Supercharger Performance and Engine Performance Parts

Our last articles about combining supercharger performance with turbocharger top end seems to have found some online appreciation. So, I’ve decided to write up a step by step on how to do the math for twin-charging your own car.

I’m going to start with a typical compact car engine, such as the Toyota Celica 2.2 liter 5sfe engine. The engine makes 135 hp at 5200 RPMs with a 6200 RPM redline.

For starters, every horsepower requires about 1.5 CFM of air (depending on the air density).

So 135 naturally aspirated hp requires a flow of 202 CFM at pressure ratio of 1.

The pressure ratio is the ratio of turbocharger or supercharger boost pressure divided by atmospheric pressure. Each 1 atmosphere is equal to 14.7psi of pressure… thus:

PR = (14.7 + Boost pressure)/14.7

So for a normally aspirated car: PR = (14.7 + 0) / 14.7 = 1.

Supercharger calculations:

Using 14psi as our target boost (and the maximum safe boost we’d want to extract out of a roots style supercharger) we get the following pressure ratio:

PR = (14.7+14)/14.7 = 1.95

New expected horsepower level: old HP * pressure ratio

New HP = 135 * 1.95 = 263 HP

New CFM = 227 * 1.5 = 395 CFM.

So now, we have our supercharger flow requirements, we need a supercharger able flow 395 CFM at a pressure ration of 1.95 (or 10psi).

Going through different Eaton supercharger maps I had available I find one available option:

1-      The third generation M62 or the fourth generation MP62 are capable of producing 395 CFM @ 1.95 PR @ 13,000 RPMs.

My final expected hp is going to be less than the original estimate263 for two reasons:

1-      The supercharger requires 35 hp to drive it at 13,000 rpms.

2-      The outlet temperature (if not managed through a proper intercooler) is going to be 88*C higher than the inlet temperature, and with every 13*C by rule of thumb costing us 1 hp of power, then 88*C would equate to a power loss of 7 hp.

Our final supercharged power figure (with no intercooler and no other bolt-ons) is

Final hp = (original hp * pressure ratio) – supercharger drive power – (delta T (Centigrade) / 13)

Final hp = 263 – 35 – 7 = 221 HP

Supercharger Dynograph:

14psi Eaton M62 on a Toyota 5sfe

More power can be made between 5000 and 7000 RPMs with bolt on modifications designed to shift the power peak to the right, mainly cams with longer duration, and properly designed headers.

With this graph it is clear that the power of superchargers is mimicking a 95% larger motor by providing linear boost across the entire RPM range. This makes the engine feel like a much larger engine which is great for OEM applications.

Turbocharger:

As we’ve seen in our previous calculations, it takes 35 horsepower to drive our M62 to maintain 14psi of boost on our motor. Because of this horsepower requirement, we find that with superchargers there is a point of diminishing returns when talking about higher boost and flow levels.

Now if my ultimate horsepower goal is 320 horsepower for this motor, then let’s do the math:

Pressure ratio = 320 hp / 135 hp = 2.37

Solving for boost: 2.37 = (14.7 + boost)/ 14.7

Boost = (2.37*14.7)-14.7 = 20 PSI

CFM requirement = target hp * 1.5 = 320*1.5 = 480 CFM ~= 33 lbs/min

(Some turbocharger compressor maps are graphed in CFM vs PR, some are in lbs/min vs PR, 14.47 CFM = 1 lbs/min depending on air temperature and density).

So to find a turbocharger that will give me my target HP goal, I need to find a turbocharger that has the point 480 CFM of flow at a 2.4 pressure ratio on its map.

Now I’ve done this search before so I know that the best turbo for this engine is a T3/TO4E 46 trim…

VERY IMPORTANT: Other trims of this compressor such as a 50 trim a 54 trim a 60 trim or a 60-1 trim, although they do have more CFM flow capacity, they cannot produce those CFM’s at the pressure ratio that I’m looking for. This is why you REALLY need to check your engine demand and flow requirements on your turbocharger compressor map. If your engine needs cannot be plotted on your compressor map then it’s not a proper turbo for your motor and it may never spool or create boost. A smaller turbo with a taller map (rather than a wider map) that produces less peak CFM, but at a higher pressure ratio, may be more adequate for your sized motor. Bigger is not better you really have to choose the right turbo for your application.

