Bumping up compression?
08-13-2013, 03:04 PM
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Bumping up compression? Im not good at math when it comes to the engine numbers. so I have a quick question if i bought 4.8 rods and flat pistons and threw them in my 5.3 being as the 4.8 rods are longer and mill down my head. will that bump it up in compression. i know milling the head down will. my main question is can i put the longer rods in and flat tops without any problems like my pistons kissing the valves. and or will it bump it too much to where i cant run pump gas? Im pretty sure the cylinder bore is the same but is the depth of the cylinder the same on both motors?
08-13-2013, 03:44 PM
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Yes using longer rods with stock piston will bump up the comp. it's pushing the piston up more. Do the math rod length + crank stroke= ?.00 inch dived by 2 gives you the compression ht.
Stock ls block deck ht. is 9.240 - .230.
My crank is 4in crank div. by 2 = 2.0 + rod leg. 6.125 = 8.125 - 9.240 =1.115 compression ht. piston is o at this deck ht. The ? is are you going to use after market pistons or the ones you have if so you will need to know comp ht. of stock pistons also stock crank stroke and the 4.8 rods lengths ....Also you will have to mock up milled heads for stock pistons and longer rods using clay and seeing how much the clay is smashed when the intake is fully open.
Stock ls block deck ht. is 9.240 - .230.
My crank is 4in crank div. by 2 = 2.0 + rod leg. 6.125 = 8.125 - 9.240 =1.115 compression ht. piston is o at this deck ht. The ? is are you going to use after market pistons or the ones you have if so you will need to know comp ht. of stock pistons also stock crank stroke and the 4.8 rods lengths ....Also you will have to mock up milled heads for stock pistons and longer rods using clay and seeing how much the clay is smashed when the intake is fully open.
08-13-2013, 04:07 PM
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Yes using longer rods with stock piston will bump up the comp. it's pushing the piston up more. Do the math rod length + crank stroke= ?.00 inch dived by 2 gives you the compression ht.
Stock ls block deck ht. is 9.240 - .230.
My crank is 4in crank div. by 2 = 2.0 + rod leg. 6.125 = 8.125 - 9.240 =1.115 compression ht. piston is o at this deck ht. The ? is are you going to use after market pistons or the ones you have if so you will need to know comp ht. of stock pistons also stock crank stroke and the 4.8 rods lengths ....Also you will have to mock up milled heads for stock pistons and longer rods using clay and seeing how much the clay is smashed when the intake is fully open.
Stock ls block deck ht. is 9.240 - .230.
My crank is 4in crank div. by 2 = 2.0 + rod leg. 6.125 = 8.125 - 9.240 =1.115 compression ht. piston is o at this deck ht. The ? is are you going to use after market pistons or the ones you have if so you will need to know comp ht. of stock pistons also stock crank stroke and the 4.8 rods lengths ....Also you will have to mock up milled heads for stock pistons and longer rods using clay and seeing how much the clay is smashed when the intake is fully open.
Thanks.
08-13-2013, 05:07 PM
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Give me a few minutes I got to smoke on this one they didn't tell me the rod len. I''l look into my GM catalog and my files and tell you what is what. We also got to find out the Specific with the pistons: Give me the crank your using , rods are 4.8 and what piston your using 5.3 or 4.8.
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08-13-2013, 05:28 PM
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Well.The stroke for the 4.8L is 83mm (3.268˝) while the stroke for the 5.3L is 92mm (3.622˝) and that crankshaft.The 4.8L engine used a "long rod" that measured 6.275˝ center-to-center because it had a shorter stroke. Look for a 121BW casting number on the side of the cap.
˝
The 5.3L, 5.7L and 6.0L all used the "short rod" that measured 6.096, center-to-center, but there were two different short rods used for the 6.0L engines.
Now we have to look at the pistons.
GM has used several different pistons in these engines because of the different displacements and applications. There are a few things you should know about them:
All of the Gen III pistons are cast,eutectic aluminum with 1.5/1.5/3.0mm ring grooves.
The 4.8L piston is a flat top, but the 5.3L is dished because they both use the same head, so the 5.3L with the longer stroke would have had too much compression with a flat top piston.
I don't no the dish or the comp. ht of the pistons to do the math you would be best to get a set of after-market cheap set of aluminum pistons.
˝
The 5.3L, 5.7L and 6.0L all used the "short rod" that measured 6.096, center-to-center, but there were two different short rods used for the 6.0L engines.
