High compression pistons

[hoggie]

Hi,
Are high compression pistons worth the expense for a naturally aspirated engine(325m20) or are they only beneficial for tubo'd engines.If they are worth it what kind of gains can you expect.Cheers

[march109]

more usable on a N/A, for F.I engines you want low CR.

you talking custom or early M20B25 pistons?

no real gain in an M20 (a good m20) early (ie. high comp engines) seem to not loose as much power over the years as the later M20's (low comp), most of the good early lumps still make stock power after 17+ years.

The term high and low comp is misleading though, compared to other manufacturers they arn't really lo or high.

IIRC
early m20 = 9.7:1 CR
late m20 = 8.8:1 CR

a search will confirm if thats correct though.

[hoggie]

I am talking custom pistons something like JE high compression custom forged pistons my 325 is an '89 model so I presume that would be a late model.So would the JE custom pistons be beneficial in terms of power

[march109]

Yes that makes it a late model, just having high comp pistons doesn't mean the engine will produce anymore power the management will need to be tweaked to suit.

But theoretically in a N/A engine higher CR will create more power since the piston is driven down in the cylinder by the pressure produced in the combustion chamber during the burning fuel and air mixture, if the fuel/air mixture is under more pressure before it ignites the more force exerted on piston and the more power produced. Think of like two springs of the same length and spring rate being compressed one more than the other, when relaeased the one compressed more will spring up higher.

Wether a higher CR in an M20 will actually make more power is debatable both 9.7:1 and 8.8:1 CR M20's made the same power when new, with the management being the only difference in the engines of note AFAIK.

With the right rods, lightened pistons, some arp hardware you could make a real screamer of an engine. However my custom pistons wern't cheap +import duty + shipping, factor in even basic rebuild parts new HG and head skimming ect wether just raising the compression gives decent BHP per £Â£ is highly debatable.

I can't recommend JE pistons customer service myself even though I have a set as I bought them through an agent in the U.S, but the product is definatly spot on.

Hopefully someone more helpfull will be along in a moment.

TBH I reckon the costs of this would be prohibitive. Unless your planning on a real mental revving engine and arn't worried about cost there are better routes to the same power.

What mods have you got in mind other than higher CR? What sort of £Â£ you looking at spending. I could PM you more info on pistons ect if you want.