Oil usage and detonation problems solved.

[jeff1234]

I have posted in the past about oil burning and that I changed my valve seals in the car. Very shortly after doing the work the engine started smoking even worse so I pulled the head and took it to the machine shop where it was verified the #4 exhaust was the source. In the process of deciding what to do extra to the head, the subject of compression ratio came up. I then made measurements of the deck, piston dish gasket etc. The things you need to know to calculate CR. As i did my calculations nothing made sense, I had over 9:1 CR. Doing a search for combustion chamber volume i found a post by 4-Lbodies which showed me that the pistons i have were for a G-head with a 55-56 cc chamber. My wiseco piston dish is 9 cc's and it should be 24 cc's to mate up with the swirl head chamber of 49-52 cc's.
Is it any wonder that I had detonation??
I think not.

[cordes]

How did the turbo spool? Pretty quickly? How much boost were you running? That's a lot of compression when you thought it was stock.

[4 l-bodies]

Hey Jeff, just saw this. Better plan on either a g-head swap, piston swap, or buy an alky injection kit for it. Or run better quality fuel. What did you use for headgasket bore and thickness when doing your calculation? I use 3.525" and between .065"-.072" compressed head gasket (depending on brand).
Well at least you know why you've been chasin your tail with the detonation for a while. Making progress!
I attached two pics, stock and modified. I know this is g-head, but you can try to soften the edges on the chamber to avoid hot spots. It all helps with detonation.
Good luck,
Todd

[jeff1234]

Yes it spooled up good. It was making great power and would get some tire spinning going from third to fourth. I had maxed out the fuel and it helped.

[jeff1234]

I'm thinking about unshrouding the chambers. I need to pick up 15 cc's. My head gasket is Cometic .084. Maybe they make a thicker one. I used 3.530 for the gasket opening. I have a .014 deck but I haven't cc'd the chambers yet.

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Yes it spooled up good. It was making great power and would get some tire spinning going from third to fourth. I had maxed out the fuel and it helped.

[4 l-bodies]

I'm thinking about unshrouding the chambers. I need to pick up 15 cc's. My head gasket is Cometic .084. Maybe they make a thicker one. I used 3.530 for the gasket opening. I have a .014 deck but I haven't cc'd the chambers yet.

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Yes it spooled up good. It was making great power and would get some tire spinning going from third to fourth. I had maxed out the fuel and it helped.

Unshrouding the chambers will help, but it will be nowhere near 15cc's. It'll seem like you're removing a lot, but it will only end up being a few cc's unless you're very, very, aggressive unshrouding. WAY more than attachment below. Note this is BV head too.

[jeff1234]

Well, I've got some thinking to do. Methanol kits are expensive and some reading I've done leads me to believe that it may not do much. I was boosting at 20 psi before I pulled the head and was only getting a momentary CEL going from fourth to fifth. I have another option of turning down the boost. Not as much fun, but spending hundreds of dollars prepping the G-head from my 2.2.. on my engine run stand would be less fun. It is interesting to note that the hundreds of detonations over the years only resulted in very minor pitting on #1 piston only.
One other note, the block is bored to .040 over, so that's it for it.

[cordes]

Dang. At 20 psi and that compression, that's actually pretty wild. Do you know what your EGTs were? I wonder how much timing it was pulling? Probably all of it.

[4 l-bodies]

Jeff your probably one of very few people running a -9cc forged piston. You must have had them custom made because virtually all our vendors that I'm aware of, stocked only swirl 2.2/2/5 pistons. I have a set of Ross forged pistons in my 85 SFB GLHT. Took like 3+ months to get them made.
Back a couple years ago I saw a massively modded G-head that had like .100"+ cut from surface to try to bring chambers back down to 50cc swirl size. This head was unused, but was done like 30 years ago by Arrow Racing. They did this because MP only had swirl pistons available. So really just the opposite of what you're dealing with.
Never really understood why people would shy away from ordering custom pistons for their application. They take longer to make, but not much difference in price over what vendors would sell "stocked' pistons for.
Todd

[jeff1234]

I don't have an egt gauge but I can tell you that regardless of whether I'm looking at my OTC4000E or my MPScan file it always goes to zero. The timing thing has always confused me because, in this case, zero is some amorphous number starting somewhere and ending somewhere in the ecm's imagination which I don't have the proper security clearance to observe or manipulate.
My pistons were sourced from one of our best suppliers as the .040 oversized piston from stock. Perhaps the manufacturer made a mistake but I don't remember being warned about which head the pistons were designed for. It seems like there was breakdown of communication in this case.
I wouldn't have ordered customs pistons because I, like many of us in this hobby, operate within the constraints of our personal budgets.
As concerns your recommendation of the G head, I have reconsidered taking the head off my 2.2, give it a clean, drill for 11mm studs, replace the guides and give it a valve job. In the end it probably is the most efficient way to do it.

