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justinc
14th December 2007, 09:21 PM
Had a play with fuel and boost pressure yesterday. I had reduced the fuelling for our trip in September towing to Albany, and thought I might re adjust it.
I had noticed up a particular hill I was getting up to 495 to 500 degrees (upstream) and flat throttle I could manage only about 75kmh in 4th.
I adjusted the fuel stop one whole turn out, and disconnected the wastegate altogether, effectively making it into a non wastegated unit. This results in 15psi at the manifold on full fuel. I now get about the same temps on the same hill, but I can ACCELERATE from 70 to 110 in 4th, and the difference in inlet and outlet temps is huge. I now feel my intercooler is actually working better with the higher boost pressures and air speeds, and the efficiency is consequently much better. Maybe if the hill was longer, I would get higher temps eventually?
Now to do a fuel consumption test. I had been getting between 24 and 27mpg with a mix of city and rural driving.
After the adjustment, vehicle is even more drivable, pulling well from bottom end and allowing for easy low speed 4th gear work.

One thing is also a plus. I luurve the inlet whistle through the snorkel and drive round with the passenger window down and a :D

JC

Dougal
16th December 2007, 10:50 AM
Do you have any specs on the turbo you're running?

My little T25 has been pushing 20psi for about 3000km now. Still no intercooler but it's on the list. I cut and threaded the wastegate rod to adjust the boost. With the wastegate line plugged the exhaust pressure might still be blowing the wastegate open and limiting your boost.

I've also refitted my exhaust manifold pressure gauge. I had it hooked up about 2 years ago when I had turbo and engine issues. Back then it would run a small range of 1.5:1 (i.e. 7.5psi backpressure for 5psi boost) then would settle into 2:1 (30psi backpressure, 15psi boost) for the rest of the rev range.
But now I've got enough fuel, good engine compression and a turbo without a shagged comp wheel it's much better.
Still get the small range of 1.5:1 and 2:1 during fast acceleration (max EMP = 40psi). But under more steady load in higher gears the backpressure drops as the exhaust gas temps rise.

When I hit 650C EGT's the backpressure drops to 20psi and the boost is 20psi. While I've read of this magical zone before, this is the first time I've been there. The wonderful place where the turbo is giving as much pressure as it's taking, taking it's extra energy from the exhaust gas heat instead of pressure.

andrew e
16th December 2007, 07:35 PM
i need to **** off my $300 ceramic turbo so i can run more than 10lb boost.......... Ahhh bugger it. I'll play with my other 110 its more fun.

justinc
16th December 2007, 07:35 PM
Hi Dougal,
I have a TB25 watercooled one, and I feel it has reached its max speed a number of times at 15psi, as the whistle becomes a bit more like a howl, and this usually denotes overspeeding/ housing restriction. I used this .48 A/R housing for bottom end response and less lag, and it doesen't seem to enjoy much more boost/ compressor speeds!
I'm happy enough with the performance, and fuel economy is important to me whn it is 155c a litre down here!!

JC

Dougal
17th December 2007, 02:22 PM
With a good intercooler, you'll be pushing about 10% more air at 15psi than I am at 20 with no intercooler. 12psi with a 60% effective intercooler will deliver the same amount of air I'm getting now, plus lower EGT's.

I ran with temp sensors in the intake upstream of the turbo for a while. Back then it showed 15psi at 3600rpm with no intercooler was still on the efficiency map so I'm happy I'm not going to blow mine up just yet.:angel:

The factory 4BD2T turbo (freefloat T25 with a T3 mount flange) I have in my bargain bin actually has the same compressor wheel and housing I'm running now. Only difference is my ex-nissan turbo has a triangular outlet flange which I had to make an adaptor for, the Isuzu one fits a hose.
I'm sure Isuzu wouldn't have specced a turbo that'd overspeed in normal use, this particular one outlasted the engine (cracked head 4BD2).

I'm still averaging 10km/l but on 225/85 BFG AT's. I hope to break 12km/l on a flat 100km/h run one day.

Diesel here's only $1.27 per litre, but we pay road tax on top of that. 91 petrol is around the $1.70 mark. 98 is butt-clenching.