How To Find Piston Dome Volume

[Logical1]

Hey guys, I recently got a SCREAMING deal on some nice forged Arias high Compression L20b pistons. However The seller does not know the +cc of the dome area :( This is all the info I got:

 

"Arias Datsun L20B +.040 Domed pistons. They are for a standard stroke, 3.386 Bore"

 

 

The seller was less than a optimal communicator, But I just couldn't resist a custom set of forged pistons for less than 1/8'th the price.

Any help would be awesome, So far I have found a method of installing one on the crank (in the block) lowering an inch, filling with inert liquid, measuring. Lowering 2 more inches, measuring again and then some math. Makes sense but I'm hoping for an easier way ;)

[Laecaon]

Since the domes are so irregular shaped, the easiest way would be to use liquid in someway. I dont think math is the answer here. Or find who really makes these pistons and ask them.

[DISLEXICDIME]

Oooooh shiney

[RoadRace]

Hey guys, I recently got a SCREAMING deal on some nice forged Arias high Compression L20b pistons. However The seller does not know the +cc of the dome area :( This is all the info I got:

 

"Arias Datsun L20B +.040 Domed pistons. They are for a standard stroke, 3.386 Bore"

 

 

The seller was less than a optimal communicator, But I just couldn't resist a custom set of forged pistons for less than 1/8'th the price.

Any help would be awesome, So far I have found a method of installing one on the crank (in the block) lowering an inch, filling with inert liquid, measuring. Lowering 2 more inches, measuring again and then some math. Makes sense but I'm hoping for an easier way ;)

 

you're on the right path: you have to assume zero deck - quench area on the piston is dead level with the deck.

 

1) volume of the cylinder for 1 inch - just use the math, v1 = piR2*1 (note: if using a burette measring CC's for V2, then use metric dims for V1)

2) put the piston/rod in there at 1 inch below deck (or 2.54 cm), seal the piston with vaseline, use the burette and measure again = V2

3) V1-V2 = dome volume

 

fluid of choice varies, but Marvel Mystery Oil has been my choice for years.

 

As a side note: the pin height looks awfully suspicious on those pistons. best to check it against the OE specs

[Braden]

ha you bought those from the machine shop going out! 200 bones wasnt bad at all bro i almost bought them myself!!! best way to do it is get some modeling clay and imprint them then cc the imprint, it will get you close

[banzai510(hainz)]

280zx non turbo cast flaptop are almost to much to run for pump gas/street in a L20

 

Personally youll have proplem running those. So I will be the first to say ,Youll regret putting those in

 

Didnt Xerox on th REALM had a super Hi comp motor and got rid of it?

[Braden]

well what if he runs these with an open chamber? i dont know how to calculate it but i wouldnt think it would be any worse than a peanet head with flat tops

[Logical1]

Thanks for the idea's guys, I plan to build a funky Frankenstein beast! (was going to build a turbo but just got laid off, so dual side drafts it is!)

 

Input on ideal compression ratio of a (L20+sss 219 head+dual SU's?)

 

1.l20b Block

2. 219 SSS head

3. L16 Rods

4. High Comp forged Pistons in the +20/+40area Hoping for +30ish (will mill down for specific compression)

 

Cant wait to put her together!

[DISLEXICDIME]

dont go over 10.1.1 its a big head ache and it gets expensive and a pain to tune.

[DISLEXICDIME]

well what if he runs these with an open chamber? i dont know how to calculate it but i wouldnt think it would be any worse than a peanet head with flat tops

 

open chamber is more prone to detonation

[Master-O-Turbonics]

open chamber is more prone to detonation

 

 

or an un-prepped closed chamber head with as cast sharp edges...

 

 

[Logical1]

With L20b block, L20b Crank, L16 Rods, SSS219 Head, & +30 Pistons I am at 10.06 Compression With Premium octane gas and and a properly prepped head (smoothed sharp edges & Ect) Does That seem reasonable for a DD SOLID high-comp L-series? I am trying to build an engine that will last till the end of time with a decent amount of power. It doesn't have to be "race prepped" just SOLID & stable. With a good amount of power for an 'L' Series -L1

[datzenmike]

You need much more than the dome volume. You also need to verify your distance from the center of the piston pin to the top flat of the piston. If there is any kind of dome other than the raised area under the valves it is going to hit a close chambered head.

