Help needed

Hi all,
Looking for some genuine help with this one,i am planning to play a little more with the engine in my 850 T5 but before doing so i was advised to install sometype of gauge to tell me how hard the engine is working,the question is which gauge gives the most important information

oil pressure gauge?
exhaust temperature gauge?
oil temperature gauge?
cylinder head temperature gauge?
water temperature?

to make the question even more difficult i dont want to have 5 gauges excluding the boost gauge already installed dotted all over the car,might take away the surprise factor sitting beside a impreza,so i only have room for 2 discreet gauges, which two would you take given the choice? .

Any help would be very much appreciated guys


Jason

[Pedro Fandango]

Hi Jason,

i've added an air/fuel ratio gauge (aswell as boost), just to make sure it never runs lean

[warthog]

Hi Jason,

for a mechanical engineer, this is an interesting question, to which I can't see a simple answer - is anything simple these days? When choosing how to instrument a system one first has to decide what data you are looking for and how you are going to analyse the results obtained. To get any meaningful data you have to closely define the environment in which the testing takes place. This can either be done by very careful control of as many variables as possible or by very extensive data logging of the variables. This is why engine test and development cells are so expensive and complex.

To take your list of gauges as a start.

Oil pressure - unless you have a mechanical fault this should remain fairly constant as the engine speed increases (once above fast idle). Oil pressure will tend to drop as oil temp rises. Depending upon the cause, either one can cause the other. With an oil cooler and thermostat the oil temp should remain reasonabley constant once up to normal operating temp. Oil temp/pressure data tends to give indications of impending mechanical failure rather than how stressed a well running engine is.

Cylinder Head Temp. Useful in an aircooled engine, pretty meaningless in a liquid cooled, well running engine as the whole point of liquid cooling is to maintain a constant controlled temperature. It will rise if the liquid cooling fails for whatever reason, but this failure should also be apparent from the coolant temp. gauge.

Exhaust temp gauge (EGT). Useful in determining how efficiently the combustion process is working. Peak EGT should be seen when the mixture is weakest ie in the cruise. At full throttle the engine is normally setup to run rich (to a greater or lesser degree) and would show a lower EGT than in the lean cruise. This is not the same as exhaust pipe temperature which will be an equilibrium between the energy delivered by the mass flow of hot gas passing down the pipe and the heat transfer out of the pipe by external cooling (radiation, convection, conduction etc.) To get a small picture of how the whole engine combustion process is working one would need to have EGT's from each cylinder seperately.

If you are wanting to see how much power the engine is producing and delivering to the wheels the 'simple' answer is a torque meter attached to the drive train. An obvious area to put one could be the top engine torque link. A simple strain gauge would suffice. That's the easy bit. Analysing the data and relating it to the power output is another major mass of work!

To make anything simple requires varying degrees of assumptions. Whilst pondering this question, I thought - Engine power output is related to mass of fuel burnt. Where does that energy transfer go? 1. Out the exhast 2. through the cylinder walls and head into the water system 3. rotary motion into the gearbox 4. heat transfer into the oil that then transfers that heat into the cooling water 5. Heat transfer through the engine cases coooled by the environment (airflow). 6. Light (mostly in the Infra red spectrum) 7. Sound

Assuming that all the losses are related to each other ie as one goes up so do the others, if one was to measure the differential water temp. (Water outlet temp. minus water inlet temp.) and also measured the water flow rate one could then calculate the amount of energy transferred to the water. This would be an indication of how much energy the engine was producing. However, with the sweeping assumptions made in the last two paragraphs, I very much doubt the figures would be of much use!


There you go. Plenty for others to tear apart and shoot down in flames :-)

Cheers

Brian

does this work like a bicycle puncture repair kit ? as long as you have one, you never get a puncture !

but seriously ... what do you do IF it shows an incorrect AFR ?

I fitted an oil pressure gauge to a old British motorcycle once - I had to remove it coz the reading was too scary when hot!

May I suggest to the poster that any additional instrumentation will probably just confuse or amuse rather than actually aid engine health diagnostics etc.

Just tune it up and change the oil & filter regularly - you wont go far wrong.

All the best

Dr D

"There you go. Plenty for others to tear apart and shoot down in flames "

Unless someone can translate all that into English for the rest of us to understand then I don't think they have any right to tear it apart. Mechanical Engineer or no Mechanical Engineer, that is a lot of info to be storing in ones head!

Surely knowing how your car drives and being able to "feel" or "sense" problems due to changes in performance etc along with regular maintenance and visual checks is surely the best way to know your car is running well. Anything else is more for show (although it does look kinda cool).

[bulldog]

How about one of those Halfords specials that show time, internal temperature and direction of travel. Its coming up to Christmas so they must have loads in stock. £4.99 I think. They even got a sticky bit to put it on top of the dashboard.

if you require extra instruments the three insrumets worth fitting are oil px this will give a good indication of the condition of the big end and main bearings and can be usefulin detecting problems before its to late a (low oil px switch is set around 7psi) a oil temp gauge and a ammeter (not engine related but use full anyway)

[Ross9]

Agree with Andy hear, Oil pressure is useful to keep an eye on, oil temp if your going to be driving it hard a lot, Cov Crew mentioned an Air Fuel guage, this is very handy to have, as to the quesiotn about the bike puncture kit etc, if it shows lean, then slow down, take your foot off the gas and if it doesnt come back up, find a safe place to stop.

Good for tuned cars running a lot of boost, perhaps the injectors get to 100% duty, or theres a blockage in one, fuel pressure drops etc, all things that might manifiest themselves, usually when approaching high rpm and flat out in the case of injectors maxing out.

So, you say 2 guages, I would go for oil pressure and air/fuel guage, if you can fit a 3rd one, oil temp.

Ross

[Ben O Brien]

Hey,
Yes i agree totally,i have them for my 242 project and also have an external temp gauge although not at all engine related just handy,these are all volvo gauges so fit in the dash
Best Regards
Ben

Thanks very much for the reply guys really apreciate the help,this one really had me confused ,now i know what i want for christmas.....wonder could i persuade her to buy blitz gauges.

Thanks again.

Jason