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Discussion Starter · #1 ·
I was wondering since I'm in the process of assemblying a blown 350 chevy, that it would be a good idea lowering the C.R. I see the pistons have a lip on the perimeter wich seems like does nothing but raise the c.r. the pistons are regular dished shape, I belive if I machine them flat I will not loose any strength and will perform better with the 7 pounds of boost my B&M blower pumps.
How deep can I cut the pistons before I weaken them? I mean, can I go lower than the bottom of the dish?
Thanks.
Augusto.
 

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The problem with cutting down that lip is that it will move the top ring closer to the top of the piston which is something that would be very detrimental to the life of the motor, especially a blown motor.
 

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Augusto said:
I was wondering since I'm in the process of assemblying a blown 350 chevy, that it would be a good idea lowering the C.R. I see the pistons have a lip on the perimeter wich seems like does nothing but raise the c.r. the pistons are regular dished shape, I belive if I machine them flat I will not loose any strength and will perform better with the 7 pounds of boost my B&M blower pumps.
How deep can I cut the pistons before I weaken them? I mean, can I go lower than the bottom of the dish?
Thanks.
Augusto.
I don't know what kind of fuel you have available, but here in the states, we would have to build the motor about 8.0:1 in order to use 7 psi boost on readily available pump gas. You say the pistons are "regular dished shape" which indicates to me that they are stock-type pistons. My first guess if that you will not be able to run on pump gas with this setup unless you have some hellacious pump gas down there.
 

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Discussion Starter · #4 ·
amigos,
first, the lip has a trapezoidal section wich means the ring is already exposed rigth now, it won't be changed by the cut.
second, this are trw cast pistons wich I was already using, I tore the motor apart to change rings, this is my third set of this pistons with no failures yet, I'm at 6.600 feet of altitude so 7 pounds is about rigth, I use 92 octane pump gas, it pings a little at wot but no big deal, I'm thinking of installing a boost retard device or a water injection or both, but lowering the cr migth be the best option, also I would like to unshroud the valves wich will give me a few extra cc's in the combustion chambers, I would like to go down to maybe 7.5:1 any thougths?
Augusto.
 

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engineczar said:
The problem with cutting down that lip is that it will move the top ring closer to the top of the piston which is something that would be very detrimental to the life of the motor, especially a blown motor.
The only safe thing to do is flycut dome pistons down to being flat without removing any ringlands correct?

Maybe you should try running heads with a few more cc's to drop your CR instead?
 

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Augusto said:
amigos,
first, the lip has a trapezoidal section wich means the ring is already exposed rigth now, it won't be changed by the cut.
second, this are trw cast pistons wich I was already using, I tore the motor apart to change rings, this is my third set of this pistons with no failures yet, I'm at 6.600 feet of altitude so 7 pounds is about rigth, I use 92 octane pump gas, it pings a little at wot but no big deal, I'm thinking of installing a boost retard device or a water injection or both, but lowering the cr migth be the best option, also I would like to unshroud the valves wich will give me a few extra cc's in the combustion chambers, I would like to go down to maybe 7.5:1 any thougths?
Augusto.

Trimming the pistons will make the ring closer to the top which might pop the head off. :nono:

Your total manifold pressure is only 4.8 psi above sea level, but.......


Ditto what Tech said......... WHAT ?

..... "no big deal"...... If you can hear it, it is already waaaaaaaaayyyy too much ping. :nono:

You have audible detonation on 92 octane with 5 pounds (sea level) of boost......

You need TIMING help.

Unshrouding will get you maybe 2 to 2.5 cc maximum bigger chamber. Mine went from about 54 to 56.0 cc.

Water injection is a viable solution. WWII airplanes used it, as well as civilian air liners, and gained 10% takeoff power.
 

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techinspector1 said:
Lack of squish with thicker gaskets might make it ping worse.

In theory, possible. :thumbup:

But people have been running extra thick gaskets on SBC blower motors for 40 years.....

seems like I read something once about increased pressure causing increased velocity that supposedly causes chamber swirl.......

whatever
 

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Discussion Starter · #14 ·
well hemi motors don't have squish, most japanese motors are hemi and I've turbocharged them succesfully, I have a doubt about dual gaskets, what would happen with the valvetrain and the intake manifold, everything will go out of alignment.
Yes you guys are rigth, if it pings it pings period, I'll check the ignition and will correct it, but I must get the CR down to take advantage of the boost pressure if I don't it will only make heat and no power.
Augusto.
 

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Discussion Starter · #18 ·
454C10
Everyting is stock, I'm quite bussy rigth now but will measure everything up and come back later for more help, if somebody's got any suggestions please post.
Thanks
Later.
Augusto.
 
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