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Dyno Tuning Part 2: Pump Gas Horsepower

Created on Thursday, 01 November 2012 20:54

Subject Comparison:  67mm CEA, 91 octane, stock intake/cams/3" exhaust vs built big intake/cams/4"

Two similar setups with some key changes show big differences in power.  Here we have two 3.0's which are setup with the same fueling, compressors, octane, and similar tunes with very different results. If you are someone who wants to make power on very low octane fuel, pay close attention.


The above chart compares torque from two very similar engines with changes that affect their breathing ability at high RPM.  They both use the same compressor turbo (PTE billet 67's), same engine size, same 91 octane.  As you can see, they're tuned to the same air fuel.  The blue line is a billet 67 torque curve on 91 octane running around 17.5psi with stock intake, cams, 65mm turbine wheel and a 3" exhaust.  The red line is also running the same billet 67, same 91 octane, but running only 16.5psi and has the added benefit of cams, intake manifold, 68mm turbine wheel, and 4" downpipe back exhaust.  As you can see, they both reach the knock limit of the fuel (since they're the same engine size on the same fuel) at roughly 400ft/lbs of torque.  What that means is that we cannot add more boost or timing in search of more torque without getting knock on this octane fuel.  Remember that the lower the octane, the less cylinder pressure we can generate before knock occurs.

The blue line shows smaller intake/cams help torque come on quicker, but also choke it up top faster.  The red line shows that adding a bigger short runner intake manifold, a bigger turbine wheel, and higher duration cams pushes spool back some, but holds power much later into the powerband.  Whereas the blue line makes peak torque at around 5krpm, the red line has torque maintaining it's peak all the way to 8krpm.  So both cars make around 400 ft/lbs on the same fuel, right?  One makes it a little bit earlier and drops off earlier (making only 350 ft/lbs at redline, 7krpm) and the other car makes the same peak torque later but holds it all the way to 8krpm. What is the horsepower difference?  Well, since horsepower is a function of torque and RPM, you don't need to make a lot of torque, you just need to make that torque at high rpm.  Look below for a horsepower comparison:

165whp difference!  Remember, both of these cars make within spitting distance of the same torque.  The red line is a car with the breathing modifications that allow it to make that same torque at high RPM, which makes for a tremendous power difference.  If you want to make lots of horsepower on pump gas, the formula is simple; airflow.  A big intake manifold, cam's, healthy turbo with a big hot side and a free flowing 4" turbo back exhaust is what you need to start.  Things like headwork, oversized valves, and efficient intercooling only help!  You will sacrifice spool and low end torque, there is no free lunch!

Here is the same car with horsepower and torque overlayed.  As you can see, as long as the torque stays flat or doesn't drop off, horsepower is going up!  If torque were to drop off at the top end, you would end up with less peak horsepower and similar peak torque numbers. Once this motor is broken in more, we'll be able to rev it higher and see how much it can make on pump!

Here is another car that makes great horsepower on pump gas!  This car is running AEM's excellent Infinity EMS, which accounts for the super clean dyno pull.  This car has big cams, a 74mm billet COMP TBB turbo, twin Turbosmart wastegates, and a Hypertune Race intake along with a 4" turboback exhaust.  All of these things combine to make for a car that comes alive late but pulls hard all the way to 8000rpm.  The engine's ability to maintain torque at 8000rpm is what makes for the 620whp on pump gas!  We're also using the COMP CT5x housings which are GT42r style.  We've found this makes for a more efficient compressor and that means lower IAT's.  Lower IAT's mean we can extract more power without running into the knock limit.

A note on pushing pump gas.  California pump gas is about the worst out there for making horsepower.  California, Nevada, and Arizona use an emissions friendly blend of pump fuel that is equivalent to 89 octane in other states. How do we know?  We test!  After tuning hundreds of cars over the last ten years, we have a very good feeling for what kind of boost and timing numbers are going to generate the maximum pump gas power.  That said, we feel it is necessary to give our clients the honest truth about pump gas;  pump gas is so low on octane, and so inconsistent that trying to push the limits of pump gas power isn't "safe" by any stretch.  Of course, any time you're modifying a car or trying to extract lots of horsepower out of a small turbocharged engine, you're pushing the engine.  However, trying to make big horsepower on low octane fuel is asking for trouble.  If you get a bad tank of gas, the engine gets too warm, or a variety of other issues, an engine strung out on pump won't have the buffer zone to survive.  That said,  every car tuned at FSR get's tuned for great power within the knock limits of the fuel.  That's why we, unlike many other tuning shops, believe in knock sensing and use knock data as feedback for our tuning session!!!  Tuning a car to best mean, which is the industry standard, on many modern engines (especially Subaru, Mazda, GM, and BMW) blows through past the knock limit window of safety.  

Extracting the most horsepower safely out of pump gas is a balance of boost, timing, and air fuel ratio.  While some tuners are only looking for smooth dyno charts, that usually comes as a result of advanced ignition timing and lean air fuel mixtures.  That's fine for race gas or in some cases E85, but not always for pump gas!  Some cars will be able to make the same horsepower in more than one way:  with extra boost and less timing, or with less boost and more timing.  The "more boost less timing" graph won't be as smooth, but in some cases it will be much safer!  Depending on combustion chamber design, IAT's, among other things, we'll make every adjustment to make the best safe.

What's best for you?  Firstly, having reasonable horspeower goals that are in line with the octane fuel you want to run is where you want to start. Secondly, just call or email us, we'll help you put together a package of parts that will achieve your goals on the fuel you're going to run.  The right combo is everything; always match your cams to your intake manifold!  Another option to alleviate the cost and risk of pushing your horsepower on pump gas is water/meth injection.  The water/meth systems inject a mix of charge cooling water and high octane methanol only when under boost.  This allows us to run more horsepower safely.  We strongly recommend the Aquamist system over any other system for its quality and controller with built in failsafes. 

Lastly, consider some of our newer selections which allow you to run a flex fuel sender and there are no buttons to push or laptops to plug into. Just pump any blend of E85 or 91 octane and the ECU will Automatically sort out how much power you can safely run.  Then you can leave your low octane woes behind!

Flex fuel compatible Engine management: