Project: Iron block 5.6 - Page 13 - PeachParts Mercedes-Benz Forum

Frank Reiner · #**181** 09-30-2022, 04:02 PM

Quote:

Originally Posted by Tony H

Next installment: I cannot increase the dish volume enough on the 4.5 pistons to get the cr under around 10:1. The dish is shallow and there is not much material left on the sides. So I am pursuing a few different directions on the custom pistons-having custom pin bushings made or having the new piston pin bores made for undersize pins and then honing for my 26mm pins. I think 10:1 would be pushing it using pump gas.

10:1 is really not a problem if the intake closes between 35 & 39 deg. ABC @ .050 valve lift. Code 16/17 cams. It's what Mother Benz did.

Frank Reiner · #**182** 09-30-2022, 08:39 PM

Tony:

If you haven't already absorbed the information in this article, it contains some very useful stuff. In addition to the thoughts on Dynamic Compression Ratio, take a look at the picture of the LS combustion chamber; that shape is effective in mitigating detonation.

https://www.chevyhardcore.com/tech-stories/engine/discussing-compression-ratio-and-pump-gas-compatibility/

Tony H · #**183** 10-01-2022, 02:12 AM

That's a great article. I am familiar with dynamic CR. There is no where in the head to remove material without losing squish pad area so the piston is where it needs to come from. Her is is a nice article that explains valve timing events.
https://www.motortrend.com/how-to/ccrp-9812-secrets-of-camshaft-power/
If I can remove enough material from the piston without losing squish pad area to make 10:1 I will go with it. Much less expensive too. All the cams seem to have retarded timing. I think it was to reduce NO x as it lowers dynamic CR.

rwd4evr · #**184** 10-10-2022, 01:36 AM

Love the idea of thinking out of the box and making your own stuff, but at this point why not just put a 5.6 with a painted block in it?

You can still use your adapter bellhousing. I have carved out a rear plate to fit over the rear main plate. I'm going to drill holes to roughly align with the dowel pin hole and install dowels, then re dial in the runout(I have it at .003+/-). I'll ten fill the holes with JB weld to keep them located in the plate. There are two holes that will also be drilled and countersunk in the plate with Allen heads into the alloy block trans holes. This is a 5.6 being prepped for my 4 speed in the drift car.

Sent from my SM-G991U using Tapatalk

Tony H · #**185** 10-10-2022, 11:23 PM

Jason, That's pretty cool and I would love to see the results but I'm in too deep to change course now. I'm right on the verge of a major breakthrough. Good news regarding the custom pistons. After a few more inquiries they will now make the pistons to fit my pins. Not sure what changed but I'm pleased. I will not have to do anything to the rods. Considering the time and uncertainty of machining the pistons and cost of rings it looks pretty good. They can probably make them lighter as well. Engine needs to be balanced already.

Tony H · #**186** 10-12-2022, 01:25 AM

I inquired if the new pistons would have offset pins like the originals. He said if you want it they will build it. After lots of research on the subject I have determined it is mostly for cold noise reduction and any performance difference one way or the other is negligible for a stockish engine. So I am inclined to go offset as it may negate some of the forged piston noise.
Edit: after some thought the amount of offset seems to vary among different combinations and I cannot seem to find out what dictates a specific amount of offset. So the offset in the originals may not be correct for the customs unless offset is a function of stroke and rod length. I have a 4.5 and a 5.6 piston. Tomorrow I will try to see if I can measure it.

Tony H · #**187** 10-15-2022, 05:42 PM

I measured the offset at .068" which is pretty much in line with the common figure of .060". Most aftermarket pistons do not have offset and the net is all over the place with information-most is just opinion. I am inclined to go with the offset since that is the way the factory did it. I think the concept is obsolete today due to the tight clearance pistons run at.

Frank Reiner · #**188** 10-15-2022, 07:22 PM

Quote:

Originally Posted by Tony H

I am inclined to go with the offset since that is the way the factory did it. I think the concept is obsolete today due to the tight clearance pistons run at.

By all means use offset. If the custom pistons are 2618 forgings, and almost all are, the expansion and the required clearance will be that of pre-hypereutectic alloys. Expect .0025-.003" clearance.

Tony H · #**189** 10-15-2022, 10:53 PM

I see no downside to offset. Especially with the 2618 alloy pistons.
I modified a dial caliper so I can reach down into the coolant passages and get an idea of the cyl wall thickness.

Tony H · #**190** 10-17-2022, 12:57 AM

Checked cyl wall thickness at 90deg intervals in 3 places. There was a great variation of thickness-from over .300" to 1 spot at .185". I will probably bore a nominal amount like .25mm or .010" and keep as much wall as I can.

Tony H · #**191** 10-20-2022, 04:47 PM

In case anyone is following this-another possible issue before I order pistons is the distance of the ring package from the piston crown due to possible interference from notches in the cylinder wall to clear the intake valve since i'm using 5.6 heads. With the 4.5 bore the 5.6 intake valve just touches the cylinder wall. I don't think the minor bore increase will provide the needed .060" clearance, so notches are imminent. I have calculated the 5.6 bore of 96.5 provides about .080" more of clearance on each side.

Frank Reiner · #**192** 10-20-2022, 05:17 PM

Quote:

Originally Posted by Tony H

Checked cyl wall thickness at 90deg intervals in 3 places. There was a great variation of thickness-from over .300" to 1 spot at .185". I will probably bore a nominal amount like .25mm or .010" and keep as much wall as I can.

Quote:

Originally Posted by Tony H

In case anyone is following this-another possible issue before I order pistons is the distance of the ring package from the piston crown due to possible interference from notches in the cylinder wall to clear the intake valve since i'm using 5.6 heads. With the 4.5 bore the 5.6 intake valve just touches the cylinder wall. I don't think the minor bore increase will provide the needed .060" clearance, so notches are imminent. I have calculated the 5.6 bore of 96.5 provides about .080" more of clearance on each side.

Tony,

Even if you were to increase the bore to 93.5mm the wall thickness would still be .125+". Valve clearance would be >.030", which seems quite sufficient from a clearance standpoint, if not from a flow standpoint.

Tony H · #**193** 10-20-2022, 10:24 PM

I'm probably conservative in my goal of .160" wall thickness especially since it is one spot and the rest of the cylinder is at least .210". The remaining cylinders have at least .200" wall thickness. I guess I need to decide how much cylinder wall thickness I am comfortable with. I can probably get by with a 2mm overbore to 94.5mm giving me a .146" wall thickness at the thin spot-probably fine since everywhere else would be at least .160" thick. I read of boosted big blocks with .100" thickness. I also read .060" is the minimum between moving parts so I was going on that. Again probably conservative.
Edit: this engine is so conservative (probably 250-275hp and 5500 rpm if I'm lucky) I could be fine with thinner cylinders in a few spots given the thinnest cylinder is .185" in one spot. I can't measure the thrust side since the coolant passages are on one side or the other. It's .185" on one side and about .240" on the other. The "T" indicates top.