Turbo Selection: Sizing a Turbo

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I want to share the formulas I've learned in my search for the best turbo selection for my application. Don't be intimidated, the formulas are easy and most of it is the same because we are staring with the same displacement.

The only variables that will affect the outcome are max RPM you plan to peak at, the RPM you desire to begin creating boost and the PSI of boost you think you will run to achieve the lbs/min of air to meet your power requirements.


Let's start with displacement. There are 16.387cc in a cubic inch.

1364cc/16.387 = 83 CI

Next we calculate the airflow of this displacement at a given RPM in it's naturally aspirated state. I'm gonna use stock "peak" rpm to show the accuracy of this exercise.

(CI x 0.5 x RPM) ÷ 1,728 = CFM

(83 x 0.5 x 5,500) ÷ 1,728 = 132 CFM n/a

Now we have to correct the CFM for volumetric efficiency or VE. 132 is in an ideal world where friction and and valve opening time are not an obstacle. However a modern 4v per cylinder engine with variable valve timing (VVT) has a VE of ~95%.

CFM x 95% = corrected CFM

132 x .95 = 125.4 CFM

Now we get to apply the boost. Turbo compressor maps have pressure ratio (PR) on the Y-axis. To establish PR you need to plug in the boost PSI you plan to run on your turbo. For this example I'm gonna use the 22psi that is achieved with the Trifecta tune.

(Boost PSI + Atmosphere PSI) ÷ Atmosphere PSI = PR

(22psi +14.7psi) ÷ 14.7psi = 2.5 PR

FYI, 2.5 is a very high PR for the smaller turbo family, which our stock GT14 is kin to.

Next we have to convert PR into Density Ratio (DR). This can be found on a density chart.

As air is compressed it heats up and it's density goes down. For the sake of simplicity we are going to assume the I/C removes all the added heat therefore making the DR the same as the PR....... 2.5

Now we multiply our corrected n/a CFM by our boost density(DR) to get our turbocharged CFM. As a general rule, every 14.5 CFM or 1lbs/min will generate about 10 Horsepower.

Here we go............

CFM x DR = CFM Turbocharged

125.4 x 2.5 = 313.5 CFM

To convert to lbs/min multiply CFM by 0.069

313.5 CFM = 21.63 lbs/min

21.63 x 10hp = 216 HP

Multiply that times a drivetrain loss of 15% and you get 184 whp. Pretty damn close to what people are seeing with a tune and basic mods.

 

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Here I plan to demonstrate how to apply these numbers to a turbo compressor map to find out if that particular turbo will meet your requirements.

 

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Here I plan to show how my selection will hopefully get me my 300hp target.

Looking likely that I will go with a GT2860RS, or "disco potato" as it is commonly referred to.

It is way larger than I was expecting to go with, but I've crunched all kind of numbers and it looks like anything less won't allow me 300whp.

However I won't see any boost til passed 4k RPM, but that is the compromise with turbos. You can't have both high horsepower and low end range....... with such a tiny displacement engine.

 

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For the 1.8 crowd..... Just for fun I applied 8psi boost to the 1.8 peaking at 6k RPM and came up with 186hp. That may not sound fantastic, but you could see 8psi boost at a much earlier RPM with the right turbo and have a low to mid range monster...... relatively speaking.

 

[Swordsmith]

A suggestion I was looking into last winter was a twin turbo setup, basically it's one turbo for the low end and then the second turbo kicks in when you get up into the mid range. The two small turbos added together produce the same air flow as a single large turbo, but you get to run the single small which means you aren't sacrificing the low end like you are with a large one.

One of the problems with this setup was that commonly the whole thing dumps the boost when going from single to double, which means... I forget, they had a funny name for the phenomenum, but basically you shift, ramp boost, drop boost, ramp it again. At any rate, it looked like there was a way to set up to avoid this.

 

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Yeah that would be great. And actually I'd be more inclined to do a twin scroll for the sake of less complicity. But the fact is, I'm actually doing this turbo upgrade in real life......

Someone needs to just do a straight single turbo upgrade. Who has two thumbs and is gonna do it? This guy.:banana:

The amount of custom fabrication and income required to just do a simple single turbo swap is enough to make me dizzy. Multiply that by two (approx. $1,100 for one high quality turbo, not including intercooler, custom exhaust charge piping and external BOV) it will just never get done in a reasonable or timely fashion.

People have been talking turbo swap since I joined this forum. But that's all it has been..... Talk. Talk is not only cheap. It's actually free. I'm ready to bite as hard as I bark.

 

[Swordsmith]

The theory would be not to install two small turbos, but rather to add one small turbo to the existing one. That -should- mean roughly the same amount of work as installing one large, although naturally it will not really work out that way. But it shouldn't be as much work as installing two small from scratch.

 

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Won't work. The turbine exducer opening of the stock turbo will restrict anything above 230hp. Adding another turbo won't change that. If you can"t get the exhaust out, adding more air flow in can't happen effectively.

