Turbo pistone compression...

mocpac · Post by **mocpac** » Thu Jan 18, 2007 10:28 pm

I was thinking, in a NA car the compression help to get more HP. But for turbo less compression is better to support the heat. Now. Is this a must for turbo? I can have some forged pistons with like 10:1 compression and get more HP or this will not make any difference in a turbo application?

Nubira2.2 · Post by **Nubira2.2** » Thu Jan 18, 2007 11:37 pm

you will make the compression with the boost

exist3nce · Post by **exist3nce** » Fri Jan 19, 2007 4:52 am

Generally lower compression lets you run more boost. 10:1 is still reasonable for a turbo application as long as you aren't trying to make a huge amount of power. I'm running 7psi on stock motor (U20SED) which has 9.5:1 compression, and there are no problems.

Audacity Racing · Post by **Audacity Racing** » Fri Jan 19, 2007 5:19 am

It's a safer bet to raise boost pressure instead of CR. If you start to knock with a higher compression ratio... you can't do anything about it. Atleast with boost you can cut it if you need to.

Here, jack the boost pressure up around 15psi

http://www.spankdu.com/icspray.pdf

wolfhound · Post by **wolfhound** » Fri Jan 19, 2007 9:50 am

You just have to be careful with the amount of boost you run.

I ran 0.7 on the std 1600 16V with shortened con rods to bring compression down to 8.5, i ran a stand alone management system with 350cc injectors, I still managed to smash the std pistons within 400 kays.

I then dropped the comression down 7.5 with the shortned rods and a 1.6mm deck plate (yes huge lag) but the set up lasted me 12000kays this time until the deck plate gave out.

I have now ordered a set of forged Pistons from IPP and this should equate to 8.8 with no deck plate.

Sean

exist3nce · Post by **exist3nce** » Fri Jan 19, 2007 4:33 pm

wolfhound wrote:You just have to be careful with the amount of boost you run.

I ran 0.7 on the std 1600 16V with shortened con rods to bring compression down to 8.5, i ran a stand alone management system with 350cc injectors, I still managed to smash the std pistons within 400 kays.

I then dropped the comression down 7.5 with the shortned rods and a 1.6mm deck plate (yes huge lag) but the set up lasted me 12000kays this time until the deck plate gave out.

Sean

kays? ummm is that km / kilometers? I can't believe the pistons died that fast. I've already put 3000+ km on my setup and no problems so far.

Post by **PrecisionBoost** » Sat Jan 20, 2007 12:56 am

Shortening rods and adding plates is a quick and easy way to reduce compression but it's dangerous because the piston will loose it's "quench pads"

Basicly.... the cylinder head has coolant running through it.... so it stays relatively cool compared to the pistons and exhaust temperatures exiting out the ports.

Normally what happens is as the piston comes close to the cylinder head at top dead center the outside areas of the pistons are almost touching the cylinder head.... which allows some heat transfer between the two.

So basicly... the cylinder head cools the pistons each time it comes to top dead center.

Adding a gap to reduce compression ( con rods or spacer ) makes the gap between the outside edge of the piston and cylinder head to far for heat transfer to occur.

So in the end.... you end up with a hotter piston.... which can create pre-ignition knocking.... which can very quickly destroy your average cast piston.

The only "safe" way to reduce compression is to change the piston design.

Some people choose to reshape the combustion chamber... in many cases they try to make the "quench pads" larger and improve the geometry around the valves.

Sometimes this adds compression... so in turn they will machine their pistons to compensate and move the compression back down to where it should be at.

Hmmmm..... I can't remember if Stefan posted his wonderfull pictures showing how he reshaped his 8V cylinder head.... that would be a good example to show you a nice geometery and the quench pads (areas that almost touch the piston)

wolfhound · Post by **wolfhound** » Mon Jan 22, 2007 9:04 am

I must admit that the 1st set of pistons I used were aftermarket KS pistons for the Opel 1600 and as such had a HUGE machined slot under the oil control rings, Those are the ones that lasted 30 odd kays (Yup KM's

)

The second set that went bang after 400 were the Std Woo ones - no slot for oil feed and generally alot stronger.

I must admit I haven't stripped the engine since I blew the headgasket and deckplate but am quite curious to see how these pistons (std woo running 7.5) lasted.

Chris, you seem quite clued up on the turbo thing. I have been told to run a bigger ring gap of 0.6-1.0mm is that true ?

Séan

wolfhound · Post by **wolfhound** » Mon Jan 22, 2007 9:11 am

I must admit that the 1st set of pistons I used were aftermarket KS pistons for the Opel 1600 and as such had a HUGE machined slot under the oil control rings, Those are the ones that lasted 30 odd kays (Yup KM's

)

The second set that went bang after 400 were the Std Woo ones - no slot for oil feed and generally alot stronger.

I must admit I haven't stripped the engine since I blew the headgasket and deckplate but am quite curious to see how these pistons (std woo running 7.5) lasted.

Chris, you seem quite clued up on the turbo thing. I have been told to run a bigger ring gap of 0.6-1.0mm is that true ?

Séan