Turbocharging FAQ

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Turbocharging FAQ

Post by esteinmaier » Sun Oct 23, 2011 4:31 pm

Turbocharging basics

I'm writing this article to finally help those that are beginning to gain an understanding of the journey they are about to get into and why things are the way they are.

Power output can be roughly interpreted by dissecting the 4 strokes of your car's motor. Intake, compression, power, exhaust, repeat. The intake stroke isn't going to be a big positive or negative for you. Getting air into the cylinder is pretty easy compared to the rest of the strokes especially when turbocharged. The next stroke, compression, costs a great amount of power. It's using the rotational inertia of the running motor to highly compress the air and fuel that were introduced during the intake stroke into a highly combustible state. The more air and fuel goes in, the more energy is spent during this stroke. This one is directly relative to the following power stroke, however, which powers the rest of the energy costs of running the motor in addition to whatever load is on it externally. The more air and fuel were compressed in the compression stroke, the more power will be made in the power stroke. The exhaust stroke, however costs lots of energy getting the spent exhaust gas that is now many times as large in volume after it's heated out.

The power stroke is the one we want to concentrate on to make more power and the exhaust stroke is the one we need to take into consideration as to not waste all that extra power on something that doesn't make the car go faster. Turbocharging will, in all cases, make the power stroke more beneficial and the exhaust stroke more parasitic, costing power. Your job in designing your system is to figure out what combination of parts will give you the greatest gains in the power stroke and the least losses in the exhaust stroke in your target power range.

The difference between a naturally aspirated motor and a turbocharged motor are that the exhaust side of the turbo is creating a restriction in the exhaust to reclaim the energy of spent exhaust gas and that the intake is pressurized by the compressor side of the turbo to force more air into the cylinders prior to the compression stroke. So looking at this, we can deduce that one of the main places we are going to look at to end up with the most positive output is the turbocharger itself. The exhaust side should be large enough that it is creating the least restriction possible, but small enough that it can effectively spin the turbine wheel hard enough with the mated compressor side. Here's where we need to look at the purpose of the car and the size of the motor. With common compressor selections and a 2.0 or 2.4 liter motor, a .50 A/R housing or so would provide great spool characteristics, but once you are trying to spin the compressor to higher boost levels or high RPM, the dropoff will become much more noticeable. We could also look at a 1.30 A/R housing, but it's likely that unless you only want to make usable power over 6000 RPM, there just won't be enough exhaust gas to get the turbine spinning.

So then we look to the compressor. We need one big enough to make all the power we want to support. So then you can either plot points on a compressor chart yourself, or do it the easy way and punch data into a calculator online. Let's say we want to make 400whp with race gas, so we can use lots of boost and we can rev the motor high enough to do it with ease. Let's say we've chosen an 83mm CompR from Bullseye Power. It can do 1200, so why not 400, right? Well, that's not necessarily a wrong choice, but probably not the best one. Our goal was 400 and it would certainly do that, but we would be hard pressed to get any usable power under 6500 RPM on a 2.0L. So maybe we should look at a 13t. So we plot that one, and the compressor wheel is so small that it just can't move that much air no matter how hard we spin it up. Then we finally settle on a 50-trim since it will do our goals plus some but isn't just a monster.

I'm not going to get too in depth with applying compressor maps, since that could be an article all it's own, but the things to look at are your pressure ratio and your flow. A compressor map looks a little bit like a winter mitt pointed up to the right usually. That "mitt" is the area that the compressor will perform satisfactorily. The farther you go to the right, the more flow the turbo is capable of, and the farther you go up, the more boost pressure the turbo is capable of. Without putting too much math into it, lb/min roughly equate to 1/10 of the wheel horsepower and the pressure ratio is roughly equivalent to 15psi per. So if you look at the 2.0 line on the pressure ratio, a specific turbo might be capable of 40lb/min of airflow. That means that the specific compressor wheel will make about 400whp at 15psi. The last thing is the surge line. That's the left limit. When you demand those pressures from the compressor, it stalls or "surges". It's not as damaging as the surge caused from not using a blow off valve, but shortens the turbo's service life regardless and doesn't benefit performance.

