21 replies to this topic

[jpc]

New pistons are cheap enough, so no point buying second hand.

As a rule of thumb, the standard 295 head on a 998 with flattops will give 9.1:1 CR. So if you can find 28cc one way or another then you'll have a nice CR for 12psi boost.

If you go +20 (1014cc) you have 9.2:1, and +40 (1030cc) 9.3:1, still fine for 12psi.

I would advise going the the 295 route, they work very well on turbo setups. If you can't find an un-skimmed one, then you can buy flattop pistons and get a slight dish machined in them. On of my 998 turbos has 26cc chambers, so I got 2cc machned in the pistons.

I got my hands on a 12G295 head and i think it has the origional 28cc chambers. But when i tried to get the cc myself i kept getting slightly more, around 29-30cc, im assuming this is because the inlet valves seem to be sitting a bit low in the chamber, this is the only logical explanation i can get or else im getting the cc test wrong. But im hoping fitting the hardned valve seats will allow me to bring it flush again. Will the valves in it be up to the job for a turbo, as iv read somewhere that its better to fit stainless exhaust ones.Is boring it out by +20 or +40 a big advantage .i was thinking of waiting till i have the engine stripped to decide what to do and then i can see what sort of wear there is in the cyclinders and if its bad ill be boring it out anyway. But if they are good would the standard pistons i take out do with new rings.

Is 28cc the ideal chamber volume or does it depend on boost, i only reason i dont want to go mad with boost is that i want to keep it as reliable as possible. Does increasing the boost reduce reliability.

[jpc]

i would never use second hand pistons.
if you have a look online there are loads of comp ratio calculators.

Bump

[Cooperman]

New pistons are cheap enough, so no point buying second hand.

As a rule of thumb, the standard 295 head on a 998 with flattops will give 9.1:1 CR. So if you can find 28cc one way or another then you'll have a nice CR for 12psi boost.

If you go +20 (1014cc) you have 9.2:1, and +40 (1030cc) 9.3:1, still fine for 12psi.

I would advise going the the 295 route, they work very well on turbo setups. If you can't find an un-skimmed one, then you can buy flattop pistons and get a slight dish machined in them. On of my 998 turbos has 26cc chambers, so I got 2cc machned in the pistons.

I got my hands on a 12G295 head and i think it has the origional 28cc chambers. But when i tried to get the cc myself i kept getting slightly more, around 29-30cc, im assuming this is because the inlet valves seem to be sitting a bit low in the chamber, this is the only logical explanation i can get or else im getting the cc test wrong. But im hoping fitting the hardned valve seats will allow me to bring it flush again. Will the valves in it be up to the job for a turbo, as iv read somewhere that its better to fit stainless exhaust ones.Is boring it out by +20 or +40 a big advantage .i was thinking of waiting till i have the engine stripped to decide what to do and then i can see what sort of wear there is in the cyclinders and if its bad ill be boring it out anyway. But if they are good would the standard pistons i take out do with new rings.

Is 28cc the ideal chamber volume or does it depend on boost, i only reason i dont want to go mad with boost is that i want to keep it as reliable as possible. Does increasing the boost reduce reliability.

If rebuilding for increased performance it is always wise to re-bore and fit new pistons to gain some decent reliability. Get top quality pistons for turbo application and do the trial build, calculations, then any machining to get everything right.
Don't try to 'short-cut' if you want a good and reliable engine. Remember 'Cheap on price - do it twice!'
The 295 head does have slightly small inlets for an FI engine. Maybe a pocketed block and a 12G940 head would be better, although you'll need to enlarge the combustion chambers quite a lot and possibly skim the tops of the pistons to get the CR down. You could just put larger inlet valves in the 295, but it may then be difficult to fit unleaded seats. A different distributor with a slower advance curve is absolutely vital.
A turbo engine needs careful building with much thought and calculation to get it right. If it's done incorrectly it may just 'blow-up' (seen it before). The higher the boost, the more potential power and the less the reliability, which is just STFO (stating the ******* obvious).

