Determining The Air Flow of an Engine

Recently I came to the conclusion that I need a bigger turbo for my truck, but since I’m cheap, I decided to commandeer a stock turbocharger from another factory built engine.   Used stock turbos are almost always the cheapest to buy because people that drive newer factory turbocharged cars upgrade them for bragging rights.  Some factory cars come with really nice turbochargers that shouldn’t be discarded as often as they are.  Newer Volkswagens, GM & Ford diesels, and Mitsubishi’s all have great turbos on them that are just asking to be mounted onto engines that they don’t belong on.  The problem is that you need to figure out which turbo is the best size for your application.  As you may guess, this is when math enters the building. The good news is that I have done most of it for you.

I created a turbo sizing spreadsheet that calculates the engine air flow with or without a turbocharger in CFM and Lbs/Min based on inputs of your choosing.  Just change the numbers in the green boxes to your own specification to display your air flow.  Once your numbers are all in there, you can compare them to any turbo compressor map.  This will help you determine the which size turbo is best for your application.

Step 1: Cubic Inches.  Type in the cubic inches of your engine into the upper green box.  In my case, I am starting with a 2.0L engine which is 122 cubic inches. If you don’t know your cubic inches, I made a conversion box for that too.  Cell H19, check it out.

Step 2: Volumetric Efficiency.  Now you need to take an educated guess at your volumetric efficiency.  In the most basic of words, this is a measurement of how efficiently your engine can move air/fuel into and out of your engine.  Unless you have a really polished high performance engine, your volumetric efficiency is probably somewhere between 75% and 90%.  I have read that most modern 4 valve engines are in the neighborhood of 85%, so that’s what I’m going with for mine.

Step 3: Boost!  This is how much pressure you expect your turbo to make when you are beating the heck out of it.  I’m going with 20 psi here as a starting point.

Step 4: Ambient Temperature is the temperature outside.  85 degrees sounds like a perfect sunny day!

Step 5: Compressor Efficiency Range.  This is found on a compressor map like this one from You can see that towards the bottom of each ring, there is a number in the 60’s and 70’s.  That is your compressor efficiency within that lobe of the compressor map.  You want to land above 70% when all the math is done. Throw “70” in that box as a starting point.  The more efficient your turbo is, the less  heat your turbo makes.  As the efficiency goes down, the outlet temperature of the turbo goes up.  Simple enough right?

Step 6: Intercooler Efficiency / Intercooler Pressure Drop / Air Filter Pressure Drop.  I have read the most “decent” intercoolers are around 60-65% efficient, with .5-1 PSI of pressure drop.  The average air filter has .5-1 PSI of pressure drop before the turbo.  Some high performance air filters actually list this stat, which is pretty awesome.   Naturally you want the least amount of pressure drop possible across the intercooler and air filter, with the highest efficiency intercooler.  Little changes in these areas pay huge dividends in horsepower.  Play with the numbers a bit and look how much the air flow changes.

Step 7: Engine Compression Ratio.  This is something that you will need to look up, or figure out mathematically for your engine.  It represents the ratio of the combustion chamber volume; its largest capacity to its smallest capacity. In my case 8.5 to 1.

With these inputs, you can see how the air flow changes in columns B, C, and D based on RPM (Column A).  If you have a compressor map for the potential turbocharger for your vehicle, you can determine if it will be within the turbochargers efficient range based on your engine specs.

Let’s see an example, shall we?

I am too scared to rev my engine past 7000 rpm, so why don’t we assume that 7K is my red line.  At that rpm, my max air flow is 31.53 lbs/min (426.24 CFM).  My Density Ratio (Cell K4 in the spreadsheet), is 2.03.  (Density Ratio is more accurate to use on compressor maps because it accounts for temperature.)  Cross those two points on the compressor map (seen in red), and you notice that I would actually be ~73% efficient at 7000 rpm.  This sounds like a great place to be, except that I rarely hang out at 7K rpm.  The majority of my time is spent at 2500-4500 rpm, which would put me below 70% on the compressor map.  This is hard on the turbo, and worse for performance.  So for me, this turbo is just too big!  I would need something smaller for my engine to be happy at lower rpm’s.


