Destroking an L61 and compression ratios
05-21-2012, 12:24 AM
#3
Well I was talking to someone the other day who brought up the idea. I did some searching and saw GM did it on a race engine. The block and bore are the same between the L61 and LSJ, its just the stroke difference between the two, one being for forced induction and the other being for natural aspiration. I think its a 94mm stroke on the L61 and an 86mm on the LSJ. L61 has a 10CR and LSJ is 9.5. Im wondering if destroking would drop it down to 9.5 or if it would be different from the LSJ.
05-21-2012, 01:46 AM
#4
WARNING: TECHNICAL BS
You would have problems with piston-to-deck clearance if you simply changed cranks, because you would increase "quench" and effectively lower your compression ratio. (Quench is the room left from the top of the piston to the bottom of the head, including the head gasket.) "Quench" would be much too large if someone swapped cranks. Here's a quick example to help: A small block chevy V8 has a deck height(from the middle of the crankshaft to the top of the deck WITH NO HEAD GASKET) of 9.025". Say someone wants to know what compression distance he needs his pistons to have. (Compression distance basically means from the top of the piston to the middle of the wrist pin on the piston.)
1: The stroke of a 350 is 3.48", but you need to divide by half so that means 1.74"
2: Connecting rods are typically 5.7"
3: Add these together and you have 7.44"
4: Subtract the 9.025" to get the remaining space. This is 1.585"
5: Head gaskets for a typical sbc are .015", and IIRC you want a quench of about .040" This means you want to have 1.560" compression distance.
Say you built this engine but are a complete ******* and put a 3.25" crank in accidentally. Now instead of a .040" quench you now have .155" quench. That's 4 times what you need! You engine will now run like **** and it isn't because the compression ratio.
____________________________________________
IN A NUTSHELL, WHAT IT MEANS TO YOU:
It can be done but you will either need to use LSJ pistons or get custom ones made. Or use a different sized connecting rod. Custom-piston is the route I was going to with a chevy V8 build. Was going to take a 307 and put a 400 crank in it but would need hella oddball pistons.
You would have problems with piston-to-deck clearance if you simply changed cranks, because you would increase "quench" and effectively lower your compression ratio. (Quench is the room left from the top of the piston to the bottom of the head, including the head gasket.) "Quench" would be much too large if someone swapped cranks. Here's a quick example to help: A small block chevy V8 has a deck height(from the middle of the crankshaft to the top of the deck WITH NO HEAD GASKET) of 9.025". Say someone wants to know what compression distance he needs his pistons to have. (Compression distance basically means from the top of the piston to the middle of the wrist pin on the piston.)
1: The stroke of a 350 is 3.48", but you need to divide by half so that means 1.74"
2: Connecting rods are typically 5.7"
3: Add these together and you have 7.44"
4: Subtract the 9.025" to get the remaining space. This is 1.585"
5: Head gaskets for a typical sbc are .015", and IIRC you want a quench of about .040" This means you want to have 1.560" compression distance.
Say you built this engine but are a complete ******* and put a 3.25" crank in accidentally. Now instead of a .040" quench you now have .155" quench. That's 4 times what you need! You engine will now run like **** and it isn't because the compression ratio.
____________________________________________
IN A NUTSHELL, WHAT IT MEANS TO YOU:
It can be done but you will either need to use LSJ pistons or get custom ones made. Or use a different sized connecting rod. Custom-piston is the route I was going to with a chevy V8 build. Was going to take a 307 and put a 400 crank in it but would need hella oddball pistons.
05-21-2012, 01:50 AM
#5
Senior Member
Why would you de stroke it?If your doing the work,keep the stroke and get lower compression pistons.The LSJ and L61 are the same bore and the L61 has a longer stroke,so if you were to run the same stroke as a LSJ it would drop it to 2.0L.Not sure what CR it would drop to though
05-21-2012, 02:10 AM
#6
Why would you de stroke it?If your doing the work,keep the stroke and get lower compression pistons.The LSJ and L61 are the same bore and the L61 has a longer stroke,so if you were to run the same stroke as a LSJ it would drop it to 2.0L.Not sure what CR it would drop to though
05-21-2012, 07:40 AM
#7
Cool. That's what I was thinking but I wasn't sure what it would do to compression ratio. We were just talking about it the other day and I was looking into it more just for shots and giggles. The biggest reason is because racecar. Lmao
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