[mini13]

Hello JPC,

first off, its good that you are asking questions, and the right ones at that, you have obviously done some digging and thought about this

the trouble wih most of the compression ratio calculators on the interweb is they assume a round head gasket, which the mini does not have,

to work out the compression ratio from scratch you need to know two things, the volume above the piston at TDC, and the volume in the cylinder between the top and bottom of its stroke,
you then add them together and divide by the volume above the piston at TDC, easy!



the volume above the piston at TDC is just the head chamber volume + the piston dish + the headgasket volume + the deck height volume.

the volumes of the cylinder and the deck height are both calculated the same way as they are just cyliders, so you can use pi* radius squared * height.

for the headgasket, just use the minispares figure, I'd use the gasket below,

http://www.minispare...=...2509&title=


I would aim for a fairly high compression ratio (for a turbo engine) if you are not going to use any more than 10 psi, i'd go as high as 9:1, and wouldnt go below 8.5:1 (I run 8.5:1 on a 1380 upto 18 psi)

Once you run through the caclulations you should see that flat topped pistons and no deck height give you about 8.7:1, which IMO is a good figure. conviniently the std 998 flat top A+ piston is sturdy and cheap i have known them to be used at 20psi reliably.

http://www.minispare...=...7841&title=


for the rest of the engine you dont need anything special, just rebuild it with std parts and tolerances as per the haynes manual, and check for wear as you go.

camshaft wise the std metro cam is good a good upgrade over the 998 one,

The fourmula seems simple enough but i seem to be getting different figure to you. Must be doing something wrong.
These are my calculations

chamber size - 28.3cc
head gasket - 2.8cc
Deck height - 0cc
Piston dish - 0cc
Volume of cyclinder - 998 / 4 = 249.5

So
28.3 + 2.8 = 31.1
(249.5 + 31.1) /31.1 = 9.0:1

Yep, your figures are right, I had an extra cc for the volume down the side of the pistons above the ring.

[jpc]

Hello JPC,

first off, its good that you are asking questions, and the right ones at that, you have obviously done some digging and thought about this

the trouble wih most of the compression ratio calculators on the interweb is they assume a round head gasket, which the mini does not have,

to work out the compression ratio from scratch you need to know two things, the volume above the piston at TDC, and the volume in the cylinder between the top and bottom of its stroke,
you then add them together and divide by the volume above the piston at TDC, easy!



the volume above the piston at TDC is just the head chamber volume + the piston dish + the headgasket volume + the deck height volume.

the volumes of the cylinder and the deck height are both calculated the same way as they are just cyliders, so you can use pi* radius squared * height.

for the headgasket, just use the minispares figure, I'd use the gasket below,

http://www.minispare...=...2509&title=


I would aim for a fairly high compression ratio (for a turbo engine) if you are not going to use any more than 10 psi, i'd go as high as 9:1, and wouldnt go below 8.5:1 (I run 8.5:1 on a 1380 upto 18 psi)

Once you run through the caclulations you should see that flat topped pistons and no deck height give you about 8.7:1, which IMO is a good figure. conviniently the std 998 flat top A+ piston is sturdy and cheap i have known them to be used at 20psi reliably.

http://www.minispare...=...7841&title=


for the rest of the engine you dont need anything special, just rebuild it with std parts and tolerances as per the haynes manual, and check for wear as you go.

camshaft wise the std metro cam is good a good upgrade over the 998 one,

The fourmula seems simple enough but i seem to be getting different figure to you. Must be doing something wrong.
These are my calculations

chamber size - 28.3cc
head gasket - 2.8cc
Deck height - 0cc
Piston dish - 0cc
Volume of cyclinder - 998 / 4 = 249.5

So
28.3 + 2.8 = 31.1
(249.5 + 31.1) /31.1 = 9.0:1

Yep, your figures are right, I had an extra cc for the volume down the side of the pistons above the ring.

O rite, i get it now. didnt realise how much of a difference a cc made,
Cheers

[Wil_h]

New pistons are cheap enough, so no point buying second hand.

As a rule of thumb, the standard 295 head on a 998 with flattops will give 9.1:1 CR. So if you can find 28cc one way or another then you'll have a nice CR for 12psi boost.

If you go +20 (1014cc) you have 9.2:1, and +40 (1030cc) 9.3:1, still fine for 12psi.

I would advise going the the 295 route, they work very well on turbo setups. If you can't find an un-skimmed one, then you can buy flattop pistons and get a slight dish machined in them. On of my 998 turbos has 26cc chambers, so I got 2cc machned in the pistons.

I got my hands on a 12G295 head and i think it has the origional 28cc chambers. But when i tried to get the cc myself i kept getting slightly more, around 29-30cc, im assuming this is because the inlet valves seem to be sitting a bit low in the chamber, this is the only logical explanation i can get or else im getting the cc test wrong. But im hoping fitting the hardned valve seats will allow me to bring it flush again. Will the valves in it be up to the job for a turbo, as iv read somewhere that its better to fit stainless exhaust ones.Is boring it out by +20 or +40 a big advantage .i was thinking of waiting till i have the engine stripped to decide what to do and then i can see what sort of wear there is in the cyclinders and if its bad ill be boring it out anyway. But if they are good would the standard pistons i take out do with new rings.

