How much compression with a Magnum?

nm9stheham said
Mmmmmm.... I just tried 3 SCR calculators with the same number inputs and they all came up within 0.1 point of each other. Most of the variations I have seen have come from entering data incorrectly; the best place to go wrong is to enter the piston valve relief cc's with a + where it should be a - or vice versa. The different pistons manufacturers will list the same valve relief as + or - in their specs, and people don't catch that subtlety; that is the most common place for an error. I've run the numbers on the 318 with the lower pistons multiple times and it always comes out at the same place; at least the Magnum block and head volumes help you quite a bit over the 318 LA. But high 8's is where it ends up for the lower CH pistons.

BTW, if you use the KB's and the Felpro standard head gaskets at .051" thick, then you will right around 9.5 SCR.

As for your goals: 20 mpg in the van is mostly a challenge of moving that big cinder block shape through the air at any speed without killing efficiency. Maybe a chin spoiler? Enginewise with the FI, it seems like an easy goal to get the efficiency; not counting the wind resistance problem you'll face. But I have never cammed or programmed one of those so I'll defer to others on that beyond any general cam experience.

Just a general comment on the cam for fuel efficiency: Higher lift, lower duration and wider LSA. Crane had a series of cams back in the early-mid 70's that they labeled their 'HE' grinds that they came out with in response to the Arab fuel embargo of '73. 'HE' stood for 'hydraulic, efficiency' (I think), and they featured the high lifts, lower durations, and wide LSA's. They did the job and made a wide torque band engine with good economy. So that makes me think you are on the right track with the cams (and is where modern OEM rollers have gone for that reason).

BTW, IMHO, the 350 peak HP WITH the 20 MPG in that van is the compromise you are going to have to fight and make a trade-off. For your broader uses, I'd forego the 350 HP peak number, which you will not use much in reality, and focus on the mileage/torque. That is also partly my personal inclination, but I have built a multi-use engine that towed too and I thought more about torque at lower RPM's and let the peak HP go where it would. That was the engine that used the 192/200 @ .050" cam, and the engine was run almost 100k miles before the forged pistons got to where the blowby was pretty atrocious LOL! You should have better durabiilty with FI and hypers in newer block metallurgy.

With the above said, then certainly don't push the SCR too hard and end up with detonation issues. If you do, you at least have the option to use premium. Keep the knock sensor if the system has one! I don't know enough about the Magnum MPFI to know how it will adapt to detonation. (IMO, Magnummopar would know much better on that.)

Thanks for the input.
You know I want to hate you for saying high 8's, dont you? Just kidding. What did the Stones say, You cant always get what you want. This may be one of those timesunless I could get the deck milled really cheap (not likely).
Using the stock size gasket, KB said to expect 9.6-9.7. I really don't want to go with the thicker gasket (though the one you chose is real close) to bring the compression back down, which would affect the size of the quench area. Instead, I will go with the 9:1's and a thinner head gasket. Maybe Ill get 9.2:1 squeezed out of it. If I can do that Ill hit the 350 mark kinda easy. If not, Peyote had the same idea as I did, but to me that is not the preferred way.
I dont think a chin spoiler will do much at 70 mph. I use to own a 70 T/A and it had to hit about 100+ before you could feel her start to squat. And the chin and rear spoilers did make it squat. Still, a chin spoiler, snorkel scoop, and a rear wing would make it look good. What I really would like to find would be a rear stabilizer bar to cut back on the rear rolling in the curves.
I read an article on an old 318 that hit 350 HP that probably ended up with the same compression and smaller cam, but they did mill the deck. The decking had to reduce the quench, so I am counting on the lower quench Magnum heads to about equal that out. I was hoping the better quench of the magnum heads would help me from detonating, but from what yall are saying the magnums arent helping out that much.
I can get 20 mpg out of the van. What is the key is getting the engine to breath from filter to muffler. Just going to the correct headers should net me 2 mpg, maybe 3. And I dont mean any of that shorty header crap that is all but a waste of money.
A compromise? No; but I am having to choose my parts really carefully this time.
Like trying to find a ductile ring set for these shallow groove pistons has been a challenge. Almost had to go with earlier 5.2/318 pistons.
I am a bit surprised you did not get more miles out of your engine as I have regularly seen 150K+ miles out of these engines; but a lot of towing (work) or high rev shifting (fun) can do that.
Peace.