compression ratio

[glenn]

i have a magazine here somewhere with the number of a company that make head gaskets out of copper.
you tell them what engine you have and tell them what compression ratio you want
lower compression ratio=more boost=more power
will dig out mag over weekend

[Ant]

comp ratio is an odd thing.

things I have come to realise wuith my toil over the last few months ( 18!!)

CR and turbos is irellevant, there are people in the US running 2 bar of nboost on stcok engines, were tlkaing 9.75:1 CR mills here dude, not low comp emmisons laden slugs

the thicker the head gasket the more chance of it failing, its a bigger weak area if you get where I'm coming from.

spacing the head from the block results in two things, 1 the CR is reduced a little, 2, the piston to head clearance is altered, less squish = less concentrated burn and flamefront= pinking.

the key is intake temp and fuelling control, they are the secrets to this, the CR is possibly the least important aspect to consider when thinking aboiut going the FI route.

I'm not claiming to know it all by any means, indeed far from it but sometimes you need to open your mind a little and take the blinkers off.

many wrong turns later and I'm coming out of the shadows, 8.8:1 CR, 30K on new gen BMW engine, 11 psi of boost , yeah baby !

[Turbo-Brown]

I'd be interested to see what sort of control strategy they're using to boost to 2bar with a 9.7:1CR!

The thing about compression ratios is that, yes, you can run more boost, and that make it sound like you get more power. But! Lowering the compression knocks chunks out of the power in the first place which then needs rebuilding with higher boost.

So in escense, modest boost on a standard CR engine can give just as much power as mega-boost on a low CR engine.

Higher CRs also give better drivability, off bosst response and economy.

[glenn]

i'm not saying that putting a thicker head gasket on is the ideal route to go for lowering the ratio, but it is a cheaper alternative to having the internals machined, to reach the required ratio.
i agree there are many things to consider when going fi, but lowering the cr is the foundations of a required power output.
most mildly tuned cosworth's are running around the mid twenty's psi,
highly tuned cosworths are in the mid thirty's. you could never achieve these boost pressures without lowering the cr

i'm not that much into turbo'ing myself just thought it may be of interest to other people

[pumacos]

Hi lads I donÔš't now where to start are we trying to gain big power on a small budget or small increase in power on a large budget.

Yes u can run 11psi on a standard engine but why spend all that money on a turbo a manifold and intercooler only to gain a small increase in power in a small power band you can go get the std normally aspirated engine mapped with an increase in fuel and air and end up with the same results and no turbo lag???

Or you can lower the compression run big boost which will result in big hp and a better 0 to 60 time, yes the car around town will be a sod to drive but if you are building a car to go fast itÔš's meant for the track we do have a 70mph speed limit u cant have both turbo charged big hp and good fuel economy.

Air temperature is also very important as u can have up to 110 oC coming out of the turbo at around 3500rpm on a normal day this is first cooled by the intercooler and then cooled buy the added fuel as it enters the engine before it reaches compression
If u is still using a high compression engine the temperature will then be lifted further and can ignite the fuel air mixture which can melt the pistons more fuel can be added to cool it but the fuel economy will suffer.

To lower the compression u can have the pistons machined which will cost a considerable amount of money or you can fit a thicker head gasket, which is where the budget comes back into it

Ps both systems will need to be mapped to gain the full potential out of the engine.

The only reason WRC cars are now going high comp turbo charged engines is because of the restrictions on HP and they need to reduce the turbo lag itÔš's only a mater of time before WRC cars go back to normally aspirated engines

So what have we achieve big power on a low budget and sod the fuel economy,

[matt535]

Water injection is good for cooling the charge in turbo motors and can allow extra boost in say a fairly high comp ratio engine that wud probably melt otherwise.

[e30_Turbo]

Yes u can run 11psi on a standard engine but why spend all that money on a turbo a manifold and intercooler only to gain a small increase in power in a small power band you can go get the std normally aspirated engine mapped with an increase in fuel and air and end up with the same results and no turbo lag???

Can't say I agree with your statement there, can you explain how you came to this point of view?

11psi on a F/I M20B25 will see easy 280-320bhp, 6psi will give some 250bhp, with or without IC. (assuming injectors and fueling have been sorted)

To obtain 250bhp on a M20B25 you will have to spend Ԛ£1000's, and TBH I'm only aware of one M20B25 N/A lump running 250 (triple webbers and a v-high spec), the rest of the 240bhp lumps are 2.7 conversions, but my point is this is as much as you can reasonably go with them. Hence why so many M30B35 conversions are around.

Dales's M20B25 has Dbilias TB's (Ԛ£2k), 272 cam, 6 branch, MBE management etc and is still shy of 200bhp, so I just can't understand how you can say it that N/A can produce as much power.

My project has cost me about Ԛ£1.5k so far and thats everything in, including ECU and turbo and manifold, if you gave Ԛ£1.5k to Bexleys and asked for more power, you would be lucky to gain 20 or 25 bhp, were as my project should see returns of atleast 1 bhp per Ԛ£100 when setup and mapped properly. The aim is .5bhp per Ԛ£100 to start as that's all I've budgeted for with the injectors I have at mo.

For the money F/I is getting a easy and cheaper option by the day!!