Is 28cc the ideal chamber volume or does it depend on boost, i only reason i dont want to go mad with boost is that i want to keep it as reliable as possible. Does increasing the boost reduce reliability.

If rebuilding for increased performance it is always wise to re-bore and fit new pistons to gain some decent reliability. Get top quality pistons for turbo application and do the trial build, calculations, then any machining to get everything right.
Don't try to 'short-cut' if you want a good and reliable engine. Remember 'Cheap on price - do it twice!'
The 295 head does have slightly small inlets for an FI engine. Maybe a pocketed block and a 12G940 head would be better, although you'll need to enlarge the combustion chambers quite a lot and possibly skim the tops of the pistons to get the CR down. You could just put larger inlet valves in the 295, but it may then be difficult to fit unleaded seats. A different distributor with a slower advance curve is absolutely vital.
A turbo engine needs careful building with much thought and calculation to get it right. If it's done incorrectly it may just 'blow-up' (seen it before). The higher the boost, the more potential power and the less the reliability, which is just STFO (stating the ******* obvious).

Just to confirm, use 33in/29ex valves on either the 295 or 940 for best results. It's no problem fitting unleaded seats in the 295 with these valves.

I use the minispares Mega pistons and they have survived so far at 180bhp, so they may be cheap, but they work. sad thing is these are NLA, the replacement is a powermax to a similar spec I believe.

And don't even bother with a dizzy, chuck it in the bin and use megasquirt/jolt or similar.

[jpc]

New pistons are cheap enough, so no point buying second hand.

As a rule of thumb, the standard 295 head on a 998 with flattops will give 9.1:1 CR. So if you can find 28cc one way or another then you'll have a nice CR for 12psi boost.

If you go +20 (1014cc) you have 9.2:1, and +40 (1030cc) 9.3:1, still fine for 12psi.

I would advise going the the 295 route, they work very well on turbo setups. If you can't find an un-skimmed one, then you can buy flattop pistons and get a slight dish machined in them. On of my 998 turbos has 26cc chambers, so I got 2cc machned in the pistons.

I got my hands on a 12G295 head and i think it has the origional 28cc chambers. But when i tried to get the cc myself i kept getting slightly more, around 29-30cc, im assuming this is because the inlet valves seem to be sitting a bit low in the chamber, this is the only logical explanation i can get or else im getting the cc test wrong. But im hoping fitting the hardned valve seats will allow me to bring it flush again. Will the valves in it be up to the job for a turbo, as iv read somewhere that its better to fit stainless exhaust ones.Is boring it out by +20 or +40 a big advantage .i was thinking of waiting till i have the engine stripped to decide what to do and then i can see what sort of wear there is in the cyclinders and if its bad ill be boring it out anyway. But if they are good would the standard pistons i take out do with new rings.

Is 28cc the ideal chamber volume or does it depend on boost, i only reason i dont want to go mad with boost is that i want to keep it as reliable as possible. Does increasing the boost reduce reliability.

If rebuilding for increased performance it is always wise to re-bore and fit new pistons to gain some decent reliability. Get top quality pistons for turbo application and do the trial build, calculations, then any machining to get everything right.
Don't try to 'short-cut' if you want a good and reliable engine. Remember 'Cheap on price - do it twice!'
The 295 head does have slightly small inlets for an FI engine. Maybe a pocketed block and a 12G940 head would be better, although you'll need to enlarge the combustion chambers quite a lot and possibly skim the tops of the pistons to get the CR down. You could just put larger inlet valves in the 295, but it may then be difficult to fit unleaded seats. A different distributor with a slower advance curve is absolutely vital.
A turbo engine needs careful building with much thought and calculation to get it right. If it's done incorrectly it may just 'blow-up' (seen it before). The higher the boost, the more potential power and the less the reliability, which is just STFO (stating the ******* obvious).

That was a bit of a stupid question alrite. ha.

I think ill stick with the 12g295, i supose i could always just fit larger inlet valves and forget about the hardened valve seats and just run with additive, and what about the exhaust valves, do you think they would be large enough as they are.
I know i have alot of thinking to do and loads of research to do too but just want to get the gist of most things so that i know where i going