Mo power for my Mopar 340...

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67DART340

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After investgating my new 67 Dart 340 4 speed, I found that I have a 340 built after april of 1972, which would make it a low compression 8.5:1 motor with a cast crank (it has a damper for a cast crank). It has 3418915 heads with 1.88"/1.60" valves, a 600 CFM Edelbrock carb, and headers with 2.5" exhaust pipes (no H crossover)... I've been told that after it was rebuilt that 9.5:1 was the compression with forged pistons... Even though it has a good sounding cam (which isn't known), my thoughts to improving the output would be: 1) 1.6:1 roller rocker either stainless steel or aluminum and or, 2) 750 CFM carb... What is anyone's thought on that?.. Is 750 CFM too much?.. They did use Thermoquads, which I would guess would be close to 750... Would I have clearance problems with the pushrods in the heads?... I believe this was a de-rated motor at 245 HP@4,800 compared to the previous high compression 275 HP in 71 and early 72... Thanks..
 
What intake manifold is on there now? Have you run a compression check on it to see what your dynamic compression is?

I'd go with a edelbrock performer RPM (one of the new air gaps) for an intake and a 750 cfm carb. The 750 shouldn't be a problem with an aftermarket intake, headers, cam and 9.5: 1 compression. Actually, it shouldn't be a problem without the intake either, but that might help quite a bit if you're running a stock 4v manifold.

On that note though, while you're in there I'd check out what cam you've before you go to the 1.6 rockers. If you don't know what you've got it may not make sense to go to the 1.6's, and you still may have to respring if you go that route depending on how high the lift is on that cam. You may be better off buying the cam you want and matching the valve springs to it.
 
The intake is an Edelbrock Performer RPM (I forgot to mention that), but not the newer air gap type... I will check the compression when the weather gets warmer and will try to measure the lift to calculate the cam specs, if possible... Thanks for the feedback...
 
No problem!

The RPM will be fine, I like the new design of the air gap but if you've already got the regular RPM I'd keep it. The cam specs will be the biggest issue going to the 1.6's, if its already a higher lift cam with the stock heads you could max out the lift pretty easily.

Most manufacturers stamp the cams too, so if you felt like pulling the timing cover you might be able to get the info that way.
 
The early 340s really only had about 9.5:1 compression even though the factory rated them at 10.5:1.
 
Skip the 1.6 rockers. There would be a gain, in most case's, but it's very little. If the engine was thought out with the lift in mind, that would be different.

T-Q's were stock. The general accepted ratings are 800 - 850 cfm.

Not knowing the cam specs makes any type of recomendation hard. But IF you want a new cam shaft, most of the power is in the duration of the cam and it's rate of lift. Quicker being better. Cams with that feature, a quick opening, ca be found in Comp Cams extreme hi-lift series, Lunati's Voo-doo grinds, Huges engines cams and specail order cams.

There a bit tuff on the valve springs, so it's your call on the installation of a cam like that. There isn't a streetabilty issue with them, FYI.

Just choose the duration that fits the RPM range of your driving and intended useage. Valve lift really should not exceed head flow abilty's.

The rest of the car specs should also be known. Car weight, converter stall, rear gearing. Etc.......

A 750 is fine if the car isn't to heavy or gearing to numerical low.
 
I don't know what rear gears are in the car yet... But I do know that the 67 Dart should be about 3,400 lbs... The OD tranny has a low 1st gear at 3.09:1, then goes to a 1.67:1 in 2nd, which is a big change (46%), then 3rd at 1.00:1 is a 40% change, and 4th at .73:1 is a 27% change... That is not what I would call a close ratio tranny, but it's what I've got... the standard 4 speed (pre 75) has a 25%, 33%, and 25% difference between gears... I would say more for performance than economy... Thanks for the input....
 
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