Big Turbo Problem
Now the last thing I want you to think is that it
was lack of planning that caused this problem. Bigger is not always
better (even for sports car drivers!) and it wasn't sheer greed for "more
blow" here.... or I could have just sought out the "mother of all turbos"
as in the photo here! (now that IS a big turbo!!!)
I actually spent many hours studying the matching
graphs (pressure Vs air flow
- it's not just all about maximum boost, you need the turbo to pump as much
air as the engine requires for a given power output too),
trim (compressor/turbine rotor diameter
ratio - affects turbo size, spool time and boost/flow),
A/R ratios (housing geometry ratio for
both turbine ane compressor - again affects turbo size, spool time and
boost/flow) etc. of the latest and greatest ball bearing turbos, to find
te perfect match for the engine.
I also wanted the turbos to bolt straight in place of the old T25s if at
all possible, so size was always a factor and also using a turbo with the
same T25 exhaust mounting flange. Understand also though, that I wasn't going
to be under or over conservative - the RIGHT turbos for the engine tune
level was the ultimate concern. I wasn't going to all the trouble of
modifying the engine to extent that I have, only to limit the final output
by the selection of turbos.
OK other people seem to use "reflowed" T25s with 360 degree bearings, or
"hybrid" T28s, but these are just comprimises to try and achieve more flow
in the existing T25 form factor - they are never going to supply the boost
and flow plus rapid spool up that I want. So it was: pick the best turbos
- if they fit, all well and good - if they don't, they will be fitted somehow,
whatever it takes!
So my fnal
choice was the GT28RS - also known as the "Disco
Potato"! (actually this name even appears on the Garrett product
catalogue and was coined by them from the shape of the turbo map - clcik
on the image on the right for the page from the Garrett Catalogue). The Garrett
GT range all have a dual ball bearing assembly instead
of the floating oil bearings of older designs like the T25. They spool much
faster and also have greatly improved turbine/compressor aerodynamic properties.
Even with these factors, it's difficult to get a "free lunch" and the more
common .86 A/R turbine vesrion gives very high power but would be some what
laggy on capacities under 2.0 litres (the 3.5 litre V8 equates to 1.75 litres
per bank). More research (and cutting though sales talk!) revealed that there
is a lesser known .64 A/R GT28RS - with this more efficient aero and
ball bearings, this should provide spool as quick as the old T25s but
hugely increased power capability. Before buying, I also emailed a few
suppliers for their views - Click here to view
the trail
So, I ordered
the turbos from the USA to take advantage of the good exchange rate and they
arrived in good time. On first inspection they looked impressive and definitely
"man enough for the job". The Exhaust turbine housing, although having a
very different outlet (a 5 bolt exit, for which I can get a V-band adapter
with larger bore exhaust - no problem there, as I will have a one off stainless
steel system done) didn't seem much larger than that of the T25s. The
inlet air compressor is much larger though - even the inlet is 3 inch, a
full inch larger than the T25s, as the photo on the right shows (GT28RS on
left T25 on right).
The alignment of the CHRA (Centre Housing Rotating Assembly) wasn't correct
for the oil banjos and coolant lines to line up, but this can be loosened
and moved without affecting the balancing. The exhaust flange was the right
pattern, but the drilled hols were smaller than that of the T25 (!?), but
this isn't too hard to correct. So I started trying the turbos in position
and, as the photos below show, they looked magnificent.....
Take a closer look at the photos though - when I offered the lovely new GT28RSs
up to the exhaust manifolds - although I couldn't put them on the studs because
of the hole sizes, it was clear that even if I could,
the compressor housing comes very close to the block
on the near side and the large 5 bolt exhaust exit would foul the bell housing
on the off side... OH DEAR!
Here's a photo borrowed from a recording of 5th Gear (the full video is on
the 5th Gear section). My contact, GW, is holding
up a GT28RS next to the T25 in situ - it's obvious how close the T25 small
exhuast exit comes to the bell housing and that the GT28RS won't fit in
the same position!
So, siiting there with the engine and turbos a thought occured (probably
at the same time as realising that the full proof, but expensive option was
repositioning the turbos completely and fabrictaing completely new, one off,
manifolds) - more clearance could be afforded by spacing the exhaust manifolds
out at their natural 45 degree angle (moving the turbos downwards and outwards).
It was time to share these findings with the PM71 project team -
Click here to read the email
trail
I made up the annotated pictures below to explain the situation and how
a manifold spacer could possibly be used:
In a positive vane, I also took these photos with the turbos in the positions
they would adopt with manifold spacers . They just might just be propped
up on mallets, but they do give a good impression of how it would look. They
certainly look a significant upgrade to T25s and clearly what's needed to
match the rest of the modofications to the induction... I know I'm doing
the right thing insisting on getting them fitted!
Now, whilst I was surfing the Net, searchin the Yahoo turboesprit message
board for
references
to Project M71, I happned across an posting about an
Esprit V8 being prepared by Garrett themselves for
a racing series in the USA.
It was clear from the photos that, as you might expect of Garrett, they were
changing the turbo installation. The shot on the right shows the engine in
the workshop, with modified plumbing for connection to their chargecooler
(photo in "Engine" section of "Design"
section)
This shot shows the car with an engine on stand at an exhibition - but click
for the enlargement and look closely at both turbos... the one on the
right has the old 3 bolt exhaust exit (probablt a T25), but the one on the
left has a new 5 bolt exit! I emailed the message board, got the Garrett
and race teams contact info and emailed them to fond out what was going on
- Click here to read the email
trail
So the answer was that they were using GT25R turbos
(that have the 5 bolt exit but are not as large as the GT28RS). The photo
on the right shows how the team have cut a slot in the bell housing to fit
the right hand GT25R.
The dimensions of my GT28RSs look like they make even drastic measures like
this less than what is needed - they need to be spaced away from the
engine/transmission. However, this means that they encroach on the chassis
tube that runs very close to the outer side of the turbo.
The annotated picture from beneath the car illustrates the problem here -
without the engine being fitted to the chassis, it is very difficult to guage
how much room I have to play with here. I may, of course, have to fit the
engine in oder to find the optimum position for the turbos and the exhaust
manifold arrangement - This isn't a step I planned on doing though
- I'd rather find a way to keep to my plan of developing the complete
engine/induction, map the ECU at the dyno/engine cell and then start the
assembly.
You'll notice from the underside view that the exhasts
have to take a fairly sharp vertical "dog-leg" over the drive shafts
too. So moving the turbos rearwards would exacerbate this problem. If
I do completely reposition, I'll probably use the position shown on
the right here - this will allow straight exhaust exits to the rear of the
car, won't present oil drain issues and provides an even shorter inlet to
the chargecooler. The diagram shows a cheaper alternative of using an adapter
pipe from the standard manifolds - but this position could also be used
with fabricated stainless steel manifolds.
Either way I'm concerned about the length of the pipework to the turbines
- Exhaust manifold design is quite complex because of the way the pressure
pulses from each cylinder combine - this is important on normally aspirated
engines to provide an "extractor" effect, but it is key on turbocharged engines
to ensure spooling of the turbine (the odd and even cylinders are combined
in the primaries and the two bores are kept seperate right up to the turbo
inlet on the standard manifolds). The thing to avoid is what's called
pulse reversion - where the exhaust gas travles in the wrong direction!
It is possible to merge the primaries at a "collector" point and then
run a secondary pipe to the turbo, but distances/dimensions are
important....
Well that's the Big Turbo Problem defined - what
the solution is, I'm not absolutely sure yet!