Now we have to look at the pistons.
GM has used several different pistons in these engines because of the different displacements and applications. There are a few things you should know about them:
All of the Gen III pistons are cast,eutectic aluminum with 1.5/1.5/3.0mm ring grooves.
The 4.8L piston is a flat top, but the 5.3L is dished because they both use the same head, so the 5.3L with the longer stroke would have had too much compression with a flat top piston.
I don't no the dish or the comp. ht of the pistons to do the math you would be best to get a set of after-market cheap set of aluminum pistons.
08-13-2013, 08:46 PM
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Hey I found you a piston it's by Wiseco 1.110 compresion ht. it's for a 4 inch stroke but you make it up with your rod here's the equation: your stroke divide by ..... 2 + rod and compression ht. gives us a deck ht at 0.............1.811+6.275+1.110=9.196 deck ht. that is @ 0 at the deck. stock LS deck is 9.240 - .230 deck ht.
This same Wiseco piston with 4 in. stroke and 6,125 rod & 1.110 compression ht=9.209 or 9.236 depending on the block iron or aluminum. iron block is some time shorter in deck ht.
This same Wiseco piston with 4 in. stroke and 6,125 rod & 1.110 compression ht=9.209 or 9.236 depending on the block iron or aluminum. iron block is some time shorter in deck ht.
08-13-2013, 09:28 PM
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Hey I found you a piston it's by Wiseco 1.110 compresion ht. it's for a 4 inch stroke but you make it up with your rod here's the equation: your stroke divide by ..... 2 + rod and compression ht. gives us a deck ht at 0.............1.811+6.275+1.110=9.196 deck ht. that is @ 0 at the deck. stock LS deck is 9.240 - .230 deck ht.
This same Wiseco piston with 4 in. stroke and 6,125 rod & 1.110 compression ht=9.209 or 9.236 depending on the block iron or aluminum. iron block is some time shorter in deck ht.
This same Wiseco piston with 4 in. stroke and 6,125 rod & 1.110 compression ht=9.209 or 9.236 depending on the block iron or aluminum. iron block is some time shorter in deck ht.
08-13-2013, 09:53 PM
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Stay with me : the 1 thing we have got to no is comp ht. we got that 1.110 for 6.275 rod......... stock piston ht is 1.321 for stock 5.3 rod and crank. Your deck ht with that combo is your stroke divided by 2... 3.622=1.811 stroke+6.275 rod+1.110 comp. ht=9.196 deck ht. you will have to mill the block from 9.240 to 9.196 to get to 0...11.2 with 58cc heads and 10.8 with 61cc heads. I want you to learn how to do to this for you......Now take this in add 1.811 (5.3 stoke divided by 2)+6.275 (4.8 rod)+ (plus stock comp ht.)1.321= will it be more than 9.240 if so stock piston won't work.....Also if you mill the block this much it will be hard to line up the intake, block,heads...Cheapest way will be to mill the heads with left over $ buy a cam and springs.....I hope you learned how to do this calculation....
08-13-2013, 10:58 PM
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my calculator reads: 1.811(this is 3.622 divided by 2) + 6.275+1.321=
1.811 + 6.275+1.321=9.407 that is how it will be and the deck ht. is what.....9.240-.230 it's out the block by 0.167 with a 9.240 deck ht.
The thing is just mill the heads and .....If you wanted to do something fly for 490-500 bucks you could turn the rods down and buy a set of rods and build a stroker I took a 4 inch Eagle crank had the rod turn down and Nitrated to 1.889 Honda rod journal ....stock is 2.10 .....it went from a 4.0 to a 4.175 crank I I will have to use Gen 3 6.125 rods but they work all day long....It was something I wanted too do....Also this for a long sleeve block not my 408.
1.811 + 6.275+1.321=9.407 that is how it will be and the deck ht. is what.....9.240-.230 it's out the block by 0.167 with a 9.240 deck ht.
The thing is just mill the heads and .....If you wanted to do something fly for 490-500 bucks you could turn the rods down and buy a set of rods and build a stroker I took a 4 inch Eagle crank had the rod turn down and Nitrated to 1.889 Honda rod journal ....stock is 2.10 .....it went from a 4.0 to a 4.175 crank I I will have to use Gen 3 6.125 rods but they work all day long....It was something I wanted too do....Also this for a long sleeve block not my 408.
