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340 low compression #2459870
03/02/18 12:41 PM
03/02/18 12:41 PM
Joined: Jun 2007
Posts: 337
Colorado
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67plymouthman Offline OP
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I bought a 340 a few years ago that was machined by a reputable machine shop. It was a 73 with new cast flat-top pistons 8.7:1, j heads with small valves, cast crank, comp extreme energy.477-.480 cam and I have been thinking about shaving the heads for more compression. Car is basically a cruiser but dynoed at 275 h.p.. My Mazda wagon dynos at 275 frown
Any tips?
I have headers, fuel injection, and older performer intake. She does run good in a 67 belvedere. 3.23s, r.v. Torque converter

Re: 340 low compression [Re: 67plymouthman] #2459934
03/02/18 03:04 PM
03/02/18 03:04 PM
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Bend,OR USA
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Maybe a Pro charger with 2 to 6 lbs. boost is the easy way work devil grin Feels good, last a long time up


Mr.Cab Racing and winning with Mopars since 1964. (Old F--t, Huh)
Re: 340 low compression [Re: 67plymouthman] #2459945
03/02/18 03:43 PM
03/02/18 03:43 PM
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USA, MI
MarkM Offline
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Either Enginequest or Eddie heads. You need something on there with smaller chambers and flows better.

Re: 340 low compression [Re: 67plymouthman] #2460058
03/02/18 07:38 PM
03/02/18 07:38 PM
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67plymouthman Offline OP
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I live at 5,000 feet and think compression is much needed. Being on a budget, if I can mill my heads .40 and probably have to buy shorter pushrods what else would I need?
Any other budget friendly ideas?

Re: 340 low compression [Re: 67plymouthman] #2460062
03/02/18 07:42 PM
03/02/18 07:42 PM
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Lincoln Nebraska
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RapidRobert Offline
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Quote:
Any other budget friendly ideas?
maybe some OE mag heads if you found a set in good shape/not needing alot of $$$. but there is is the intake/rockers etc for that swap. Is the 8.7 actual measured?


live every 24 hour block of time like it's your last day on earth
Re: 340 low compression [Re: 67plymouthman] #2460195
03/02/18 10:16 PM
03/02/18 10:16 PM
Joined: Jun 2007
Posts: 337
Colorado
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67plymouthman Offline OP
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Yes-8.7 measured

Re: 340 low compression [Re: 67plymouthman] #2460224
03/02/18 11:10 PM
03/02/18 11:10 PM
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ahy Offline
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I do think the combo could benefit from more compression... and still run pump fuel. Rule of thumb, low 9's CR is OK with open chamber iron heads at low altitude. If you stay at higher altitude, higher compression should be OK (unless the gas is lower octane which may be the case).

Gains would not be huge. Is it worth it? Up to you.

Re: 340 low compression [Re: 67plymouthman] #2463799
03/09/18 03:40 PM
03/09/18 03:40 PM
Joined: Jun 2007
Posts: 337
Colorado
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67plymouthman Offline OP
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What about a Hughes whiplash cam? They have been out for awhile. Will it help with the lower compression much more than my extreme energy cam?

Re: 340 low compression [Re: 67plymouthman] #2463801
03/09/18 03:42 PM
03/09/18 03:42 PM
Joined: Jun 2007
Posts: 337
Colorado
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67plymouthman Offline OP
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How much can I take off j heads and be safe?

Re: 340 low compression [Re: 67plymouthman] #2463818
03/09/18 04:00 PM
03/09/18 04:00 PM
Joined: Oct 2007
Posts: 3,092
New York
polyspheric Offline
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Choosing a cam to adjust the compression ratio is ineffective.
To get an idea of the effect, advance yours (if you have the intake P-V clearance).


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Re: 340 low compression [Re: 67plymouthman] #2463828
03/09/18 04:12 PM
03/09/18 04:12 PM

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That is one deal where the only answer is more compression--IMO I would drive that good running combo forever and keep that $$ in your jeans but--if you want it to pep up do it right and swap the pistons leave the rest as is
Milling the heads just can't make up for a piston that is so far down in the hole that is like water in a well
Just drive it or.... put a 125 shot of NOS on it and for $400 and two hours you have done the most with the least and made it a real Hot Rod that drives easy otherwise
Perfect candidate for a 125 shot I mean Perfect!


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