Why Do Forced Induction Engines Have A Lower Compression Ratio?

[love ke70]

hey everyone.

ive recently been thinking about this, and to me it seems to defeat the purpose. see the way I'm looking at this is if we take a turbo'd engine. with a compression ratio of 8.5 the bottom end is gonna be rather gutless and only be any good once it comes onto boost. now the way i see it the lower compression ratio means slower acceleration until the turbo spools up and a longer time to spool up because the engine isnt pumping as much air and it will also mean that the turbo has to push more air to create the same pressure in the cylinder as it would with a higher comp. engine.

so from this thinking to me it seems that to have a decompressed version of n/a engine turbo'd ie. 1gge (9.5:1) vs 1ggte (8.5:1) if the compression ratio had been kept the same greater power would have been delivered with less boost from the turbos.

just after some thoughts or an explanation of this. feel free to point out the flaw in my thinking :)

[Dyzza]

No deal!

[love ke70]

No deal!

would you care to explain that comment ????

Edited December 15, 2005 by love ke70

[Gaijin Doru]

ever think that forced induction engines need to fit more air into the cylinders?

[love ke70]

ever think that forced induction engines need to fit more air into the cylinders?

 

i may be very dum but i don't understand what the point is that you are making?

[Super Jamie]

you have static compression and you have dynamic compression

 

static compression is the bore and combustion chamber divided by the combustion chamber. this is the 8.5:1 and 9.5:1 you have described above

 

dynamic compression is the compression the engine sees when it's running, this is related to the above two figures, but also to stroke of the crankshaft, profile of the camshaft, and air density

 

air density is the big point on a forced engine, as you're making the air more dense than one atosphere. this is called boost

 

now i'm going to get all mathematical on you

 

 

let's say you have an imaginary engine. this engine is constructed in such a way that you can put 10 units of air into the combusion chamber at full compression stroke before it blows up

 

naturally aspirated, with 9.5:1 compression, let's say you're forcing 8 units of air together. no worries

 

now slap a turbo on it, and put half a bar of boost on, and you're at 10 units of air. your engine is on its stress point. it's still nice to drive off-boost because of the high compression, it's just like normal, and it's got a little bit of punch up top, but you can't go any further

 

take the turbo off and drop the compression to 8.5:1, and let's say you're only compressing 5 units of air now. the motor is a little bit slower but still quite driveable

 

bolt your turbo back on and wind a whole bar of boost in. you're up to 10 units of air again, except you are making a shitload more power when the motor comes on boost

 

this is roughly how air works. the way the numbers work is not linear. one point of compression doesn't equal one bar of boost. rather, if you plotted it on a graph, it would be exponential

 

in other words

the more you are willing to drop static compression, the more room you have to compensate when you increase dynamic compression by adding boost

 

i hope that helps, it's about the simplest way i can think of describing it

 

i just pulled those numbers out my ass, they are roughly correct, the idea is right anyway. there do actually exist fomulas out there for calculating dynamic compression based on boost, i just can't be bothered remembering them, that's what spreadsheets are for

[Felix]

sorta....

 

static compression ratio is as jamie said.

 

dynamic compression is how the static compression is affected by the cam timing.

 

the effective compression ratio, is the combination of boost coupled with your static compression ratio. below is an effective compression ratio comparison chart:

Edited December 15, 2005 by Felix

[love ke70]

sorta....

 

static compression ratio is as jamie said.

 

dynamic compression is how the static compression is affected by the cam timing.

 

the effective compression ratio, is the combination of boost coupled with your static compression ratio. below is an effective compression ratio comparison chart:

 

ok thats more or less what id thought, but maybe theres something just not connecting in my brain. but with jamies example of the engine that blows up with more than 10 units of air...wont it produce the same amount of power whether its being fed .5 bar or 1 bar boost, as long as its compressing 10 units of air.

I'm assuming theres something flawed in my thinking...

anyone wanna try and drill it through my thick skull? :)

sorry guys but this just isnt quite makin sense in my head yet

[Felix]

ok, well look at that chart i posted.

 

if you have an engine with 7.5:1 static CR on 10psi, it would have pretty much the same effective compression as a 10.0:1 static CR engine on 4psi.

 

in general a lower compression engine on higher boost, will deliver higher power and be more detonation resistant, with a much safer tuning margin. it will be less flexible off boost, however.

 

a higher compression engine on low boost will have more offboost performance. it will be a lot closer to the ragged edge of detonation, and will need more precise tuning to keep it safe. with modern efi it is not so much a problem these days.

[love ke70]

ok, well look at that chart i posted.

 

if you have an engine with 7.5:1 static CR on 10psi, it would have pretty much the same effective compression as a 10.0:1 static CR engine on 4psi.

 

in general a lower compression engine on higher boost, will deliver higher power and be more detonation resistant, with a much safer tuning margin. it will be less flexible off boost, however.

 

a higher compression engine on low boost will have more offboost performance. it will be a lot closer to the ragged edge of detonation, and will need more precise tuning to keep it safe. with modern efi it is not so much a problem these days.

 

okies that seems to make sense, so its basically to try and prevent detonation.

and if you got a good tune you could up static compression with much the same effect as up-ing boost, only wih stronger off boost performance.

thanks for the help. makes a bit more sense now :)

[irokin]

i may be very dum but i don't understand what the point is that you are making?

 

don't worry...I don't think he does either...

 

Ive seen an SR20DE turboed SSS pulsar versus a 180sx with an SR20DET. Pulsar was limited to 10psi boost and this was pretty much his boost limit. The 180SX was running 12psi but was no where near its limit (Ive seen them take 20psi+). They were very evenly matched (the 180SX was probably slightly faster in the top end), but the 180SX had heaps more power potential.