Help . . Compression ratios & AFR ratios

T

turbospool@1krpm

Member +
Im trying to get my head around understanding both CR's & AFR's. AFR's I seem to understand quite nicely now however CR's not so much. Now so far here is my understanding of them in my own words:

AFR's :

Air fuel ratios. This ratio determines how much air & fuel are being combined in the the combustion chamber during combustion. An AFR of 14.7:1 is considered a balance between rich & lean mixtures, with lower being rich and over being lean. This again is also best determined via EG temps as higher temps mean lean and lower rich.

Now also when for example controlling fuelling on cars via piggybacks, how I understand this is that the piggyback taps into the stock MAP sensor etc and then displays flow of air to the eye by its screen or through laptop, right? If so from here onwards, how do you balance fuel / air in combustion chamber as we go through the RPM range ? Please someone explain in simple words.

NEXT . . CR's :

I understand that an engine CR is a ratio to define what volume of air there is in a combustion chamber when the piston is at the bottom of its stroke compared to the remaining air volume when the piston is at the top of its stroke?

Now so from this, would I be correct also in assuming that if for example a thicker headgasket is added to an engine its CR would increase as there would be space for more air when the piston is at the bottom of its stroke and when its at the top of its stroke? Please someone explain this too. In addition also please explain in own words how low comp pistons work.

P.S. Please can people who can explain from their own knowledge and in their own words reply, just to help me understand, as I dont want anyone assuming this and that or copy & paste from websites, hope you dont mind.

Thanks
 
Last edited:
T

Timmy

Member +
The map sensor read boost pressure and has nothing to do with afr's really (it does but only in terms of fueling maps). However the lambda sensor aka o2sensor read afr's. The afr is reading the unburnt fuel in the exhaust gasses.

CR = Compression Ratio. As in a 10:1 ratio compresses the air fuel mix to 1/10th of the volume. I thicker head gasket will reduce the CR.
 
T

turbospool@1krpm

Member +
Timmy said:
The map sensor read boost pressure and has nothing to do with afr's really (it does but only in terms of fueling maps). However the lambda sensor aka o2sensor read afr's. The afr is reading the unburnt fuel in the exhaust gasses.

CR = Compression Ratio. As in a 10:1 ratio compresses the air fuel mix to 1/10th of the volume. I thicker head gasket will reduce the CR.
Click to expand...

Ok mate, thanks for the attempt but thats ^^^ very very unclear as im sure many will agree.
 
TO82

TO82

Member +
Basicaly the lamba sensor senses the amount of oxygen in the exhaust gases, which if lean the ecu can correct it with more fuel, vise versa if its rich.

Also if you were to use a thicker headgasket this would lower the engines Compression Ratio as the engine has more area to compress the fuel and air mixture meaning less compressed, usually in n/a tuning a higher CR is used, but to allow a turbo'd engine to run more boost the compression ratio needs lowering! ;)
Hope that helps you ;)
 
Last edited:
T

turbospool@1krpm

Member +
TO82 said:
Basicaly the lamba sensor senses the amount of oxygen in the exhaust gases, which if lean the ecu can correct it with more fuel, vise versa if its rich.

Also if you were to use a thicker headgasket this would lower the engines Compression Ratio as the engine has more area to compress the fuel and air mixture meaning less compressed, usually in n/a tuning a lower CR is used, but to allow a turbo'd engine to run more boost the compression ratio needs raising.

Hope that helps you ;)
Click to expand...

First paragraph about the AFR - thanks mate, is helpful.

Second paragraph on CR - OK. Wouldnt in that case a lower CR allow you to run more boost ? I mean If the CR is lowered ( more space created in the chamber ) then you can put through more volume of air/fuel during each complete stroke right ? I also know that knock comes into this, as meaning that, when the piston is on its way towards the top of its stroke, there isnt too much air/fuel in there that it ignites itself before the spark plug ignites it.
 
TO82

TO82

Member +
i put that wrong mate! n/a use higher compression and turbo use lower therfore allowing them to run more boost! ye your right.
:homer:

Duno what i was thinking there :confused: lol
 
Last edited:
TO82

TO82

Member +
Also the bit on knock this is when the ignition is retarded to prevent knock, therefore stop it igniting before it should. Depending on the type of engine and use. you need to find a balance between retarding the ignition as if you want the maximum power from the engine you need to have less retardation, but to be safe have more!
 
T

turbospool@1krpm

Member +
^^^ Thanks mate. Edging towards mastering these elements now I hope lol.

Also, so say when CR is lowered in an engine, by for example sticking on a thicker headgasket, what method(s) is used to determine how much extra fuel / air we can stick in the combustion chamber during each complete stroke? Would you just increase fuelling via a piggyback / fpr in accordance with how much boost your running (hence the increase in air vol) and again this would be via AFR ratio & EG temps right ?
 
Last edited:
TO82

TO82

Member +
You can use the A/f ratios to monitor the fueling and then would need to increase or decrease it in certain areas on the rev range, but it wont tell you what boost to run you would need to decide a certain boost level and get some kind of fuel managment mapped to correct the fueling and ignition, for the given boost.
 
T

turbospool@1krpm

Member +
So wots the best way to define what max boost you should run on a engine ? Just go by theory or is there something to look out for through piggyback / EG's / AFR's ???
 
TO82

TO82

Member +
Well depends on what turbo your running as for it maximum boost, as after a certain point the turbo will be creating excess heat for example, and therefore make less power, also depends what has been done to your block, if it has had the compresson lowered, if its running forged pistons and rods? it all depends what you want from the engine, but looking at the afr ratio this will only tell you weather you have enough fuel for the amount of boost you are running.
 
T

turbospool@1krpm

Member +
TO82 said:
Well depends on what turbo your running as for it maximum boost, as after a certain point the turbo will be creating excess heat for example, and therefore make less power, also depends what has been done to your block, if it has had the compresson lowered, if its running forged pistons and rods? it all depends what you want from the engine, but looking at the afr ratio this will only tell you weather you have enough fuel for the amount of boost you are running.
Click to expand...

Yes thats ok, but what I mean is, say for example on a un-forged stock EP running on stock turbo upgrades to say a T25 or something, how then when tuning this new setup would a tuner know what max boost the engine can run on this turbo?
 
T

Timmy

Member +
Expiriance is the only thing that tells us what the stock internals can take.
I personaly based on reading the forum think they take alot with all the supporting mods and a good map. But i think if you intend on tracking it the revs will kill it pretty quick.
 
C

clys

Member +
Another thing to take into consideration is that the ECU only "uses" what the O2 sensor has to say in open loop AKA part throttle or idle conditions. Once you mat the pedal, the ECU goes into closed loop mode and rely's on only the more important sensors to make adjustments, the MAP sensor being one of them. Basically a faulty O2 sensor will not make your car stop running, whereas a bad MAP will.

Another thing about AFR's is that in turbocharged cars, you want MUCH more than a stoich (14.7:1) AFR, as I'm sure you know. I prefer to run in the 11.8-12.3 range myself as it provides more power and detonation protection. In fact tuning my FPR tonight I noticed a MASSIVE difference between 10:8 and 11.3 in power, just something to keep in mind.

Remember those rep points. :)



Clyson
 
You must log in or register to reply here.
Top