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tuning for severe performance.. part1.

Hi guys n gals.

as part of the turbo conversion I am in the process of calculating the math to show performance gains on mk2 172.

Later parts will calculate ideal turbine characteristics for this engine (to be overlayed on compressor maps available from the manufacturer of the turbo (Mitsubishi), a volumetric efficiency map, and the theoretical torque and power curves that should result. most of this can be converted into theoretical performance figures.

ok.. this bit looks at a std mk2 172 running a standing quarter mile. after the standard result.. check out the results for increasing power at the wheels (flywheel est shown for those who like that sort of thing ).

It is a sobering thought isnt it.. ?.. look at the power increases you need to bring the times down a fraction..

Now you know why you find it easy / hard to keep up with / pull away from both cars with lesser power to weight ratio and greater power to weight ratio..

Enter the world of the harsh realities of performance tuning lol...

(do you still want that induction kit and exhaust ?)

145 bhp @wheels (est 177.2 @flywheel) , 15.628 s, 87.221 mph

155 bhp @wheels (est 189.4 @flywheel), 15.284 s, 89.181 mph

165 bhp @wheels (est 201.6 @flywheel) , 14.969 s, 91.059 mph

175 bhp @wheels (est 213.9 @flywheel), 14.678 s, 92.863 mph

185 bhp @wheels (est 226.1 @flywheel), 14.409 s, 94.599 mph

195 bhp @wheels (est 238.3 @flywheel), 14.158 s, 96.274 mph

205 bhp @wheels (est 250.6 @flywheel), 13.924 s, 97.892 mph

215 bhp @wheels (est 262.7 @flywheel), 13.705 s, 99.459 mph

225 bhp @wheels (est 275.0 @flywheel), 13.499 s, 100.98 mph also = (0-100 in 13.5 seconds)

235 bhp @wheels (est 287.0 @flywheel), 13.304 s, 102.45 mph

245 bhp @wheels (est 299.4 @flywheel), 13.121 s, 103.88 mph

255 bhp @wheels (est 311.7 @flywheel), 12.947 s, 105.28 mph

My ultimate aim with a sutable compressor wheel / housing is apx 230 bhp at the wheels for the turbo 172.


oooooooops missed the standard bit and figures used.

here ya go !


estimated weight = 2464 pounds.. (172 mk2)


fuel= 100

total for calculations = 2765 .. call it 2800 lbs

BHP at wheels, estimated standing quarter mile time, terminal velocity (speed at end of run)


135 bhp @wheels (est 165 @flywheel), 16.004 s, 85.168 mph

hiya mate....waiting with baited breath for the theory!!!

BTW, if VE of 100% is full cylinder filling, then, if a turbo can squeese an extra 20% more in, does it make teh VE 120% or does it stay at 100% etc, or is it not linear?

Can you mod the turbines? or would you get specially manifactured ones? or get the nearest similar one?

I thought a standard 172 could run around 15 seconds? Ive had a couple of goes 1st time and made a mess of the starts and still got 15.8

Hi all..

Ben, the ve can never excede 100 - ie - the cylinder is competely filled.

it is simply the fact that the charge is now twice as dense - twice as much fuel (presuming a 2:1 boost ratio after inneficiencies) and twice as much power.

the ve can be caluclated from a plotted torque curve.

take the figues of say. 3000, 4000 , 5000, 6000, 7000 rpm. Find the peak torque on the graph.. call this 100 minus the standard inneficiencies on even a tuned engine - say 4%.. (this can be calculated accurately by looking at theoretical max torque based on cc and rpm, but for this case its near enough )..

so, the peak is at 96%, find out where the graph peak is at what rpm and note it..

all the rest of the figures for a ve curve can now be calculated from the graph by looking at a paticular rpm.. seeing where it is on the torque cuve, and calculating the ratio of that torque to the paek shown.

for the 172 you end up with a figure of apx.

3k 83%, 4k 85%, 5k 91% 6k 96%, 7k 86%

from that you can plot a ve to rpm graph for that engine yep ??.

Ben, I will show you how to plot a ve to pressure ratio graph and then use it to select a turbo and the resulting influence on the power curve.

ps.. I could do with as many rr torque plots of the 172 as I can get.. have you any you can send ??

Mathew. quite possibly .. the figures are comparative, the percentage difference is accuarate.. so is the effect on times.



ClioSport Club Member
  E90 LCI 330d


Feel the power!!!!!!!!!!!!!!1

Joe, hurry up and get it done mate, I wanna go!

Mathew, just recalced the theoretical run with hardly any fuel, no spare wheel , and a lot lighter driver than me lol..

weight plotted total 2550 pounds.

bhp 135 @ wheels.

87.865 mph terminal

time= 15.513 s.

There ya go..


but.. to bring it DOWN by 1 second (to 14.513 s) needs an increase of 30 BHP at the wheels !!! -

Now, if you reduce the weight by 200 pounds from the new caclulated figure.. the difference is that the time is now 15.1 s @ 90.290 mph.

so as you see, weight is very important - but there is only so much you can lose..


ok, got it.
but would love to meet up when i get back to the uk and discuss how you calculate inefficiencies.

oh Ben, I forgot to mention. a good ve plot from a torque curve will enable an accuare assesment of WHERE in the engine the inefficiencies lie..

by looking at the change in torque after a mod, you can see where you have improved it and that will point to the next area of work.. ie.. induction or exhaust. (when the ve remains constant after mods.. its time to look at the cams )


overall inefficiencies at WOT can be deduced by calculating a theoreticall torque curve for an engine of same cc as yours and same rpm assuming a VE of 100%.

then transpose this over your torque plot and the difference from the theoretical line to the max torque peak is the inefficiency of the engine at its max flow rate.

all other points on the plot then are simple math as described above.

the inefficiency will be in the range of 2- 9 % for this type of engine.


it aint really that complicated is it, i guess you just gotta know it.

but i suppose the calculations are a bit iffy for the a newbie.
I suck at maths, eventhough i love it. especially when it explains my passion(read:life) and when i first found out (by myself) how much force was being exerted on the gudgeon pin on the wright brothers first airplanes engine!
was stoaked for about 4 mate confirmed it by lookin though his files (engineer) andhad the actual wright brothers engine graph!!

worked out at about 1/3 ton @ 3000 rpm.

i just can imagine that appearing and dissapearing 3000 times a min!!!!

love to know some more practical tech stuff off ya....jus goes to show how little you know when somebody like you turns up!!

