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

[AndyTTR]

I've always been under the impression that low compression ratios are better for turbo engines but never really understood why. I recently was looking over the specs on the 2006 WRX STi Spec C. and saw that it runs a 13.5:1 compression ratio!? Also, was it Saab that recently started producing variable compression ratio engines?

I guess I'm really asking what the relationship between compression ratios and forced induction is, which is better (high vs. low CR) and why?

[nick.parker]

Hi,
That 13.5:1 is not correct. It will be more like 8:1. Basically once you get to a high enough cylinder pressure (this pressure is dependant on the head and piston design, fuel type, fuel mixture and ignition timing and mixture temperature etc..) you can get spontaneous combustion, or detonation.
With forced induction engines since you are putting higher pressure air into the cylinder to start with, that pressure threshold is reached more quickly (once you further squish things by the compression ratio of the engine). Any engine can produce more torque and be more fuel effcient with higher compression ratios. But you can make more power with more boost, which may require a lower compresion ratio.

At the end of the day a suitable CR depends on your application and power goals.

Regards, Nick

[Nim]

Lets get some terms defined. You're talking about static compression ratio.

Static compression + boost = dynamic compression.

Higher dynamic compression is better, and produces more power. The problem is that if your compression is too high, it will detonate before your spark, and cause problems like exploded engine. This is why a lot of turbo engines run a bit rich up high, to help stop that from happening. Better designed combustion chambers, higher RON fuel and more intelligent computers all mean you can run a higher compression without exploding your engine. This is why modern engine compression just keeps going up and up.

EDIT: In WW2, the Germans had problems with their aircraft engines as they didn't have access to American fuel. The English did, and the RON on the petrol just kept getting better and better and higher and higher. As such, the English just used higher compression, and more boost. The Germans had to run larger and larger pistons to keep up with the English power delivery. As such, the engines got heavier, and they needed even more power than the English fighters to keep up.

The reason for turbo lag (why a turbo 2.0 feels slower off boost than an N/A 2.0) is that not only does the engine have to spin the turbine, but it is also running at a much lower dynamic compression. It's static compression is lower than the NA, and the cylenders haven't gotten boost yet to raise the dynamic compression. This is the purpose of Saab's variable compression engines. Keeping the compression higher off boost, will produce more power, and spool the turbos quicker, meaning less time in the 'dead zone'.

I'd guess that the 13.5:1 ratio on the WRX is actualy dynamic compression, 'cuz it sounds far too high if you're adding boost on top of that. The engine would get very unhappy, then die. Unless you never wound the boost up, and got it regularly serviced (every 2-3 months) by a machanic that took as much care of your engine as a race engine builder.

[oldcorollas]

umm..when you are talking about compression ratios, people quote "static compression" as it is easiest to measure.. however, depending on cams and overlap etc, the dynamic compression can vary considerably.

in a very basic (and possibly incorrect ) way to say it...
1) compressing a mixture raises it's temp. higher compression ratio = higher temp rise (and pressure rise)

2) with FI, more air/fuel is in there, so you are starting from above 1atm before compression stroke, so if you have same static CR, then you may get a rise in pressure/temp that is too high for the fuel, resulting in spontaneous combustion (pre-ignition.. not pinging which is too much advance).

3) combustion chamber and piston shape, and squish area/design significantly affect the ability of a motor to resist pre-ignition.

higher CR will be better off boost and probably spool quicker than low CR.
the GZE runs 8.9CR safely with high boost.. but the WRX may not eb able to take much more boost than standard perhaps?
ppl can and have boosted high CR motors before (eg, 1UZ) with 10 and 11:1.... just depends how much boost, and all the other mitigating factors, liek intercooler discharge temp and tuning/safeguards

anyway, thats a start

[AndyTTR]

Thanks for the replies guys. So the limiting factor here is the strength of engine and the auto-ignition point of the fuel right? Theoretically, if you could build an engine that could sustain it, you would get more power from an engine with high dynamic compression and boost than an engine with lower dynamic compression and boost? OR would an engine with low dynamic compression and high boost be more reliable while still making similar power?

Kinda off topic, but what effect does increased dynamic compression have on the torque output of the engine?

[Nim]

If you could run 200ron fuel, you could make much much higher compression ratios.

Boost is esentialy creating displacement. More air = more fuel. At 14psi, a 2.0 engine is burning about as much air/fuel (depending on tune) as a 4.0ltr engine. Then there's the thermo efficiency of turbo, but that's complex and I wont get into it (mainly 'cuz I don't understand enough about it). So lowering your static compression, then just ramming boost in to raise your dynamic compression up to say 13:1 is much more effective than just raising your static compression to 13:1. Problem is, ram too much boost in there, and you run into conrods bending, crank trying to escape the block and bearings falling appart land. Also off boost, at some point the engine simply will not run (at for example, 3:1 compression, I don't think it's gonna work at all untill you hit boost - which you can't do 'cuz the engine wouldn't start).

[kingmick]

i was bored so i wrote a few basics down! so dont flame me im sure everyone knows most of this!

A few combinations that are used for specific and general turbo setups!

Street
Because of the fluctuations in fuel quality and Temperature you need to be careful how high you go with static compression. To high and you run the risk of having detonation on hotter days and then doublie so if the fuel isn’t of the highest quality. Intercooling is also a limiting factor on the street for a number of reasons, Limited space to fit a larger cooler in either air/air or water/air, less than optimal access to clear cooling air because of front bars etc, legal reason such as getting defected !
You can run higher compression and higher boost if you have very good cooling but if you use a car day to day you will run into problems with hot weather and bad fuel.
The beauty of having lower compression is that the times you want the extra power on cooler days you can increase the boost with ease.

