decompressed pistons

SMOKIN-U

SMOKIN-U

Comp Diesel Sponsor
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Jan 13, 2008
Messages
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what are the benifits and down side to running a decompressed piston? If you did decompress i think stock on my 05 is 17.5-1 could a guy go down to like 15 or 15.5-1 thanks
 
IMO High boost numbers is the only reason for decompression. I can see if u want mass amounts of giggle gas but then you could just shave the top some to make room for the gas. I think it would be sweet to see some numbers from the same motor 1 not decompressed and 1 that was u can bet their would be a power loss.....
Brandon
 
really you would lose power? how is that i am new to the concept i have always heard of high hp trucks being decompressed. you would think if it hurt power they wouldnt do it. what would be the benifit then?
thanks, Ryan
 
I would say HIGH boost numbers and cylinder pressure is the reason they do it. If u cant fill the void what would be the point?
Brandon
 
Same engine same everything, except for lower compression, the higher compression engine will make more power.
 
Which contains more oxygen?
140cc combustion chamber/pocket volume or 170cc's of combustion chamber/pocket volume.
Now which will hold more fuel mix?
Early nitro burners learned to reduce the piston domes so they could squirt more fuel and still avoid hydraulic lock.
 
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Well, we just did this

You may have seen Our build thread...Garmon's Interceptor build...My truck but his Know-How..

We decompressed my truck.....Cut the piston a hundred thousandth's off the top...

Reasons:

Other than minor ones like valve clearence after cam and decking, and for more cyl. volume.......The main reason is so the engine will LIVE at higher HP and RPM levels.....

Most race engines are spun at higher rpms and make huge power, so I would think it would live longer with not so much compression..

I do notice a little loss in low end snap right off idle and It does white smoke when cold, but otherwise it make awesome power......Like my dyno thread said...We just had a few minutes and didn't get the chance to change much in the way of tunning or gate settings and it made 766HP for a high....That's with STOCK HEAD BOLTS and around 60+ psi.....
 
97' CTD said:
IMO High boost numbers is the only reason for decompression. I can see if u want mass amounts of giggle gas but then you could just shave the top some to make room for the gas. I think it would be sweet to see some numbers from the same motor 1 not decompressed and 1 that was u can bet their would be a power loss.....
Brandon
Click to expand...

what about cams with lots of duration and high lift??
 
Clunk said:
Which contains more oxygen?
140cc combustion chamber/pocket volume or 170cc's of combustion chamber/pocket volume.
Now which will hold more fuel mix?
Early nitro burners learned to reduce the piston domes so they could squirt more fuel and still avoid hydraulic lock.
Click to expand...


Yeah, they got to compress air AND fuel and about 8 TIMES more fuel than a regular gasser...........

We only have to compress air.....
 
rpainter said:
what about cams with lots of duration and high lift??
Click to expand...

That is fly cutting not decompressing a motor. Yes u will loose a hair of compression but fly cutting is not specifically designed to lower compression numbers, it is to clear big grinds. Swole is right it will live longer but u have to take into account your specific application and needs as his intent as far as I know is to DD his and have a nice little racer all in 1 where longevity is ideal.
Brandon
 
97' CTD said:
That is fly cutting not decompressing a motor. Yes u will loose a hair of compression but fly cutting is not specifically designed to lower compression numbers, it is to clear big grinds. Swole is right it will live longer but u have to take into account your specific application and needs as his intent as far as I know is to DD his and have a nice little racer all in 1 where longevity is ideal.
Brandon
Click to expand...



:clap: Agree$.02
 
97' CTD said:
Swole is right it will live longer but u have to take into account your specific application and needs as his intent as far as I know is to DD his and have a nice little racer all in 1 where longevity is ideal.
Brandon
Click to expand...


BINGO!!!!!

:clap:
 
97' CTD said:
That is fly cutting not decompressing a motor. Yes u will loose a hair of compression but fly cutting is not specifically designed to lower compression numbers, it is to clear big grinds. Swole is right it will live longer but u have to take into account your specific application and needs as his intent as far as I know is to DD his and have a nice little racer all in 1 where longevity is ideal.
Brandon
Click to expand...

also fly cutting creates hot spots on top of the piston I decompressed my pistons in order to clear my cam. I talked to three different machine shops and they all told me not to flycut pistons. decking the whole piston is much better. my truck is a daily driver also and also a sled puller.
 
rpainter said:
also fly cutting creates hot spots on top of the piston I decompressed my pistons in order to clear my cam. I talked to three different machine shops and they all told me not to flycut pistons. decking the whole piston is much better. my truck is a daily driver also and also a sled puller.
Click to expand...

U tell me what would make a fly cut create a hot spot? Would u mind to tell who shops these were? Were they high performance gas motor shops?
Brandon
 
97' CTD said:
U tell me what would make a fly cut create a hot spot? Would u mind to tell who shops these were? Were they high performance gas motor shops?
Brandon
Click to expand...

I was told at the point of where the piston is cut will creat a hot spot?? and no not gasser shops. but I figure the $270.00 I have in cutting the 6 pistons and coating the tops and skirts shipping included I could not go wrong. maybe they were wrong but this being my first diesel rebuild I could not argure with them.
 
Leaving a corner will always give you a spot that will heat more easy....for example take your cutting torch and begin a cut right on the corner, takes almost NO heat, try it again in the center of the plate.....you'll see the difference.

Does it make a diff in our diesels.....doubt it.....might light up meth a little early..duno
 
As I go back to thinking it was a gas shop due to if it were a hot spot it would cause the air/fuel mixture to ignite prior to the plug firing if it were to retain or store enuff heat to ignite. Or did they tell u it would melt the piston? I do not see it causing any problems in our motors and please do tell if I am overlooking something. But hey me and Red are just thinking out loud.......
Brandon
 
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