Does any of that make sense? Should I try again?

New Video: Tanner Foust Street Drift

Can’t see the video? Here is the link:

Tanner Foust has a new drift video out that you definitely need to watch.  It takes place on a section of the Mulholland Highway, known as “The Snake”.  I dunno how you land a gig like that, but I have to assume that he and Ken Block are two of the happiest guys on earth.  I mean seriously, is there anything better than beating the snot out of a car that somebody else paid for?

The Reincarnation: My Turbocharged Minitruck

Yesterday we reviewed the riveting tale of the triumphant rise and epic fall of my first truck. Today I am presenting you with what I am calling “The Reincarnation”.  With my first truck solemnly resting in a junkyard far beyond the point of no return, I found myself in desperate need of that missing flame that my Dodge Ram 50 and I once had.  I needed to have the heart and soul of my first truck, but with heaping gobs of horsepower, more interior space, and a clutch pedal.  To me, that would be pure perfection.

Being a fairly typical gearhead, I get a thrill when I open my hood for people and surprise them with a power plant that they don’t expect.  I’m not talking about Chevy engines in Fords though, that is still bizarre to me…  I am talking more about upgrades within the same genre.  Like a Dodge minitruck with the same make fuel injected, turbocharged, DOHC engine where a carbureted loaf once puttered.  Doubling or tripling the factory horsepower and getting more than 1 horsepower per cubic inch intrigues me. It’s even better when it appears like the vehicle came that way.  Onlookers begin to question their own eyes.

As you may or may not know, a Dodge Ram 50 truck is really the same as a Mitsubishi Mightymax truck so most parts are interchangeable.  Mitsubishi built them for Dodge in the 1980’s and early 1990’s using all Mitsubishi parts, and Dodge decals.  So, I figured that if Dodge equals Mitsubishi and Mitsubishi built 220 horsepower turbocharged Mitsubishi Eclipses, then I could potentially get a 4G63 turbo Mitsubishi Eclipse (DSM) engine swapped into a Dodge Ram 50.  Simple! With some bolt on upgrades, I could be in the neighborhood of 250+ horsepower, and I liked that thought, a lot.

One day, many years ago, I was hanging out on some local automotive internet forums when a kid asked if anybody needed parts from a 1989 Dodge Ram 50 Extended cab.  I wasn’t actually looking to buy a truck at that time but I sent him a message anyway asking if he had the whole truck and what was wrong with it.  He quickly responded letting me know that he had the whole truck, it was in great shape, but had a hole in the engine block.  It was currently sitting in a parking lot and it needed to be gone ASAP.  Could this be my new Ram 50? I thought.  “Oh and 1 more thing” he said, “it’s free and has a clean title”.   Yes, everything had officially fallen into place.

I picked up the truck the following weekend, and it was exactly what I had been dreaming about for years.  Clean original paint, macro (extended) cab, low mileage, straight body, very little rust, it was perfect in my eyes.  Within days, I had ripped the junk engine out and grabbed an engine from a 1991 Mitsubishi Eclipse.  I dropped the 4G63 into it’s new home with custom motor mounts, and a variety of other custom accessories.  Wiring, plumbing, cutting & welding was intense for a couple months, and at the 3 month mark, the truck was drivable.

Driving it brings back all the happiness and memories that I had in my first truck, but now with more tire burning gross displays of horsepower, and 5 gears to shift through.  I have been driving it for several years now, and it never lets me down (unless I deserve it).  It starts every time, runs smooth, and generally loves to be a usable truck again.

Current list of modifications:

– 1989 Dodge Ram 50 Macro Cab 5 speed truck
– 1991 Mitsubishi Eclipse 4G63 Engine. DOHC, turbo, intercooled, ~18psi.
– 1985 Mitsubishi Montero Limited Slip Differential, 3:55 gears.
– Cadillac Escalade 17″ wheels
– Custom clutch
– 1987 Dodge Ram 50 2.0L 5 speed transmission
– 1991 Mitsubishi Eclipse FWD Flywheel
– Custom driveshaft, motor mounts, radiator pipes
– 1991 Mitsubishi Mightymax waterpump
– 1993+ Mitsubishi Mightymax Grille + Bumper

After my first truck’s lesson, I can’t see myself ever getting rid of this truck.  The fact that it will get sideways at 40+ mph is enough to scare me to death and make me keep it forever.

