"1971" Duster 340 true compression ratio?

[Duster_340]

I know the advertised comp. ratio for this engine was 10.5. Was it an actual 10.5 Compression ratio?

They were a powerful engine even with the shi-y passenger side exhaust manifold.

Mopar to ya,

[SUN RA KAT]

I'm pretty sure that they still had the 10.5 compression ratio & this was the last year for it, too. The 1970 340's has a Carter AVS carburator and the 1971 340's has the Carter Thermoquad. I had both engines - the 1970 Duster 340 and later my mom sold me her 1971 Duster 340.

[dave571]

That's a question i've been wondering about myself.

As an example, my 400 is advertised at 8.2(8.3?) in the service manual.

When actual measurements are taken of the deck clearance, and head volume, the actual compression is 7.4 or 7.5. Nearly a full point different than advertised!

The 9.9:1 I have achieved is by milling the deck down. With the head gasket installed there is only.054" piston to head clearance and using the older small chamber heads(72cc? I knew when I calculated it at www.kb-silvolite.com)I get 9.9 to one. I also know that with my camshaft I have about .130"piston to valve clearance. I also know that only 440 sixpacks had valve reliefs cut in the pistons from the factory. That in mind, how did the performance Big blocks of the late 60's and 70's, have more than 10:1 compression, when they used the 906 head, which had a larger head volume, yet the pistons weren't so tall that they needed valve reliefs?

Aftermarket 10:1 pistons have valve reliefs, fairly beefy ones at that. It would seem that the factory bigblocks, couldn't have had the compression ratios's they advertised. I'd like to take one apart and make some measurements to verify this. My dad has a 69 TNT motor on his garage floor, that is "virgin". The thing is, taking him into letting me take a head off of it.

I don't think he'll like the idea

Back to the small block, the head volume is smaller, but so is the displacement. Were they 10:5 to one? I seriously doubt it. If they were, I don't think they'd run on pump gas with the relatively mild camshafts they had from the factory.

Anybody got some factory measurements(not spec's, actual measurements from the engine) to try at kb-silvolite?

[DartGT66]

That's very true with Mopar engines; from the factory they were nowhere near the blueprint figures. The true CR's are usually waaay below the advertised CR. By the way, '72 340 already should be 8.? from the factory; '71 was the last year for 10.5:1 advertised CR. You really can't tell what there really is without measuring, the deck heights and the head CC's are usually far greater than the advertised specs. Even the closed chamber BB heads tend ot in the 80cc range and the open chamber heads near 90cc.

[pishta]

That is a great question. answer is as close as your handy compression tester. By taking Mopars word for it is buying into the sales lore of the muscle car era. By doing the math from a blueprint, yeah it may be 10.5:1 with a steel shim gasket, but we dont run them anymore, and the 63cc for an 894 and the 65cc for a 915 head is WAY off, try more like 70-72cc. For example, look at a heart head (302) with the large quench area that is flush with the gasket surface. These blueprint to 62.6cc's, smaller than the 340 heads chamber, dont you think? I thought they might have measured the 340 head with the blueprint .045 pop up, but even the lo-po 73 with the .054 clearance is 63cc's. My V6 Mitsubishi says 10:1 and use premium fuel, BS, it runs fine on shit gas...when in doubt, whip it out! (compression tester!)

[DartGT66]

Compression tester? My engine gives 120 PSI cranking pressure and the CR is.......11:1.

[c318cuin]

compression testers are only good for relatively mild cams. Wild cams tend to mess with the readings making a high compression engine have next to no cranking compression. Like many have allready stated the only true way to know is to tear it down and mic it.

just MHO

[E Strong]

I believe that most of the compression ratios advertised by Mopar are close. I have a 68 Chrysler 300 with 440 HP. It has 10.1:1 Advertised compression. I had the engine rebuilt in 98. When they pulled the heads off at the shop they were very interested in measuring the true compression. The guy who had his 440 done just before me got stung with a 8.? compression. He was told in was a magnum. When they measured the pistons were down about .040 in the cylinders. When they measured mine they were above the cylinder. They calculated the compresion at 10.3 to 1, wit thin MP gasket and .010 milled off heads (906).
Hope this helps!!!

Quick Claims Adjuster!!!

[Tarrbabe]

Mopar usually stated the compression height correctly. But the cyl. head volume was usually off some. I have heard of heads that were advertized as 65 cc's and had 68 - 70 cc's or even more. Bottom line was that the compression was .4 to .7 lower that advertised. Depending on the engine you choose.

[diego-ted]

So with all the above info given, how does the 340 run on 91 oct. pump gas????

diego-ted