340 build advice needed

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Mr Man

Mr Man

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Hi all,
I have a new project in the garage and I'm looking for some expert opinions on what I should do.

Car details: '72 Duster with 340 swap, 727 w/Jegs 3000 stall converter, and 8 3/4 with 3.91 sure grip. It's intended to be a fun street car, not daily driven. Full interior and A/C. I'll be using some cheap full-length 1 5/8" headers.

Engine details: '71 340 block bored .030, original forged crank, fresh unported J heads w/2.02 valves, KB 243 pistons, purple shaft 284/484 cam. Engine was rebuilt years ago by the previous owner but never fired, so I decided to tear it down and check clearances, etc. I have corrected the previous owner's mistakes of terribly narrow oil clearances on the mains (less than .0005!), incorrect ring gaps, pistons installed to rods in the wrong orientation, and a crank that was over 90 grams out of balance. Good thing the previous owner never tried to fire it up...

Anyway, I'm ready to reassemble the engine and have 2 questions:

1. The current compression ratio is 10.36:1 with a 0.039 head gasket (I cc'd the heads at 65cc, 6cc valve reliefs on the pistons, and the pistons go above the deck by 0.019"). Is 10.36:1 too much for pump gas? What should I aim for?

2. The car came with 2 intakes:. The original '71 cast iron #3512100 and matching Thermoquad carb in excellent shape, and a Torker. I'm leaning toward the Mopar iron intake and TQ carb. What do you think?

Keep in mind I'm trying to keep costs down, that's why I'm sticking with the old school purple cam and intakes that I have on hand. I may switch to an RPM Air Gap and Holley carb in the future if I decide to keep the car.

Thanks in advance!
Scott
 
I, myself would use the TQ & OE intake.
10.36-1 is a little high. Again, I would use a thicker head gasket.
Try for a 10-1. Octane could be an issue at that level and the iron heads.

Otherwise, it looks good.
 
I would run it.
I run 10:1 340 j head at sea level with no issues. Looks like you are at 1000 ft (less air available to compress.
Your dynamic with a 70* intake valve closing event will be approx 7.85:1, well within pump gas limits.
A good tune is the key. I have never run that cam but I would install it at 106 or 104 to help the bottom end torque.
 
I get about the same Static CR: right around 10.4:1. With the larger cam, the DCR is going to be fine as is: 7.8 at and ICL of 110 and 8.0 at an ICL of 106. I would set up the latter, just to keep a bit better low RPM torque. (And I just now read the above post so looks like we are at the same spot.)

It will be fine; I've run 10.3 and a higher DCR on pump gas with iron heads. Just sneak up on the total ignition advance slowly and don't worry if you can't get total advance up to the numbers some do with lower compression engines.
 
And yeah, OP that thing was a train wreck ready to be fired up! Good thing you went through it in detail.
 
Compression depends on which cam. Make sure you degree the MP cam. I would prefer it to be under 10:1 and run a milder camshaft but that's me. I ran a similar 9.?:1 340 with the '71 intake and carb, a Comp HE268 cam, and headers. Was a GREAT street package in an E body. Would be even better in a car with more gear and 500lbs less.
 
I suggested what I did due to a previous engine I had. A 9.8-1 ratio 360 w/J Heads, 2.02's and a purple 292/.509 cam. It did run on 93 octane. But nothing less. A bad batch of gas and I was screwed.

Ignition timing has to be spot on and fuel quality good at all times. Your cam is smaller which builds more compression which needs just exactly what I said. IMO, I would look for that thicker head gasket for a 10-1 ratio. Or slightly less.

My being super close to sea level may have mena but nervous for you so if there is an error, it is in the side of caution.
 
moper said:
Compression depends on which cam. Make sure you degree the MP cam. I would prefer it to be under 10:1 and run a milder camshaft but that's me. I ran a similar 9.?:1 340 with the '71 intake and carb, a Comp HE268 cam, and headers. Was a GREAT street package in an E body. Would be even better in a car with more gear and 500lbs less.
Click to expand...
I got curious...... With the 268 cam and an SCR of 9.7, your engine's DCR would have been at or a hair above the same DCR at the OP's with a 284 cam.... assuming the OP's purple cam is truly a 284 duration at around .005"... and assuming 9.? = 9.7 LOL.

The OP can easily drop 0.20 to 0.3 points off of both SCR and DCR just by putting the standard Felpro 8553PT head gaskets. That'll add some margin against detonation.

And good a well cooled engine and engine compartment is going to be a part of the picture, either way.

For total perspective, I DID have quench in the 10.3 engine (which the OP will not have) and was cautious (and limited) on ignition timing. The cam was 254 intake duration and a 114 LSA (for fuel mileage) and was installed with around a 60-62* ABDC intake closing so DCR was up there in the 8.3 range. It was run from sea level to 6k feet. But, in 7-8 years and 95k miles, I only got one bad batch of gas, and never touched the ignition timing once I got it set (stock point distributor). And this engine was used with a stock converter and I even towed with it.... both of which will tend to promote the occurrence of detonation.

The high stall converter and high rear gear that the OP has will help him to stay away from detonation regardless of his CR.
 
Last edited:
Thanks for your input, guys. I think I'll go the cautious route and drop the compression to about 10.0. I'll also shoot for 106 ICL when I degree the cam.

I appreciate the help!
Scott
 
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