9ff 944tt

[John Sims]

Ah! But then it's a 16 valve head so I guess it's one valve from each cylinder to turbo one and one valve from each cylinder to turbo two.

Very cunning.

And... as the valves are smaller the gas velocity will be higher.

.....but only half as much gas to turn the turbo.

This is obviously a "suck it and see" situation to find out if it realy works or not.

 

[Mark_Bennett]

ORIGINAL: eastendr

ORIGINAL: Mark Bennett

ORIGINAL: Hilux

OOOhhhh I love the wheels, they look like the ones on Ricks car??? What are they?

They are just the 18" 5-spoke carrera wheels off the 996/Boxster

Mark, I believe the boxster versions are much narrower. Probably best to keep them as 'space saver spares'

ROFLMFARO [] [] [] []

(that'll teach you not to have bribed me with a pint last night at the Dysart [] [])

I nearly added - no offence to Rick's wheels...[]

You are correct - the Boxster ones are an inch narrower at the back as - like the 944, they don't need to compensate for all the weight being in the wrong place...[]

I'll buy you another lemonade next month [8D]

 

[eastendr]

Ta, mate []

 

[944Turbo]

ORIGINAL: John Sims

ORIGINAL: Mark Bennett

They are just the 18" 5-spoke carrera wheels off the 996/Boxster

I wait with baited breath to see how Tony is going to sell the idea of spending 20,000 Euros to Mrs 944Turbo; particularly for a power hike of 20 bhp on what he has currently. []

I appreciate that the more power you get the more additional power costs but that is pretty ridiculous.

My car won't make 350Bhp at 13Psi add 5 psi to that we could make say another 75Bhp with a better torque curve then it could be justified - at least after the gearbox has been rebuilt and the body sorted []

 

[sawood12]

ORIGINAL: 944Turbo

Lower compression piston,

Would this work?? Surely to reduce compression ratio you need to change the stroke of the piston and not the volume of the combustion chamber at TDC?? If you simply machine a pocket in the piston crown you will just fill this extra volume during the induction stroke which will be compressed by the same amount dictated by the stroke of the engine. Maybe 0.9 bar boost and 375hp is the max you can achieve with std S2 compression ratio and they don't intend to increase it anymore - unless of course they have shortened the stroke.

 

[Fen]

Pockets in the pitson crown (or head) make a difference to compression ratio. For example imagine your cylinder is 10:1 compression with 200cc at BDC and 20cc at TDC. Take out a pocket of 10cc in the top of the piston crown and your cylinder becomes 210cc at BDC and 30cc at TDC or 7:1.

edited to correct a typo that sort of disproved the theory - well done to Scott for spotting it!

 

[Hilux]

Surely to reduce compression ratio you need to change the stroke of the piston and not the volume of the combustion chamber

You are right on both counts however its the amount of reduction you need that dictates the method.

You end up either way with a swept volume that is larger allowing more air under pressure greater than atmospheric which when compressed gives the correct fuel/air ratio and compression pressures.

Turbo engines are around 8:1 whereas n/a engines are 10-12ish and diesels 18-20:1 ish (I think) so it doesnt take much to reduce the ratios hence dished pistons are easier and cheaper.

 

[eastendr]

Fen's explanation is spot on

It's quite usual to cut valve pockets in piston crowns when running bigger valves or higher lift with a high compression engine.

Rick

 

[Diver944]

They should have just bought some shiny JE pistons and Carillo rods like I did []

 

[Mark_Bennett]

ORIGINAL: Diver944

They should have just bought some shiny JE pistons and Carillo rods like I did []

Ohhhhh - hope you haven't done anything that will get them dirty...[]

 

[sawood12]

ORIGINAL: Fen

Pockets in the pitson crown (or head) make a difference to compression ratio. For example imagine your cylinder is 10:1 compression with 200cc at BDC and 20cc at TDC. Take out a pocket of 10cc in the top of the piston crown and your cylinder becomes 210cc at BDC and 20cc at TDC or 10.5:1.

With a 10cc pocket in the piston crown surely you'd end up with 210cc at TDC and 30cc at BDC or else where does your 10cc pocket go at BDC? In this case you still have the same difference in capacity at TDC and BDC i.e. 180cc but squeezed the same amount, so with the pocket all you've done is to increase the capacity of your engine but maintain the same compression ratio.

 

[Fen]

It's a typo, should be 30cc at TDC. Correceted now.

 

[Hilux]

In this case you still have the same difference in capacity at TDC and BDC i.e. 180cc but squeezed the same amount, so with the pocket all you've done is to increase the capacity of your engine but maintain the same compression ratio.

True, but with a turbo you have `say` 2 bar pressure (1 bar over atmospheric) into the cylinder ie; more air under pressure so the swept volume is the same but there is a larger amount of compressed air in the cylinder before the compression stroke start to achieve the correct compression uopn firing. More air means more molecules for fuel to cling to hence more power.

I think []

 

[Fen]

Basically it's a ratio of volume at BDC vs volume at TDC. If you pocket the piston to add more volume that is a constant at all piston positions and therefore it makes more percentage difference to the TDC volume than the BDC volume, hence the ratio is reduced.

My typo in the original explanation didn't help as pocketing the piston seemed to increase CR, but it's fixed now.

 

[eastendr]

This is becoming very 'open university'

No further contributions please, unless you've got a kipper tie, wing collars and flares. Oh, and sideburns and a mullet would definately help [] [] []

Rick.

 

[sawood12]

Drat, I tick all those boxes apart from the mullet! If I stitch some leather elbow patches to an old chequered blazer does that count instead? Come to think of it I could get the wife to make up a blazer from the chequered material off an old 944 seat.

 

[eastendr]

I bet you could sell HUNDREDS of those in the 'retro' area of e-bay

[] [] [] [] []