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Old 06-21-2007, 07:33 PM   #1
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Iím trying to get the theory of turbo sizing down. Iíve read Maximum Boost and the tech articles on Garrettís site and there are still some huge gaps in my understanding.

On the efficiency charts the pressure ratio is easy enough to grasp ((Boost + 14.7) / 14.7) but Iím still confused what to use for lbs/min. According to Mr Bell I should use engine displacement multiplied by the pressure ratio to get CFM but that seems arbitrary. Shouldnít I aim for the CFM at my target rpm? If I do that, though, Iíd also need to know the volumetric efficiency at that rpm and pressure ratio. Also, do you multiply CFM by 0.0807 to get lbs/min or does that only work at standard temps and pressure?

BTW, my target pressure ratio is 1.5 (7.5 psi). I currently live in a climate similar to Las Vegas with temps of 115 and single digit humidity common in the summer. Iím shooting for 200hp but itís a DD so reliability and drivability are more important. 185hp might me a more realistic goal for the short term.
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Old 06-21-2007, 08:08 PM   #2
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2556. You need to look at the CFM flow of the turbo, and then equate it to the theoreticall output. A 2860 would be an easy choice here as well. Not to much lag, decent power levels and flow rates. These turbos should not overheat your air at that power level and also, make 7-9 psi gaol of 200 whp doable IC'ing or WI is a must for those kinds of results also.
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Old 06-21-2007, 08:38 PM   #3
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I'd like the extra head room of the potato. I don't plan the live in the desert for more than a couple of years. I may just go with a SR20DET turbo (GT2560R?) in need of a rebuild for economic reasons, though. Theory's great but new potatoes are going for $1000.

I was thinking about one the the 28x2.5x5 ebay intercoolers but with A/C, I see how it will fit. I may be better off with one of the MSM intercoolers BEGI has for $50.
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Old 06-21-2007, 09:11 PM   #4
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the CFM on the X axis will be around 25-30 at 7000RPM
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Old 06-21-2007, 10:08 PM   #5
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This is all very budget dependent, what is your turbo budget and what manifold are you going to use?

If you are stuck with a T2 cartridge then that's a constraint, or equally true if you need a T3 based turbo. The cheap chinese turbos are interesting if you get a decent one. There is some hit and miss to this. I am less happy with the newest one (although not disappointed) and was pretty happy with the first one.

So what really matters? Well getting a turbo that is sized correctly is an important part of the equation. Big turbos make more power with less boost but they spool later. Garrett and others focus too much on only the compressor side of the equation in my opinion. The turbine is nearly and sometimes more important than the compressor (especially if you're way off.)

I like larger turbines. Sure they spool slower, but not that much slower, in a Miata any turbo that can fully spool by 4000-4500 is going to be a good match for the car. Once spooled they tend to remain spooled with a good bypass valve.

So you want to use the largest turbine that will allow you to spool the compressor at the RPM required. i.e. If you need to spool at 3500, then you need a turbine that is small enough to meet this requirement, otherwise step up to the next larger turbine... part of this is experience from others and "feel" as most of us don't have the resources to test everything ourselves. You then match that to a compressor that will flow the lbs/min of air closest to the center island of the turbo.

Obviously there is a sweet spot. Take the following compressor maps:

T3/T4 46 Trim and the T3/T4 54 Trim.

You have decided that you want 200 WHP (230 CHP) and to make that much power you will need roughly 21 Lbs/min of air. You want to make ~7 PSI of boost or a pressure ratio of roughly 1.50.

So you need a turbo that will spool freely and flow a decent amount of air at a low pressure ratio. So you'll want a compressor with a larger A/R and the largest A/R turbine that will spool the compressor at the RPM you want while delivering the peak power you want.

The 46 trim can flow 21 lbs/min at a pressure ratio of 1.5 with 74% efficiency. The 54 trim can flow 21 lbs/min at a pressure ratio of 1.5 with 78% efficiency.

It's easy to pick the right compressor now. Unfortunately the turbine is where a lot of the guesswork comes in. Having some experience helps, but bolting the damn turbine in place and having other housings available to "tune" it helps more.

Mark
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Old 06-21-2007, 10:12 PM   #6
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Crap, that's nowhere near what I get. With a pressure ratio of 1.5 and 112cubic inches I've got 170 * 0.0807 = 13.7 CFM. Assuming a 70% NA VE at 7000, I'd get about 10. Multiply that by 1.5 and I have 15 CFM.
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Old 06-21-2007, 10:39 PM   #7
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Old 06-21-2007, 10:47 PM   #8
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Old 06-21-2007, 11:49 PM   #9
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I wasn't trying to argue in my previous post. It just feels like high school algebra again. I read the book and listen to the lectures but by formulas never give me the answer in the back of the book.

I just ordered a mild steel dev. gases DP and an old style T25/28 manifold that was on sale at BEGI. My hp/boost goals are modest so it seemed a good choice. My turbo budget is flexible. I have $300 available right now but I'm in no rush. I set aside a little car money each month. I have never driven or built a turbo set up so I'd like to keep it cheap enough that if I have to tune by swapping in other parts it not quite so painful.

Thanks a lot for the info and the links.
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Old 06-22-2007, 01:34 AM   #10
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Quote:
Originally Posted by arga View Post
I wasn't trying to argue in my previous post. It just feels like high school algebra again. I read the book and listen to the lectures but by formulas never give me the answer in the back of the book.

I just ordered a mild steel dev. gases DP and an old style T25/28 manifold that was on sale at BEGI. My hp/boost goals are modest so it seemed a good choice. My turbo budget is flexible. I have $300 available right now but I'm in no rush. I set aside a little car money each month. I have never driven or built a turbo set up so I'd like to keep it cheap enough that if I have to tune by swapping in other parts it not quite so painful.

Thanks a lot for the info and the links.
Well the DP forces you into the T2 cartridge. That sorta sucks, but isn't the end of the world. The manifold could have been redrilled for the T3 flange.

I would seriously consider the GT2876R with the .86 A/R housing. It will spool a little late, but it should make some pretty serious power even at 7 PSI, although you might have to port the wastegate to control boost. There is nothing wrong with the .64 A/R housing but you'll limit the HP per PSI boost.

The other option is the 3071R, while the turbine section does flow a good deal more, the 2876R compressor map is more favorable. On the 3071R I would stick with the smallest A/R turbine housing, while on the 2876R I would use the larger turbine housing.

Piece of advice... spend the extra few dollars and get the stainless downpipe. You'll have a much sturdier piece.

Mark
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Old 06-22-2007, 09:10 AM   #11
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Arga my car is also my first turbo car ever. I hope your experience is as good or better than mine you have done the research and have good products so far. just take your time and buy what you want instead of what you can get. A nissan t25/28 isn't the best turbo ever. Several people report them to be abit laggy like Cjernigan etc. but i think they have exhuast leak issues but oh well. GL.
also no matter what your material of choice i highly recomend ceramic caoting caoting the entire system and turbine housing for maybe 200-300$ Ask aeround maybe someone in your area can get this done for you i found a local guy who gives me same day turn around on most parts.
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Old 06-22-2007, 04:20 PM   #12
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Called and upgraded the DP to stainless. The manifold is already ceramic coated. Will look into the rest.
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