GMPP CT350 Dyno Testing Part 1: Budget Options For Added Horsepower

When selecting the next engine for your race car, GM Performance Parts (GMPP) has a extensive crate engine catalog that deserves a look through before making your decision. For circle track racing, GMPP offers four powerful ready to run options that offer simple drop in horsepower with long lasting dependability to go with it. Many regional and national series have adopted these CT (Circle Track) engines as the spec power-plant for their series, but their fit into other local and regional series makes them appealing to numerous Saturday night racers that might not need to keep the engine sealed from the factory.

“We wanted to provide a lost-cost, durable engine that would allow nearly everyone to race, and to keep the weekly and seasonal engine costs low.” – Bill Martens

Over the next two articles, we are going to take that approach of evaluating a GMPP Circle Track Crate Engine by dyno testing a number of different options that may fit into your class’ rule book. For this part, we are going to take a look at the CT350 Crate Engine(or ‘602’ engine as it is also known by) and how it reacts to different carburetors, air cleaners, and carb-spacers so you can have the advantage when it comes to that final lap.

For many, the Small Block Chevy has been the weapon of choice for cars both on and off the track. While in a new age of fuel injection, bluetooth, and anti-lock brakes, the popular “Mighty Mouse” engine still claims a large chunk of the cheese as far as whats under the hood of most dirt track cars. Today, GM Performance Parts carries on the orders of providing top notch engines in the form of their massive crate engine line – including four specifically for circle track competition.

CT350 In Depth

GMPP worked hard to create an affordable, yet powerful engine package that could be used in a number of different classes. That hard work paid off in the CT350 featuring Vortec style cast iron heads, and is capable of 350 hp and 390 ft-lbs right out of the crate. It’s also the only CT engine that uses a flat tappet camshaft. This is the engine that we’ll use for our testing as it has the best fit into a number of racing series.

“We wanted to provide a lost-cost, durable engine that would allow nearly everyone to race, and to keep the weekly and seasonal engine costs low. Once you start “upping the ante” by allowing more expensive components, or building a high-maintenance engine, you are going to lose cars, “Said GMPP’s Special Programs Manager Bill Martens. The complete engine retails for just below $4,500, which is a huge bargain for 350 horsepower and nearly 400 ft-lbs of torque.

The basis for the CT350 was to create a economical engine package that could still deliver the power the racers were going to crave.

Martens explained to us,”It’s pure and simple economics for the entry-level racers. As it turned out, the CT350 combination also provides a really good show when racing against it’s peers, whether it’s in a Sport Mod, Hobby Stock, or even a Late Model car.  The engines are so close to each other right out of the crate, that it really boils down to the car setup and of course, driver skills.”

The valvetain is conducted by a GMPP camshaft with .435 lift on the intake, .460 on the exhaust, and 212 degrees duration at .050 on the intake, and 222 degrees for exhaust.

The basis for this engine was to create a economical engine package that could still deliver the power the racers were going to crave. “The CT350 is very versatile in that it can compete successfully  in almost any class, if the rules provide some balance between “open” motors and the GM Circle Track Crate engine,” said Martens. “They have been ran in hobby stock, stock cars, Modifieds, Sport Mods, Late Models and even Truck series.”

GMPP decided to borrow the short block from their popular 350 HO high performance street engine, and spice it up for circle track racing. Assembly began with an all new, non-remanufactured 4-bolt block that is machined to the exact production specs of that old junk yard block you might have sitting around the garage. It is filled with a nodular iron crankshaft, powdered medal steel connecting rods, and 9.1:1 hypereutectic aluminum pistons before an 8-quart circle track oil pan is slapped on the bottom end.

The valvetain is conducted by a GMPP camshaft with .435 lift on the intake, .460 on the exhaust, and 212 degrees duration at .050 on the intake, and 222 degrees for exhaust. According to GMPP, this cam combination promotes plenty of mid-range power for strong pulls off the corners, with some still left over for the top end of the straightaways.

The CT350 Circle Track Engine from GM Performance Parts has been around for 9 years now, and remains the best racing value in the game,” Martens explained. “Just remember that ‘it is, what it is’, and don’t attempt to drive it like a Super Late Model, and you’ll have a great experience with it.”

