Let me say it this way.
I’ve seen absolutely no correlation between cranking compression and detonation or more accurately detonation resistance.
I have seen engines bumping 155 on the gauge rattle like crazy and 200 not rattle at all. I haven’t checked my engine yet with a calculator (it’s sitting on the dyno as I type this, but I had a PAYING job come in so my junk comes off tomorrow so I can do that then mine goes back on and I suspect I will make 75-80 pulls minimum unless it craps itself) but I will here in a bit get on Wallace and see what mine should be, but it better be 195. 200-205 is where I think I want to be. Over 210 I may wet myself a bit.
What I see related to cranking compression is the higher it is the smaller the tune up window is.
That means that you have less margin for error. Things like your timing curve, engine coolant temperature, spark plug heat range and much less obvious things like your overall first gear ratio, rear axle ratio converter stall speed or clutch tune up and most importantly how the driver drive the car become critical.
So if the engine temp climbs 15 degrees (talking sticks here or manually shifted slush boxes) and the driver doesnt down shift and is running a gear high you will rattle the engine where it wouldn’t if the coolant temperature was where it should be.
Another example is spark plug heat range. If it’s a drag car the plug can be a bit hot (it can never be a glow plug but it’s criminal how many guys run a plug thats too hot) but a car that is on a road course you can’t be any too hot.
That’s why I call unorthodox pump gas compression ratio engines (I suppose that would be depending on who you ask anything over 10:1 but I consider anything over 11.5:1 on pump gas unorthodox) some of the most difficult to teach the end user how to use it. I’ve been teaching it for better than 35 years and guys still refuse to believe it can be done.
The short answer is I look at cranking compression as how wide or big the tune up window is. So I suppose in that sense cranking compression can be related to detonation sensitivity but as I said before, I’ve seen pretty low cranking compressions rattle bad.
I almost forgot. RING SEAL and OIL CONTROL is beyond critical in unorthodox pump gas compression ratio engines. If the engine is using a quart per 1k miles and you are hitting the gauge at 180 it’s going to be hard to keep it from rattling. At 195 you just about cant do it. You’ll be running plugs so cold they fouls at low loads and the timing will be so retarded the exhaust gas temperature are screaming and it will still rattle.
Oil control is absolutely critical if you want to bump more than 185 on the gauge. You really need at a minimum a fully functioning PVC system. If you can use a Panevac over that all the better. Or if you are a bucks up dude you can use a vacuum pump. But you have to control the oil getting to the cylinders.
To that end, if your machinist isnt using a profilometer to measure the source geometry of the bores get a new machinist. If he still thinks a ductile iron moly filled top ring is the ****, take your parts and run.
I’ll go run my numbers on Wallace and see what it says my junk should do and post it and then when I get my trash checked I’ll post that follow up here.
