Jump to content

Pressure Ratio formula, for tuning turbos


dennhop

Recommended Posts

I got this formula off another site, but figured I'd toss it out here in case anyone else was looking at tuning either single turbos, or compounds...it's really more for balancing pressure ratios between compounds... Pressure Ratio is (atmospheric pressure + gauge pressure)\atmospheric pressureSo an example would be as follows (and I'm using my current setup as the example)Right now, my overall boost is maxing at about 55 psi, and the primary (atmospheric turbo) is maxing at 20 psi. Pressure Ratio is (atmospheric pressure + gauge pressure)\atmospheric pressure, then my two pressure ratios are as follows:(14.7+55)\14.7=4.74 overall Pressure Ratio(14.7+20)\14.7=2.36 Primary Pressure Ratio (PR)Then to reverse engineer the pressure ratio and boost of the secondary:overall PR\Primary PR=Secondary PR4.74/2.36=2.01 Secondary PR.then, reverse the original formula, so 2.01*14.7-14.7= 15 psi at the secondary. What this tells me is that with my current setup, my atmospheric (primary) charger is working harder than the secondary, which means I need my secondary to hold the wastegate closed longer, in order to get the secondary to spool more. Also you can see, the pressure ratios aren't quite matched up, which also shows the imbalance between the two, and the fact that my secondary is acting more as an airflow blockage, than anything else. (I really need to get my drive pressure gauge hooked up!)I just figured I'd toss this out there, for you guys to play with, in case anyone was curious. The atmospheric pressure is going to be different according to your location and elevation, so make sure you look it up where you do most of your driving, to get the most accurate numbers!

  • Like 1
Link to comment
Share on other sites

Well, after a little test drive today, with the DP gauge running, on a WOT pull, I got 55 psi on overall boost, 20 psi on the primary (atmospheric) and 60 psi drive pressure...pretty close to a 1:1 ratio. Only problem is my secondary still needs to spool faster. I'm going to have to go with a boost elbow or something.

Link to comment
Share on other sites

  • Moderator

I don't think it's a bad thing having the primary work a little harder. The point in twins is to get the spool of a small turbo, with the flow of the big turbo. With being nearly 1:1 I would say you are setup very well. If you look at the maps for both turbo's I bet you find you are in the sweet spot for each one, which means the best, and coolest, compressor flow you can get. That also means the turbine isn't spinning too fast. What boost is your WG opening on the secondary?Do you have partial throttle numbers? Like at 20 psi overall what's the split, or at 40 psi overall.If you want to increase the work on the secondary you need to reduce the WG flow, which will increase drive pressure. I would consider leaving it, it's not like the primary is at 4:1 and the secondary at 1:1.

Link to comment
Share on other sites

It's kinda hard to watch all the gauges, but best I can tell, the primary is running consistently up with the secondary, so if I'm at 20 primary/55 secondary, I'm pretty much at 10 primary/27 secondary. I'm NOT happy with the EGT's though. EGT's are still running up faster than they should, and I can't get the boost past 55...before I was pegging and wrapping a 60 lb gauge. I installed a needle valve into the wastegate reference line, so I need to keep playing with that, and seeing if I can get that tuned in. I honestly have no idea what the wastegate is opening at on the secondary...I need to check that out. Drive pressure is still pretty much a 1:1 ratio though.

Link to comment
Share on other sites

Turbos were the only thing that changed at that time....however, I did realize I was starving the truck for fuel, so I increased the preboost fuel screw amount on the back of the AFC. However, its going to be a bit before I can test it out, as my tcase decided to explode yesterday.....

Link to comment
Share on other sites

×
×
  • Create New...