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L18 build

Posted: 19 Nov 2022 01:22
by Wilse
I know there’s heaps of info on engine builds, etc, but can’t find much on L18’s performance wise. I’ve got L16.18 and 20 engines and parts, but ideally want to build an 18 so that I don’t need to get engineers certification etc here in Aus. My question is this. For a fun street car build, what would people recommend.? I’ve read some use the L18 crank, L16 rods and then mill the L18 pistons to flat tops. Or am I better to bore L18 to oversized and leave it at that. Or further, obviously more costly option is to line bore the big ends and stick the L20 crank in.. ?
Also what head will work best. I’ve also got all the options of heads available to me as well, including a 219..

Re: L18 build

Posted: 19 Nov 2022 07:32
by datzenmike
I guess using L16 rod is to get a longer rod on the L18 crankshaft but this pushes the L18 pistons out of the bore so they need the tops milled almost 3mm to fit. Much easier to just bore the L16 block to 85mm and fit the L18 crank/rods and pistons essentially making an L18 wearing an L16 block.

L18 bored to 1mm oversize gains you negligible results.

With over boring the L18 main bearing saddles to L20B size...
An L20B crankshaft does have drawbacks. The L18 block is 2cm shorter in height so when the rod and crankshaft form a 90 degree it happens farther down the bore and the rod is exerting more sideways force on the piston. The ideal rod/stroke ratio is considered OK between 1.5:1 and 1.8:1 with 1.75 often considered perfect. The lower the number the more side loading on the piston and the more friction.

1/ L20B crankshaft + Z20E pistons (flattops) and L16 rods fit. 31.75mm pistons + 133mm rods + 86/2= 43mm stroke = 207.75 and you have room for 207.85mm Rod/stroke ratio 1.54:1 Basically an 1951cc L20B displacement and 9.9 compression with open chamber head and 10.7 with closed.

2/ L20B crankshaft, 89mm KA24E (or Z24) pistons + 130.2mm L16 rods + 86/2=43mm stroke = 207.2. L20B and L18s have the same 85mm bore. L20Bs have been bored to 89mm and KA pistons fitted. That's somewhat scary on an L20B, might be worse on an L18s cylinder wall thickness. Would produce a 2,136cc displacement. With open chamber head.... KA-E pistons have a 2.8cc dish for 10.17 compression. Z24 are 15cc. so 8.5 compression. Rod/stoke ratio 1.51:1

If you have access to a Z22 crankshaft...
3/ Z22 crankshaft + L18 rods and Z20E pistons, 92/2=46mm stroke +130.2mm rods + 31.75 pin height flattop pistons = 207.95mm just 0.1 above the deck . Displacement is 2,086cc compression about 10.56 on an open chamber head, but the rod stroke ratio is a poor 1.42:1. Perhaps friction reducing coatings to the piston skirt would be an advantage here.

Re: L18 build

Posted: 19 Nov 2022 14:44
by Wilse
Thanks Mike. So am I waisting a lot of effort for not much gain to do any of this? Would it more beneficial to make gains in the head, Cam, Induction and exhaust side of things and leave the L18 bottom end alone, apart from say new rings and a balance.??
Interesting that you say about L18 internals in the L16 block rather than L16 rods on the L18 crank and mill the pistons, wouldn’t it be best just to leave L18 as is? IE no real benefit to L18 internals/ L16 block.?

Re: L18 build

Posted: 19 Nov 2022 15:56
by datzenmike
If you needed to fool an inspection then you could bore the L16 and put the rotating parts from an L18 in. Otherwise just rebuild the L18 as is.

Datsun rotating parts are individually balanced very well so a rod from one engine will swap into another without worry.

Here's another combination....

Bore the L18 block to 87mm (2mm over) Z22 pistons from 720 truck, L16 rods, 78/2= 39mm stroke, 35.56mm 9.32cc dish Z22 pistons and 133mm L16 rods = 207.56 just under 207.85 deck height. Makes a 1,853 displacement engine with 8.5 compression with open chamber head or 9.04 with close chamber head. Call it a big bore L18 with 1.7:1 rod stroke ratio

Re: L18 build

Posted: 19 Nov 2022 23:26
by Wilse
Thanks. I should have clarified at the start. An L18 block is acceptable without engineering certification, so that is my preference to use that block.
That combination sounds good, as you say, close to that ideal rod/stroke value that you mentioned..
I presume a 2 mm oversize bore on the L18 block wouldn’t create too many issues..??

Re: L18 build

Posted: 20 Nov 2022 07:15
by datzenmike
Well 2mm is rally 1mm all the way around so only 0.040" removed. I believe replacement oversize pistons come in 1mm over, so twice that should be fine.

Beware there are two Z22 pistons, you want the ones from the 720 truck.

Best you confirm all this math.