stock "J" heads 1.88/1.60 (did not do a valve job, just cleaned everything up).

New pistons and rings - silvolite 10.2:1 compression, but since they're being used with the lower compression 72 heads, I'm only getting about 9.4:1

Cam is 280/480, 110 deg (I think)

Cast iron manifolds with 2 1/2" exhaust and flowmasters

Edelbrock RPM Air Gap and 600 cfm Edelbrock carb

Still running points distributor, about 12 degrees advanced.

It's connected to a 904 with a shift kit and the torque converter only stalls at about 2300 RPM. 8 3/4 rearend with 3.91 sure grip.

This engine should be somewhat of a screamer, correct? I'm dissapointed with it's performance. Help!
 
put the timing up about 18.it will run better right there....
 
bad440 said:
put the timing up about 18.it will run better right there....
Click to expand...

I wouldn't just say that without also asking about the total timing - which is a much more important number. Total timing should be close to 36 BTC with all advance in. Makes sure it is not detonating or you will kill the engine quickly.

Are you sure about the cam initial setting? I understand that the recommended initial setting for a Mopar Performance purple cam should be 3-4 degrees less than Ma Mopar says (though I'll leave this recommendation to more experienced FABO members). You can't guess on this setting, it must be measured with a degree wheel or tape! Having this number off will certainly kill power development.

Yes, in my opinion, your car specs should be a screamer! If its not, then something is far off. Good luck in tracking down the problem!
 
headers.

easiest fix you could possibly do.

stock '70 hemi cudas. after purchase, by only adding headers the cars would do wheelstands. everything else stock, the headers added 50hp as the only upgrade.

stock manifolds are quiet. but at a heavy price.

the other two items holding her back are the torque converter and the carb.
750 double pumper or one of the top end thermoquads would do better.
torque converter needs to be a 3k... you could do one at a time, or all three upgrades at the same time for the biggest change lol.

if you were doing it all over again from scratch, you would use 2.02 heads, solid lifter cam with adjustable rockers (steel type from a 273 would be fine) and about .550 lift
 
finallygotadart said:
Just installed a budget rebuilt 72 340 in my 67 dart. First time ever rebuilding an engine. It sounds great and runs pretty smooth, but it does not have the grunt I was expecting. Please help me tweak it. Here are the details of the rebuild -

.030 over 72 block with stock "J" heads 1.88/1.60 (did not do a valve job, just cleaned everything up).

New pistons and rings - silvolite 10.2:1 compression, but since they're being used with the lower compression 72 heads, I'm only getting about 9.4:1

If you used the 72 type pistons, you have about 8.1. They were 4 eyebrow pieces. The heads were the same except valve size. The compression difference was made with the pistons, not the heads. The early 68-71 340 had 2 eyebrow flat tops. The 72-73 engines had 4 eyebrows with a large trough that dropped compression drastically to about the 8.1 range. Unless you used the early style 2 eyebrow flat tops, you have much less than 10.2.

Cam is 280/480, 110 deg (I think)

Cast iron manifolds with 2 1/2" exhaust and flowmasters

Headers would help you tremendously. The manifolds are holding you back greatly.



Edelbrock RPM Air Gap and 600 cfm Edelbrock carb

Not a bad choice.

Still running points distributor, about 12 degrees advanced.

As mentioned, probably not enough, especially if you used the later piston style.

It's connected to a 904 with a shift kit and the torque converter only stalls at about 2300 RPM. 8 3/4 rearend with 3.91 sure grip.

That's not much above stock. Stock was about 1700 and the factory high stall was about 1000 more.

This engine should be somewhat of a screamer, correct? I'm dissapointed with it's performance. Help!
Click to expand...

Possibly not correct. If compression is where I suspect, you only have around 8.1. For that compression, it is overcammed. You can advance the cam timing and it will help greatly. I honestly think your biggest problem is compression........or the lack thereof.
 
RustyRatRod said:
Possibly not correct. If compression is where I suspect, you only have around 8.1. For that compression, it is overcammed. You can advance the cam timing and it will help greatly. I honestly think your biggest problem is compression........or the lack thereof.
Click to expand...

I agree, the lack of compression is killing the engine. I think the best solution for this would be to use a better head. Find a set of cheap factory cast heads with less cc's. Maybe even a head with 2.02's in it too. Then the engine would breathe better and you'll be really taking advantage of the cams potential.

also ditch the old cast iron exhaust logs. You can get a set of headers for $120 shipped last time I saw on Jegs' website. They may still have the sale going on.
 
Thanks. I'll check the timing. The 3,000 converter and headers will have to wait for a bit.

I am afraid the biggest problem might be the compression. They have 4 eyebrows (I assume eyebrows are valve reliefs?) The kit (through Mabbco) advertised the pistons as 10.2:1 for 68-71 340s. Since mine was a 72 block I called the manufacturer of the pistons to get a little more information. They said my heads have about a 65cc combustion chamber and that the pistons should provide approximately 9.4:1 compression. How can I check this figure now that the engine is already installed?

Sounds like I might have gotten screwed.

I assume a set of Edelbrock heads would be the cure all?
 