[jeff1234]

So, I did some more calculations and it looks like I have a 9.8:1 CR, presently. However my static compression tests show an average of 118 psi across the board. I should be showing 150 psi. Therefore I have alot of blowby in spite of the fact that i show less than 10% leak down across the board.This is probably why the situation as is works. Bearing in mind that the block is bored out to max and I can't afford new pistons I think I should put it back together with the existing head and a thicker gasket If its available, or not, as I can go back to 16 psi with no detonation and make it live as long as it will. I think that because of the low static CR, going to the G head, that the engine would be down on power and going to 20 psi boost wouldn't make up for the low compression.
Opinions please.

[4 l-bodies]

Changing the head back to the g-head will normally require a calibration change for spark advance. Sounds like you are able to do this, but will require some calibration work for optimizing performance.
I have found a big variance on compression gauges from gauge to gauge, and technique used. The spring used in the schrader valve is far weaker than the spring used for say a tire valve core. I once had a compression gauge read low, and after changing out the valve core, it gained like 30-40 PSI! I use hot engine, all plugs pulled, Disabled HEP so fuel isn't washing down cylinders, fully charged battery (even throwing battery charger on it at times), TB wide open, and turn engine over like 5-6 times by reading pulses on gauge, for each cylinder.
Camshaft selection and camshaft timing can certainly alter compression readings. The more overlap and LSA will alter compression readings. Obviously turbo selection will dramatically alter spool time as well. Sounds like you have a nice sized turbo for your application.
Can you post a snapshot of the compression calculation. I'm assuming you're using the Stan Weiss calculator. Here is a stock swirl 2.2 and 2.5.
Todd

[jeff1234]

Todd, excellent procedure, which I missed doing a couple of items, ie, hep, throttle body. Clearly these would make a difference. I have a Mac gauge. I will follow your procedure when it's back together and report back. Thank you.

[cordes]

Oh yeah; not going wide open with the throttle during the test will drop it right down to where you were even for a strong engine in my experience. I think when you retest you'll have significantly higher numbers.

What are your goals for the car? 9.8:1 with 16PSI would be a monster on the street I would think.

[jeff1234]

I have to tell you guys that human error on my part caused my piston dome numbers to be wrong. I used a 6cc syringe to measure the piston dish. The number I had in my head was 9.4 cc's when it was actually 3x6 cc's + 4cc's. The piston is correct and talking with the person who sold me the pistons put me out in the garage taking a picture of my piston top to send to them for comparison. That is where i realized my error. Using the website compression calculator that Todd sent me results in a CR of 8.38. Alot safer than my
previous calculations.
While it is embarrassing to have to admit this error it's better than spending a ton of money on bigger more expensive gaskets and fly cutting the chamber only to discover that money resulted in a way low unusable compression ratio.
Thanks to all who helped me find my own error.
Jeff

[cordes]

That sounds much more reasonable. That CR is a little higher than stock. Depending on how all the parts come together tolerance wise, I wouldn't be surprised if you were octane limited at 16PSI or so for no knock at all tuning.

[jeff1234]

That's quite possible. I can tell you I'm going to find out.😝

[jeff1234]

Hello Todd/Brian, update! I took the head to the machine shop. The stock 782 "as is" chambers cc'ed at 51 with the machine shops burrette.The head had been milled .013 once. Factory deck to valve cover sealing surface was 4.133 mine was 4.121. I had measured another head i used with a 60 cc syringe at 52 cc's.What I think is that these 782 castings likely were nearly all the same cc's To recalculate my CR I measured the volume between the deck and the piston at TDC. Instead of using a 6cc syringe I used a 25 cc one and was surprized to see that the actual volume was 3.5 cc's larger @ 25. I performed the procedure twice to verify. Using these numbers on the Stan Weiss calculator gave me 8.125:1 cr for a 2.5 overbored to 3.48 with the 51 cc chamber. To get the cr to stock the machist unshrouded the valves to result in 54.5 cc chambers. On the cc calculator I zeroed the piston to deck clearance in favor of the true measurement. The 54.5 cc chamber results in 7.85 :1. Stock is 7.8 :1. What psi am I theoretically going to be able to boost up too?
Jeff

[4 l-bodies]

Jeff,
I have found a lot of variation from one application to another in terms of detonation tolerance. One seemingly identical engine being prone to moderate amounts of detonation, while another seems to have no issues whatsoever. I just think they're too many variables at play here. I just don't think there is a hard fast # of lbs. of boost for pump fuel.
Since this thread involved oil usage, make sure that your intercooler is cleaned of any oil residue. Are you running a FMIC? What plugs are you using? The reason I ask is when I run into one of those combos that is prone to detonation, I switch to a plug with a non-projected tip. That seems to help the issue.
Todd

[cordes]

If everything is in great shape, and your timing is spot on, I could easily see 20PSI being no issue. While I'm surprised that it held together given the timing, my silver Shadow with a 2.5 saw 21-22PSI routinely and that short block is alive and well in my NY today. A lot of it will come down to injector, turbo, IC, etc. etc. but high teens should be a non-issue with stock compression IMO.