If you’ve read our guide to GM heads (If not here is the link, it’s a MUST read if you wanna learn what heads can best fit your class and power goals), the GM Vortec style heads offer a competitive advantage over some of the older GEN 1 designs. For the CT350, GMPP uses the 62cc chamber version available from the Vortec line of heads. They use the common 1.940 intake, 1.500 exhaust valves making these heads an easy fit into almost any track rules out there. The heads are then topped with an a GMPP aluminum high rise dual-plane intake manifold and a HEI distributor.

GMPP makes these ready to run engines easy to complete.

Completing The Crate

GMPP makes these ready to run engines easy to complete. Here is the quick list of everything we needed to get our engine firing on all eight:

  • Water Pump & Thermostat
  • Fuel Pump & pushrod
  • Starter
  • Spark Plugs
  • Spark Plug Wires
  • Carburetor

Dyno Testing

For this part of our testing, we wanted to take a look at how this engine would perform in the different rules that are available out there for teams to race in. We loaded our engine into a late 70s Camaro and strapped it down to our DynoJet chassis dyno to test what the real world horsepower results are between the most common differences in class rules and personal choice. After bolting the 500 CFM Holley 2-Barrel (PART #: 0-80583-1) to a new Summit Racing adapter/spacer, we were ready to break in the engine. To insure proper break-in, we filled our GMPP crate with 8 quarts of COMP Cams Break-In Oil of the 10w-30 variety, before swapping our engine with Mobil 1 10w-50 Racing Oil after the break in.

Our 1970s Camaro has spent the last few seasons sitting untouched after the previous owner dominated the local track, winning multiple A-mains and track championships, before moving on to a higher class.It arrived in the powerTV shop with no engine or transmission. A few fresh coats of rattle-can-black, and a few new stickers later, our test mule was ready to receive the GMPP crate.The Camaro also features:

  • TCI Powerglide transmission,
  • Ford 9-Inch rear end with 31 spine axles
  • 6.33 gears
  • Stock power steering
  • Stock water pump
  • 100-amp alternator
Testing Standards and Conditions

Weather in the powerTV shop was a little warm this day with an average temperature of 80°, with 31% humidity. All of our testing was performed on the same day, run 2-3 times to confirm data, and with 15 minutes off cool down between dyno runs. The Camaro used in testing is equipped with a TCI Powerglide, Ford 9-Inch rear end with 31 spine axles, and 6.33 gears filling it’s third member. Sadly, we were limited to run only CA 91 octane fuel. Those lucky enough to be back east, GMPP recommends you opt for 92, though we weren’t expecting a large decrease in power.

Here is the quick breakdown on what we are looking to test and why we it could be relevant to your engine:

2-Barrel vs. 4-Barrel Carbs – This is one of the most common class limited factors out there. Entry level classes tend to limit racers to the 4412 Holley 2-Barrel, while mid-range and top level class engines are found using the 4-Barrel Holley 4150hp. We’ll find out just how much of an advantage the larger double pumper has over the smaller 2-Barrel.

Carburetor Spacers The common thinking among racers is, adding a carb spacer is a great way of adding some ponies. Simple as that. But how true is this? We are going to add a carburetor spacer and find out just how much of an increase, if any, you can expect.

Air Filter Size and Design – Many over look just how important the air cleaner is in an engine combination. Most class rules dictate a max size, but we’ll be testing the difference between different heights as well as K&N’s open top filter design. We will attempt to answer the question, does adding more height to the Air-filter increase horsepower?

Test #1: 2-Barrel Carburetor

For our base line run, we opted to use the Holley – Keith Dorton Signature Series carburetor. We kept with Holley’s and GMPP’s recommendations of 73 size jets, the stock power valve, and 32 degrees of total timing.

Carburetor Test #1:
Parts Used:

  • Holley 500 CFM Two Barrel Racing Carburetor (Keith Dorton Signature Series)
  • Summit Racing 2-barrel Carb Adapter
  • K&N 3-Inch Air Filter-Closed Top
“Following Holley’s decision to make the original 500 CFM (0-4412) a more race ready carburetor free of all unnecessary components like the choke, trans linkage, spark ports, PCV tube, etc. A number of beneficial race features (including screw-in air bleeds & stainless steel throttle plates) were added along with a new sales number,” explains Bobby Writesman of Holley Performance. “With the race features aspect complete, Holley then contacted Keith Dorton who is one of the industries most accomplished circle track engine builders. Keith was extremely successful in developing a new and effective fuel curve that produced immediate “out of the box” winning results.”