Hey dood, dont sweat, you do fine.. dont forget I have been at this for 25 years lol


Each bit of work on the turbo mod I will produce the data and how to calculate it.


ClioSport Club Member
  E90 LCI 330d

Boost pressure is generated at the compressor side of the turbo, then fed from there via an intercooler (but not always) and into the plenum chamber, then through into the manifold.



ClioSport Club Member
  E90 LCI 330d

Yes, a dump valve goes before the plenum chamber, in the pipe that goes from the intercooler to the plenum chamber.

Ideally a dump valve should be placed as near to the plenum chamber as possible.



the dump valve is for the following reason..

you are bobbing along at 6000 rpm with 22psi of boost pressure

Suddenly, you lift off the throttle.. the turbo is still spinning at maybe 150 - 200,000 rpm (yes hundred thousand rpm !!) and generating pressure..

where does it go ??

answer - out the dump valve.

without this you can actually have a reversal of airflow in the compressor stage.. aghhhhhh !..


Chavvy, just to clarify a wee bit more what the guys have said.

the turbo is 2 propellers to all intents, connected on a common shaft..

exhaust out the engine is fed to one side and turns the propeller or turbine.

the other end of the shaft turns turbine completely seperate from the exhaust gasses.

air is drawn into this turbine from the atmosphere (eg outside air), the air is compressed and pumped out to the inlet manifold plenum.

Fuel is added by the injectors to the quantity (mass of air) that we have going into the cylinders and bang !. the exhaust rushes out and makes the turbine on the exhaust end spin again and the whole situation repeats.


Oh Ben.. lol..

Let me tell you how to make your own gas turbine too...

You HAVE to try this, its entertaining...

get an old turbo of something... any...

the intake for the compressor stage (air) is left open to air...

the output of this stage goes to the exhaust inlet on the turbo...

the outlet is left as an outlet...

now, insert a few largish bore hypodermic needles into the inlet to the exhaust compressor and rig up a simple electronic pump (12) with a potentiometer to control the flow..

feed the output of the pump to the hypo needles... hee hee hee hee

the pump supplies parafin...

spin the inlet (compressor side (air)) with a propane supply at medium pressure so that this time the air turbine is actually driving the exhaust... as it spins up.. and starts whining add a spark..

(you can put an RC glow plug in the exhaust turbine housing...)

there will be a point in the rpm range of the turbo where rotation is self sustaining as the exhaust turbine is spinning the air turbine to produce enough feed BACK to the exhaust turbine .

at this point add the parafin through the pump and remove the propane.. you vary speed by controlling the parafin supply with the pot,

instant gas turbine engine..

ps (you will need to hook up an oil supply too).

Way ta go doods.. diy gast turbine engines made from an old turbo..


unless you are slightly mental !


ok, the needles go into the inlet of the exhaust compressor. So since its connected to the outlet of the inlet (air) compresor, i will need to drill holes for them to sit? how do they seal?

do i have to get well out of the way when i add the parafin? will it make that cool noise?

oil supply to the norma oil channels on the turbo? can i use a gravety feed? and just dump the used oil into a container?

will a bigger turbo be more mental!?

how about a tiny turbo, are they cute?

man, could i power anything with it?

this is gonna be soooo cool!!!!!!

Ben, do a search on turbocharger gas turbine on

there are quite a fe wlinks with pics.. and some download sound files.. hee hee

there are several pics with different methods of mounting the injectors..

YES... stand back when you add parafin LOL...

oil feeds are discussed on some of the web pages..

Have fun..


if i die, you had better maake a good obituary!!!

cant do it till i get back to the UK

But will read up on it, cheers.

my mates into making DIY hydrogen bombs so i think hell like it!

we used to make bombs when I was younger lol..

a group of us.. a Cisco Specialist, me and a local policeman..

Photographic flash powder, strontium nitrate and a hot wire fire in a close container JEEZUS CRIZE !!!

also, Sodium Chlorate and Sugar (Before they added burn inhibitors to the weedkiller ...

Thermite is good...

Capt just a quick question When you fit your Turbo inlet maniford, you will remove the standard inlet maniford as well as that big lump as well?. Without removing the big lump below the inlet maniford is there enough room?.



First I must correct my spelling And Ive just asked a stupid question havent I ? HeHe. That big lump holds the injectors?. So you would remove both the big lump and normal inlet manifold. Then bolt on the turbo manifold so it would end up with the mapp sensor on the left side of the engine bay?.




Hi Chun..

yes.. its all removed

then the manifold bolts to the head, the inlet is at the gearbox area with the MAP sensor in the middle of the manifold.


oh, BTW... the reason the big lump is in 2 parts on a std 172.. is simply so you can remove the plenum chamber unit from the lower inlet manifold to change the spark plugs..


yup, cheers mate. read it all, but at work so will have to get my head in that mode after work.....
Cheers though!

You noted that it was with no turbo, why is there 1 turbo in the no. of turbos then?

sorry, didint look a teh calculations yet. Looks interesting though.