The Triumphant Rise and Epic Fall of my First Truck.

I would expect the editor of any automotive blog to have owned and driven a massive number of vehicles to give some kind of legitimacy to the blog itself.  For that reason, I feel that it is my duty to the readers to own and drive as many vehicles as possible, and for that, I thank you.   Between my own personal automotive obsessions and working as a technician, I have driven hundreds of cars, each with it’s own intricacies, oddities, and personality.  How trustworthy are my gearhead opinions though?  Why continue on this automotive journey with me if you don’t even know where I have been?  I think at this point in our internet relationship, it’s time that you learned about the triumphant rise and epic fall of my first daily driven vehicle.  I didn’t realize it at the time, but I really loved that truck more than I can explain.  Something about it was special to me, though I am still unsure of what.

Triumphant Rise:

When I was 15, I had about $500 to my name from dogs chasing me my newspaper route days, and I knew that the pinnacle of life was knocking on my door.  I was getting my license soon.  Sure I already owned 1 car, which will undoubtedly come up in future musings, but I needed a car that I could actually drive daily.  A car that moved under its own power, maybe something with wheels and a running engine.   After harassing my parents for a while, they finally agreed to let me get a second car a little early.  (Thanks Mom and Dad).  I immediately ripped through the “Vehicles for Parts”, and “Antique Autos” sections of the local Want Advertiser searching for the perfect older car to drive daily. A Dart would be cool, I thought aloud… or maybe an Iroc, OH how about a 1939 Studebaker, that would be really cool too..

The weeks steadily passed and my parents shot down every dumb idea I had, thankfully.  Then my mom came home one day and said that a person that she knew had a 1987 Dodge Ram 50 for $500 that needed some work.  Initially, I wasn’t so sure, but within a day or two I had found pictures of some Ram 50’s in MiniTruckin magazine that made me fall in love.  I scrounged up every last penny I could find, and I bought it.  It was the beginning of a very special relationship.

The first thing I did when I got it home was to rebuild the carburetor.  That’s what you’re supposed to do right? It took a few tries, but I got the carburetor back on the truck and got it running again, albeit only slightly better than before I had touched it. Nevertheless, I pressed on.  I changed fluids, fixed exhaust, and even fixed the leaky power steering.  After an eternity (a few weeks) had passed, I got my license and began driving my newly improved Ram 50.  We quickly became great friends, and each weekend, I would spend endless time improving different areas of her:

– Removed rust and rot
– Straightened all the metal
– Added bucket seats
– Lowered 3 inches
– New tires, polished chrome wheels
– Absurdly loud stereo system
– Tinted window
– Bug guard (they were cool then)
– Tonneau cover
– Custom frenched license plate in the tailgate
– Painted neon green brake drums and calipers

The Epic Fall:

It was beautiful on so many levels, and perfect for cruising the beaches. Rust free, dent free, and just a really astonishingly clean truck.  Although the local law enforcement were not fans of my tailgate, it really did set my truck apart from the rest. Mine was special.   After about a year of driving the truck, I began thinking irrationally.  For some unknown reason, I decided that I could not handle the slowness of the truck any longer, and I needed a faster vehicle.  What an idiot! I put the truck on the front lawn for sale for $1000 obo, and a guy I knew gave me $800 cash for it.  He then drove away my original $500 purchase price, about $1000 in upgrades, and thousands of hours of making it perfect. All because I was an idiot.

Years pass, and the truck vanishes from sight and everyone’s memory but my own.  It was only a year after selling it that I had realized the mistake that I had made. I constantly dreamed about turning back time, but without a Delorean, I just couldn’t.  I jumped from one car to the next to the next, desperately trying to find the magic that my little Ram 50 and I once had, but the magic just couldn’t be found.  It was gone for good.

