What's a safe effective compression ratio for running 89 octane?

My stock Merc 5.7 EFI runs on 87 and has 9.3:1. A lot depends on how much timing you are running and what the heads are made out of. With cast iron heads and 9.5:1, start giving it some timing and you will need to bump up the octane. There are lots of variables, how hot you run the engine and cam timing effects it too.

Aluminum heads you can bump compression up to 10-10.5:1 and run 93 with a mild cam.
I have a 402 LS engine with 11.8:1 that runs on 93 octane, but the cam is a 242/250 113 LSA.

Great point..
in a 525SC, with .040 head gasket, 8.0 lbs static, 7.5 lbs boost (177), around 12.0:1 effective, je 2618 blower Pistons, .010 below the deck, 6.358 rod, Lunati roller cam @ 234/[email protected] \ 629/612, I can either go with enginequest 320 iron heads or Brodix dragon Slayer 320, big question is is 89 safe to running this set up or is it way out of the question even with the aluminum heads?

Thanks

Id be careful relying on the concept that aluminum heads can allow for a more aggressive tune..

as stated, theres more to it than one simple mathematical equation. Things like intake charge temp, coolant temp, spark plug type, ignition curve, fuel curve, combustion chamber design, camshaft timing, exhaust system, altitude, and so on.

That cam, with the eq heads, is prob gonna put you in the 650-700hp range. The 177 is gonna be working its azz off to make 7.5lbs of boost on that combo. Stock 525sc, tossing a smaller pulley on nets about that much boost. But thats a stock 525sc. Boost is just a measurement of restriction. My personal opinion, is for a 600-650hp engine, the 177 will work. Above that, its a good idea to start looking for a larger blower.

the smaller blowers when spun hard, take more power to turn, than a larger blower spun slower , at the same boost level. Plus the single carb starts holding things back. Basically, youre out of the blowers efficiency range.


why not dyno the engine on 89 octane. Let the dyno tell you what it likes for boost, timing, fuel, etc. You can start with a pulley combo that nets 5lbs . Then install a smaller pulley, and see if the gain in boost is a worthwhile one. If you go from 5 to 7psi, and only pick up 20hp, its a waste. If you go from 5 to 7, and it picks up 50-60hp, it might be worth it to go with the smaller pulley.

That's a great concept but I have one pulley and can't find another (unless I drop $600 on a keyed shaft and 2 pulleys and I'd rather drop that coin on setting up my 6V-71 instead).....so it's 144% over driven or nothing on the 177 and with a 468 and a super chiller "according to the charts" that puts me at 9.5-2 = 7.5 Lbs boost. It'll probably be less because I'll probably run the engine around 3,600 to 4,000 on average and 5,000 max until I swap blowers, so maybe 5lbs on average cruising under normal conditions???

So given the restrictions I have, run at least 91 unless I dyno it and see how it reacts to 89?

I really doubt you will make that much boost with the pulleys you have.
I think mine were the same size or very close and I made just shy of 8# with no chiller.
I think the smallest upper made was 2.75 and that made 8# or so. Boost pressures show the restriction. An engine that flows better with more cam and better flowing heads will have less boost pressure but will still be moving the same amount of air.
As far as running it on 89 octane, who knows. It may be ok but its a big risk.