Street: 8.5-9.5 on engines like the 4a,1g,1j and 2j with boost from 5psi-22 and a bit higher if you have a good tune and the conditions are right. Bit higher compression will make it much nicer as a day to day car.
Note, there are engines that can run higher and lower than this, there just examples!

Drag: The sky is the limit here because you are only tuning there engine over a limited amount of times in a ¼ mile run, this means you can run intercoolers with dry ice etc and use Nos and or fuel as well as exotic fuels to control the inlet temperatures.
Combinations range from 7-1 compression with 45psi of boost to engines with 11-1 compression and 20-35psi of boost. The reason for the difference is the weight of the car, setup and budget. It would take a big book to go into all the reasons and advantages for each, but budget usually plays the biggest role in choices that are made.

Circuit racing: There are some of the same reasons as above for choices for circuit racing but there is also the main one as apposed to Drag racing in that you need them to be durable as the distance travelled is much further than drag racing. This brings into play things like heat soaking like a street car, with the only added bonus that you are travelling at much higher speeds to cool things down.
Different setups will suit different drivers, highly skilled drivers can run lower compression and much higher boost {45psi etc}, were it is fast for someone that is not as skilled to have a higher compression and lower boost { 8.5-10.0 compression and 10-25psi}. The other advantage in higher compression and lower boost, is in mixed engine classes were you have higher torque, lower in the rev range engines against you. It all comes down to corner exit speed.
Combos used are 7.0-8.5 compression with 25-45 psi boost and 8.5-10.0 with any were from 5-30psi etc!
Just thought id write some basics out! There all examples there are way to many variables to go into every thing! I don’t feel like sitting and writing a book at the moment! lol

[AndyTTR]

thanks again guys, i think i'm starting to understand the relationship between compression ratio and boost. Cheers!

[30psi 4agte]

I have always thought varying thoughs on this subject. There is a the tech info around to explain things but to put it in its simplest form this is what i think.

I think of it in terms of volume
In a high comp engine the area between the piston and the head is going to be smaller than it would be if it was a low comp engine.

1.
Now lets say you do a comp test on a engine with a static comp ratio of 10.5 :1 and the gauge reads 200 psi
You are running a boost level of 15 psi

Then you will have a static compression pressure of 215 psi ( 200 +15 )

2.
Now do a comp test on a low comp engine with a static comp ratio of 8.0:1 and the gauge reads 180 psi
you run a boost level of 35 psi

The static compression pressure 180 + 35 = 215 psi which is the same as the high comp engine with 15 psi boost but the amount of fuel and air in the cylinder is greater due the low comp engine having a larger area in the cylinder.

More air and fuel in the low comp engine should make more power than the high comp engine even though they both have the same cylinder pressure
MORE AIR/ FUEL = BIGGER BANG

This is just my theroy and is not proven however i would love to have two identical engines one high comp and one low comp to do a test to see if it can be proven

there is more to it than this and pros and cons for each setup like response etc but if anyone can tell me why this wont work like this please do.

This is why im led to believe that the F1 cars of the 80s run such low comp and sh1t loads of boost.

[AndyTTR]

Just thought i'd share a bit of info on the saab variable compression engine that i googled up.
http://www.saab.co.uk/main/GLOBAL/en...lon/index.html
http://www.sae.org/automag/techbriefs_05-00/03.htm

It appears that the whole head tilts back 4 degrees on demand, increasing the size of the compression chamber (as the crankshaft stays stationary, the top of the chamber is pulled away from the piston at TDC). 20 years in development... those crazy sweedes.

edit: Its a 'mono-head' design where the compression chamber and cylinder is part of the head (so there's no issues with head gaskets). I guess the oil and coolant lines run through flexible pipes on the outside of the engine. That must be one mother of a seal keeping the oil pressure inside the crankcase~

[kingmick]

Just thought i'd share a bit of info on the saab variable compression engine that i googled up.
http://www.saab.co.uk/main/GLOBAL/en...lon/index.html
http://www.sae.org/automag/techbriefs_05-00/03.htm

It appears that the whole head tilts back 4 degrees on demand, increasing the size of the compression chamber (as the crankshaft stays stationary, the top of the chamber is pulled away from the piston at TDC). 20 years in development... those crazy sweedes.

yes they came up with that engine around 6 years ago from memory! good idea but more mving parts and sealing issues means it needs plenty to perfect in the higher HP type engines.

[30psi 4agte]

think of it in a turbo setup . you would be moving the Whole exhaust turbo EVERYTHING that would def be hard to do !!!!!!!!!!

[AndyTTR]

yeah, i just added a bit to my previous post about the sealing issues. That's gotta be hard on things like the flex joints and other connections like that. I know engines normally rock around a bit normally, but considering it'd have to contend with those normal motions plus an extra 4 degrees of displacment... i'll bet one of the key factors keeping it out of mass production (for the moment) is long term reliability issues.

[jeffro ra28]

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 8.7:1. That is why camshafts determine what the DCR is because the earlier it closes after BDC the higher the DCR.

[Nim]

30psi 4agte, isn't the compression ratio the compression... ratio... :S

14 psi air pressure + 15psi boost = 29spi * 10.5 (:1) compression = 304psi
14 psi air pressure + 35psi boost = 49psi * 8.0 (:1) compression = 392psi

I thought that was how it works, not calculating compression, then adding boost as psi post compression...

EDIT: This is a rough guide based on static compression ratio. You'd need to factor in compression loss from valve overlap (as explained above) to find dynamic compression, then multiply ( air pressure + boost ) * dynamic compression.