[Formulajones]

I'll give some backround on that deal Limelight, and maybe it will help.

That particular car has 11:1 compression, and is zero decked, tight quench, original 186 small combustion chamber iron heads etc... The cam is the original copy of a DZ solid flat tappet cam with the only real change being the LSA is tightened to 112 and the ICL is only 110, so it's not advanced a ton. Still 254 degrees @ .050 duration.

I set the timing up with 16 initial, and 20 degrees of centrifugal for a total of 36 degrees. I figured if it complained at 36 I could simply knock that down to 32 or 34 and have 12 or 14 initial as a fail safe. It likes 36 and doesn't complain so it's left there. It's all in at 2600 rpm.

The vacuum advance is limited to 10 degrees, and I have it coming in at 6 inches of vacuum and all done by 9 inches. I did it this way because this engine, even with that timing in it, only makes about 10 inches of vacuum up here at 5,000 feet idling at 1,000 rpm. It's a very rambunctious camshaft in a very small engine.

With the M-20 and 3.55's, and a lot of time spent on the 780 holley, it would return 17 mpg highway, cruising at 65 mph and 2900 rpm.

I installed a TKO600 and dropped that cruise rpm down to 2,000 at 70 mph and now the car gets 21.5 mpg.

That was simply because the rpm dropped. If I had to guess I'm probably losing 5 degrees or so of mechanical timing from 2,000 to 2,600 where it's all in, but it's likely still in the low 40's all said and done at the lower cruise rpm. Something I would have to go back and check to be sure but interesting point.

What I could likely do in this scenario is add a little more vacuum advance to the equation, maybe 3-4 degrees and light throttle cruise would be right back where it was when buzzing along at 2900 rpm. That's probably how I would approach it, and it might pick up a little more mpg. But as it sits right now I'm very happy with how it runs on this 91 octane cat pea and the engine is also very happy so I'll likely leave it alone.

[pastry_chef]

Originally Posted by pastry_chef

454 with 8.2 compression - 567 HP

https://www.hotrod.com/articles/bolt...ome-big-block/

I'd want a 3500 converter.

I've only seen a 454 with AFR heads below 600 HP once, usually 650 and up.

A 427 with 265 heads and 238 @ .050 hyd roller - 632 HP.
https://www.youtube.com/watch?v=Rtj1KIn_HCg

[Formulajones]

On those smog 454 era engines, as mentioned it's a slightly different approach. They love timing.

My 79 1 ton 454 pickup started out bone stock. I had tons of timing in that thing. 16 initial and a total of 38 degrees, and then dialed in 14 degrees of vacuum advance, and it all came in pretty early, so it drove around part throttle with a bunch of timing in it.

This truck weighed 5500 lbs. (scaled it) and with a 400 turbo and 3.73 gears out back it would knock down 14 mpg highway cruising at 65 mpg at 2800 rpm and around town it would get 11-12 mpg. Not bad for a big truck shaped like a brick and weighing almost 3 tons. I towed a 10,000 lbs. enclosed trailer with it across country and still got 10 mpg highway with it.

I later refreshed that engine with simple rings and bearings, left the pistons, no milling of anything so compression stayed the same. I installed a Melling Class-1 SPC-6 camshaft and new valve springs, did some mild pocket porting on the heads, and modified the factory intake just like the Pontiac iron intakes are done, and a set of headers. Still a low compression smog motor. That thing was like night and day and would outrun some muscle cars on the street, absolute ball to drive and hilarious. Probably made all of 350 hp but made a boat load of grunt.