Actual 70 340 Compression?

[YO7_A66]

I know that the factory rated the above at 10.5:1 in 1970. I have heard of numbers that range from 8.9:1 to 10.4:1. Can anyone confirm actual compression numbers on a stock motor?
Thanks

[perfmachst]

hello, well most were in the 9.1 to 9.5 range. by cutting the block, heads to the minium specs, you end up with a true 10.71 or more. factory tolerances had plenty of leeway.

[YO7_A66]

Thanks,
I am calculating 9.02 to 9.44:1 with my current stock rebuild setup depending on head gasket thickness.

[cuda66273]

If your looking for more you can use a Cometic .025 gasket.

Look at the bore on the Felpro gasket...4.180 your bore is 4.040 now think about that big compression robbing gap all around the combustion chamber....Cometics are bore specific.

...and they won't push...PERIOD

[YO7_A66]

Don,
My 340 was bored .040 over. Do they make the .040 over gasket at .020-.025?

[Mr.Mopar]

I once asked Dick Landy this question in a Mopar Muscle tech question article and he says that they were a true 10.5:1 with the steel shim .024 gasket. Thats what he says.....

[Bigdog340]

I spoke to the folks who make Keith Black pistons for the 1970/71 340s'. He says the same as the info from Dick in the post above. Those engines were all 10.5:1 or very close there of. They used a raised (not dome and not flat top)2 valve relief piston and a thin head gasket. Keith Black makes the exact replacement piston in various oversizes.
Other vendors make them in a flat top 4 valve relief, but they will not give you a true 10.5:1 rebuild if that is your desire.

71 Duster 340
3.23 Posi

[YO7_A66]

I currently have the Silvolite #1267, flat top with four reliefs with an advertised compression at 10.2. But, they have 10cc in the valve reliefs. My latest check (without cc the heads) show a best of 9.06:1 compression with my current setup.
I will be pulling the heads this Winter to have them cc'd and then I will be able to determine where my actual compression is at. I have the receipts from the last owner who had the motor rebuilt and the receipts show "resurface sylinder heads" at $70.00. So I have some unknowns in my formula right now.
Thanks for the information.

[wheels2]

David,
Go to www.nhra.com, down to the middle of page. Look for blue print specs, this should give you what the factory said it used. They are sorted by car modle and year. There is alot if info at this site. Hope this helps a little.

[YO7_A66]

wheels2:

excellent reference material.

Thanks

[dave571]

The problem is, the factory said a lot of things. I've never measured one, so I don't know what they were.

If dick landy says he measured one, then who amI to argue.

Not just saying. My 400 was rated at 8.2. Measured at 7.5