Turbocharging and Decompression Plates

[Alan Whitaker]

Hi All
I am looking for a general opinion on the following
You can lower compression in several ways i.e. lower compression heads if available from another engine in the group.
Lower compression pistons with a larger bowl, if available from manufacture.
Also decompression plates.
I have been told the best way to go is to find pistons with a lower compression height so if you were looking for a decompression plate and you wanted it 0.150 inch and you could find pistons with the same lower 0.150 that would be better, the piston would be 0.150 inch from the top of the bore.


Alan

[SteveNZ]

A decompression plate is a poor method in most cases to reduce compression. It alters the squish bands, which will affect the combustion properties. Low compression pistons are better.

Lower compression pistons dont simply move the pistons down the bore at TDC, that would be similar to a decompression plate. The piston crowns will be altered, either smaller domes or a dish etc. Similar work done to the combustion chamber will have the same effect.

[nabilhpe]

SteveNZ what did you mean by the last sentence, working on the cylinder head to enlarge the combustion chamber will have the same effect as using the head spacer? or as using the lower CR pistons?

removing material from the (combustion chamber) will also alter the squish bands right? if done correctly can it have a positive effect?

Nabil

[Guy Croft]

Alan, hi

I think this might be re the V6 Essex (Ford) unit you wrote to me about?

My advice is simple. Forget quick fix things like shop-and-swap cast pistons or decompression plates. What you need is forged pistons designed with the right bowl volume and crown height. If the std pistons are press fit get some race rods too.

If the pistons used are not directly a turbo type of suitable compression height, bowl etc (adn of course with thicker crown and stronger lands than normally aspirated) they'll be liable fracture under the pressure of turbocharging whatever donor engine they are from.

GC

[SteveNZ]

SteveNZ what did you mean by the last sentence, working on the cylinder head to enlarge the combustion chamber will have the same effect as using the head spacer? or as using the lower CR pistons?

Nabil

Depending where you add or remove material it could have either effect. It may or may not alter the squish. Good work will have a positive effect.

[Alan Whitaker]

Hi Guy
I know you have the right answer, so it is new rods, the pistons are TRW forged for turbo use with Total seal gapless ring set.
I know it is the way to go as the cost of a set of rods is nothing to a new engine, many thanks guy.


Alan