Embarrassing Question

[Zrush]

Please don't laugh but I must ask this question. OK here goes. When you modify the internals of an engine, does it change the bore and stroke? If it does, how would one go about finding out the new figures.

Vicky

[MikeW]

Originally posted by Zrush

Please don't laugh but I must ask this question. OK here goes. When you modify the internals of an engine, does it change the bore and stroke? If it does, how would one go about finding out the new figures.

Vicky

Depends on what you mean by modification. Increasing the stroke requires a new crankshaft. This is what Nissan did to go from the L24 to the L26. You may have seen people talking about using the crankshaft from a diesel Maxima for this purpose.

The only thing that will increase the bore is literally over-boring the cylinders and fitting larger pistons. This what what Nissan did to go from the L26 to the L28.

[ChrisA]

Hey Vicky, only dumb question is the one not asked. Bore you'd measure with a bore micrometer. Stroke would not change unless crank and or rods are different than what was in stock motor. There is a formula for determining cubic inches and compression using the variables of bore & stroke & combustion chamber chamber cc's but again you'd have to determine what crank/rods/pistons are in the engine.

You'd have to see an engineering diagram of the crank to determine the difference in stroke. I think the service manuals tell the stroke lenth on the cranks.

When I was a younger gear head I could have quoted you all the formulas but I've forgotten most of that stuff.

Chris

[MikeW]

Originally posted by ChrisA

Stroke would not change unless crank and or rods are different than what was in stock motor.

I don't see how changing the rods alone could affect the length of the stroke.

[ChrisA]

Originally posted by MikeW

I don't see how changing the rods alone could affect the length of the stroke.

Right, crank stroke is unaffected by rod length, but if you change the rod length from stock havent you changed a variable that determines cubic inches or compression or something?

Chris

[Zrush]

Isn't a "stroker" motor when one takes the rods from a L24 and puts them into the L28? Shortens the stroke right?

Vicky

[billcapp]

If I remember right, Bore X Stroke = Displacement (can be in cubic centermeters (cc) or cubic inches (ci). A 240Z stock = 2400 cc (Actually I think it is 2399 cc).

[Zrush]

I'm trying to figure out the compression and or hp in the red Z. I know the cylinders measured 215 each. The crank is stock, but he changed the pistons, cam and did a 40 over bore. Smokemup.com has the calculation tables. I just need to know what the stock crank stroke is in a 1977 280Z then I guess. Anybody?

Vicky

[MikeW]

Originally posted by ChrisA

Right, crank stroke is unaffected by rod length, but if you change the rod length from stock havent you changed a variable that determines cubic inches or compression or something?

Changing the rod length would not change the displacement. All it would do would move the piston closer to or farther from the cylinder head but the piston travel is determined only by the crankshaft. Of course, you wouldn't want to move the pistons closer as they already come to the top of the block. Shortening the rods would indeed decrease compression.

[ChrisA]

Stroke would still be the same.

I just re-read this and its really helpful (I think).

http://zhome.com/rnt/L28conversion/3031FAQ.html

Chris

[MikeW]

Originally posted by Zrush

I just need to know what the stock crank stroke is in a 1977 280Z then I guess. Anybody?

Looks like 79mm. Here's a nice engine calculator that should help you out:

http://www.classiczcars.com/forums/showthread.php?s=&threadid=1292&highlight=L6

[Zrush]

Originally posted by MikeW

Looks like 79mm. Here's a nice engine calculator that should help you out:

http://www.classiczcars.com/forums/showthread.php?s=&threadid=1292&highlight=L6

Very Cool. Thanks. I was getting pen and paper ready to do manual figures but that link saved me from massive amounts of calculations.

Vicky