according to my calculations a a reduction of 0.020" from the combustion chamber is equivalent to losing 3.1cc's from the combustion chamber volume, which when starting at 55 cc's is about a -6% change. Im not sure if im starting with 55c's though. This is based on a bore diameter of 3.475" (2.5 turbo oversized 0.030") which im not sure is correct either. lol
I used to run an .060 shim in my 2.5 Shelby Lancer, it allowed me to go to around 21psi boost on pump without having to crank the ignition timing way back That was big boost back in those days I also threw a 3 degree offset cam key in there to make up for the timng change.
acannell ... detonation can be caused by many contributing factors. Lets step back a second and review your set-up.
You don't say how much was removed from the deck or head ... do you know?
Have you gone over your cam timing? Over advanced cam timing can raise internal engine temperatures, causing detonation.
Internal heat is a big killer.
Cooling system up to snuff? What range t-stat are you using? Running a 50/50 mix in there or something else? Radiator in good shape? Water pump new or used?
Oiling ... basic questions. Type and weight?
14 psi isn't a ton of boost. I know from your other posts that you're using a WB (and EGT?). Looks like you're getting plenty of fuel but other than the WB reading on that how do you know? Did an injector skip a beat? <----- EGT reading would usually show this as low temp.
Injector wiring good shape? ... connections solid?
What fuel pressure are you set at?
Does it rise while boosting?
Fuel filter new?
Your issue may have been just a bad tank of gas.
Trick I'd try to trace detonation issues that might be fuel related is this. Get (9) gallons of fuel in the tank and head to a paint supply store. Buy a gallon can of either Xycol or Toluene. Pour it in the tank.
This will raise the octane of the fuel pretty high. If he detonates under these conditions you definitely have another problem .... if the fuel delivery system is OK otherwise (wiring, pressure and volume).
If he doesn't detonate could be a change of gas stations is in order. Or, a reduction in boost level because of a area wide fuel issue in your locale.
Didn't go over the ignition too much ... assuming you're at stock (12*) timing ... stock plugs? Again, not a lot of boost so stck plugs might be fine but dropping a heat range given your problems might be in order.
I know you asked about MBC's vs comp controlled boost ... don't chase your tail. This is a non-issue. I would get the ECU sorted out for nothing else but timing and fueling tables to match your goals.
.020" is quite a bit. That'll raise your CR about .6 or so. No way of knowing on top of what exactly because we're working with nominal factory CR statements. You could be lower than that already ... you'd have to have measured it to be exact. Being realistic though ... if your wanting more boost in the future ... a shim is probably in order. If you're happy at 14-15psi I'd leave it alone. 8.6x:1 responds nicely.
Just a gut hunch on my part leans toward some bad gas or fueling issue as the cause of your problems (cracked ring lands). BTW, What product states a 6RON increase in octane? That's an assload to be sure ... especially if it comes in a 4oz bottle (lol). It takes a gallon (9to1 ratio) of the xycol or toulene to raise the octane (4-5RON) or there about. The side benefit of those chemicals is the increased power at the flywheel.
Skip that garbage in a can stuff and just put a few gallons of Union 76 100 octane in with your fill up. 3 gallons of that in a tank can work wonders with our lousy 91 octane premium Then test it all the way to the SoCal meeting a week from today
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