 

I worked out the compression for your motor (L20B block and crank, L16 rods, closed chamber head and using stock L20B pistons.... 4.10. You are going to need a huge amount of material above the piston pin to raise the compression like 80 cc!!!!!!

 

Need more info on these mystery pistons... pin height mostly and diameter.

[Logical1]

You need much more than the dome volume. You also need to verify your distance from the center of the piston pin to the top flat of the piston. If there is any kind of dome other than the raised area under the valves it is going to hit a close chambered head.

 

I worked out the compression for your motor (L20B block and crank, L16 rods, closed chamber head and using stock L20B pistons.... 4.10. You are going to need a huge amount of material above the piston pin to raise the compression like 80 cc!!!!!!

 

Need more info on these mystery pistons... pin height mostly and diameter.

 

Pistons are 84.8mm and pin height is standard 38mm. Wow! +80cc to work with that setup eh? I must admit to being lazy and using the helix engine design application which obviously has flaws. If we guesstimate that the pistons are in the +30cc range, with a standard L20b block & 219 head. What would you suggest for a crank/rod combo for optimal compression. That being said, I currently have a L20 crank and Rods. Swapping Rods is a bit cheaper than looking for another crank ;) Also considering getting the pistons milled. I know I could swap the head/pistons out for an easier set-up but that just seems like such a shame with the gorgeous 219 head and the forged pistons I have! Thanks for the help Mike :)

[banzai510(hainz)]

Ghetto

Open chamber is prone to detonation.

A SSS or closed chamber will put the Comp Ratio so high he cant afford the Aviation gas.

 

This set up will not happen. Even if Logical got these piston free it will still cost him to run this set up correctly w/o proplems.

 

 

These piston are in the Motorsport book

[Braden]

ok guys this has been discussed before, somebody school me on this because i must be missing something, lower compression less than 9:1 can run 87 octane fuel fine without pre-detonation because obviously there is not enough compression force to auto ignite the fule mix. compression ratios above 9:1 need to run higher octane fuel to prevent pre detonation. also it is fact that the lower octane fuel burns faster thus aiding to the pre detonation problems, the higher octane fuels burn slower and produce a more complete burn. now we are talking about this guy building a high comnpression engine and having to run high octane av gas. yes this i understand but how is it possible that the lower the compression ratio (open chamber y60 or u67) is more prone to pre detonation than a higher compression sss or peanut head? that makes no sense its contradictory. its like a diesel they ignite their fuel off of compression, so having a diesel with low comp[ression would be worthless because it wouldnt run!! so if i have this straiught this is what you are saying, if he runs the pistons with the peanut head he is going to have an engine less prone to detoanation problems vs an engine with an open chamber head and lower compression? this cant be right :blink:

[banzai510(hainz)]

for some reason its the combustion chamber shape and the way the gas explodes and how the flame reacts.

Dont dwell in it too long . Not worth woorying about.

 

Logical is thinking of using the L16 rods to lower the overall piston height. I never heard of using L16 rods in a L20. But to me why bother and could have got L18 4cc dish pistons and be done with it with the same comprression ratio.Dont get me wrong . These are Awsome Pistons. But forged piston you need more clearnace when boring them out as they expand and if you dont do it right will pop a top ring. Also it may smoke more at start up cause they are looser in the bore.another thing cam timing is going to be more critical esp w a longer /higher lift cam

 

these pistons will get be used at least in a L20. Maybe L18 is more possible.

 

Ask Pull apart Dave that hangs with Frank/Ichouse. he had a LZ with Flatops. Nothing but detonation proplems and have to run race gas a 8$ a gal. he doing a turbo swap now

[datzenmike]

Pistons are 84.8mm and pin height is standard 38mm. Wow! +80cc to work with that setup eh? I must admit to being lazy and using the helix engine design application which obviously has flaws. If we guesstimate that the pistons are in the +30cc range, with a standard L20b block & 219 head. What would you suggest for a crank/rod combo for optimal compression. That being said, I currently have a L20 crank and Rods. Swapping Rods is a bit cheaper than looking for another crank ;) Also considering getting the pistons milled. I know I could swap the head/pistons out for an easier set-up but that just seems like such a shame with the gorgeous 219 head and the forged pistons I have! Thanks for the help Mike :)

 

L20B bore is 85mm stock so a worn L20B will be slightly more. Minimum stock piston diameter is 84.985mm so I would think the 84.8mm would be loose in the bore.

