We examine the relationship between exhaust sizing, flow, and engine performance.

The factory delivered exhaust system on your vehicle is probably the great unsung hero of the car world, it dampens the roar of combustion to a pussycat purr, and generally works without fail for a very long service life. The real question remains, is your OEM exhaust holding back performance?

You can be certain it is.

The humble cast-iron manifold offers a reliable, cheap, and thermally stable option for

OEMs and is typically found on most production engines. While these designs will work well, it is often not the best choice for the performance minded enthusiast. The bottle neck in exhaust flow comes from less than ideal runner design, and subsequent losses in exhaust gas flow.

The header, or tubular manifold is the clear choice for performance, these lighter weight series of tubes are selected and arranged for optimum flow, and promote exhaust scavenging. The selection of a tubular exhaust header for high-performance OEM engines and racers alike is evidence of this enhanced performance over the cast-iron manifold, and dyno numbers easily support these claims.

Header Design: Manifolds, Shorty, Tri-Y, and 4 into 1

Obviously headers will differ by manufacturer and by application, but a few basic rules of thumb can be remembered.

A shorty header is generally an improvement over a cast iron or log manifold, but it will not promote good scavenging due to the collector/tube combo which tends to develop exhaust turbulence.

The longer primary tube length of the “long tube” header preserves the exhaust pulse velocity for longer and promotes excellent scavenging at the engineered collector where they meet. This almost always produces a noticeable gain in horsepower and typically a bump in torque.

There are two major types of long tube header, the standby Tri-Y, which has two of 4

cylinders (per bank) merge halfway to the collector and a final time at the collector. 4 tubes into 2 tubes, into 1 tube, or Three “Y” pipes, Tri-Y. The second common type is the 4 into 1, more simply it maintains all exhaust pulses separate until meeting at the merge collector.

There is a lot of discussion as to which long tube design is best, and for the majority of street and general performance cars a test by Super Street Online is relevant.

Testing a 200 HP Honda 1.6L B16 engine spun to 8500 RPM revealed that both designs offered good gains over the stock (195 HP) manifold. But the Tri-Y out performed both the Manifold and 4-1 under 6500 rpm (201 HP 3.1% over stock) after 6500 the 4-1 stole the show but only by a small margin (204 HP 4.6% over stock). In either case, the mere 1.5% difference between tri-Y vs 4-1 is not massive, as 1.5% is only 15 HP on a 1000HP engine. Chose what suits your application best and drive happy.

How much does primary length and collector design matter?

On Engine Masters Episode 15 a Blueprint engines Chevrolet 6.2L LS V8 was tested with a series of manifold and header designs provided by Hooker exhaust.

The manifold was an optimised cast iron design which exhibited good flow, but had short primary runners and no collector to speak of. It produced a baseline of 474.2 Horsepower @ 5700 rpm and 453.2 ft-lbs Torque @ 5300 rpm.

A set of Hooker mid-length headers were tested next, these are very similar to a long tube header but have slightly reduced length primary tubes for ease of installation in vehicles with fitment considerations. These headers produced 488.2 HP and 464.3 TQ, a solid 3% gain over the manifolds.

The final test was used a full length header with full collector, typical of many domestic muscle car applications, it developed the best power overall, 494.5 HP and 474.3 TQ, nearly 5% gain over cast iron manifolds.

It is very clear that header design and primary tube length does have an impact on engine performance.

Beyond the Headers, to the Tailpipes!

Exhaust piping serves to evacuate spent engine gasses away from the occupants of the passenger compartment, while the muffler serves duty reducing noise output. Sizing of exhaust components, both header and tubing has long been a point of discussion, even argument, among the performance community.  This of course brings the discussion to “Back Pressure”.

Back pressure is the presence of a significant restriction to airflow through an engine. Since an engine is at its core, a complex air pump, the importance of flowing fresh (and fuel) in, and spent air/fuel out cannot be understated.

While maintaining good flow and exhaust velocity is key, it should not exceed 250 ft/sec, and never be confused with “back pressure”. Back pressure of an exhaust can be measured, either with a hand held anemometer to check velocity, or with the installation of test ports and a pressure measurement of less than 1 psi is ideal.

This chart from exhaust videos.com can prove helpful when choosing the correct size of pipes for your own system:

Pipe Diameter (inches)	Pipe Area (in2)	Total CFM (est.)	Max HP Per Pipe	Max HP For A Dual Pipe System
1 1/2	1.48	171	78	155
1 5/8	1.77	203	92	185
1 3/4	2.07	239	108	217
2	2.76	318	144	289
2 1/4	3.55	408	185	371
2 1/2	4.43	509	232	463
2 3/4	5.41	622	283	566
3	6.49	747	339	679
3 1/4	7.67	882	401	802
3 1/2	8.95	1029	468	935

 

On Engine Masters Episode 9, a Chevrolet 454 cubic inch V8 was tested with a variety of exhaust equipment and the net result was bigger, is better, defeating the oft repeated requirement for “back pressure”.

The engine was first tested with a long tube header and 8” inches of pipe, it produced the highest power figures, 620 Horsepower and 580 ft-lbs of Torque.

The second test was conducted with a 2.5” dual exhaust system and Magnaflow mufflers and tail pipe to simulate installation in a typical muscle car, using the same headers as the first test. Power numbers were reduced, 601.2 HP @ 6200 rpm and 554.1 @ 4600 rpm, a loss of 3.1% horsepower and 4.5% torque.

The third test used the same arrangement but increased the tubing and muffler size to 3.0”, unsurprisingly power improved, and nearly reached the numbers achieved in the bare header test, 614.2 HP and 569.8 TQ.

The science and data collected has clearly defined that there is little to be lost by choosing an “oversized” exhaust or header combo.

Often, promoters of backpressure exhaust systems will claim torque will be lost by reducing backpressure, but this is not at all the case. This belief that oversized exhaust reduced power and made a car slower, likely dates from the time when dyno testing was not commonly available, and exhaust system changes can change air-to-fuel ratios, especially in carbureted vehicles. If these changes to the AFR are not corrected it could be possible to experience a power loss, which might lend some creditability to the old backpressure belief.

“Show me a race car with exhaust gas back pressure and I’ll show you a loser. – Corky Bell, Supercharged! – Pg 183”

Exhausted!

An entire technical book could be written on this topic, and for the sake of time I’ve pared the subject down to a basic overview. This of course comes with some compromises; we did not study the entire horsepower or torque curves of the examples above, however only at extremely low engine speeds did any performance loss from improved exhaust systems occur, and none of the tests explore part throttle operation. Also omitted was any discussion on sound levels, ease/difficulty of installation, local laws, and other realities of fitting larger than OEM exhaust components. However the takeaway here is that a good header and high flow exhaust will always help in extracting maximum performance of any engine combination.

Sources:

Supercharged! Design, Testing and Installation of Supercharger Systems by Corky Bell ISN:978-0-8376-0168-7

Hot Rod Engine Masters Episode 9 on Youtube

Hot Rod Engine Masters Episode 15 on Youtube

Super Street Online

Exhaust Videos.com