Months turned to years, and my brother and I needed something to do one hot summer day. We decided to challenge the junkyards in a game of mud, oil, and grease.  It was always a great time and would have been again, but destiny had other plans.  There she sat, my beloved 1987 Dodge Ram 50.  Smashed on every corner, missing wheels, and another truck sitting on her roof.  I felt sickened.  Something I had worked so very hard on was now crushed right before my eyes.  It was truly an awful feeling that I would not want any other gearhead to go through.  I reluctantly approached the truck knowing full well that I had done her wrong.  A silent heart felt apology was in order as I slowly meandered my way around the back of the truck.  My custom tailgate, the green drums, the lowering blocks, the interior, the tint. It was all still there, just horrified in every sense of the word…. I spent a few minutes thinking about all the great times we had, snapped off some pictures for memories, and then shamefully walked away.

Amazingly, this saga didn’t end that day in the junkyard, but “The Reincarnation” is a story best left for another day.  Anybody interested?

Name That Car!

For a little fun today, I decided to test out your automotive skills.   The game is as simple as it sounds, all you have to do is Name That Car!  Naturally, I am trying to make it challenging, so all you get is a picture of a stripped out floor pan.  Good Luck!

UPDATE:  The first person to determine the make, model, and engine size of this pictured vehicle, I will send you a free t-shirt.


MAP Sensors

MAP is an acronym for Manifold Absolute Pressure.  The MAP sensor is a key component in a Speed Density fuel injection system, and measures pressure and absence of pressure (vacuum) at the intake manifold.  MAP sensors typically have 3 wires: a 5 volt reference signal wire, a ground wire, and the wire that goes back to the ECU for all that sweet, sweet air related data.  The ECU (a.k.a PCM, ECM) then calculates the air / fuel ratio based on VE tables within the computer. We will cover Volumetric Efficiency tables at a later date.  Just imagine a magical grid in the computer that says “if you see this voltage from the MAP, then do this…”.  The cool thing about these sensors is that they are simple, and can be easily used for higher performance applications.   The bad thing about these sensors is that they are part of the speed density fuel injection system that doesn’t know exact amounts of air going into the engine, it just makes educated guesses at it.   These guesses are all well and good, but solid numbers are always better.  Or are they?  Naturally, there is much more to a speed density fuel injection system than just a MAP sensor…..

IAT Sensors

IAT is an acronym for Intake Air Temperature.  The IAT sensor measures the air temperature that is going into your intake manifold.  The colder the air, the more dense it is, and the more fuel you need to keep your engine happy.  Coool ….literally.  Almost all IAT’s are simple two wire devices that measure resistance.  As the air temperature changes, the resistance in the sensor changes and the ECU knows to change the A/F ratio based on this.  Combine this data with that of the MAP sensor and your computer can now give a pretty accurate guess of the volume of air moving through your engine.  This is great news, but it’s all based on calculations, instead of real solid numbers.  This is where speed density is tossed aside and big Mr. MAF enters the party…..

MAF Sensors

MAF is an acronym for Mass Air Flow.  These sensors are pretty impressive because they measure air volume, along with temperature, all in one (no IAT necessary!).  Remember, the MAP sensor above measured intake manifold pressure / vacuum and then estimated air volume with computer software.  A MAF actually measures real air volume so that the computer doesn’t need to guess what it might be.  As you can imagine, MAF’s are typically more accurate ways of measuring the amount of air that goes into your engine.  This sounds great, right? Well…. it is, on a stock vehicle, and even lightly modified ones.  However, if you decide that you want to make way more horsepower than your car was ever intended for (where do I sign?), then MAF just isn’t going to cut it.  MAF’s quickly become bottlenecks in your air intake system because they can only measure “X” air at once.  If your engine needs more than “X” amount of air, then your MAF freaks out causing your engine to run lean and quickly turns rotating engine parts into liquid hot magma, (Hellooo magma).  This is where Speed Density is welcomed back into the party, and brings along a couple of lovely friends, known as MAP and IAT.

So as you can see, there are pros and cons to each, and it really depends on your vehicle and its modifications to choose who is the cooler sensor at the engine party.  Luckily, choosing one or the other only needs to happen when you modify your car to the extreme.  If you have a stock car, none of this really even matters.

For those of you that are attempting to make crazy horsepower,  what do you guys & gals prefer?