Mr. Keith Dorton knows and thing or two about building carbs, and details he helped work into this 2-Barrel such as stainless steel throttle plates helped this combination lay down 223 horsepower and 251 ft-lb torque on the DynoJet. Not bad considering GMPP recommends a 650 CFM 4-Barrel with the use of this engine.

After making a few pulls to confirm the data, we noticed that our air-fuel ratio was a little rich, and thought there might be some power being left on the table. We swapped to 71 jets for our final numbers. No matter which carburetor you are using, having a full jet kit like the one we used from Holley, makes keeping track and changing jets a very simple job.

2-Barrel Baseline: 223.76 horsepower, 251.56 ft-lbs of torque.


Carburetor Spacer Test
Parts Used:

  • Holley 500 CFM Two Barrel Racing Carburetor (Keith Dorton Signature Series)
  • Wilson Manifold  2-Barrel Carb spacer
  • K&N 3-Inch Air Filter-Closed Top
Test #2: Carburetor Spacer Test

Many class rules allow the use of a carb-spacer with the common height being around 1-inch. We pitted our 2-Barrel Baseline results against the addition of a Wilson Manifold Spacer. It is a 1 inch, 2 barrel to 4150 tapered spacer, “We spent time working the design to not only perform at full throttle, but under partial throttle as well. It is extremely important for a circle track engine to perform well under these conditions. No matter what the driver says, they are never at full throttle for very long,” says Keith Wilson of Wilson Manifolds. “They might think they are, but drivers spend a lot of the time modulating the throttle coming off the corners.”

But how much gain could we expect? We replaced our Summit Racing carburetor non tapered adapter with Wilson’s 1 inch tapered spacer. The CNC’d walls of this spacer brought us some very interesting results

Test#2 Results:

2-Barrel CNC Spacer: 223.69 horsepower, 246.94 torque.

Peak Power Gain: 0HP and -5ft-lbs of torque(but an additional +10ft-lbs of torque and +5HP during power curve)

By simply looking at the max horsepower and torque numbers, you would never see that the true benefit of running a tapered spacer. While torque was down at the peak by 5 ft-lbs, one quick glance at the graph shows that our engine held torque and horsepower higher throughout the most important area, the power band.

The 5ft-lbs drop in torque was only seen between 3750 and 3900 RPMS. It quickly showed an increase of almost 10ft-lbs during the curve. In an engine that is only putting 223 horsepower to the ground, having the gain in horsepower and torque during the power curve is a huge advantage.

Wilson explained to us, “Especially on the GMPP crate engines or any engine using a dual plane manifold, the more volume you can create above that split, the better fuel distribution you are going to have. Tapered spacers provide more volume to accomplish this, than a smaller straight through design. Regardless of what manifold you have, the carb spacer should be treated as a tuning device. Different heights and designs are going to give you different results. Simply put when you use a spacer you are adding CFM to the carburetor.”

Carburetor Test #2:
Parts Used:

  • Holley 650 CFM Four Barrel Racing Carburetor (4150 HP- Series),
  • K&N 3-Inch Air Filter-Closed Top
Test #3: 4-Barrel Carburetor

To see what an extra 150 CFM and better fuel control could do, we bolted on our Holley 650CFM Four Barrel Racing Carburetor. Obviously we expected to see horsepower and torque gains, but if you have the option of choosing either a 2 barrel or 4-barrel carb how do you make the decision?

“Track rules pertaining to optional carburetor selections are usually factors of weight breaks, RPM ignition limits, tire widths and sometimes even a gear rule. Track size & configuration, along with the condition of the racing surface are also key factors,” explains Writesman. “Cars running on narrow tires, tight or flat corners will in most instances opt for the 2-barrel to reduce tire spin. A 4-barrel carburetor used under these same track conditions will overpower the track and burn your rear tire(s) off.”

Given the compression ratio of the engine (a conservative 9.11), the 4-barrel proved it could be well worth the swap if the conditions were correct. We picked up an additional 22 hp and 6ft-lbs of torque. The key here is how much better the 4-Barrel holds torque throughout the power band. Right when the engine needs to be pulling the most around (4,000-5,250rpm) the 4-barrel shows it’s muscle pulling over 15 ft-lbs more torque at times.

