Low compression pistons
 

Whats the advantage of low compresion piston?

Cheers!

You can run more boost for more power.
tabetha

don't go to low as there crap off boost , i run 2. bar of boost from a T38 on my car with a comp of 8.2:1, kin awesome !!!

get good pistons, most good tuners don't go low comp anymore as its an old school way of reducing the risk of detonation. most good tuners have solved the problems with all the new technology we have today...

i know of a few cars that run big boost with higher / std comp now days..

unless you cant find a good tuner to set it up or you are going crazy with the boost, keep it std comp with good quality pistons

Yeah dont want any less power off boost! laggy enough as it is :banana:

So what psi can you squeeze out of a standard T3? (reliably!)

Reliably ?, I would GUESS around 17/18psi.
The T3 was never designed to boost much more than they do std, as they do not ahve a 360 bearing or step gap seal, they are a double overhang design, good for what it is but not ace with more expected from it.
With 360 bearing 25/28psi all day long.
Comp has nothing to do with lag, rpm dictates this, a ROLLER BEARING will have virtually ZERO lag, due to it not having so much friction to overcome.
There is no magic potion to prevent detonation.
The biggest consideration to CR is the effeciency of the engine.
Another consideration is the ADIABATIC effeciency of the i/cooler, basically the cooler you can keep the charge the more igntion you can run, all things being equal.
tabetha

Sorry i didnt mean lag in the turbo sense of the word, i meant with the pistons already beeing lower comp than NA CVH engines its already slow enough from standstill! So lowering the comp again must make it proper slow until on boost!

"Comp has nothing to do with lag, rpm dictates this, a ROLLER BEARING will have virtually ZERO lag, due to it not having so much friction to overcome. "

yea right !! i have driven cars with almost identical spec`s one on 7.2:1 and the other on 8.5:1 and the difference is shocking !!!
roller bearings "help" to reduce lag but don't stop it !!
you need a variable vane turbo for that !!

having a lower CR WILL give it more lag as you reduce the aspirated side of the power band, that's old school knowledge !!!

as i said in my first post, lower comp USED to be used to reduce the chance of detonation

old school trick that's well out of date now days !!!!

DENNIS

You are wrong on so many levels ;)

Is it any wonder two cars are different on different specs by your own admission ?, ie virtually is not the same is it ?
The air standard effeciency of a 7:1 CR is 54.1 versus 56.5 for a 8:1, and you can notice this, have you not stopped to think that therreason you can feel something is because the lower cr one was running more boost, as this is what "old school" used to do ?
As I did not use the word ELIMINATE, what is your point ?
A sleeve bearing(normal type) has a typical bearing loss of 800 WATTS, ie it takes 800 WATTS of energy just to rotate it.
A ROLLER BEARING one takes 200 WATTS.
The shaft movement at 150,000rpm is typically 0.025mm for the SLEEVE BEARING versus 0.13-0.25mm for the ROLLER BEARING.
The FLOATING(SLEEVE) bearings DO NOT ROTATE AT SLOW SPEED at all, but the ROLLER NEARINGS ONES DO, so VIRTUALLY ZERO LAG.
When NISSAN did some testing with traditional SLEEVE bearings versus the ROLLER bearings the results were, time to boost to 40Kpa 5.3 seconds on the standard bearing as opposed to 1.6 seconds on the ROLLER bearing, so a clear improvement might be what some would consider this!!
The ONLY reason lower CR went out is to do with EMISSIONS ,a nd nothing else, the lower the CR the dirtier it is.
VARIABLE VANE TURBOS still have lag, nowhere near as much, but bearing design plays a very big part, as does seal type.
tabetha

I beleive the sleeve bearing (is it not a fluid bearing as it does not make contact with anything but oil) not laggier in the respect most people state.

Lag is used by most people as a term that includes both the minimum rpm to produce boost, and the time from opening throttle to boost while in the rpm range of the turbo.

Now, i haven't driven a roller bearing turbo'd car, but i see it like this. When on full throttle from low in the rpm range, the difference in when the turbo will boost between roller and non roller is negligable - as the volume of gas needed to drive the turbo is not much different between the two.

However, from a lag point of view - what u are waiting for is the time taken for the turbo to accelerate to 100k+ rpm when more than sufficient energy is available to drive the turbo at that speed. This is where the roller bearing has an advantage. BUT - this advantage becomes less as turbo size increaces - due to the inertial becoming the majot factor, to the point that when you get to T4 size the difference between roller and non roller is marginal.

Its only the sprung loaded thrust bearing that creates the friction, wet floating bearings fall out of the core when you pop the circlips out! Not a tight fit at all!

i don't want to get into a comp argument, every body has different ways of engine tuning ( it would be no fun if they was all the same !! ), incidentally both cars ran the same boost from a T34, and i was talking about lag time before full boost was achieved, not on boost performance..

and more and more high power cars use higher comp ( i noticed the one on the cover of fast ford @ 1094 BHP and has a comp of 10:1 !!! just one off the top of my head )

variable vane turbos are a good thing and i feel that they ( when they can make them last !! lol ) will replace most turbo set ups !! ( the Astra VXR uses a large turbo ( well almost !! ) and they have no noticeable lag and power all the way to the limiter !!

a T4 with T2 lag anyone !!

Intresting topic :cool: