JDEstill
Probationary Member
- 4
- 0
- Nov 21, 2002
doug -
sorry, it's a little too easy for us engineer types to get way out there instead of keeping things simple!
Anyway, if it was me, and I had no previous experience to guide me, and there wasn't anyone to ask about it, what I would do is:
1. figure out the max engine air flow as per my article, using:
a. max rpm I'd expect to spin the engine to
b. max boost I ever planned to run
c. my best guess at the engine's VE
d. my best guess at the IC outlet temp (if an IC is present)
The key there is to be honest about those things. If I was planning on a pump gas street car, I would not want a compressor that would be good to 30 psi. I'd probably say my max boost was 20-23 psi maybe, if I thought I'd be using water injection or something, or maybe 17 psi if I wasn't going to do any of that. Whatever, honesty is key.
2. Once I had that air flow I'd convert it to acfm at my local conditions. For a given mass air flow you'll need more cfm capability up in the mountains than you will at sea level.
3. Then I'd find a compressor where that cfm is about... I dunno... 80%? 90%? of the max that compressor could do. If I needed a 400 cfm compressor, I'd look for a compressor that was rated up to 450-500 cfm. Or if I was looking at a compressor map I'd want all the curves to start falling off at 450-500 cfm or so.
That should give you a turbo where you are getting the maximum use out of the whole map, not just part of it, and it should keep you out of any part throttle surge problems you might get with something that is way oversized.
To me, the key to all that is good honest estimates of you want it to do, and what you engine really needs. It's too easy to fall into the bigger is better trap. We all do that with cams, heads, everything, but sometimes what makes for a good race car doesn't make a good street car. In the same way that 250deg cam is great at the track, you may be a lot happier with a 220deg cam on the street. Being honest about the turbo size you need might also make for a better street car. But hey, if you are going to be running 25 psi, by all means get something that can do it efficiently!
4. On the turbine side, there is so little info out there (at least for GN turbos) that I would have to consult a vendor to get their recommendations on what would match my chosen compressor. If I had a performance curve for the turbine, I could pick one out on my own. It's a bit complicated though, so I won't go into it here. Once I got one though, I could, if I had the desire to optimize it, start measuring backpressure at the turbine inlet. That would tell me if it was undersized or not. If I was making 20 psi boost, and saw 50 psi backpressure, then I could say whoa! I need a bigger turbine! And if I saw 20 psi backpressure, I could figure I might be a little oversized, and might get a little faster spooling with something smaller, and it shouldn't hurt my performance too bad. Sad to say, with the limited info out there, recommendations and experimentation is the best way I know to pick out a hot side to match the cold side.
John
sorry, it's a little too easy for us engineer types to get way out there instead of keeping things simple!
Anyway, if it was me, and I had no previous experience to guide me, and there wasn't anyone to ask about it, what I would do is:
1. figure out the max engine air flow as per my article, using:
a. max rpm I'd expect to spin the engine to
b. max boost I ever planned to run
c. my best guess at the engine's VE
d. my best guess at the IC outlet temp (if an IC is present)
The key there is to be honest about those things. If I was planning on a pump gas street car, I would not want a compressor that would be good to 30 psi. I'd probably say my max boost was 20-23 psi maybe, if I thought I'd be using water injection or something, or maybe 17 psi if I wasn't going to do any of that. Whatever, honesty is key.
2. Once I had that air flow I'd convert it to acfm at my local conditions. For a given mass air flow you'll need more cfm capability up in the mountains than you will at sea level.
3. Then I'd find a compressor where that cfm is about... I dunno... 80%? 90%? of the max that compressor could do. If I needed a 400 cfm compressor, I'd look for a compressor that was rated up to 450-500 cfm. Or if I was looking at a compressor map I'd want all the curves to start falling off at 450-500 cfm or so.
That should give you a turbo where you are getting the maximum use out of the whole map, not just part of it, and it should keep you out of any part throttle surge problems you might get with something that is way oversized.
To me, the key to all that is good honest estimates of you want it to do, and what you engine really needs. It's too easy to fall into the bigger is better trap. We all do that with cams, heads, everything, but sometimes what makes for a good race car doesn't make a good street car. In the same way that 250deg cam is great at the track, you may be a lot happier with a 220deg cam on the street. Being honest about the turbo size you need might also make for a better street car. But hey, if you are going to be running 25 psi, by all means get something that can do it efficiently!
4. On the turbine side, there is so little info out there (at least for GN turbos) that I would have to consult a vendor to get their recommendations on what would match my chosen compressor. If I had a performance curve for the turbine, I could pick one out on my own. It's a bit complicated though, so I won't go into it here. Once I got one though, I could, if I had the desire to optimize it, start measuring backpressure at the turbine inlet. That would tell me if it was undersized or not. If I was making 20 psi boost, and saw 50 psi backpressure, then I could say whoa! I need a bigger turbine! And if I saw 20 psi backpressure, I could figure I might be a little oversized, and might get a little faster spooling with something smaller, and it shouldn't hurt my performance too bad. Sad to say, with the limited info out there, recommendations and experimentation is the best way I know to pick out a hot side to match the cold side.
John
But seriously, anybody know how/where to get this number? I wouldn't even know a ballpark figure to use. Who among us has an intake air temp gauge probe mounted after their IC and could share some insight?
