Cylinder Compression Ratio
Turbo Mini Forums

Aright Guys,

I did a search 'Boost Pressures' and have answered my questions !

But inevitably now I have more....

Just to confirm I have the right understanding...

Your cylinder cr and the octane you run dictates the boost you can run, but you also need to sort you ignition settings to avoid detination. You fuelling also needs to match. ie needle and spring. So a session on the rr.

I think I am gonna get a mega jolt set up, and start with a 10PSI needle, spring and actuator(Intercooled of course)


Peter, hows that program coming on ?

Cheers Brocky.

Edited by Brocky on 17th Mar, 2006.

BTW - I took my buddy VTEC Pete out for a spin this weekend, and even though I am running her in, he gave it a name, the "Brocket"...cause I'm a brocket maaannn, nah nah nah nah nah nah nah nah nah nah nah....

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I think that the answers to your qestions are yes and yes.

BUT, why not just increase the combustion chamber volume a bit more and stick with the stock turbo cam.

My theory, and it is just theory at the moment, is that the power you can produce from a turbo engine is directly proportional to the combustion chamber volume at TDC.

Think about it, the more air and fuel you can get in the cylinder, the more power you can make. The main limit to this is the pressure in the cylinder at the point of ignition. Too high and you will get detonation.

Hence, the more you can cram in without reaching detonation pressure the better.

High overlap cams only undo the work that the turbo has done.

Is this bollocks or what????

Saul Bellow - "A great deal of intelligence can be invested in ignorance when the need for illusion is deep."
Stephen Hawking - "The greatest enemy of knowledge is not ignorance, it is the illusion of knowledge."

Sounds a good point...I imagine there will be other restricting factors which goven how full you can get your cylinder in the first place ie volumetric efficiency, due to ports etc.

Higher lift will be good for getting more mixture into the cylinder, and more overlap actually helps to clean the exhaust out before the next inlet stroke, which improves things also. but also more importantly reduces the dynamic cr which means you can up the boost ? also I think you need to retard the ignition though ?

Cheers

Brocky

Edited by Brocky on 15th Mar, 2006.

BTW - I took my buddy VTEC Pete out for a spin this weekend, and even though I am running her in, he gave it a name, the "Brocket"...cause I'm a brocket maaannn, nah nah nah nah nah nah nah nah nah nah nah....

Build thread - http://www.turbominis.co.uk/forums/index.php?p=vt&tid=7483

Katy Mini Owners Club Thread - http://www.turbominis.co.uk/forums/index.php?p=vt&tid=505552

Let's make sure you embarrass at least one VTEC MINI today !

https://www.facebook.com/groups/17557298589...096464253807802

Updated original questions

BTW - I took my buddy VTEC Pete out for a spin this weekend, and even though I am running her in, he gave it a name, the "Brocket"...cause I'm a brocket maaannn, nah nah nah nah nah nah nah nah nah nah nah....

Build thread - http://www.turbominis.co.uk/forums/index.php?p=vt&tid=7483

Katy Mini Owners Club Thread - http://www.turbominis.co.uk/forums/index.php?p=vt&tid=505552

Let's make sure you embarrass at least one VTEC MINI today !

https://www.facebook.com/groups/17557298589...096464253807802

Larger combustion chambers will allow you to produce more power because you can run higher boost. You are still aiming at a pressure before ignition just below that that will cause detonation.

In a turbocharge engine, the compression of the charge is shared between the turbo compressor and the cylinder. You can get the turbo to do more as long as you enlarge the combustion chamber.

Hence, my statement "the power output of a turbo charged engine is directly proportional to the volume of the combustion chamber".

I do not agree that it depends on the inlet valve closure. However, the inlet valve timing should be optimised so that it closes when cylinder pressure rises above boost pressure. Problem is, this point must move about dpending on boost, revs, throttle position etc.

I'm just trying to make up some simple rules to try and understand what is happening in a turbo charged engine. The one area that we do not seem to have fully developed is camshaft selection and unless we understand the science we will not get it right.

I am not happy to use a cam because someone else claims to have good results. I want to know why a particular cam works well.

Since my original post above, i have dug my old text books out of my attic and have been refreshing my memory on the Otto cycle!! I now have a nice sketch of the Otto cycle and I have identified the valve opening and closing points plus the point of ignition. I have also considered the effect of higher inlet and exhaust pressures.

It becomes fairly obvious then why NA cams no good for what we are doing.

I thought your idea of using roller rockers was a bit extreme, but I now understand the logic behind it.

Saul Bellow - "A great deal of intelligence can be invested in ignorance when the need for illusion is deep."
Stephen Hawking - "The greatest enemy of knowledge is not ignorance, it is the illusion of knowledge."

Thanks for the reply,

Being a senior subsea engineer myself, and the author of several Technical Proceedures pertaining to installation of oil and gas pipelines, i find myself trying to write the technical procedure for spec ing a turbo a+...

What are the main constaints.?

1). Budget

2). Fuel you want to run

3). Bottom end mods which again is linked to budget...in terms of handling increased power outputs ie at what point do you need to strap the centre main if you limit your rpm to 6k, 140 ish ??same story for the cross pin etc. Its kind of the chicken and the egg I suppose.

I like your philosophy of max power from min boost..

Once you get this program running is there scope to produce a spread sheet type data base which will give you a spec for a given application for a given budget which gives the optimum power out put ...or am I just far to optimistic...

Cheers

Brocky

BTW - I took my buddy VTEC Pete out for a spin this weekend, and even though I am running her in, he gave it a name, the "Brocket"...cause I'm a brocket maaannn, nah nah nah nah nah nah nah nah nah nah nah....

Build thread - http://www.turbominis.co.uk/forums/index.php?p=vt&tid=7483

Katy Mini Owners Club Thread - http://www.turbominis.co.uk/forums/index.php?p=vt&tid=505552

Let's make sure you embarrass at least one VTEC MINI today !

https://www.facebook.com/groups/17557298589...096464253807802

BTW - I took my buddy VTEC Pete out for a spin this weekend, and even though I am running her in, he gave it a name, the "Brocket"...cause I'm a brocket maaannn, nah nah nah nah nah nah nah nah nah nah nah....

Build thread - http://www.turbominis.co.uk/forums/index.php?p=vt&tid=7483

Katy Mini Owners Club Thread - http://www.turbominis.co.uk/forums/index.php?p=vt&tid=505552

Let's make sure you embarrass at least one VTEC MINI today !

https://www.facebook.com/groups/17557298589...096464253807802

im afraid hard enidance has sugested that high lift does bugger all for outright hp at the top with losing some at the bottom for any given cam.

t3tone used a 276 cam running 12 psi on 1.5 rockers and got the same hp for the same boost at the same inlet temp but gained a few bannanas at the bottom buy swapping to 1.3's

thus sugesting depending on cam choice lift is not the be all and end all anfact it looking likely to be a waist of money using the 1.5's at the info we have at hand at the moment.

alot of testing is requiered first before we all can say this does or doesnt work.

last year i ran 10 psi on a 9.8 cr intercooled on my 1380 and got 140 bhp at the flywheel using a static 24 deg on 99 ron fuel from tescoes with out any det at all as the operater was using det cans.

and with my own modded head which lowrered me 8.1 to 1 cr i had to run 13psi to get the same power at the top but lost a bit at the bottom in the process.

my view is decide what hp you require then use the higest cr and lowest boost to sfly obtain it as this will always make for the best drivable road motor going.

but we always want more dont so that proberly blew that theoery out of the window.

always looking for them bigger bunches of bannanas