Where it gets tougher is when you have to put the motor into the equations, but a 2.0L neon motor would have to be spinning at ridiculous RPM to make 400whp at 15psi whereas a 6.0L LS motor would do that at much more docile RPM. When they say "There's no replacement for displacement", they are right and they are wrong. The replacement for displacement is forced induction or RPM. But combining all 3 is always the winning combo, all else equal. (note, the 2.0L can safely spin over 20% faster than the 2.4L with only 20% less displacement. You'll have to decide which you like better, as they both have advantages.)

We see that the common selections for turbine housings on the 50-trim t04e Garrett are .48 or .63 A/R. We decide we want to drag race this car, and since it's a 2.4L that will see 7000 RPM, the .63 A/R seems to be the better choice. The reason again is that the .63 is less of a restriction in the exhaust than the .48 A/R, and therefore will cost us less power during the exhaust stroke but still be small enough to make sure we have all the power we need at the RPM used during drag racing.

Let's change the application a little bit and say that it's not a drag car, but an autocross/street car. Let's leave the 400whp goal there, but we have to realize it's going to be a greater challenge to do it since have a very early spool is now of great concern since we want to have power roll on around 2500RPM. We'll need to get the compressor size down closer to our goal, use a smaller exhaust housing, and expect to be required to run more boost pressure to overcome the additional losses expected when pumping the exhaust through the smaller housing. The smaller t04e 46-trim compressor and the .48 A/R housing now look more attractive, don't they?

It's not just size either that should affect your selection. The newest (And unfortunately more expensive) units use technologies that help create wider powerbands. Some examples are ceramic ball bearings, billet compressor wheels, ETT (Extended Tip) wheels, contoured wheels, race cover compressor housings, and more. Typically, it's worth having some of these features in your turbocharger, as it's pretty cheap power compared to head porting, etc.

So then we have seen the effects of different turbochargers, let's look at the other components that that make it possible.

Of course, you need a manifold to connect the turbocharger to the cylinder head. Generally, there's 2 types: log, or tubular. The log's advantages will be durability since there are less joints to crack or warp, and typically faster spool since the exhaust gas won't have enough time to cool much and lose it's advantage of being many times larger than normal. But the tubular has the advantage of directing exhaust more precisely into the turbine housing to keep the pulses from losing energy by hitting each other and cancelling each other out at higher RPM. From what we can see here, the log might be better for the autocross car, while the tubular might be better for the drag car.

The intercooler also needs to be looked at as well. For something that's going to be driven on the street, an air to air front mount intercooler is almost always going to give you the best results, so I'm going to concentrate on those. A larger intercooler has more surface area to keep the intercooler as close to ambient as possible and more material to keep from heat soaking quickly. That's at the cost of having more volume to fill with pressure before the boost pressure reaches the engine. In almost all cases, the larger intercooler's benefits will far outweigh the benefits of the smaller one. So as a general rule, get an intercooler as big as you can fit and afford.

The exhaust after the turbo will also benefit from changes as well. In the same motor's N/A form, too large an exhaust inside diameter could become a perfomance hinderance because velocity of each exhaust pulse drops when exhaust size goes up. Well, in the turbocharged exhaust, the turbine has already absorbed that energy and we've accounted for it in our turbo selection. So if there's no pulse to worry about, that simplifies our calculations. Bigger becomes better. So you use the biggest piping you can fit and afford.

We've covered the air going in and out now. But what makes more power? That's right, burning more fuel. The addition of the extra air from the turbo system is just the way we get enough oxygen in to facilitate the burning process. You can only burn as much fuel as you have the air to support, but there's far more to it than that.