I enjoy the brainstorming of ideas, but for the intent of this thread, the direction is a single turbo upgrade. The best analogy I can summon right now is that a woman can't skip losing her virginity and jump straight to being pregnant, even if the bible tells you otherwise. But as far as I'm concerned, the bible is just talk and isn't very productive.

 

[Dragcamaro]

I actually spoke to some people on the turbo forum about a compound setup, and after some number crunching you would have to be running about 53 pounds of boost to make close to 300 hp. that's a big turbo running into the smaller stock one, compounding boost.

id like to see those iats and how the plastic intake handles it haha

 

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Hahaha. My vision is 300 without going over 25psi. But even at that I don't plan to use any of the stock coldside piping.

 

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The final selection takes some leap of faith commitment. I am going with the T25 flange, so that will afford me the comfort of swapping later to any of the Garrett mid-frame turbos.

The stock small-frame is a GT14 equivalent. the GT28 is HUGE by comparison. I just have to accept that I'll be driving a 90hp n/a half the time during daily driving.

But my goal is clear. Big horsepower at the top. If someone wants a road race warrior. A GT22 is a small-frame and would have excellent advantages over the stock without adding much lag. But the trade-off is for all that trouble I'd hate to have a limiter of about 250hp.opcorn:

 

[DevilSonic89]

Mmmm good stuff here Pre. Can u do the math for the 1.8 again but at a lower RPM like 3500 or 4500 at 8psi??

 

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At 3,500 @ 8psi I come up with ~ 112hp.
At 4,500 @ 8psi I come up with ~ 143hp.

(109cu x 0.5 (every other revolution in a combustion) x 4,500) / 1728 = 142cfm

142 x .95 (95% VE for 4v/cyl, with VVT) = 135cfm

(14.7 + 8) / 14.7 = 1.5 PR(DR)

135cfm x 1.5DR = 202cfm

202cfm x0.069 = 14 lbs/min x 10hp = 140 hp

As an example of the relation of boost. If I plug 10psi into the formula, at 4,500 it jumps to 158hp and at 6,000 makes 211hp.

 

[cndctrdj]

what about efficiency range of turbo selection, compression, timing, cam retarding, 2step, exhaust, head flow? thats a great step and a very scientific way to start. but so many other variables come into play.

 

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This formula is for ballpark matching for target horsepower. If you can show me a formula that can take all those extras into account, I'll listen.

I think the only formula for that is to guess, using this or a similar formula, install, and dyno.

Also this formula by no means tells you what hp your car will make. Instead it gives you a target air flow (in lbs/min) for turbo selection. Then you plot the PR and the AF on a company's compressor map to see if it puts you in that compressors efficiency range of about 72-78%.

I will show how to read compressor efficiency maps soon, for those that don't already know.

Applying this formula has revealed to me that the GT25, while better for mid-power, won't get me my goal peak horsepower. The boost that would be needed falls right off the compressor map. It's looking like a GT28 will better suit my needs. But the turbine trim is a whole 'nother science and is currently hanging me up.

 

[DevilSonic89]

Would a GT25 work better for the 1.8?? Trying to understand the formula lol

 

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Look at the Compressor map for the GT25.

Your to plots are where 1.5 PR (8psi) meets 11lbs/min and where 1.5 meets 18.7lbs/min.

That shows you where your target flow happens with that compressor. you want to be right near the meaty island in the middle. So at a glance the GT25 appears a little big for the application of 8psi on a 1.8.

It's looking much better at 10psi, or the line across 1.7PR in relation to the 2 AF numbers above.

 

[DevilSonic89]

Wow 0_o lol mind blown so if i wanna run the gt25 it will be at 10psi...but i kinda wanna stay withn the 5psi to 8psi range...what turbo will be needed? Sorry for the questions.

 

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Actually, looking at the other Garrett options, it looks like the GT25 I posted above is a decent choice. I'm still learning myself. But you want the 2 AF numbers to plot close to the island. That way when the boost comes on at your desired RPM it will run right through the compressors most efficient range as you rev up to max RPM.

If you go to http://www.turbobygarrett.com you can click on every different model of GT25's and see the displacement and hp recommendations. Or you can also call them and tell them your goal and they will enlighten you better than I can.

But if you at least narrow your choice down to a GT25 or GT22, you can start choosing your flange and your downpipe size before you pull the purchase trigger.

 

[DevilSonic89]

Actually, looking at the other Garrett options, it looks like the GT25 I posted above is a decent choice. I'm still learning myself. But you want the 2 AF numbers to plot close to the island. That way when the boost comes on at your desired RPM it will run right through the compressors most efficient range as you rev up to max RPM.

If you go to http://www.turbobygarrett.com you can click on every different model of GT25's and see the displacement and hp recommendations. Or you can also call them and tell them your goal and they will enlighten you better than I can.

But if you at least narrow your choice down to a GT25 or GT22, you can start choosing your flange and your downpipe size before you pull the purchase trigger.

ok thanks. i been looking at the website and im going to choose between a gt2252 or a gt2560r