Test #3 Results:

4-Barrel Baseline: 245.28 horsepower, 257.45 torque.

Peak Power Gains Versus 2-Barrel Baseline: +22HP and +6ft-lbs of Torque

We found that where classes that allow the use of a 4-Barrel carburetor, it would well worth the 30 minutes and tuning time spent to make the swap and see how your car reacts to the change. But Writesman offered us one more warning about the 4-Barrel carburetor, “If you are allowed wider tires or the track has a little more banking you may want to check your lap times with a 4-barrel. However you must remember that over-revving a crate engine will just float the valves or at worse, shorten the engines life. Crate engine suppliers usually make excellent recommendations regarding carburetor selection but it is ultimately your racing conditions and guidelines that determine what is best for you.”

Air Filter Height Tests:
Parts Used:

  • Holley 650 CFM Four Barrel Racing Carburetor (4150 HP- Series),
  • K&N 2-Inch Air Filter-Closed Top
  • K&N 3-Inch Air Filter-Closed Top
  • K&N 4-Inch Air Filter-Closed Top
Test #4: Air Filter Sizes – 2, 3, and 4-inch breathers

Determining Correct Air Filter Size And 2-inch Breather Test

More is better, is words that many racers live by when it comes to building their engine. More lift on the cam, more compression, ect. What about when it comes to air cleaner size? Most rules state a max size, usually around 4 x 14. We’ll stick with the 14 inch size for this testing, and test the difference in 2, 3, and 4 inch sizes. (To be exact the sizes have been rounded from their decimal form to be a cleaner read.) We conducted the testing with our Holley 4-Barrel combination. For test #1, we started with the smallest air filter.

This got us wondering though, there has to be a easier way to determine the correct Air filter size right? First we need to determine the maximum volume of air your engine will require. Bob Harris, K&N Corporate Accounts Manager showed us the process to establish that,”To determine the maximum volume of air your engine will use, take the displacement in cubic inches and multiply by the maximum RPM you operate the engine at.  If the engine is supercharged or turbocharged, divide the pounds of boost by 14.7, add 1 to this answer, and multiply this value by the CIDxRPM value.  Divide by 3,456 for a 4 stroke engine, or 1,728 for a two stroke engine.  This will calculate the maximum airflow required by the engine.

Sounds really complex but when you look at the equation it’s not that difficult:

____________Cubic Inch Displacement x Max RPM x (B)_____________lbs. boost
CFM =______——————————————————-________B=__—————-   +1
__________________ 3,456 or 1,728______________________________14.7

So for our engine the equation looks like this.

________________350 x 5500
557 CFM =_____————————-

So now that we know the volume of air that our engine requires, we can determine how big of an air filter to run. “To determine what volume of air a given filter is capable of flowing, multiply the diameter of the filter by its height, multiply by pi (3.14), and multiply by 6,” Says Harris. “If the filter is a tapered cone (different diameters at each end), add the diameter from the top and base together and divide by two for an average diameter.

Again it sounds more complicated then it actually is. Since we are not using a tapered cone air cleaner, our 2-inch air cleaner equation would look like this:

527CFM = (dia.)14 x (height) 2 x 3.14 x 6

Harris suggested to exceed the CFM required by the engine by approx. 100-150 over what the engine requires. So we might be starving our engine of air with our 2-inch air filter. Based on the equations that K&N gave us we shouldn’t see a whole lot of difference in the numbers produced by the dyno.

2-Inch Air Cleaner 527CFM: 245.09 horsepower, 258.62 torque.


There has to be an easier way to determine the correct size of air filter to use right?

3-Inch Test:

Parts Used: Holley 650 CFM Four Barrel Racing Carburetor (4150 HP- Series), K&N 3-Inch Air Filter-Closed Top

By using the equation that K&N had given us, by adding one inch to our air-filter we increased our CFM from 527 to 791. But what would the results show?

The temperature in the powerTV shop hadn’t changed that much at this point in the day. (Our Dynojet software was still reading an ambient air temperature of 81 degrees.) As K&N predicted, we didn’t see much of a change at all. We did see a increase of .2 hp. But the 2-inch air cleaner was pretty close to what our engine’s required CFM. This would explain why we didn’t see any change. Maybe bigger isn’t always better? Next lets try a 4-inch!

