question: Low CR vs. High CR for forced induction?

[AndyTTR]

the thermodynamic definition of compression ratio is;

(maximum volume @ BDC)/(miniumum volume @ TDC)

if i'm not mistaken?

[30psi 4agte]

Ahh im just using it as an example.

Forget about atmospheric pressure it just complicates things.

my example is not how you calculate comp ratio but just a basic guide to explain things

[Nim]

Just thought i would explain that the dynamic compression ratio is the cylinder area above the piston after Intake valve closes over the area in the chamber at TDC. It is generally much lower. For eg, the 2jz engine has a Static compression of 10.1, well the dynamic compression ratio would be 8:71. That is why camshafts determine what the DCR is because the earlier it closes after BDC the higher the DCR.

Yeah, so using a VTEC / VVTL system when off boost to raise the dynamic compression would probably be easier than moving the whole head like the Saab system.

2ZZ-GE, with turbocharger, re-set the VVTL to switch over at about 3000rpm, run a lower bump on the low profile, and a standard bump on the high profile. Bam, anti lag without blowing your exhaust manifold up. (Well, sorta anti-lag).

[thechuckster]

my 2c...many modern turbo engines run with higher compression ratios so they are more drivable for every day plebs. Modern engine management allows factories to up the CR and wind back the boost.

For instance, the ford turbos only boost to about 6psi - not much in the scheme of things - but the engine is already a twitchy powerplant. Likewise, the factory 4wd turbos tend to run fairly high compression ratios and moderate boost levels so they get a lot of torque early on and are drivable for day-to-day use.

You should also remember that emissions targets are harder to achieve with low CR engines.

The genuine rally engines are far more unpleasant to drive than every-day sluggers - they delivery torque from faily low down, the turbos use anti-lag enabled to reduce launch/accell delays and when they do come onto power, it's brutal and fucking fast. They also run exotic race fuels so the whole CR/boost level argument gets skewed.

[mullett]

Yep, that's most of the technical stuff as I understand it (and a bit more ). The end result as you drive the car, is that a higher compression ratio in a boosted motor will be more fuel efficient, and will have more torque when off boost, as well as building boost faster (all other things being equal), where a lower CR motor will be capable of making more power (if you run more boost), but will be laggier etc...

RM.

[oldcorollas]

just to add a little more from a different angle.... there7s two ways of lookign at it

CR and boost in......

or

shape of the cylinder pressure vs angular displacement graph..

i can't be bothered finding any atm, but they are often in papers about knock detection etc...

anyway, higher SC or DC results in a steeper rise in pressure and a higher peak, whereas lower compression has slower rise and lower peak.
comparing NA vs turbo, the peak cylinder pressures might be lower in turbo.. but the area under the graph is greater, ie, there is more oomph to push the piston down for longer.

think of it this way...NA and 1atm boost (twice as much volume).
when piston is halfway down bore and has greatest leverage on crank, with 1atm boost, you still have much more force acting on crank.....
when you increase compression in a turbo.. you effectively move the whole graph upwards, and make it a bit steeper... resulting in more area under the graph, and more total force on piston = more torque...

maybe i'l find graph later

[oldcorollas]

links..
http://www.fs.isy.liu.se/~larer/ISIS...d_2002_SAE.pdf

http://www.popularhotrodding.com/tec...ression_ratio/


this kinda sums it up...

[Nim]

Woh, I never even thought of that! Nice one oldcorollas! (Talking about the boost thing)

This is an interesting tool:
http://www.rbracing-rsr.com/comprAdvHD.htm

Calculates dynamic compression. Anyone know what a 4A-GE rod length is? I can't find it.

[myne]

http://theoldone.com/archive/quench-area.htm

Get a professional to re-work your combustion chamber, and you might be able to run high boost with more or less stock compression.

Here's a 4a head those guys apparently worked on http://homepage.mac.com/dgiessel/.Pi.../4a/index.html

[go_the_datt]

. Anyone know what a 4A-GE rod length is? I can't find it.

C-to-C: 4.803"
B.E. Bore: 1.772"
B.E. Thick: 0.859"
P.E. Bore: 0.787" (20.0mm)
P.E. Thick: 0.860"

4.803 inch = 121.996 2 millimeter

[Nim]

http://theoldone.com/archive/quench-area.htm

Get a professional to re-work your combustion chamber, and you might be able to run high boost with more or less stock compression.

Here's a 4a head those guys apparently worked on http://homepage.mac.com/dgiessel/.Pi.../4a/index.html

That's pretty interesting. See the way they've put a swirl in the 'grain' of the polish on the back of the valve and up the valve stem?

[oldcorollas]

That's pretty interesting. See the way they've put a swirl in the 'grain' of the polish on the back of the valve and up the valve stem?

thats very questionable as to benefits the height of the texture, vs the amount of air going thru... but it looks pretty

[urantia]

And what about diesel. What if you shove more boost into a diesel. Take for example an older mercedes turbo diesel that doesn't have an ECU of any sort. If i force more boost into a diesel am i limited by the strength of the motor? too little fuel and too much boost in a diesel what will happen? will the combustion tempratures soar thru the roof? i'm not sure but i'd like to try with a spare turbo diesel i have. (and the effect of higher boost on a turbo diesel running straight vegatible oil)

[Nim]

thats very questionable as to benefits the height of the texture, vs the amount of air going thru... but it looks pretty

Interesting none the less. Although yeah, they do slightly rough polishing to cause a 'cushon' layer of air, increasing intake velocity if I'm not mistaken. This cushon layer would move slowly, and not really add much swirl to the air layer above it.

["Z" UTE]

Someone bought up the emissions problems with low CR engines.

The Toyota 1JZ-GTW VVTi is an LEV engine, and runs 10.5:1 (fading memory). The combination of VVTi higher compression ratio, and moderate boost worked very very well. The power delivery from the engine was very linear, and it had a mountain of torque.