21 replies to this topic

[jpc]

Hey,
I am in the process of beginning to collect bits and pieces for my 998 turbo build and i came across second hand pistons on the net,

http://cgi.ebay.co.u...=item3cb93fff56

they seem to be in good condition and cheap enough. I was wondering if these pistons would be suitable for my 998 turbo build because i dont know what kind of CR i would get from them. i have a 12G295 with 28cc chambers but dont know how to figure out the compression ratio these would give me and the engine is still in the car so cant go measuring anything. Would the 6cc dish on the pistons bring the CR too low. I want the engine to be as reliable as possible so dont plan on goin mad with boost.

Also what engine cc would a +20 pistons give me. Would i be better off sticking with the standard pistons and bore. And is getting the cyclinder bored out very expensive.

Cheers
JP

[ANON]

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

[new_zealand _minis]

i agree dont buy second hand ones .. maby if it was a non turbo std road motor . but nothing else

there is a comp ratio calculator on itunes free app for ipod iphone etc . real easy to use .....

i dont think it will be to low

over boaring is not thant expensive . just get some new 6cc dish +20 and get it boared

[jpc]

Ill stay clear of used pistons so. Im in no rush with them anyway as the engine is still in the car and ive got loads of body work to do yet. I just want to know exactly what im going to do so i can be looking out for bits.

Still trying to figure out the compression ratio,I tryed this site (http://www.csgnetwor...m/compcalc.html) but dont know some of the information needed like Head gasket tickness and deck clearance. I'm assuming the head gasket bore size is the same as the bore size of the cyclinder.
Also how would i find out the bore size if i was to rebore the engine by +20 or +40. Does increasing the bore size give good power gains?

What kind of compression ratio would i be looking for for a turbo mini. I dont plan on going mad with boost and i want to make it as reliable as possible so would a slightly lower cr be better. Around 8.5 : 1 at 10 psi. or 9.0 : 1 at 10 psi. (getting these figures form here- http://www.turbo-mini.com/70947.html)

[jpc]

Anyone know what to allow for deck clearance and head gasket tickness when calculating compression ratio.

Edited by jpc, 24 June 2011 - 06:00 PM.

[jpc]

bump

[ministudio1275]

i've got a few turbo bits. pm me if interested

[PhilipGCaldwell]

agree with the above, if its turbo, buy new pistons
the dish is prob about 6cc so take that into account on a CR calculator

[Cooperman]

Anyone know what to allow for deck clearance and head gasket tickness when calculating compression ratio.

Piston top to block deck level has to be measured when you do the trial build prior to final machining and final assembly. It can vary by a lot depending on piston type, whether it's been 'decked' before, manufacturing tolerances on crank and rods, etc. Then you can calculate the volume required in the combustion chambers and make that correct.
For gasket volume allow about 3.2 cc on a 998.
Don't forget that when going to forced induction you must have a distributor with a slower advance curve or you'll get 'detonation' and blow the pistons apart.

[jpc]

Anyone know what to allow for deck clearance and head gasket tickness when calculating compression ratio.

Piston top to block deck level has to be measured when you do the trial build prior to final machining and final assembly. It can vary by a lot depending on piston type, whether it's been 'decked' before, manufacturing tolerances on crank and rods, etc. Then you can calculate the volume required in the combustion chambers and make that correct.
For gasket volume allow about 3.2 cc on a 998.
Don't forget that when going to forced induction you must have a distributor with a slower advance curve or you'll get 'detonation' and blow the pistons apart.

What i had planned to do was to get pistons with the correct dish to give me the CR i would be looking for and leaving the head as it is, As im assuming it lower if anything ill be making the CR. Im no expert so correct me if im wrong. But from what i have read most people who turbo a 998 get an untouched 12G295 with the 28.3cc chamber (which i have) and just use standard flat top pistons and run at 10psi boost but was tinking of lowerning the CR slightly to be safe with the same boost level or is this necessary.

And as for timing, i plan on going with a megajolt or megasquirt setup so getting the correct advance shouldnt be a problem.

[Cooperman]

It's up to you, of course, but I would never, ever build an engine without doing a 'trial-build', measuring everything, doing the calculations and then getting the final machining done in order to ensure that everything is matched and correct.
I had a 1275 turbo engine brought to me which a so-called 'expert' had built. It did 4 miles before it blew up and, as it was a cash deal apparently, the builder didn't want to know. I had to completely re-build it with the correct newdished Omega pistons, the correct C.R. and the turbo re-calibrated with a certificate confirming the max boost. It cost a lot but was/is correct now and goes very well with around 117 bhp at 6.5 psi boost.
You really can't afford to just 'guess' at the figures or go on heresay if you want it correct.

[jpc]

It's up to you, of course, but I would never, ever build an engine without doing a 'trial-build', measuring everything, doing the calculations and then getting the final machining done in order to ensure that everything is matched and correct.
I had a 1275 turbo engine brought to me which a so-called 'expert' had built. It did 4 miles before it blew up and, as it was a cash deal apparently, the builder didn't want to know. I had to completely re-build it with the correct newdished Omega pistons, the correct C.R. and the turbo re-calibrated with a certificate confirming the max boost. It cost a lot but was/is correct now and goes very well with around 117 bhp at 6.5 psi boost.
You really can't afford to just 'guess' at the figures or go on heresay if you want it correct.

Im sure you know better than me, i havent even stipped down an engine before so its going to be fun trying to figure out all these measurements and calculations,
its definiately going to be a challange but wouldnt mind giving it a go. I dont plan on using it as a daily driver but still want it as reliable as possible,
The biggest problem is my budget, want to keep costs as low as possible.
Anyway im a long way from actually tackling the build so plenty time to do some more research to give me a better idea what to do when the time comes.
I supose theres no set template for turbo builds as every engine is going to be slightly different,
You said you went with dished omega pistons, how did you know what dish to go with, i no you did a certain bit of maching to get everything perfect after. Do you do some rough calculations first to give you the CR your looking for and then do the fine tunning after.
Any ideas what sort of CR would suit my engine at 10psi boost (with intercooler).
Appreciate the advice,

[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,

[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.

[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