3-Inch Air Cleaner: 245.28 horsepower, 257.45 torque.

4-Inch Test:

Parts Used: Holley 650 CFM Four Barrel Racing Carburetor (4150 HP- Series), K&N 4-Inch Air Filter-Closed Top

For our final air cleaner height test we used K&N’s 4 inch tall air cleaner. The extra inch took our air-filter CFM from 791 to an amazing 1055. Remember according to our earlier calculation the maximum volume of air our engine will need is 527CFM. So we are doubling what our engine requires.

4-inch Air Cleaner: 246.30 horsepower, 258.57 torque.

Air Cleaner Height Test Results

The final inch came up as the top performing air cleaner. But it only added roughly 1hp versus the 2-inch air cleaner. We’ve proven exactly what K&N explained to us, All of the dyno’s were within one horsepower of each other.

it’s not the size of the air filter that matters. It’s the CFM required by the engine. Regardless of the size of air-filter, if you are not fulfilling the CFM requirement of your engine, it will suffer. But once this requirement is made, adding additional CFM to the air filter will produce very minimal gains.

Peak Power Gain: +1HP and +1ft-lbs of torque with the 4-inch breather.

Open Top Air Filter  Test:

This seemly simple design of converting the normally solid pie tin we're all a custom to, and taking full advantage of the space available to add additional air flow. How would adding even more air through the top compare?

Parts Used: Holley 650 CFM Four Barrel Racing Carburetor (4150 HP- Series), K&N 4-Inch Air Filter-Xstreme Top (Filter Top)

One product that is hit or miss on some rule books is air filter designs such as the K&N Xstreme Top Filter. This seemly simple design of converting the normally solid pie tin we’re all a custom to, and taking full advantage of the space available to add additional air flow. How would adding even more air through the top compare?

After making a few runs to confirm the data, we saw a gain of 2 wheel horsepower. While it was a minimal gain, the CT350 seemed to like the open top as it produced our highest numbers of the day. Whether you choose to go with a open top or a closed top Harris did give us some additional information for dirt track racers, “I strongly suggest the air force foam wrap. I know a lot of circle tracks limit what a racer can run. Usually, they can only run a 14×4 air filter, so they choose the Flow Control brand and then the foam wrap to protect against dirt and debris.”

Open Top Air Cleaner: 248 horsepower, 260 torque.

Air Cleaner Height Test Results


Taking all the data in, we saw some surprising results. The common thinking we addressed earlier of “more is always better”, funny enough isn’t necessarily always true. When we added 150CFM by attaching the 4-barrel carburetor, we saw our biggest gain of the day of 22HP and 6ft-lbs of torque. But when we started increasing our air filter size and let our crate engine breath a little more we saw very minimal gains. We learned that this was because we had already fulfilled the engine’s required CFM.

Coming up in part two of our series of article, we'll be upgrading the valvetrain with the help of COMP Cams

Now that we’ve consulted the experts and found what combination works best for the top of our engine, we still wanted to see how much more horsepower is lurking inside our crate. Coming up in part two of our series of article, we’ll be upgrading the valvetrain with the help of COMP Cams by pulling out the sealed bolts and exploring the true potential of this engine for use in classes that don’t require it to remain sealed. Next, we’ll strap on a set of long tube Headman Hedders to see just how much power the average dirt track engine is giving up by running stock manifolds.

While today’s results might seem like a very minimal gain, but when you are battling for tenths of a second on the track, every little bit of horsepower and torque increase adds up! The other important aspect to remember is these gains were accomplished by affordable products. This is something that any race team who is using the CT350 or any engine for that matter, could perform on their engine to find some extra power!

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About the author

Tom Bobolts

Tom started working for Power Automedia in early 2008 at the young age of 20. Starting off as an intern spinning wrenches in the PowerTV garage, Tom cut his teeth helping us build the very project cars we feature. Since moving inside the office, most of his time is spent writing and shooting installs - but he still finds time to get out in the shop. Outside of work, Tom enjoys a variety of different motorsports from Street Bikes, Muscle Cars and just about anything that demands high amounts of horsepower.
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