yes you would need a custom made topped piston. You will not be able to move the pin bore in the piston down that far into the pin boss, so the only option is raise the heigth of the top of the piston. You might want to reconsider what you are thinking. The piston will be longer noving the rod connection lower cxould cause a binding of the piston in the bore. Also the shorter rod/stroke ratio will cause excessive side loading on the piston that may result in a cracked piston. You would be much better off using a shorter stroke crank a longer rod and piston with the pinboss moved up into the oil ring area to locate the pin under the oil ring(racer option, not utilized much on a street engine) That way you will have a greater rod/stroke ratio, less side loading on the piston. You will have a higher rev engine, but you will also have less torque overall until you get into the higher rpm range.

 

Rods do not have a stroke....

Rods have a length.
Crankshafts have a stroke. You are refering o the differences in crank stroke rather than rod lengths.

If you were to use 240 rods you'd have to use a piston with a shallower pin height.

L28 rods are 130.3mm
L24 rods are 133.0mm

 

RE: Maybe just use an L24...

n/c

 

No. Your pistons will stick out the top of the block. Unless you get different pistons or shorter throw crank the longer rods will cause the pistons to hit the head. It is possible to use the L24 rods, because those are the rods used in the 3.1L stroker. They just have a longer throw crank and really short pin height pistons along with a serious overbore. I looked at doing the same sort of thing.

 

RE: Don't raise the bridge, lower the river!

An easier solution than the pistons is to take the 10 mm off the top of the block (deck it 10 mm). An added advantage of this solution is that it gives you a lot more room to install a strut brace, but you do have to shorten the exhaust down tube a bit.

- John

 

RE: MSA Adjustable Length Connecting Rods

Call Sam at MSA and order a set of their tubular adjustable length connecting rods. Their length can be manually adjusted from 65 to 85mm. They are also working on some V-Tech connecting rods that shorten above 4,800 rpm.

- John

 

RE: MSA Adjustable Length Connecting Rods

If the rods shorten above 4500rpm, wouldn't it have the same stroke (determined by the crank) and significantly lower compression?

 

RE: MSA Adjustable Length Connecting Rods

You guys are joking right?

 

RE: MSA Adjustable Length Connecting Rods

thats what I was asking... wtf?? ive never heard of any of this stuff, decking 10mm? talk about timing chain slackness. And adjustable connecting rods? you've got to be kidding...

 

RE: MSA Adjustable Length Connecting Rods

i agree! talk about a loose chain. what have you guys been smoking?

 

Hey Nate! Don't let all the "chain yanking",make you afraid of asking questions!In
general you can't change the rod length without a coresponding crank stroke,block
deck height,or piston pin height change.It's desireable to have longer rods in the same stroke motor,but can be very expensive to acheive. 240 Al

 

if you would likethe facts about your change, I can give you all the specs. I dont have my books here but mail me at home and I can get a work up for you.mrjeff@worldnet.att.net I know what I am talking about I have donethe engine math, I built my stroker with custom rod lengths ect ect

 

RE: MSA Adjustable Length Connecting Rods

Makes ya think, don't it...

- John

 

War and Peace...

Agree with response above. There are a lot of responses which I haven't read, so mine may be redundant.
Do your math calculations using 1/2 the stroke length + rod length + piston compression ht (distance from pin center to top of piston).

e.g. 240Z L24: (73.7/2) + 133 + 38.1 = 207.95mm which is the distance from the center of the rod journal to the uppermost point of the piston's travel @ TDC. Any difference in the top of the piston and the block deck is the deck ht ( this is usually about zero). If deck ht is too high (+) then the piston hits the head, if too low (-) then combustion takes place down in the block as well as the head and bye-bye head gasket, plus it's poor quench efficiency. Piston domes or dishes are not included in deck ht, it is the surface of the piston edge as reference.

280Z L28: (79/2) + 130.4 + 38.1 = 208mm

I think you were initially asking about using L24 rods in an L28 to improve the rod/stroke ratio. The easiest variable to control in this is the piston and what to get it from if you don't want the expense of custom pistons. e.g. in this case you want to use a rod that has the same journal diameter and cap width as an L28. You've chosen L24 rod, which is 133 -130.4 = 2.6mm longer, so you need a piston of your bore size which has a compression ht of 38.1 - 2.6 = 35.5mm.

So, 86mm bore and 35.5 comp ht (with a pin offset same as Nissan and a pin dia of 21mm to direct fit with no machining of rod small end). Nothing comes to mind from OEM Nissan parts. However, with a 1mm overbore to 87mm, a piston from a Z22 engine would work. Be careful though because the truck engine has a different rod/stroke ratio than the car (Z22S vs Z22E). As expected, the truck is the lower ratio and has a slightly shorter rod and longer comp ht than does the car source (different casting numbers on the rods too). I'll let you figure out the best one to use as source of the piston.

Pistons can be machined a bit to alter compression ht, but only by a small amount, otherwise it brings the rings up too near to the deck @TDC.
I have taken L28 pistons (75-78) with the 10.9cc dish and have them machined in a lathe so that the dish rim is just machined away and it has been tuned into a flat-top with an altered comp ht, enough to allow using a 133mm rod from an L24 or L16. This trick increases the rod/ stroke and the compression ratio in a single step on a 75-78Z.
Use post-'72 rods to get the beefier bolts (9mm vs 8mm) and don't use the later Maxima rods as they have a smaller rod journal size.

DAW