Do you know how to properly set up points (reading dwell, not gap?)? Did you degree the camshaft? can you get cylinder pressure readings?
If the heads had no valve job, and the ignition is not perfectly set and tuned (as far as timing curves & initial) it may feel doggy because you don't have headers or a convertor.
I would get it running better before you buy anything "downstream" in the power train. Fix the cause, not the symptom.
 
No, RPMs won;t fix anything. Tuning and headers will go a long way...
The heads will be much larger than 65cc, and I'd wager the deck height is taller than stock too. So you're not high compression but that's not fatal rpoviding you can tune for it.
 
Sounds like the voodoo bracket master II I put in a 9.8:1 340. I had to install it @ 104* (ground on 107*) to get any bottom end out of it. What is your cylinder pressure? May need to advance your cam. Does it stall in drive at idle? You need more stall. I ended up with a 3200 in my car and the voodoo 280/480
 
Do you remember off hand where the piston sat at TDC? A good solid 10:1 should be above the deck roughly .012 where an 8:1 should be below the deck damn near .0812 give or take.
 
Get the initial timing figured out, then total. Don't go beating on it with too much total. Verify what you have so you can make adjustments.

IMO, the engine initial timing is ~2/3 of what it should be.

It will run a lot different once sorted out.
 
crackedback said:
Get the initial timing figured out, then total. Don't go beating on it with too much total. Verify what you have so you can make adjustments.

IMO, the engine initial timing is ~2/3 of what it should be.

It will run a lot different once sorted out.
Click to expand...

Good call... My 340 like 18-20* initial
 
swinger340 said:
Good call... My 340 like 18-20* initial
Click to expand...
a set of headers witch are only nabout 120 dollars and three inch exhaust would help. do away with the points go to ether stock electronic witch came out in 72 or go to msd I run all the above on my 340 and i'm only pushing 8-1 compression and it will go sideways in second
 
Thanks everyone for your advice. I really appreciate it.

Is this "timing tape" available at any auto parts store?

Also, thinking about changing to an electronic distributor, maybe the Summit brand that has everything under the cap. I'm pretty sure the vac advance is not working on my points distributor because all I can hear is air when i suck on the hose....and i have not been able to find a vac canister that will fit. Pertronix was an option, but if I can't get the vac advance right then it won't do any good. So for a few extra bucks I thought I would get the summit distributor....good or bad idea?

Leadfoot - the piston sat just about even with the deck at TDC.

Moper - I did not degree the cam, as i was told it does not really benefit a smallblock (bad advise I suppose). I have a friend that can help me get cylinder pressure readings. Will this help determine compression ratio?

Thanks again to everyone who has commented!
 
I'm going to co-sign on the timing issue as well. 18*-20* initial w/ 34*/36* total, all in by 2400-2600. Big choke point with the manifolds and as others stated, can be cured relatively cheaply. Keep us posted on what changes you make & how impactful they are.
 
Search "hillbilly timing tape"

I don't ever use a timing tape, maybe to mark a balancer... never install them
 
Is this "timing tape" available at any auto parts store?
Mr. Gasket sells them for under $5. Summit/Jegs have them.

Also, thinking about changing to an electronic distributor, maybe the Summit brand that has everything under the cap. I'm pretty sure the vac advance is not working on my points distributor because all I can hear is air when i suck on the hose....and i have not been able to find a vac canister that will fit. Pertronix was an option, but if I can't get the vac advance right then it won't do any good. So for a few extra bucks I thought I would get the summit distributor....good or bad idea?
In my opinion - buy the MSD ready-to-run with it all under the cap. If that's too much the simple MP conversion kit or distributor is my 2nd choice. I worry about quality control over time with the Chinese copy stuff. Regardless- you need a functioning vacuum advance (in my opinion) and adjutability in the centrifugal is always nice to have. The MSD has adjustable switches for those plus multi-spark.

Leadfoot - the piston sat just about even with the deck at TDC.
"Just about" is not even and as little as .020 can make a big difference in static compression. However - IMO - it's irelevant atthis point. You can work with what you got provided you have the right support stuff and know how to tune it.

Moper - I did not degree the cam, as i was told it does not really benefit a smallblock (bad advise I suppose). I have a friend that can help me get cylinder pressure readings. Will this help determine compression ratio?
It won;t tell you any compression ratio. It will tell you how much pressure the cylinder is developing - which can be a help in figuring out if the cam is in right or needs to be looked at. Just out of curiosity - what engine does benefit from degreeing if a small block doesn't need it? The answer is every engine (including my Neon DOHC) benefits from degreeing if only to give you peace of mind that it's in where it's supposed to be.
 
in lieu of the new information of the distributor vacuum advance not working I would definitely say upgrade your ignition system. You'll be so much happier just going to a stock electronic setup. I thought I heard that the summit all in one HEI setup won't fit against the firewall???
 
There are two types of stock exhaust manifolds: standard and hi-po. If you have the hi-po type, don't worry about switching to headers (they will only gain about 15 extra HP IIRC). If you have the standard type, then they are definitely hurting your HP development.
 
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