4350 replies to this topic

[chris_uk]

Lower comp pistons mean we can run higher boost safer. Also means the engine isnt as stressed. Matrs right tho, lower compression = lower power

[ArticMonkey]

Thats about right, the pistons won't add any power.

If you've fitted lower compression pistons, they actually drop the power.

[Mike (Cliffie)]

Thats about right, the pistons won't add any power.

If you've fitted lower compression pistons, they actually drop the power.

Not at all. The lower comp pistons allow higher boost which will in turn give higher power. My issue isn't pistons, it is my cams that are letting me down restricting intake duration at the top end.

[techieboy]

Instantly dropped the power of my original high compression conversion 12bhp moving to low compression pistons despite being remapped with them. What it did mean was being able to run more ignition advance and gave a bigger safety margin. Power only increased again with smaller pulley, different injectors and new stoopid exhaust.

[Mike (Cliffie)]

Instantly dropped the power of my original high compression conversion 12bhp moving to low compression pistons despite being remapped with them. What it did mean was being able to run more ignition advance and gave a bigger safety margin. Power only increased again with smaller pulley, different injectors and new stoopid exhaust.

Which is the full stage three conversion Techie. You are on a 2.9 I assume?

[techieboy]

Indeed I am, the screaming little b4st4rd.

[siztenboots]

if there weren't other silly bhp in the class, then just stage 2 would have been sweet.

[Mike (Cliffie)]

if there weren't other silly bhp in the class, then just stage 2 would have been sweet.

Agreed Steve and Jimmy won class last year with 263BHP so proof that it is just about right. Sadly everyone has chased power and as much as I can get the thing round corners and brake late and get the power on early, there are a good few long straights where lack of puff will be the points breaker such as Snetterton, Cadwell and Anglesey.

[luke.]

Steve Fitzpatrick in his m3 hasn't touched the engine at all so will also be interesting to see how he gets on. Hopefully the guys who have chased big numbers have problems with reliability.

[Exmantaa]

Instantly dropped the power of my original high compression conversion 12bhp moving to low compression pistons despite being remapped with them. What it did mean was being able to run more ignition advance and gave a bigger safety margin. Power only increased again with smaller pulley, different injectors and new stoopid exhaust.

Low CR & Eaton Supercharger is not a very good combination as Techie experienced... Better keep the CR around 9.5 and the SC boost not too high. (Avoid SC power loss + high temps.)
And if you fitted hotter cams that keep the inlet valves longer open (lowers dynamic CR) it may be wise to go 10.0 CR.

But you do need a good mappable ecu to tweak the ignition tables...

[VX-GT]

Can somebody confirm standard pulley dia I have seen on US forums quoting 3.35 and UK forums quoting 3.2

[The Batman]

3.35

[VX-GT]

Instantly dropped the power of my original high compression conversion 12bhp moving to low compression pistons despite being remapped with them. What it did mean was being able to run more ignition advance and gave a bigger safety margin. Power only increased again with smaller pulley, different injectors and new stoopid exhaust.

Low CR & Eaton Supercharger is not a very good combination as Techie experienced... Better keep the CR around 9.5 and the SC boost not too high. (Avoid SC power loss + high temps.)
And if you fitted hotter cams that keep the inlet valves longer open (lowers dynamic CR) it may be wise to go 10.0 CR.

with some simple calcs

If one lowers CR from 10:1 down to 8.9:1

to negate this the pulley is reduced to 3.0 that returns you to status quo

So 2.9 pulley is only a 3% increase (almost linear growth)

I agree with Exmantaa Infact I think with reduced dynamic CR due to Cams, it may be advantageous to Raise CR to over standard 10:1 as we know that standard (10:1) is acceptable

Edited by atom x, 27 April 2012 - 06:40 PM.

[The Batman]

lee had standard compression and a 3" pulley and got exactly the same power although was blowing map sensors left right and centre so since moving down to low comp and a 2.9 it is now the same bhp but not blowing map sensors (yes i know the simple thing would have been to get a bigger map sensor but then it cant be mapped to it ) my CR is 9.5:1

[VX-GT]

lee had standard compression and a 3" pulley and got exactly the same power

same power being?

altering CR should not affect MAP as it does not alter Manifold pressure's

Edited by atom x, 27 April 2012 - 06:51 PM.

[The Batman]

im on the bigger charger, havent tried it with standard m62

[Exmantaa]

Instantly dropped the power of my original high compression conversion 12bhp moving to low compression pistons despite being remapped with them. What it did mean was being able to run more ignition advance and gave a bigger safety margin. Power only increased again with smaller pulley, different injectors and new stoopid exhaust.

Low CR & Eaton Supercharger is not a very good combination as Techie experienced... Better keep the CR around 9.5 and the SC boost not too high. (Avoid SC power loss + high temps.)
And if you fitted hotter cams that keep the inlet valves longer open (lowers dynamic CR) it may be wise to go 10.0 CR.

with some simple calcs

If one lowers CR from 10:1 down to 8.9:1

to negate this the pulley is reduced to 3.0 that returns you to status quo

So 2.9 pulley is only a 3% increase (almost linear growth)

I agree with Exmantaa Infact I think with reduced dynamic CR due to Cams, it may be advantageous to Raise CR to over standard 10:1 as we know that standard (10:1) is acceptable

Not complete Status Quo, as the smaller pulleyed SC takes now more power to spin (less crank HP) and the higer boost/revs pushes it into a less efficient zone (higer intake temps).
But it's tricky trade-off... => Use E85 or Meth injection and you can still run ~25* advance on a high-ish compression.

[Bargi]

Instantly dropped the power of my original high compression conversion 12bhp moving to low compression pistons despite being remapped with them. What it did mean was being able to run more ignition advance and gave a bigger safety margin. Power only increased again with smaller pulley, different injectors and new stoopid exhaust.

Low CR & Eaton Supercharger is not a very good combination as Techie experienced... Better keep the CR around 9.5 and the SC boost not too high. (Avoid SC power loss + high temps.)
And if you fitted hotter cams that keep the inlet valves longer open (lowers dynamic CR) it may be wise to go 10.0 CR.

with some simple calcs

If one lowers CR from 10:1 down to 8.9:1

to negate this the pulley is reduced to 3.0 that returns you to status quo

So 2.9 pulley is only a 3% increase (almost linear growth)

I agree with Exmantaa Infact I think with reduced dynamic CR due to Cams, it may be advantageous to Raise CR to over standard 10:1 as we know that standard (10:1) is acceptable

I have low compression pistons and 3 pulley inch = 255bhp (required VXR Injectors)
Sutol originally std internals 249bhp, changed to low compression went to CMS an no difference in BHP or map

[The Batman]

your exhaust is hugely restrictive

[Bargi]

your exhaust is hugely restrictive

true, nowt wrong with my Milltek Straw and std manifold