So I began to put this together and the pistons seem to be sitting pretty low .072 actually. 70 455 block and heads bored .030. Speed pro pistons that summit says are +23cc but I believe them to be +26 on stock rods. By the calculators online that gives me 9.06 static and 7.73 dynamic compression with TA212 cam. Does that seem right? looks a bit low to me. If something needs fixing it needs to be done now.
9.06? You might not even have that. What are you using for combustion chamber cc? Head gasket? Standard 455 heads are 69cc, and 70 Stage1 is 66, but I can tell you my 70 Stage1 iron heads measured at 72cc. You really need to measure everything if you want actual accuracy for your static compression ratio. As far as pistons go, every time someone asks about rebuilding a 455, I try to steer them towards a custom piston like the ones sold by Jim Weise, Autotec, and Diamond. The reason is that the off the shelf pistons sold in the past have a compression height that puts them in the hole at least .050 or more. If you want your engine to be all it can be and be resistant to detonation on pump fuel, you need 0 deck. Your only option now is to take it back apart and mill the block. Not only is that not cheap, but I would not want to take that much off a block and then deal with the intake no longer fitting correctly.
At 9.06 SCR, I get 7.48 DCR. Piston dish on the Speed pro is listed as 23cc. How would that change with rods?
haven't measured the heads just using the advertised 69 cc the gasket is felpro , I got the pistons from a member here and they are marked 26cc. Not really interested in milling the block as the cam bearings would have to be changed again? I'm sure they would get grime in them.
I still get 7.48 as a DCR. 9.06 SCR and 7.48 DCR is just fine. You can run it like that. There isn't anything else you can do about the .072 in the hole. Different pistons or mill the block. I'd measure those heads to be sure. They probably are bigger than 69cc.
The cam bearings could be removed and then reinstalled. I would do the work otherwise you're leaving a lot of power on the table and asking for detonation issues.
The head gasket is .040, twice what I thought and if the heads are more than 69 cc it gets worse. I guess I need to measure the heads and see what they really are and go from there. Probably just need to get some quality pistons.
Yes, resell the pistons and buy some custom pistons. They are not as expensive as in the past. You can specify compression distance and dish/valve relief cc's to zero in on your desired SCR and zero deck. Call Jim at Tri-Shield Performance. He can help you decide what is best in your case. First get an accuate size for the chambers. http://www.trishieldperformance.com/ http://v8buick.com/index.php?members/jim-weise.11/
Why would low compression cause detonation issues? I would've thought the opposite. Got a feeling after reading this post that the forged pistons that went into my engine could account for the suspected lower compression and ET of 13.78@99.6
I just purchased a set of autotec pistons in a custom .008 over from Jim last week. 1.995 compression height, 26cc dish, 1/16 ring pack. Less than $700. He said with these pistons they should sit .015-.020 down. He also said most decks require close to that to clean them up getting me a zero deck height. 10.3:1 with 65cc ta stage2 and a .040 gasket. I would recommend something like that before running .071 down.
It isn't the low compression, it's the amount the piston is in the hole that causes the engine to be more prone to detonation. https://www.dragzine.com/tech-stori...ormance-squished-between-the-piston-and-head/
Talk to Jim about that. He may be able to account for that so that you can hone the cylinders to fit if you need to do that. Might be just fine. Maybe check with your machine shop for exact measurements.
Ahh ok I get it now, thanks Larry, so should I take a little timing out of it for insurance against detonation? Or just not drive the car hard? It's not really practical for me to tear the motor down and change the pistons. Hypothetically would turbos or nitrous offset the negative of reduced quench?
Offset-grind the crank? More stroke pushes the existing pistons higher at TDC and lower at BDC. More stroke = more displacement = more torque. Best of all worlds...IF (big IF) you can offset-grind far enough, and get rod bearings to suit. With the pistons that far down, you might be into "custom crank" territory; or at least welding before grinding. Either of those will be expensive. You'd also have to assure piston skirt to crank counterweight clearance at or near BDC; but that's likely OK. Longer connecting rods are another option, but that won't increase the displacement. 0.072 in the hole is just a disaster all-around.
I'm not even sure how they could be that deep, the other side (only have 2 in) is almost the same at .070. No matter how many times I measure it it 's the same.
have a dial indicator on a mag base, measuring depth with a depth micrometer. Took measurements all the way around anywhere from .069 to .072
If you go the new piston route they should be forged since they run cooler and since your likely getting them custom made the top ring land should be no more then .160" down for your motors level of power. This will insure more power , less ware from fuel wash and better octane tolerance! While your at it a gapless ring should be used also.
It's not Too Late. Get the right Pistons. both the Autotech and Diamond pistons are forged. http://v8buick.com/index.php?threads/new-470-482-piston-options.324745/
