Help please, yes I searched can't find any solid info

[jwerty1]

I came across an l20b built in the eighties with forged venolia domed pistons, I'll post some pics here as soon as I can. Anyhow the builder was unsure of some specifics. Measuring the rods they are stock l20b, so the block is all l20b I know. Bare with me as my measurement skill sucks, however the pistons are 85.67mm. The pin height to the top of the dome is 41.78mm the pin height to the bottom of the dome is 35.81mm. The head is a u67. Basically I wanted to get an idea on the compression ratio. I am finding the apparently these pistons were made in conjunction with Nissan motorsports and put the pin higher up to run longer rods in the l18's but I can't find good info on them in an l20b. I didn't reinstall the pistons as of yet, but I'm pretty positive the top out below the desk but I'm not sure how far. So if anyone can help me out a bit here it would be greatly appreciated.

[jwerty1]

[jwerty1]

Well one piston installed and the dome is above the deck for sure. Hmmm this seems pretty high compression. The non domed section is below the deck maybe .1"

[jwerty1]

Also just for confirmation, the distributor that came with this motor is junk. So I could use my l18 unit with pedestal correct? Also this engine has a crank pulley/ timing indicator on the pass side of block, I could swap my l18 crank pulley and timing indicator over as well right?

[datzenmike]

You could but the one on the pass side works just as well now, why change it?

 

If it has the stock piston rod the distance from the crank center line is 1/2  86mm stroke... 43mm + rod 145.9mm + pin height 38.1 =  227mm

 

The block is 227.45 so the piston stops short 0.45mm of the deck.

 

Now if you have the stock rod and a pin height of 35.81 then the piston will stop at 224.71mm leaving a huge 2.74mm space above the piston which would amount to well over 15cc of extra space added to the combustion chamber. Naturally the dome volume would have to be subtracted from this

 

Well one piston installed and the dome is above the deck for sure. Hmmm this seems pretty high compression. The non domed section is below the deck maybe .1"

 

0.1" is about 2.54mm so this observation bears out my 2.74mm math almost perfectly.

 

 

I also see the piston diameter is oversize so the volume goes up even higher. I don't see this being a huge compression motor. Well not that high.

[jwerty1]

Well I was just wondering a round about compression ratio and wasn't sure on the pin height calculation seeing as the piston is domed.

[jwerty1]

So based on seeing numbers put out for an l20b with flat tops and a u67 head being a little higher than 10 compression, these pistons will create a higher compression ratio than that say 10.5-11? And I like the sawtooth timing indicator, so I kinda wanted to keep it.

[datzenmike]

Yes flattop pistons in an L20B is about 9.94 but don't forget the piston goes up almost to the top of the block. Your piston stops about 2.7mm short and this makes the combustion chamber effectively larger by about 14cc

 

 

 

So:

 

85.67 pistons with stock L20B rods and crank and U-67 head.

If we assume the piston top was flat the compression would be no higher or lower than about 8.42.

 

 

If I was to guess the displacement of the dome in this piston... I would say 10cc. Just for argument sake lets say 10cc. Then the compression would jump to 9.73

 

If the piston dish were 15cc it would jump to 10.57.

 

You tell me the dome volume and I can work out the compression in 30 seconds.

[jwerty1]

Thank you for the reply, that was what I was looking for. Much appreciate it

[Robert]

Those pistons were a common performance upgrade from the 80s. I have a book someplace that has specs on them....I will try to find it. From memory, I would guess they and up at about 10.5:1

 

Dome area is between 15-20cc's off the top of my head.

 

Easy method to measure them is to punch them (well not physically) into a large piece of clay to create a mold. Them fill the domed portion of the mold with water from a pipette to the flat portion of the piston.

 

 

Its possible they can be from 10.5:1 all the way to 12.0:1.

 

 

From the phot0 I am guessing 10.5

[Robert]

Just a thought....if someone spent that kind of money on the pistons, I wouldnt doubt the camshaft is not stock either. Anymore info you have on the motor? Origin? Car? Owner?

 

 

Now that I think of it....I am bettint those pistons are even higher than 10.5. Going from 9.5 or so to 10.5 is not much and if they spent that money and included a camshaft, you may be upwards of 11.5

[jwerty1]

They did have performance cam of some sort, but Im not sure the specs and neither was he, the lobes definitely measure larger than my stock l20b ones.  I got the whole motor, plus he had a bare head with a full port job, bigger valves to go in it, and racing springs, although he wasn't sure the specs or origins on those either.  I know he built the motor back in the day for a turbo setup and he said he couldn't get it to run without pinging unless on race gas (ie at least 10:1) He was a pretty cool guy, but I don't think he knew all the ins and outs.  He seemed like he built everything out of the how to modify your 510 book. I tore everything down and everything was pretty clean, reassembled, and assembled the bare head with the bigger valves and springs.  Just need some gaskets and I'll be swapping out my l18 in the next few weeks.  O yeah, he still had the turbo kit installed on the motor, it was pretty cool to see the cartech setup that I've seen so many pictures of.  I had no need for it, so he dropped the price to remove it.  I have my own turbo setup :devil:

[RTB]

I think you may need longer rods to use the pistons to their full extent. possibly z20e pistons, which were another common upgrade in the 80s. Usually those domed pistons were designed to be 12:1 to 14:1 CR.