might have been answered a load of times before, ive had a search but cant find what im looking for..

basically - why would you want a cossie engine with low compression pistons.... what are the benefits, why would yuo need them and what does it affect with the running - are they only for running immense boost engines?

what would happen if i fitted a complete low comp bottom end, i take it my car would need a different chip and setup...?

views appeciated

 

low comp means low cylinder pressures which is easier on the engine

 

i know its lower compression - but to what gain...? are they only necessary over a certian spec etc..? i dunno?

 

lets you run more boost and more igniton adv but does have a detremental effect on response etc.

have mark or stu explain it...

 

this is the question to gain a undriveble flat engine off boost what a great gain

 

By decreasing the compression ratio you are reducing the engine efficiency, and the transient response as well as performance when the turbo is out of its opperating range.
The benifits of decreasing the compression rato are that by decreasing it, you are reducing the pressure and temperature to which the intake charge is being compressed in the cylinder. This allows for more 'boost', and ignition advance (both of which improve performance in the turbo opperating range generally speaking) before the compressed charge gets to autoignition temperature.
This is when the fuel air mixture will self ignite at hotspots in the cylinder (usually around the wall, or on sharp intrusions in the pistons or combustion chamber) this is called Knock. This causes unsteady burning of the charge, rather than a nice steady flame front across the cylinder ignited by the spark plug like we want. Knock is a bad thing, also called detonation, and it leads to molten pistons!

So if you were planning on running a high boost engine, with a limited effective opperating range to when the turbo is spooled up (ie rallying where antilag is adopted to keep the turbo spinning) then you want low compression ratio to enable high boost, and therefore high on boost power.
For most road applications its a balance between keeping the off boost response of the engine relatively good, whilst maintaining good on boost performance.
Depends wether your able to put up with worse lag for more on boost performance.

 

 

Ok bad example, any application where off boost performance is not paramount. Drag racing.

Although i do stand by my comment, that in 'some' rallying applications low comp is benificial to be able to run higher boost and then use anti lag to reduce the detremental effects. If you can achieve the desired boost levels without decreasing compression then yes this is better. I thought i made that clear with my explaination. Just a bad example, especially as you have a rally cossie by the looks of things, and im guessing that it aint low comp!

 

you wont find any big output rally engines that have what is termed 'low comp' pal because then the performance off boost is terminal for a rally car mate, even with antilag, we try to optimise the engien to use as little als as possible to try and keep stress to a minimum, just a small instance for you, my main turbo has just come back from service, it has done 2 events since its last service, so 150 competetive miles and probably 300 road miles, and it needed , new thrust, new oil bearings, and full rebalance, thats because i run a fair amount of als and i run high comp as well,
in a fair many events now als is banned for club rallying, so the high comp is essential to maximise torque, even at high comp without als the car feels flat to me

 

I stand corrected

I had heard antilag was harsh on turbos, i didnt realise quite how harsh!!
Compression ratio is a non linear relationship with efficiency though, so i'd say there is merit in keeping compression ratio high to a point. I should find out in the near future anyway as i help try to turbocharge (only a mimo Gt12) a 12:1 yamaha yzf600 (20mm intake restricted) for my universities formula student car.

 

nice small blower wouldnt be a problem pal, infact the intake size will only help the torque figure arrive sooner

 

Yip, should be over 100 Nm by 6000-7000 rpm (theoretical computer program outputs) and then reducing boost once the restrictor is 'choked' up to the limiter of 13500 rpm, should be fairly nippy as it was quick in NA format last year. It only weighs in at 250kg's

 

wheres the car being built ?

 

so in summary: a waste of time for a stg 3 road car with std turbo and cams etc..

 

if its stage 3 i would opt for standard comp.

 

yep - it was just an option - as there was a low comp complete bottom end for sale... just getting all bases covered...

 

i had my ratio lowerd slighty for my stage 3 conversion.

 

but reading teh above - i'd ask why as it makes an alrady laggy car even less responsive... unless you had a different turbo and had it mapped to suit..?

 

its was no more laggy IMO...

even my new spec with very low compression at that bad

 

1stly, sorry for hijacking the thread!

The car is Imperial College London's entry to the Formula Student competition this year. So its being built in a basement lab in central london, by myself and a load of other 3rd and 4th year mechanical engineers as part of our degree.
More info: http://www.cgcu.net/icracing/

Cheers

 

Also all rally-cars running in FIA event (I know UK is slightly different) will be rescriced and not really big power, and they are all high compl. WRC as much as 10:1 or 11:1 but they run proper sport fuels.