Chemically correct, non-ethanol enriched gasoline burns completely at a mixture of 14.7 units of air to 1 unit of gasoline by weight. Leaner means more air as compared to fuel, and richer means the opposite. So a 10:1 Air/Fuel ratio would be extremely rich. So your first thought should be that anything leaner than 14.7:1 would be a waste since you aren't burning any more fuel and you would be right. And your second thought is that any richer wouldn't make any more power since there's not enough air to burn the fuel. You would be right there too. But if that's all there was to it, tuning wouldn't really be that complex, would it?

When we start adding power, the combustion becomes hotter. This poses a few problems: Fuel wants to light without the help of a spark plug, and the inner workings of the motor get softer and more vulnerable to physical damage when they get hot. That puts a limit on exactly how much power we can make, right? Keeping everything the same, yes that's true. A perfectly tuned stock motor can handle exactly 0 boost pressure.

But don't despair, we have weapons to fight this war. First, is adding extra fuel. The fuel itself absorbs massive amounts of heat. Have you ever walked through a mist of water at a theme park on a hot day? Well, make that hot day 500 degrees and you're starting to get to that level. So we add a little extra fuel to cool things down, and that helps lots. We can also use higher octane fuel that doesn't auto-ignite until higher temperatures. Again, a huge help. And then we can also delay when the spark plug lights in the cycle. Remember, the exhaust stroke follows the power stroke, so if we light it off a little later, we lose a little power, but gain lots of safety and the ability to add more boost again.

Any time you change something in the setup that affects the pattern of air going in or out of the motor, the tune needs to change as well to accomodate. So if we switched from 93 octane gasoline to 108 octane ethanol, even though we have the same air going in, ethanol burns chemically correctly at a 9:1 ratio instead, making the additional cooling far better since more goes in to cool, and still burns more completely. And then we don't have to delay or retard timing as much to keep the motor safe, so less power is wasted. A car making 300whp on a 62lb/min turbo with 87 octane fuel and tuned to it's limits could pretty easily double that number just by changing to e85, retuning fuel and spark, and increasing boost pressure.

But how do we do this tuning when we put a turbo kit on our car? Simply put, you can't. Our engine computers aren't tunable. They don't understand that boost exists, much less have that activity mapped.

The original way to do it was to put a device on the pressure sensor that hides the boost from the stock computer, but then use a tunable fuel pressure regulator so that when boost pressure starts increasing, fuel pressure does as well to keep more fuel going in as air increases. It's a pretty crude tuning method with many drawbacks, but it works when you don't need to change ignition timing and kinda close is close enough for the fuel ratios.

Piggyback computers have been a recent introduction into the neon world. (At least ones that can effectively tune boost) The AEM FIC is an example. But it still has many drawbacks as well, as you have the stock computer constantly adjusting to correct for conditions, and the second one just using percentages +/- of the stock computer that is constantly varying. It's still better than the RRFPR, but requires more setup and still doesn't give you a consistant tune.

03-05 Neons can have their ECU transplanted with a programmable version from a different car. This is not proven effective as of yet, so we won't cover it here, but it has potential to be an option in the future.

The most accurate way of tuning on our cars is a standalone engine management computer. Almost any will work with an external trigger wheel installed, but MegaSquirt and AEM both have decoders for our stock crank position sensors. These are the most difficult setups to get working right, but allow you to tune anything you want and keep it most consistant. Here, you have 1 computer in control of all aspects of the motor, and you can adjust each one, unlike the other methods where you are at the mercy of a computer constantly trying to adjust in ways you can't control.

So now that you have the means to tune, that doesn't mean you have the range to do so. Our stock fuel system is designed for years of reliable service at stock power levels. For that, it does pretty good, despite the occasional issue. But we don't want to make stock power. We want to make more. Remember that burned fuel = power? Well, we need the range to tune more. Our stock fuel systems, properly controlled, can support up to about 300whp with just a change of fuel injectors. So for a mild street setup, a change to a MegaSquirt standalone system and 440cc drop-in injectors might be just the ticket to reliably boost your stock motor to 250whp.

