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MantisX620

L20b 86mm flatop pistons. Conflicting compression numbers

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Hi all,

 

My brother has an extra 280zx engine and said I could have the flat top pistons out of it. As I search around though, I get different numbers on what the compression would be with the stock U67 L20b head. On one post, I see 10.87, on another I see around 10.11. I have access to 93 pump gas in my area. Would this setup be possible? I also intend to add a 480/280 cam from Isky and do some port work. I have the ability to control the timing electronically via timing map.

 

I also read about L18 pistons but I am not able to find any of these at all, let alone in 86mm.

 

Thanks
MantisX

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If you really want to know what your compression is, you need to buy a calibrated burette, and measure head CC, gasket CC, and piston CC, and do the math.

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Hi all,

 

My brother has an extra 280zx engine and said I could have the flat top pistons out of it. As I search around though, I get different numbers on what the compression would be with the stock U67 L20b head. On one post, I see 10.87, on another I see around 10.11. I have access to 93 pump gas in my area. Would this setup be possible? I also intend to add a 480/280 cam from Isky and do some port work. I have the ability to control the timing electronically via timing map.

 

I also read about L18 pistons but I am not able to find any of these at all, let alone in 86mm.

 

Thanks

MantisX

L18 pistons are 85mm, ie, 86 mm is .040" over bored.  Your brother may have the pistons, but the rods/piston pins are PRESSED on and most times they break while being removed.. L20B rods are 5.75" long, L18. L26 and L28 are much shorter WON'T fit the L20B.  Just build the motor and limit the total timing to 32* BTDC to deal with the compression.

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L20B with open chamber U67 head over bored to fit 86mm flattop L28 pistons..... 9.11


This assumes the L head has never been off and shaved to remove warps or to raise compression.

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Thanks doctor510, I’ll have the machine shop do the rod switching. They might have better luck than me doing it. Hopefully out of 6 I can get 4 useable.

 

Thanks Mike, in one of your other posts, was the 10.87 you mentioned calculated for a peanut head? What about the 10.11? Also I would be doing the 86mm flat tops, not 87.

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Typo. Meant 86mm.

 

 

Yes those other numbers were... well they must be wrong. I checked 3 times. 10.9 is the low compression L28 piston dish. Must have put it in the wrong column. This would be the minimum compression. Shaving the head 0.5mm or 0.020" will give you 10.44 compression.

 

Only way to be sure is to cc your head. Stock should be 45.2cc per chamber.

 

http://community.ratsun.net/topic/358-how-to-cc-your-cylinder-head-for-free/

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The deck of the block and the head will both need to be corrected for warpage.  Expect .020" total to be shaved to get it flat.  The last block I decked had a .006" "dip" between cylinder 3 & 4 on the driver's side.    

How far are the flat top pistons down the hole?  .020"?  Can't calculate compression without that number.  

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Wow, this engine build seems unstable haha. Maybe ill stick with my turbo setup and build a flattop engine out of my spare L20b for fun. I have access to E85, maybe that would help with detonation.

 

MantisX

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All L series pistons (that will fit an L20B) are 0.45mm below the deck. Or 1.4cc of volume. The gasket thickness is 6.9cc. The cylinder is 499.3cc 

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If its of any significance, I threw a small chamber head on a "very used" L20b with stock pistons and that fell in close to 11:1.  Runs great on pump premium.  If you're just going from a stock dished piston to a flat top and you KNOW the deck height of the piston is the same (could easily be custom), then you'll go up maybe 1/2 point.  Personally, for what replacement pistons cost, its not much extra to upgrade to custom at $100 a piece so you can zero-deck the piston without as much machining.  

 

You can easily remove pistons from their rods without damaging them.  You just need a cradle in the press, which can be as simple as a couple 2x6's notched with a jigsaw.  Hell, you have 6 so if you screw up 1/3 of them you still have a set.  

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Agreed!  The last set of custom rods I bought, I had them leave the bushing out of the small end so I could install my own and offset the hole for maximum rod length and minimal deck shaving. That allows for boring the holes perfectly perpendicular to the rod, as well as at the same precise distance from the center of the big end.  When aftermarket manufacturers hone rods, many times that length wanders a bit and you can have 4 rods that are each a thou longer or shorter than the last.  

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