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Discussion Starter #1
I got my Exhaust Manifold pressure gauge hooked up again last week.
I used to have it connected about two years ago and had it well benchmarked then.
But since then there's been both an engine rebuild, fuel pump issues uncovered, turbo swap and turbo rebuild. This is on an Isuzu 4BD1T.

Back two years ago my max EGT's were about 600 deg C and the turbo had dust erosion to the compressor wheel. Backpressure to boost ratios had a small range of 1.5 (e.g. 5psi boost for 7.5psi backpressure) before settling at 2:1 where it sat for the entire speed and load range. I was running 15psi which would deliver 30psi backpressure.
I had a T25 which was fractionally bigger, it would provide an even 2:1 backpressure to boost throughout the range.

Now I'm running a rebuilt T25 with a good compressor wheel and run EGT's up to 750 deg C with the occasional accidental excursion over that.
Now the ratio starts out in the 2:1 region and drops as EGT's increase. At 650 deg C the backpressure to boost ratio drops to 1. 20psi boost and 20 psi backpressure. While I'd heard of this mythical zone where the turbo boost meets or even exceeds backpressure, this is the first time I'd been there.bounce

At cruise on the open road I'm pulling about 8psi boost and 15psi backpressure with EGT's of approx 430 deg C. No intercooler at this stage, but it's going to happen.

Oh yeah, this is using a small 2 1/4" exhaust which extends to the back corner. I plan to shift the pressure tap to after the turbo soon to see what penalty the exhaust is extracting.
At the moment as I approach and pass 3000rpm the manifold pressure increases and brings the ratio back to about 1.5 at full load. This is the point where a better flowing exhaust would help me. But it could also be the flow limits of the turbo. The extra pressure tap will tell me.
 

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Great in-depth info with details, as usual. Dougal, you are a valuable asset to this site. Thank you.
Terry
 

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That is cool. So many turbos are sized with a turbine that is too small so that they can get that instant boost off idle. If you give up just a little on the bottom the entire system will work better in the end.

Another thing that your measurements prove and many have a hard time understanding is that heat energy does in fact drive the turbine. The greater the delta across the turbine, the better it works. This would make sense that as deltaT increases drive pressure would decrease.
 

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Discussion Starter #4
Thanks for the kind words.

This turbo is very small. It's a Garrett T25 with a 0.48 A/R turbine housing. Originally it was off an 1800cc nissan car. A larger A/R turbine would give lower manifold pressure and more top end, but the low end boost isn't something I can give up.

I get some boost from around 1000rpm, can get 10psi by 1400rpm and 20psi by around 1800rpm.
Under full throttle conditions at low rpms the backpressure does spike to about 40psi before dropping as the manifold and turbo warm up.

I can't wait to get a variable geometry turbo on there. I've got one but I have to fab up a special neck adaptor to make it fit.
 

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Discussion Starter #6
What kind of parts did you use to rig it all up?
Just drilled and tapped the manifold in much the same way as for an EGT probe. From there I had a short steel tube extension to lose a little heat, then a brass compression fitting and a metre or so of copper tube.
Then rubber hose through to a pressure gauge on the dash. The length of hose damps out most of the pulsing, but I'll put a restriction into one of the joins to smooth it out a little more.
 
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