How do you know which turbo is suitable for application?

i know the compressor maps can tell you, but how?

I read the thing chip put up, but still im not 100% sure how it works.

im assuming the only real way is to have experience of the engine / turbo?

for instance an engine that makes 200bhp in nasp form, how would you use the compressor map to get say 400bhp from a turbo, when you're going to lower the c/r etc?

 

ie this thread.

http://www.vauxhallsportforum.com/ph...urbos-t92.html

How do you know your say 2.0 engine is going to make 200bhp at say 1 bar?

i dont know if its my migraine stopping me understanding or that im just thick lol.

 

You have two options mate.

1 = Assumption
2 = Experience



Assumption works like this:

Your engine is making 200bhp, and is doing so from 13psi absolute pressure

Therefore its making 15bhp per psi you put into it.

So at a bar of boost, it will have 29*15 = 435bhp


Then you have to take into account the restriction the turbo offers, the extra heat in the engine etc.

So you knock a bit off, say 10-15% or so.

So you estimate that at a bar of boost you are going to get 380bhp or so.



That sounds very high for a 2 litre engine running only a bar of boost, but obvioulsy the thing is you told me it was 200bhp from 2 litres in the first place, which is also very high, a cossie motor N/A on a typical restrictive inlet (and hence 13psi absolute versus potentially 14+ for TBs) and on the cams it runs etc even with the compression higher would be lucky to see 150



Now you know you are going to need therefore just over a bar to see your required 400bhp.

So you find a map with 2 bar absolute on the left, and 40lbs/min (its roughly 10lbs per BHP) along the bottom landing you on a decent effeciency island and hey presto you have found a turbo in the right ballpark on the compressor side.
Then pick a fairly standard sort of turbine housing, like a .63 as its not a massive power output per litre so surge isnt likely to be an issue.

 

PS

As you can see, thats option 1, assumption

option 2, experience of that specific engien and having tried it with lots of turbos etc, is what you get if you got to Stu and ask for a turbo choice for a YB

If you ask Stu for a turbo choice for a vtec, you probably get something in between the two!

 

cheers chip.

would dropping the c/r make much difference in terms of starting power?

 

It would, but ignore it in terms of this "getting a ballpark figure", its not really relevant here.

 

Ok, that was part of what i was struggling with.

so say you take an engine of 2.3 litres and 5 cyls, making say 280bhp at 2 bar absolute, subject to everything else being suitable around it, and you wanted 400bhp, you could specify a turbo that would do the job, say at 3 bar absolute, simply because lbs/min is a known constant, regardless of capacity?

 

oh and by lowering c/r, as you wouldnt know the power you've dropped to, 400bhp at 2bar absolute may become 350bhp at 3bar absolute?

 

Yes mate, thats spot on correct, and is probably about the right amount of extra boost to pick up the extra 120bhp too

 

Yes thats the effect, although its unlikely to be 50bhp less, more like 5bhp less as you advance the timing too to make up for the lower CR anyway if you go too far.

 

cool..

im only half as stupid as i thought i was

cheers for that, i understand more about it all LOL.. shame i couldnt pick it up properly first time round, but nevermind LOL.

Ref c/r - ah, should've realised really, as with the discussions of lowering the c/r on cossies doesnt affect bhp because of the timing! DOH!

 

PS

For that example you are giving there, in terms of garretts, a UT38 would be a really good option

 

Cheers chip. not looking at changing the turbo, but was using a different engine type to see if it made any difference, which it doesnt really seem to, as long as the engine doesnt produce that power already that the turbo can supply (ie the 8litre V10 example)

 

Yes if you try and put a 400bhp turbo onto a 400bhp engine, you will just end up with an engine thats crap and responsive comapred to the original but with no more power.
Although potentially you could see a torque increase in the midrange, but it really wouldnt make any sense and the turbo needs to be bigger.

 

subject to everything else being able to take it

cheers again chip, it makes alot more sense now than it did first time round.

now to investigate cam choice and port sizes

 

TRIAL AND ERROR

 

Personally I like to use the mildest cams and smallest ports I can up to a point.

Its a turbo engine, so let the turbo do the work.

 

big boost to keep torque up like Rod's old 504bhp engine?

shame the rods seem to bend at higher torque in this engine

 

Yes, i like the way that Mark spec's engines

Bad news about the rods though, thats where this experience business comes in, fat better to know that first!

Bet you can get a steel set of eagles or similar for about 400 though.

 

tis really.. though one engine is running 315bhp at 1.35bar of boost with std rods..

Gotta love T5 engines LOL

yea, we like boost n torque LOL