Is it true that on a set engine, you can produce the same or maybe more power using a bigger turbo but less boost?? Ie keeping all the rest of the engine the same, changing from a T3 to a T34, you can run less boost pressure but produce the same power??

 

Yes basically this is correct!but there are many factors i.e how much air your engine can consume etc!but in general if you have a T3 on a stage 3 cossie running 23psi you would see around 300-310bhp but with a T34 maybe 330-340 due to the higher volume of air the T34 can move!so you could prob run the T34 at say 21 psi for 300 bhp!

 

right, this is were im stuck, surely the more power comes from more air being forced into the engine, and as the size of the cylinders stay the same volume, the only way of having more air in them would be to increase the pressure it is under, ie if you have a 2.0 engine then at atmospheric you would have 2.0l of air (not quite 2 litres i know but close enough) regardless of how much air you can flow up to the inlet valves, the only way of actually forcing more air into the cylinder is by increasing the pressure??

sorry if this is a bit babbly,

 

Where does the pressure come from?

From the turbo yes?

And where does it get the power from to make that pressure?

From the exhaust gas yes?


So a bigger more efficient turbo, will provide less back pressure in the exhaust, and hence the engine can breathe better for the same boost level, and hence make more power.

 

correct more air being forced in the engine = more power!

You are looking at it that 23psi is 23psi and the same volume of air no matter what the turbo!

compare the size of a T3 to a T4 the reason for it being bigger is so it can shift more volume of air per psi!

So take a T3 running 23psi a T4 wil only have to say run at 17psi to shift the same volume of air!

 

You need to think of it more as a dynamic process, not just one static volume of 2lt that is being filled. The larger turbo can supply the same volume of air but at a lower pressure, up the boost to the same level and a greater volume of air can be supplied.

 

and what chip said lol!

 

there is a massive essay on this in the tech essays section, and also i beleive on MSD's website from stu's fast ford articles.

well worth a read of both, you need to remember to stop concentrating on only the inlet side of the engine and take it all into account to get your head around it

 

Agreed

The engine and turbo both interact with each other on the inlet side AND the exhaust side, and both can effect the engines power potential.

 

right, i understand chips comment, that its the breathing efficiency, but if you take one cylinder at the bottom of the inlet stroke it will have a volume of .25L, if you then wanted to increase the amount of air (say double to make it easy) then the size of the cylinder hasnt changed, the amount of mass of air has doubled then surely pressure has to be doubled??

im not being awkward i just dont understand lol

 

right, just read what youve put and il try reading that lol

 

If the inlet stroke involved it pausing at the bottom of the stroke for a while yes.

In an engine when its moving quickly it doesnt get a chance to fully fill with air, also it doesnt empty fully on the exhaust side either, and increased pressure in the exhaust manifold increases that effect.
If its not fully empty, it breathes in less.

Then there is the fact that breathing out against a higher exhaust back pressure means the piston has to do more work to push the gas out, so therefore there is less power left at the crank.


You just arent thinking of an engine as a whole enough, you are focusing on one little thing and missing the bigger picture.

 

Just another thing for you. Before getting my car live mapped i was running 18psi on a T4, this was alot faster than a T34 car running 27 psi peak and 23 holding.

As chip has said you need to look at the bigger picture, it comes down to which turbo can flow the most air. The way you are looking at it would mean that you could run a T25 instead of a T4/GT30 and get 500bhp on a YB just by upping the boost to the same level as what they would need.

 

exactly my point nash, i was gettin to the point that with a vacum i could run unlimited horsepower!! many thanks to all that have exlained, confusion over,

cheers all

 

In a vacuum you would run NO horsepower.

 

i know!!! but from my first point you could run a bigger turbo on lower boost for the more horse power, if you folowed that trend you could run a massive turbo on vacum for infinite horsepower, thats why i asked the question because it didnt make sense,

cheers anyway chip

 

Well by your logic, for infinite power it would need to be an infinite sized turbo too

 

true, and that aint ever gonna be practical

 

practical? it would consume the entire universe and all life and energy would caese to exist

greenpeace would go MENTAL