By starting with 2.4 PR and plotting my flow requirements at that pressure ratio on my compressor map I find that this motor is a good match for my engine, it will be fully spooled to 20 PSI by 3000 RPMs! It is also capable of supporting my peak power requirement of 480 CFM @ 2.4 pressure ratio at its peak efficiency of 76%. This means that the turbocharger outlet temperatures will be acceptable, since it is working within its peak efficiency and thus should be most power friendly.

Note: Again, I already knew this was a good turbo for this motor because I’ve been through this process before, if you’re doing this for the first time you want to plot your engine demand requirements on several different compressor maps, and compare both spool (the minimum RPM that the engine will make your peak boost at) and your compressor efficiency at peak demand to make sure that the turbo you choose will spool early and give you good power efficiently at higher rpms.

Now for the three rpm points that are not on my compressor map, I iteratively reduce my pressure ratio, recalculate my CFM flow requirement at that pressure ratio, and look to see if I can plot that point on the compressor map. Essencially I am trying to find the maximum boost that chosen turbocharger can support at that engine RPM.

Here are my results:

One thing to note here, my turbocharger can no way produce any boost for this motor below 2000 RPMs. At those lower RPMs the turbo is more a drag on the engine trying to spin itself up to its operating RPM to produce enough CFM to pressurize the engine. In a typical turbocharged application it is good to use a bi-directional bypass valve (rather than a unidirectional blow off valve) because the valve will bypass the turbocharger at lower RPMs feeding the motor directly from the intake system. This will prevent the unspooled turbo from choking the motor, and also help the motor produce more horsepower at lower RPMs which will help spool the turbocharger faster by providing more exhaust gasses to the turbines side of the turbocharger to spin it up.

As you can see the major difference between our supercharger and turbocharger is that our supercharger was able to produce boost at any rpm, but did not shine at higher RPMs where it’s efficiency and required drive power increased. On the other hand, our turbocharger is unable to produce any power boost (but rather a power drag) at lower RPM’s trying to spool up, but it shines at higher RPM reaching its peak efficiency at our power peak.

Once the turbocharger is fully spooled, the wastegate begins to open bypass the turbine restriction on the exhaust, which means there is virtually no horsepower loss driving the turbocharger at this point. As far as thermal losses are concerned, I went and factored in the turbocharger efficiency and outlet temperatures, and if the system is un-intercooled, the turbocharger is only costing us 9hp at peak power due to its outlet temperatures.

So the final HP figure = original hp * pressure ratio – temperature loss

Final HP = 135 * 2.4 – 9 = 315 hp.

Turbocharger Dyno Graph:

TO4E 46 trim @ 20psi on a 5sfe motor

As you can see the turbocharger produces a non linear power graph that is heavily weighted towards higher RPMs providing us with no power advantage at lower RPMs.

The thing to note here is that I started with a car with a low redline of 6500 RPMs and a great turbo for it. As the engine’s peak rpm gets higher and as our peak power target gets higher into 400 and 500 hp range, the choice of turbo to support those power figures will be larger, and naturally the power graph as well as the first RPM that the turbocharger will spool at will all shift to the right another couple of thousand RPMs.

The higher your HP goals, the more need there will be for twin-charging, because you will be using a larger turbocharger that takes more RPM and more exhaust gas to be able to spool and create positive boost for the motor.

Twin-charged:

Now if I overlay the two charts on top of each other we can see the potential benefit of twin-charging:

Twincharged 2.2 litre engine

As you can see, until the turbocharger has spooled, the supercharger provides us with 15 more horsepower and possibly more if the engine were not completely stock, or we had higher power aspirations and used an even larger and later spooling turbocharger.

The other advantage will not show up on a dyno chart, if you’re doing 50mph in top gear (5^th or 6^th gear) then the amount of time you spend going from 1500 to 2500 RPMs may be a very long time at such a long gear, even though it seems like a small part of the dynochart posted above compared to the 3000 RPM’s of turbo goodness above 3500, it will be a significant disadvantage during a quick passing attempt where you are left without any boost. Now being twincharged, no matter what the RPM and what the situation, when you stop the gas you will get a power boost to help you pass. If you stay in the gas (to go from a pass to a full on drag race) then the RPMs will rise, the turbocharger will come online and carry you through impressively to your redline in a flash… This is also a huge advantage on the track coming out of corners in the wrong exit gear.

Now do this for your own cars… if you need help with it leave me a comment.

You can easily find compressor maps for your chargers using google image search…

Other twin-charger articles:

The Twin-Charged Hyper Car

Twin-charged Performance

Technorati Tags: boost, compressor map, pressure ratio, Supercharger, turbocharger, twincharging

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