[Half-Inch Stud]

Originally Posted by Formulajones

Dad successfully ran a flattop 455 (bored .040 over if I remember correctly) with a set of ported #12 heads. It was zero decked and a typical .039" head gasket.

I don't recall the exact compression ratio (I'm sure it was pushing close to 12:1 or so) but I do remember it pumped 210-215psi. He ran a Harold Ultradyne solid flat tappet in it that was pretty healthy, duration in the 250/260 range. That engine ran on the street for a few years on Sunoco 94 octane and I was able to make it work. But the tune had to be spot on and routinely checked. It would not have worked with todays 91.

Anytime he went to the dragstrip with it he did mix a little 110 in the tank for good measure.

These days with more knowledgeable people and better parts I've pushed the pump gas deal with a few engines on the crappy 91 we have now. One is 11:1 with iron heads and a pretty healthy camshaft running on 91 octane for 5 years now (daily driven to 50k miles so far) and it acts like it would run on less. I keep the tune pretty spot on but I don't have to monitor it like it's life or death.
My bird I also drive regularly and it's 10.13:1 with it's #12 heads and does fine on 91 with a very small 068 cam (albeit a 400) put about 25k miles on it in the last 3 years.
Neither car acts finicky even in the AZ summer heat but I don't know if I'd push the issue any more than that. Aluminum heads I would, but iron heads that's about as far as I want to go. I don't think pump gas is going to get any better in the coming years.

Yes, all this matches my experience with Hi-Compr. Add humid summer days cause my HC to ping. Shows what "we-all" used to do 30 years ago. Aluminum Heads have the 0.5 to 3/4 point advantage. I'd rather stay with Iron and shoot for 9.3:1 thru 10.0:1

Since 2003 i have had 9.00:1 and have relied on munerous little things and a big cam to realize good results, in either compression.
Fear going back to Hi-Comp though. Is 10:1 to 10.5:1 of any use? I suppose a chart is helpful: Correction welcomed!

Octane. Compr on a 455 (Race/Tow-Street)
87. 9.0:9.5:1
89. 9.5-9.7:1
91. 9.7-10.1:1
93-94. 10.2-10.7:1

[Jay S]

I would not at all go to the point that 10.0-10.5 is of not any use in a 455. There are just to many factors to narrow the compression ranges in a chart like that, to me the higher octanes on the list are getting to high with respect to the lower octanes, ranges look to wide for the 87 and the 92,93.

I did a SR cam 9:0 462 with ported D ports that wants 91, I think it is borderline in some conditions to want 93, he is gunning for 10s 1/4 mile. Even though it is 9:0, you would swear it has 11:1. IRC, I think I have less cylinder pressure on my 461 at 11:1 than he would have. He is going after all the mid range he can find to launch his car, wants to drive it to the track. My car is a semi daily driver and I wanted efficiency @ low rpms, not a huge mid range, or crazy low end tq, but strong power up high. All things I did so my 4 speed would have a longer life. Complete different combos, same CID, 2 points different on SCR, basically the same octane.


FWIW. Power is in the heads. Compression more for efficiency. As long as the compression is adequate for the chosen cam, there is much more power with better head flow than compression. Nothing wrong with 9:0, depends on the rest of the combo. Not enough info in this thread to begin to determine where the perfect SCR should be. If in doubt, stay on the low side and sleep better at night.