Explain - Forged Pistons, Compression Ratios, Decomp Plates etc

[Chr15]

This is the one side Ive never really looked into so have little understanding of it!

Could you give examples of what you would use, ie which pistons, for turbo/supercharging, n/a, etc. Also what psi you could run compared to standard.

Try to keep it 'simple' too!

[tarzan]

try a search mate. theres hundreds of threads on it. yes some threads are a load of bull but progress threads are full of detailed set-ups to give you an idea to start from

[Ross]

In short, when boosting you increase the compression of the air in the cylinder. The cylinder was "setup" to have a certain amount of compression. More than that amount can cause pre-ignition/knock.

Decomp plates / low-comp pistons / two head gaskets effectively reduce the stroke of the piston compared to the combustion chamber decreasing "compression". Thus decreasing the chance of knock.

You can run boost on standard compression ratio, but anything over a small amount runs the knock gauntlet. Once you run a reasonable amount of boost, it's generally considered better practice to run forged low comp pistons rather than two head gaskets or a decomp plate as the actual cylinder temperature starts to increase with increased boost pressure, and this can cause a failure of the piston crown as it gets too hot. Standard pistons don't like it too hot, and forged are more resistant.

Conversely you can also get more power from raising the compression (to a point). This is generally done with a performance tuned NA engine, and knock is controlled better by good combustion chamber design as part of the engine build, and good management (ECU).

Engines leave the factory at set compressions for best average conditions - performance Vs reliability Vs emissions. The factory has to meet all of those. When we start fiddling with the engine, we can disregard one or more - generally economy and emissions bite the dust in exchange for performance :)

That enough?

[Chr15]

Thats great! And I presume the various sizes depends on what headwork you have done/plan doing? Bigger sizes leading to more power too?

[Ross]

It's quite a complex question you're asking (and I don't have all the answers!). Lots of things are affected with a piston change - the shape of the piston crown affects the burn of the charge, the shape and size of the valves affect the VE of the engine, all sorts.

In short, Bigger (as in wider) pistons are used when the engine is rebored (we're talking .'s of a mm increases, so not really measurable from a performance perspective. Engines can be "stroked" (increase throw of the crank/different rods/pistons) to allow an increase in internal volume (1800cc is popular for these engines using the same 1600cc block).

Power boils down to VE (volumetric efficiency) The best NA engines only ever get up to about 85% VE - IE, in a 2000cc engine they're only "using" 1700cc of their potential. A turbo'd car can run anything from 110-150% VE, meaning it's effectively a much bigger engine. The NA car can only suck in air based on a pressure difference (cylinder pressure as piston goes down the bore Vs atmospheric air pressure). A boosted car "rams" air into the cylinders, filling it effectively "over capacity".

VE can be increased a number of ways - headwork is a very important factor. The easier the air can get in (either NA or boosted) the more VE you allow the engine to be. Better air filter, better cams (valves open for longer = more air in), or boost = power :)

[Chr15]

It's quite a complex question you're asking (and I don't have all the answers!).

haha sorry! as for the rest of your answer, thats helped loads! (as did the first reply) :)

[Ross]

haha sorry! as for the rest of your answer, thats helped loads! (as did the first reply) :)

Sorry - I'm being far too nice recently.
Meanwhile, back on the ranch...

fuckin' tick the thanks button then. Prick. ;)

[Chr15]

patience fellow cowboy ;)

[willsy]

Definitely well worth the thanks, Ross those are easily the most constructive and helpful posts I've read in a long time.

Boost certainly seems to be very popular lately so it would be great to see an ever improving wealth of information and knowledge available to help out with up & coming projects.

[RyanSaxo]

Love it when a thread has some good answers rather just the usual use the bloody search.

A good read and well explained answers.

Cheers!

[yr51ocw]

Power boils down to VE (volumetric efficiency) The best NA engines only ever get up to about 85% VE - IE, in a 2000cc engine they're only "using" 1700cc of their potential. A turbo'd car can run anything from 110-150%


While most of what you have said is very sensible advice, I will point out that most good "oem" 4V/cyl engine can achieve nearly 95% VE. A recent N/A race engine I was allowed access to some info on was as high as 140% VE!!

I work on a mildy boosted OEM engine and it runs a 180% VE at peak BMEP, I assume other manufactures using higher boost levels (VW TFSI comes to mind) would be easily pushing on the door of 200% VE

[Greedo]

Some great advise posted above, thanks guys.

With regards to upgrading pistons, connection rods and rod bolts on a turbo conversion - do people suggest I upgrade the big end bearings and shells too?

[Greedo]

And, if so - should I opt to have increased oil clearance bearings? Like offered here:
http://www.telematicagroup.co.uk/companies.php?supplier=AU-01&manufacturer=ACL_Performance&view=list&cat=ACL_2_Main_Bearings&category=Main%20Bearings&sub_cat=ACL_2_16_Peugeot&sub_category=Peugeot

[Ross]

While most of what you have said is very sensible advice, I will point out that most good "oem" 4V/cyl engine can achieve nearly 95% VE. A recent N/A race engine I was allowed access to some info on was as high as 140% VE!!

I work on a mildy boosted OEM engine and it runs a 180% VE at peak BMEP, I assume other manufactures using higher boost levels (VW TFSI comes to mind) would be easily pushing on the door of 200% VE

It's been a long time since I studied Engineering (ALMOST 20 years ago now!) so thanks for the comments. You're quite right - a quick google shows some updated results.

I'm making over 250bhp/litre myself so we've got the VE dialled in pretty good I expect but it's never been actually calculated.


Some great advise posted above, thanks guys.

With regards to upgrading pistons, connection rods and rod bolts on a turbo conversion - do people suggest I upgrade the big end bearings and shells too?

Standard bearings are very good to be honest (a new set of course) but if you have the budget I would recommend changing to ACL bearings.

[Quick]

IIRC, lowered compression will reduce the engines performance off boost, making it feel more laggy!

You can buy forged pistons usually in a lowered compression (forced induction), standard compression or high compression (usually a high spec n/a engine).
Forged are manufactored in a different way which gives a stronger pistion.