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Bad vibration, please help

B

Bt Doctur

Silver Medal Contributor
Started vibrating all of a sudden
ropemussels.jpg
 
Seems to be a "Knotty Problem". I'm at the end of my rope and therefore can't help you untangle the problem!
 
definitely an issue with cylinder quench caused by wrong pistons :)
also, you have enough rope to hang yourself so be careful
 
hystat said:
definitely an issue with cylinder quench caused by wrong pistons :)
also, you have enough rope to hang yourself so be careful
Click to expand...

Not only wrong piston profile but the quench is not correct and the TA is not to spec. A Quench style SBC piston ..................... TGM full dished SBC piston.
Quench is possible with the above piston .............. a quench is NOT possible with this piston.

The SI may be malfunctioning and the LPCP should be at about 12*- 14* ATDC for optimal torque. For simplicity combustion in the chamber takes X amount of time. As rpms increase that time remains the same but degrees of advance needed to correctly time the spark event needs to increase. This is because as the rotations are happening faster or maybe better stated traveling a larger distance in the same amount of time. The timing advance is the very thing that allows for this. The curve is the thing that sets how aggressively or lazily this happens.

Your 3.750" stroke will give you the torque that you're after and at the correct RPM range for a Marine Cruiser SBC, but only to it's MAX if you get the LPCP where it must be. The rope maybe a deterrent.
The Q/E (as well as the cam profile) will give you the efficiency
a typical Marine Engine ignition advance curve (for use with the SBC which uses the GM full dished pistons), is going to be rather conservative. It will be conservative due to the increased Detonation Potential that this piston creates (of which you won't need to be concerned with).
While being conservative, it will affect the LPCP...
I can't stress enough the importance of the Q/E build, especially for this engine.
The average Joe Engine Rebuilder will use the dreaded GM full dished pistons!
Why..... because they are inexpensive, 1 p/n fits all 8 bores, ignition advance is held back, and the average customer doesn't know the difference.
A good SBC Marine Engine builder will steer clear of the GM style full dished pistons!









Oh that was a joke.................now give me back my rope!
 
hystat said:
definitely an issue with cylinder quench caused by wrong pistons :)
also, you have enough rope to hang yourself so be careful
Click to expand...

kghost said:
Not only wrong piston profile but the quench is not correct and the TA is not to spec. A Quench style SBC piston ..................... TGM full dished SBC piston.
Quench is possible with the above piston .............. a quench is NOT possible with this piston.

The SI may be malfunctioning and the LPCP should be at about 12*- 14* ATDC for optimal torque. For simplicity combustion in the chamber takes X amount of time. As rpms increase that time remains the same but degrees of advance needed to correctly time the spark event needs to increase. This is because as the rotations are happening faster or maybe better stated traveling a larger distance in the same amount of time. The timing advance is the very thing that allows for this. The curve is the thing that sets how aggressively or lazily this happens.

Your 3.750" stroke will give you the torque that you're after and at the correct RPM range for a Marine Cruiser SBC, but only to it's MAX if you get the LPCP where it must be. The rope maybe a deterrent.
The Q/E (as well as the cam profile) will give you the efficiency
a typical Marine Engine ignition advance curve (for use with the SBC which uses the GM full dished pistons), is going to be rather conservative. It will be conservative due to the increased Detonation Potential that this piston creates (of which you won't need to be concerned with).
While being conservative, it will affect the LPCP...
I can't stress enough the importance of the Q/E build, especially for this engine.
The average Joe Engine Rebuilder will use the dreaded GM full dished pistons!
Why..... because they are inexpensive, 1 p/n fits all 8 bores, ignition advance is held back, and the average customer doesn't know the difference.
A good SBC Marine Engine builder will steer clear of the GM style full dished pistons!






Oh that was a joke.................now give me back my rope!
Click to expand...


Had the correct piston profile been used (creating a good Q/E), the increased torque would have prevented the rope from being wrapped around the propeller!


Oh.... that also was a joke! :D

 
The problem seems to be far too complex for this forum. I can recommend a few local dealers who may be able to diagnose and correct the problem for as little as $3000.
 
RicardoMarine said:

Had the correct piston profile been used (creating a good Q/E), the increased torque would have prevented the rope from being wrapped around the propeller!


Oh.... that also was a joke! :D
Click to expand...


BTW the new 4.5 and 6.2 ltr EC engines from Mercruiser have a flat top piston. These engines are completey cast and built by Mercruiser. The reason why they used the full dish piston is because that is what GM was using, no other reason.

FYI, if you are rebuilding a EFI Mercruiser engine you must use a "full dish" piston for the programmed fuel mapping and timing curve.

And thanks for finding my Rope.
 
...........


Docksidemarineservices said:
BTW the new 4.5 and 6.2 ltr EC engines from Mercruiser have a flat top piston. These engines are completey cast and built by Mercruiser.
Yes, and I'll bet you that the reason for this is to ensure that there is no place for the flame front to hide (as there is with the GM Full Dished style piston).
All of this equals less chance of Marine Load Detonation! (and the engineers know this!)

The reason why they used the full dish piston is because that is what GM was using, no other reason.
GM brought the SBC full dished piston into existance back in the early 70s as to accommodate Uncle Sam's request for emissions control.

As FI became the "norm", GM should have abandoned it...... but because it is cheap to produce and use, the GM "bean counters" insisted that it be kept in production!

FYI, if you are rebuilding a EFI Mercruiser engine you must use a "full dish" piston for the programmed fuel mapping and timing curve.
I would argue this one!
Get rid of the GM FD pistons, and everything becomes more manageable.



And thanks for finding my Rope.
Har har!
Click to expand...
 
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