I have been reading in the Mopar Six Cylinder Book and others and have seen the head volume stated to be 54cc's (at least that is where the numbers start for the compression ratio charts and the NHRA head volume). It was raining outside Sunday, so I went to the garage and CC'ed my bone stock '72 Duster head (casting 3614850). I checked each cylinder and everyone of them was 58cc's. That puts the original compression ratio very close to 8.0:1.0. How do you get to 8.4:1.0, as stated in service manual) with this engine?

bwhitejr

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'72 Duster (Performance 360)
'83 Ramcharger (Performance 318)
'80 TrailDuster (360)
'80 D-150 Truck (See Below)
CompCams 252S, Holley 390cfm, Offy manifold
Ported, Polished and Gasket Matched
P4286813 Springs,0.040 Overbore,
0.090 Shaved Head

Just as an FYI, that book is outdated and has lot's of typos in it...This is a prime example of "Real World" meets "Looks good on paper"

I just had a 1963 Truck head planed not too long ago and Took .080 off it to get it to work(stock compression) on my '67 Valiant...

Production tolerances for our vehicles weren't that great, I've had some '70s heads that "the book said" should be 53-4 CC chambers, but in reality they were anything from 53-51 CC's...(or like the drool tube heads they could be 54-68 CC's...)

Combine this with a deck height that isn't regular either (0- 0.2), and the combination of parts could be anywhere from "Turbo motor" to "Hey look stock compression!"...

When tearing down the engine, measure everything and note it so you have a baseline of what it was like originally, and then you can work on what needs to be decked, bored, etc... and how much to what part...


For the most part, you can't blame Mopar, the slant six was the "economy" grunt engine, and not afforded as much "love", as long as the thing got the customer from home to the grocery store, ran on the cheapest gas, and did it reliably for 20 some odd year, it did it's job....

-D.Idiot

The early head (think it was 61) I did also had a much larger volume
than the books claimed. On this head, volume varied from 58 cc to 64cc.
The other thing I learned was the slant six racing manuals ballpark
number for how much to cut to remove a cc of volume was not accurate
for this particular head. I needed to cut more than the original estimate.

Since there is so much variation in the parts, seems to me the only way
to get it just right is by iteration. Measure, cut, measure again, cut
again .... until it is just right.

The Direct Connect Racing Manual says: "...the '72-'76 low compression engines....". I guess the large head volume for those years explain what I measured for my '72.

bwhitejr

_________________
'72 Duster (Performance 360)
'83 Ramcharger (Performance 318)
'80 TrailDuster (360)
'80 D-150 Truck (See Below)
CompCams 252S, Holley 390cfm, Offy manifold
Ported, Polished and Gasket Matched
P4286813 Springs,0.040 Overbore,
0.090 Shaved Head

Huh, I am heard of some people having stock slants with the compression ratios in the 7's. That is about like a smogger Ford 400M. Will it take more than just .090 shaved off to get the compression ratio to the 9's? Will 2.2 pistons raise the compression ratio any more?

How much of a difference will 1 point of CR make anyway?

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82 D150-225/727
02 Dakota-3.9/5 speed
87 GMC C7000-8.2 Detroit Diesel/5+2

dusteridiot and relic lover pretty well covered it. heads are just going to vary, no way to tell what you have until you measure it.

mine cc'd out after head work in the low 40's.

as far as 1 cc making a difference, it could mean the difference between running higher or lower octane. I noticed right away after I raised it just a point and a half

-dave

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what part of "illegal" is so confusing???

As far as being an outdated and erroneous publication (Six Cylinder Engines Manual) it, the Slant Six Racing Manual and the Direct Connect Racing Manual are all I know of. I, of course, have FSMs, but they are about fixing a bone stock engine, not about performance. Is there more performance information on the Slant Six out there?

PS: There is also some coverage in the Mopar Racing Secrets Manual too!


bwhitejr

_________________
'72 Duster (Performance 360)
'83 Ramcharger (Performance 318)
'80 TrailDuster (360)
'80 D-150 Truck (See Below)
CompCams 252S, Holley 390cfm, Offy manifold
Ported, Polished and Gasket Matched
P4286813 Springs,0.040 Overbore,
0.090 Shaved Head

There are a couple really good websites you can visit that have lots of really knowledgeable folks, as well as search functions:

www.slantsix.com
www.slantsix.org

But seriously, the slant six was always an underdog motor that never really got much attention. There were a couple books put out in the early 80s by 7-L engineering titled "Two Barrel Conversions for the Slant Six" and "Performance and Economy Modifications for the Slant Six". These were writting by slant six afficionados so the info is pretty good, but I haven't seen these for sale for years.

Your best bet is to just search through the archives on this site. I think just about every topic related to building up a slant six has been discussed at some point or another, and you can always just ask a new question.

I think there is a fundemental misunderstanding on the cc numbers in the \6 manuals. Those numbers are the minimal chamber size for NHRA class racing. They have no realation to anything off the production line.

kesteb, I agree. They are the blueprint specs. I have never found an untouched RG, LA or RB motor that is close the the blueprint deck or head CC. Blueprint specs are used as the minimum, or maximum (depends how you look at it), not the mean.
If these specs were the mean, there would be a lot of damaged 170 and 440-6 motors. The 170 blueprint/min is .000 deck, the '70 440-6 is +.001. If the min was more than this, there wouldn't be enough piston to head clearance. The RGs I've checked are .010-.030 down from the .141 spec, the RB motors are .030-.050 down from the spec for that piston application.

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Slant Cecil