What compression??
Turbo Mini Forums

I am currenty in the process of striping a metro turbo block down with the intension of rebuilding it.

The current spec that im looking at is:
1293
Phase 2 Turbo cam
ultralight flywheel & backplate
titan 1.5 roller rockers
s/c gearbox

basicly im wanting around 160-180+bhp, to get these kind of figues what cc combustion chambers will i need and what dish would be best on the pistons, I was looking at maybe a BENROSS high boost head and accralite pistons but on avonbar website they state 3 different dishes, any ideas?

what kind of boost would I need to achieve this bhp?

Ive been doing some searching around on here, but not found what im looking for can anyone help me?

Cheers

Matt

my choice would be head with 28cc chambers

and why not make it a 1380 using forged accralite 73.5mm pistons with 18cc dished there the same price as the other sizes from AVONBAR
compression would be around 8.3:1

this togeather with all the other good guy bits .....would make a very powerfull motor if set up correctley
running around 14psi of boost will see "no sweat"! ....180 BHP +...

More than enough to have fun with

Edited by BENROSS on 10th Dec, 2007.

Cheers, Thats got me thinking!!

Matt

Bore the block to the next size. Then the block won't be scrap in 10000km or one piston failure... Ok 1380s may give more power with no more grief, but they still render the block useless in almost no time. Blocks ARE getting more expensive...
I would NEVER fit oil squirters or have a 4bolt on a 1380. To much hassle and expense for 10000kms. Not for 30`!

Edited by Vegard on 10th Dec, 2007.

you have a point V

but........... 6000 miles ia a shade short

more like 20k +

Don't forget to add SC drop gears and a X pin diff to your shopping list.

In the 13's at last!.. Just

Megajolt is also worth considering.

Oh and the "cake" as BENROSS calls it...lol AKA intercooler!

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1/4mile in 13.2sec @ 111 terminal on 15psi

Cheers thanks for all the help, i'll just have to decied wether to go 1310 or 1380 now!?

Matt

1310 every time if you can. 1380 is for later!

Cheers for all that info ive decided on 1310 I think, with the spec below:

Accralite Pistons 0.040 18cc
Avonbar Phase 2 Turbo cam
ultralight flywheel & backplate
titan 1.5 roller rockers (is this the right choice??)
s/c gearbox, s/c drops & x Pin diff
Benross Hi-boost head 28cc
Intercooler

What kind of power would I be looking at running about 12Psi of boost? and what would I be able to run upto

Am I right in thinking the lower compression ratio the less lag, i dont want it to be slow off the mark before the turbo kicks in.

what kind of Compression ratio would this set-up give me

just want to get everything right before I start!

Cheers Matt

Lag depends on the size of the turbo mainly, the compression ratio has some effect.
You are better off deciding upon a CR and then working backwards to dictate what components e.g. piston cc head cc you need.
28cc head and 18cc piston roughly gives 7.5:1, this is FAR too low.

am i right in thinking that at 12psi you want to be between 9:1 and 8.5:1 as a rough guide?

think i read it on here a few days ago, and it was one of you posts Tom.

James.

That is something that will work for sure, there are no hard and fast rules, at the end of the day its your engine and you make your own choices. My engine is 8.02:1 compression ratio, but I want to be able to run BIG boost.

Just been reading a few earlier posts, alot of people seem to be running about 8.3.1 at 16psi, that sounds like a good starting point for me.

Basicly I want to embarrass Impreza Turbo's & Evo's!!

Found this compression chart while I was looking too, just thought i'd post it again

Matt

Attachments:

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yeh i think you said in the post its just a rough guide.

a car running 15psi of boost on a CR of 9:1 will drive nicer/more responsive off boost but will put more strain on the components used than a car running 15psi at 8.5:1.

its just getting a decent balance which (through searching this forum) seems to be a guide of 20psi = 8:1 and 10psi = 9:1

Megajolt can help reduce lag as it allows you to advance the ignition alot more off boost then retard it when on boost.

What turbo have you decided to use?

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1/4mile in 13.2sec @ 111 terminal on 15psi

Tom why is 7.5:1 too low ? thats what mine is .

Does that mean i can run silly silly boost

Rick

7.5:1 will make it very sluggish off boost, ask Jimster.

mines running 8.7:1 @ 15psi, kept it fairly high cos it's a daily driver.

Hi,
I think I'm right in saying the higher the compression for the same amount of boost will give you more power.
The downside is you have to have precise control of timing, mixture, boost and intake air temperature to keep it all together!
Higher compression will also make the off / on boost transition smoother which makes it easier for the tyres to keep a grip
Cheers,
Gavin :)

Edited by Bat on 20th Dec, 2007.

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My Mini build diary

I'm Goona be running the standard Metro Turbo, the one I have seems to have no play in it what so-ever, which is good!!

Was thinking about running 8:3.1 but i might try and get it a bit higher than that so its more powerful off boost, maybe 8:6.1 is better

Mine will be 8.4:1 once Mr Benross has done the head.

In the 13's at last!.. Just

What cc head and pistons you using?

If my calculations are right to run 8.3:1 I will need 14cc pistons(accralite) and 24.5cc head or 23cc head for 8.6:1