turbo 5.0litre V8 need turbo help again

Tuzza wrote:
 >
 > made the sparkplug gaps bigger and setup the fuel pump for boost reference.
 > the car seems to run nicely now however the turbo is still not boosting
 > (checked the wastgate, it is fine). I took off one of the vacuum hoses and
 > revved the car up a bit and it was only sucking (the size of the turbo on
 > the size of the engine it should spool around 2400rpm). This setup has
 > been done before on a holden engine (4.2litre, 253) but i cant get in
 > contact with the person who done it.

Taking a stab in the dark the website used to be :

<a rel="nofollow" style='text-decoration: none;' href="http://www.geocities.com/luke111vh/" target="_blank">http://www.geocities.com/luke111vh/</a> with the page title psi.253 ?

there's a msg that said the new site is at

<a rel="nofollow" style='text-decoration: none;' href="http://members.optusnet.com.au/turbogsxr/index.html" target="_blank">http://members.optusnet.com.au/turbogsxr/index.html</a>

From memory (and the only reason it seems to have 'stuck' is that myself
and a couple of friends have also used these and the series 5) it had
twin turbos from series 4 rx*7s. The main difference between the s4 and
s5 is the twin flap dual port wastegate in the s unit. With boost much
above stock (or with other mods that netted more flow at the same boost,
some people have anecdotally indicated that they were at the threshold
of boost creeping up because there was sufficient exhaust gas (and the
gate was borderline small) that it started to overrun the gate.
Presumably that played some hand in thw dual version in the s5. Some
have said that the s5 also had a larger exhaust housing (in terms of
exhaust trim, or whatever the correct term is) but I suspect that the
much larger external dimensions to accomodate the extra port and flap
give the false impression that they are different, but in fact with
respect to what really determines the trim, the clearance and 'shape' of
the housing around the wheel are almost certainly identical. I wouldn't
swear to it, but I am basing it on the fact that they (on identical size
and spec motors) seemed to have no difference in their spoolup rpm
points.

So moving on from all that torturous explanation, based on the engines I
have some experience with that particular turbo with, and using the very
basic 'rule of thumb' of extrapolating the cubic inch difference and
what rpm would net the same 'constant' with a bigger engine (and there's
a shitload more factors involved, but it's at least a ballpark figure) I
don't think you are going to get meaningful boost until more like around
the 3000-3500 rpm mark (probably a rough idea of the onset of boost rpm
and rpm it reaches full boost by, very roughly..

The turbos (if it is the s4 ones) have to the best of my knowlege a
..8something exhaust housing spec. From what I have surmised from guys
running twin turbo 302 or 350/51 ford or chevy motors with ford 2.3
turbos (which are cheap and readily available there, the mazda sourced
ones are one of the few that are really 'bargains' second hand in
Australia in contrast, I think because a lot of people are frightened as
to what size engine they might fit coming off a rotary. On that front
they have a relatively larger exhaust spec for the comp spec they run,
at least in comparison to what's more typical on piston engines. Not
that it's a big deal for the price, and they can be relatively
affordably fitted with to4 compressor wheels and housings (and you need
the to4 backing plate). but they are actually quite capable in any case,
esp for a streeter..... But to the point, the guys running the ford
sourced turbos generally use the 0.48 housings on the 5 litres, and the
0.63 exhausts on the 350 odd cubed ones, you are running looser housings
yet, but it's not worth losing sleep over. From what I gather the 0.63s
on the 351 engines are little restrictive above 5500 or thereabouts, so
it's not as if the 0.63s are perfect for a 350, and the 0.8somethings on
your 308 are ludicrous, it's a little less of a step than that.

As has already been said, they won't make boost without a load - you
need to be in gear and with considerable throttle input and rpm, but I'd
suggest you might end up not seeing significant boost until the motor
spins above 3000rpm.

I meant to reply to this earlier, but forgot. I visited your page, but
didn't spot a pic with the carb bonnet. Sometimes the bonnet design
itself can be problematic, in particular but not limited to how low the
air entry is, and what diection it comes in from. if it's low and
directly from the front or rear, the air rushing across the fuel bowl
vents can actually start to depressurise the fuel bowls. It can show up
as anything from a slight leaning out at higher and higher rpm (not
necessarily with any boost increase) right through to where it
momentarily almost fully disrupts main metering and it pops and coughs
right through the inlet manifold making it nearly impossible to maintain
any boost and drive it (that's aside from the potential damage of a lean
out)

If it's the setup that was on the car before, I'd have to be inclined to
think that it was working with the 253, so it's not likely to be a carb
bonnet issue.