 

Is the top surface flat? or domed slightly??? If flat you can run p-nut or open head as they shouldn't touch. If lightly rounded you will have to run an open chamber head. Now you need the volume of the raised 'dome'.

 

Get some clay or plasticine and press an imprint of the piston top into it. Go to a pharmacy and ask for a 10cc syringe, no not those kind... get a plastic one that is graduated for giving oral doses of medicine to children. I got one for free. Fill and inject into the clay mold keeping track of how much you use. Fill until just level.

[banzai510(hainz)]

They are domed pistons

My Motorsport book says Piston dome and cumbustion chamber must be modified to achive the desired comp ratio. Accept Deves rings 1/16 compression 5/32 oil

should come with 2 teflon buttons.

 

hes not going to run these anyways

[Logical1]

Arts & Crafts Time on a Sunday!

 

Well I got motivated today to figure out what these pistons will work for and the actual specs.

 

Step one: Clean level working area, Caliper, Clay, Piston, 5cc Syringe, water with the surface tension cut with soap, & a Stiff drink ;)

 

 

Step two: Take actual piston measurements without being braindead from work!

 

 

Step three: Mold piston top into clay and fill with 5cc of water at a time.

 

 

So It looks like the actual piston size is for an 86mm bore and after trying the syringe water process 4 times, The Pistons look to be +20cc

With this new information I went back to designing a L20block & 219 head combo with these pistons. I ended up with just swapping out my crank with a L18. L20 block, 219 head, L18 crank, L20 rods, & 86mm +20 Pistons get me in the 9.2/9.1 range (just being lazy again even with fixed specs on parts, I don't trust the design utility 100%) I also get a much lower rod angle (15.5) Only thing that is making me nervous is the deck clearance. The utility says I'll have about 4.4mm, the peak of the 'dome' is about 7.8mm. Nice thing is its a offset dome and looks like it tucks nicely into the chamber on my head...

[datzenmike]

L18 crank won't fit the L20 block

[Logical1]

L18 crank won't fit the L20 block

 

Grumble!

[datzenmike]

I know I hate that. All 6 bolt L and Z series cranks have the same main bearings and they are different from the 5 bolt cranks.

[Logical1]

Update: I emailed Arias pistons about the pistons and included a link to this thread. This was the reply:

 

"unfortunately, I can't find the correct information for the pistons, no serial number to find it and the only thing I can tell you is that the pistons for these engines don't usually come with high compression and the most you could have on these pistons is about 3.5cc... which would give you about 11:1 compression ratio. I have seen the pictures and with a 45.0cc combustion chamber, you could have as much as 5.0cc but that would be about max. to give you a point of reference, 86.0mm bore with 86.0mm stroke and 45.0cc combustion chamber, .040" gasket, would give me about 10:1 with a flat top piston..."

 

So ether I royally F'd up my piston mold CC check or these are some truly wacky pistons. I'm thinking now the only way to check them 100% is to throw them in the block on a crank and do the actual measurements and math. Being that I would have to bore my block to 86mm to fit them in I am hesitant to bore and find out I wont be using them short of milling them down which = $$$

[hang_510]

I royally F'd up my piston mold CC check

 

 

I wont be using them short of milling them down which = $$$

the fluid will form a bubble, which adds to the measured volume.

youd need to have a flat surface(plexiglass) at the interface to get a true reading.

 

i sold the same set, it would produce ~11:1 in a stock L20b w/an open chamber head.

couldnt use them as i already had the block bored to my custom pistons.

 

 

dunno what you paid($200???) but a machine shop visit shouldnt be more than $100 to mill flat. i sold mine for more than that ;)

a worthy venture for the investment.

 

 

im running ~9.5:1 and it will run fine on 87, with an occasional ping under heavy load, none when i use 91.