You can't neglect the drivetrain either. I'm not going to cover that here, but even a mild turbo setup will need a stronger clutch, while the "ultimate" drivetrain might cost $10k. Read the drivetrain section to get more tips.

Let's calculate some costs, since it's a common concern for the kind of people that would be reading this.

New parts:
Turbocharger - $600-$2000 ($850 average)
Manifold - $300-$1400 ($400 average)
Intercooler - $300-$900 ($400 average)
Fuel Injectors - $350-$1000 ($400 average)
Management - $350-$4000 ($600 average)
Fuel system upgrades - $120-$3000 ($600 average)
Oil lines, boost pipes, and exhaust parts - $600-$2000 ($600 average)
Everything you forgot - $0-$1000 ($600 average)

Total - $2620-$15300 ($4450 average)

So as you can see, it's not cheap, but if you are willing to risk it by going with used parts, you may be able to keep costs down. But when you see that ebay turbo kit that says it's a complete kit for only $900, you need to ask where's the exhaust manifold? Where's the management? Where's the fuel injectors? And chances are pretty good that the parts included in the kit are of sub-par quality and you will be replacing most of it with parts that won't fail so easily.

Feel free to ask more questions and I'll try to get more in depth for this FAQ.
Last edited by esteinmaier on Mon Oct 24, 2011 7:41 pm, edited 2 times in total.
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Post by Johny » Sun Oct 23, 2011 5:26 pm

Awesome write up, nice cost calculation at the end. Thanks esteinmaier!
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Post by ZeroChad » Sun Oct 23, 2011 6:14 pm

Would it be worth also discussing how to interpret compressor maps?
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Post by Hudson_Neon » Sun Oct 23, 2011 6:41 pm

aaaaaaannnnnnnddddd this is why erick is the master of boost

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Post by esteinmaier » Sun Oct 23, 2011 11:03 pm

ZeroChad wrote:Would it be worth also discussing how to interpret compressor maps?
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Post by kalvin » Wed Apr 25, 2012 10:54 am

Wow Erick actually knows stuff! :laughing3:

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Post by NeonOn19s » Wed Aug 27, 2014 12:22 pm

"The original way to do it was to put a device on the pressure sensor that hides the boost from the stock computer, but then use a tunable fuel pressure regulator so that when boost pressure starts increasing, fuel pressure does as well to keep more fuel going in as air increases. It's a pretty crude tuning method with many drawbacks, but it works when you don't need to change ignition timing and kinda close is close enough for the fuel ratios. "

Erick - From reading your sticky thread, I'm still having some issues, and I am using the quote from above to "hide" the pressure since I was working on a budget and couldn't afford a piggyback system.

I'm using s BEGi RRFPR with SRT 4 S1 injectors. I have upgraded the Fuel pump as well. I am running at idle 50 psi on the RRFPR, but getting an engine light P0172. At first I though it was my original Bank 1 O2, so I replaced it and the code still exists..

With talking to some of my Neon guys here in Houston - Should I back the RRFPR to a lower pressure? I can't get it inspected because of this code issue. If I have to go to a piggy back system then I will figure out where the $$ will come from.

Let me know if you can help out.

here is my cell # if you have time to talk.

Thanks - Nate
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Post by Hudson_Neon » Fri Aug 29, 2014 12:17 pm

pretty much what i've figured out with boosting my car is that it boils down to $$$. you gotta pay to play and it's best to do it right the first time. that being said, doing it right usually costs more

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Post by blackout » Tue Feb 03, 2015 6:56 pm

I found a kit on ebay that comes with everything but fuel system and computer it is a preset 8psi kit talked to a dodge mechanic that deals with the performance he said I can bolt it on and it will work without being tuned and not needing to change fuel system or computer but still not sure im ordering the kit this wk pls need help before I waste my money

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Post by sidepipe87 » Sun Feb 08, 2015 2:04 pm

What do you need help with? Any post that starts of with "I found a kit on ebay..." should give you a clue right away. good luck....
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