Turbo selection and compressor maps....

[InlineUK]

Hi Guys and Gals,
I am planning on adding a blow-through twin turbo set-up to a 250 inline six and dropping it in my 68 Stang. It'll be a street machine, low-mid range power is more important that top end and i want a fast spool. I'd like to run 10-12psi if possible.
I've doing some calculations and comparing these to some compressor maps, but i'm not really sure whether i'm doing it right or where my results should fall on the maps. I was hoping someone here might be able to offer some advice....

Here are some maps i prepared earlier....
Vertical lines are 2000-6000rpm, in 1000rpm steps, calculated at 12psi.

Do you think any of these would be good for a twin turbo set-up? Too big or too small?

Any advice greatly received!
Thanks
Nathan

 

[XPC66]

Well from my calcs you will get choke with the GT22 between 3600 - 5000 rpm, if you fit an intercooler.

This is on the basis your peak power rpm is 4600 (25.7lb/min, exhaust 9.9 lb/min wth I/C; 17.4 lb/min, exhaust 7.8 lb/min without I/C) and peak torque is 3600, have a decent cam and decent exhaust. Three cylinders 99.31 dia X 92.25 stroke?

The 40 trim will choke between 3200 and 5200 rpm, once again with intercooler.

The 45 will choke between 3800 and 4900 rpm

 

[InlineUK]

Well from my calcs you will get choke with the GT22 between 3600 - 5000 rpm, if you fit an intercooler.

This is on the basis your peak power rpm is 4600 (25.7lb/min, exhaust 9.9 lb/min wth I/C; 17.4 lb/min, exhaust 7.8 lb/min without I/C) and peak torque is 3600, have a decent cam and decent exhaust. Three cylinders 99.31 dia X 92.25 stroke?

The 40 trim will choke between 3200 and 5200 rpm, once again with intercooler.

The 45 will choke between 3800 and 4900 rpm

Thanks for the reply. What happens when a turbo 'chokes'? How do you work out the choke point? Does this mean i need to select bigger turbos?

 

[InlineUK]

Would something like this be better suited?
Turbonetics T3 Super 60

 

[XPC66]

There are few things you need to consider before you settle on what compressor you'll run with:

What rpm the engine makes best power per unit time (= max torque rpm) without any boost. This is typically a couple of hundred rpm greater than peak engine VE. If your compressor centre island doesn't fit the mass air flow at those rpm, it will result in the peak torque shifting up or down because of the error... although not accurate you can arithmetically visualise it as VEengine + VEturbo.

When you are selecting the boost pressure you have to decide if you are going to use an intercooler. In this instance of course you will, because your heat of compression at 12 psig nominal is going to produce sensible air temperatures of over 100°C. The intecooler is a series impedance so you have to factor in a pressure drop .... I suggest you use 2 psig. So to get 12 psig in the plenum/manifold you need to increase the compressor operating pressure point to 12 psig + 2 psig = 14. On top of that if you are running a 2V carby you will have around 3- 5psig drop and with a 4V carby around 1 -2 psig. So your operating point could be 12 +2 + 5 = 19 psig. You could throw in 0.1 - 0.2 psig for the filter and intake pipes, deducted from the atmos denominator in the PR calc. All of a sudden you're seeing why so many guys get it wrong.

You must select the turbine to suit. The turbine affects the operating point and range of the compressor. In my previous post I gave you the exhaust flow

" (25.7lb/min, exhaust 9.9 lb/min wth I/C..)"

. When looking at the turbine maps you will need to correct the 9.9 lbs.min adiabatically back from the 800°C - 900°C to ~25°C. Once again the exhaust equivalent of density ratio, the expansion ratio (ER) will need to take into account the dump pipe pressure drop ,which I would factor as 2 psig for the exercise :- ER = (Manifold psig + 14.69) / (Dump drop (e.g. 2) + 14.69). I like to select based on 1:1 exhaust manifold Vs Plenum pressure.

 

[InlineUK]

This is all very complex!! lol
So this means that i need to factor in the pressure drops into the equations from the start, e.g. using 19 instead of 12psi. And also i need to work it out using the rpm where max torque is achieved? Am i understanding this corretly?
Ideally where should the results fall on the compressor map? Should the point for max torque fall in the middle of the island?

I'll give the calculations another go.
Thanks for your help.
Nathan

 

[XPC66]

You should try to find out what the pressure drop on the carby is at your desired rpms e.g. if you are sizing the turbo to operate at high rpms, low rpms or midrange.

It's not that hard... just give it a go.

 

[InlineUK]

You should try to find out what the pressure drop on the carby is at your desired rpms e.g. if you are sizing the turbo to operate at high rpms, low rpms or midrange.

It's not that hard... just give it a go.

Trouble is i'm not sure what carb i'll be running. Probably a 4bbl, but this is still all in the design stage, i'm trying to match all the bits together before buying too much!
I'm sizing for low-mid for the street.
I'll give it a go!
Thanks

 

[XPC66]

Well I'm not full bottle on Ford sixes, but maybe you don't need a 4 barrel if you're running a turbo. A couple of twin throat Aisan turbo carbs might be the go.

You'd have to give some thought to an appropriate cam too, maybe something like a Nissan 348T profile, big exhaust ports, low rod/stroke ratio and corresponding lowish DCR.

Is your head a twin valve or single valve POS?

 

[InlineUK]

Well I'm not full bottle on Ford sixes, but maybe you don't need a 4 barrel if you're running a turbo. A couple of twin throat Aisan turbo carbs might be the go.

You'd have to give some thought to an appropriate cam too, maybe something like a Nissan 348T profile, big exhaust ports, low rod/stroke ratio and corresponding lowish DCR.

Is your head a twin valve or single valve POS?

I think i'll need something around 350-400cfm for a carb. But then again i'm not sure how adding a turbo alters the cfm requirements.

Yep, cam selection is also something i'm trying to work out!

The head will be the Aluimium head from Mike.