 > I really dont know what to do
 > anymore. Had a mechanic look at it yesterday and he was stumped, he
 > basically told us all the things we knew already... might have to find a
 > turbo specialist to look at it,

If you are in Melbourne, best of fucken luck. When I did one of my first
blowthrough setups, I didn't know what foam to use to fill / reinforce
the floats (regular hardware store rapid expanding foam is fine if
anyone cares). On holleys and some other carbs, you can actually buy off
the shelf (and very low cost) nitrophyll floats, and they are
practically indestructible. So I went to a local turbo specialist (I
actually live not to far from a fairly well respected one). I was out of
luck, as their carby specialist was on holidays. I went back two weeks
later, and just missed him. Finally managed to catch him there and he
then disclosed that he didn't know as they had never actually even tried
a blowthrough carbed setup, they'd only ever worked on a couple of
drawthroughs using SUs. To be fair, he then actually rang around most of
the turbo workshops in Melbourne and no bullshit, not one of the places
he contacted (while I was standing there) had done a blowthrough carbed
setup either. He didn't ring every place out there, but quite a few.

There was one place that I recall that did have experience with sorting
and optimising blowthrough setups, they aren't specifically a turbo
specialist, they are a dyno tuning / performance engine building setup.
Someone brought it up a while back. If you still have no joy, I can
track the details down.

If I could give you one single 'mod' that would be of great benefit (and
cheap insurance) not to mention practically $0, then it would be
concerning the high speed air bleeds (called air correctors on webers
and dellortos, which is ironically where I first became aware of it
specifically concerning blowthrough setups. Carbs aren't perfect, but
they aren't hopeless. They do however have limits as to the extent
they'll be able to meter fuel with changes in atmosperic pressure (which
is essentially what you do in a blowthrough, supply the fuel bowl and
mouth of the carb with what is more or less increased atmospheric
pressure. air rushing through the venturi will still cause a relative
pressure difference between it and the fuel bowl (both of them are way
higher than in an normally aspirated setup). the thing is though, as the
boost pressure goes up, the carb doesn't really fully realise that the
air is denser. It does most of it's fuel metering (across the rpm and
throttle position spectrum) via the velocity of the incoming air.

What this means is that the higher the boost goes (for the same general
rpm range) the carb will actually end up not ramping up the fuel flow
proportionately, so the jetting that gave ok a/f ratios at say 7psi will
get progressively leaner and leaner with more boost.

You could of course just run big jets and that would make the a/f ratio
ok for full boost conditions, but at every other time, it would be way
to rich for most other stuff (involving more than say 1/2 - 2/3 throttle
input.

You could leave the jets as is, and drill out the power valve channels.
The power valve will open under boost (just before it in fact) and it
would be like having richer jets whlst on boost. Problem remains,
though, that it would still be a little 'unrefined' and you'd still
likely have it richer than ideal (to the point it would slightly reduce
acceleration, since you don't actually spend much time at peak rpm, you
spend more acceelrating through the rev range then repeating with every
gearshift.

Ok, now if a carb didn't have high speed air bleeds, as airflow volume
and velocity (not density at the moment) increased through the carb,
with the same jets, the mixture would actually get richer and richer and
richer the higher the rpms went. It's a result of the method they
utilise to meter fuel, so can't really be done away with altogether. So
to compensate they have the high speed air bleed. As the air and fuel
flow increase these strategically placed (and sized) holes allow a small
amt of air to be drawn into the main fuel metering circuit, and it
bascially 'dilutes' the fuel being discharged. The right size bleed will
actually not lean it, but perfectly (well adequately) manage to
proportionally influence the fuel delivery so the a/f ratio is able to
be tailored across the operating range, not just ok at one point, and a
compromise everywhere else.

The 'background' was a pain, but the actual 'mod' is the opposite. All
you basically have to do is to reduce the size of the high speed air
bleed orifices. It won't affect the mixture at part throttle cruising on
the freeway at all. But as the rpm and boost rise, it basically has
'disabled' a greater proportion of the high speed air bleeds function,
so now instead of it stopping the mixture going to rich at the top end,
it's striking up a new balance and allowing more of the natural
richening up tendency or 'phenomena' to take place (or no longer
preventing it) and can in doing so offset the tendency for things to
lean out as the air density (i.e. boost pressure) rises. See iwth a
blowthrough you have both happening as you spin up through the rpm range
at full throttle, increased velocity through the venturis, but also
increased density from boost pressure.

As a ballpark (and I haven't heard of anyone who needed to part company
from this range much) look at reducing their area by 50% . Not the inner
diameter, the actual cross section area. So you'd have to work out (or
look up) the current i.d. of the bleeds and then calculate the area and
what the reduced diameter to aim at will be to provide half the area.

Again going from a very vague memory of that site, the guy was running a
spreadbore 650? Some people only reduce the primary side air bleeds and
compensate not doing it to the secs by richer secondary jetting (which
is still better than no air bleed mod at all) , but I'd strongly suggest
that seing as the secondaries on a spreadbore are providing fae more
flow (far more than the closer to 50 50 with squarebore holleys) that
you do the secondary side as well. Maybe meet in the middle, and reduce
the secondary high sped air bleeds by only 30 % or something.

If you want ideas on how to do this without costing stuff all, and
without it being a huge engineering 'challenge' or even where they are
on the carb, email me, I'll send a pic or two and details, but I've just
about worn out the keyboard typing this post.

--
John McKenzie

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