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.
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.