Big-Block Mopar Header Performance Comparison:
2 inch Equal Length Tubes versus 1 5/8 inch Tri-Y design

Steven Charette Imperial Services 1485 South Dehmel Road Frankenmuth, MI 48734
10/20/2002

Summary
This study is the result of testing conducted to address questions regarding the performance differences between large tube equal length design headers and those of a smaller tube tri-y design. Conventional wisdom and past experience suggested that the smaller headers could result in the car slowing from ½ to one full second in the quarter mile. Test results showed only a .3 second difference in heavy headwind conditions. When corrected for environmental conditions, the loss in performance was only .2-.3 seconds, and 15-20 rwhp. Verification of the corrected results is pending.

Contents

Introduction
Test Equipment Specifications
Findings
Conclusions

Introduction
This report examines fitment, functional and performance characteristics of the 2" equal length headers as compared to those of the 1 5/8" tube tri-y headers.

Background
Many questions arise when an enthusiast begins the process of exhaust header selection. Issues of fitment, suitability of purpose, and overall performance are carefully balanced against the intended usage and appearance of the vehicle. While 2" equal length headers are preferred for general racing use, the attendant clearance problems and reduced low-end torque must be carefully weighed against the overall performance when comparing with a smaller tube header.

Scope
This report will provide quarter mile test data and calculated rear wheel horsepower comparisons relating specifically to the subject test vehicle. While the test results indicate an incremental degradation in quarter mile performance, this difference will be more or less pronounced on vehicles that are more or less radical, respectively. Additionally, please note that all testing was done in "street trim"; full exhaust and on DOT tires, as opposed to the use of unrestricted exhaust on a race prepared vehicle. Testing was not done with open headers; any comparison to race prepared vehicles is beyond the scope of this document.

Test Plan
The test was designed to be conducted in three discrete phases. The first phase would produce baseline test data. The second, incidental phase focused more on the functional and fitment characteristics of the test subjects during the process of changing the headers. The third and final phase involved testing of the new combination, with an opportunity for some minor tuning after the comparative portion of the testing was complete. The experiment was planned to minimize the effects of weather by scheduling the testing for two concurrent weekends. No changes were to be made to the test vehicle between the tests with the exception of the exhaust headers and the required changes to the exhaust to accommodate the smaller headers. (Note 1)

Test Vehicle
As stated in the Scope, the test vehicle selected for this test is at the high end of what is generally accepted as a streetable vehicle. This type of vehicle was selected specifically to illustrate the results of a comparison in a "worst case" type scenario, as this is where the majority of potential customer questions are focused.

Test Vehicle Specifications

Findings
The subject test vehicle was run at Lapeer International Dragway Saturday September 28, 2002. Lapeer International Dragway is a ¼ mile track located in central lower Michigan at approximately 830 feet above sea level. Conditions were clear and calm, air temperature 70-75º, with humidity levels in the mid 60's. 4 runs were made, with a best e.t. of 11.40 @ 119.88, with the highest mph reading an 11.43 @ 120.28 pass, and a best 60 ft time of 1.61. (Appendix A)

The following Thursday the large headers were removed, and the 1 5/8" tri-y headers were installed. After an unsuccessful attempt to remove the headers without removing the engine, it was decided that the headers would be cut out due to time constraints. The smaller headers were installed with little effort, and the following morning the car was driven to the exhaust shop to tie the tri-y headers to the existing exhaust system.

The vehicle was again run at Lapeer International Dragway on Saturday October 5, 2002. Conditions were somewhat cooler than the previous week, with temperatures in the mid 60's and a stiff headwind of 10-20 mph with gusts up to 23 mph, adding an unexpected variable to the testing. (Note 2) A total of 7 passes were made with a best et of 11.80@115.24 mph, with the highest mph reading on a 11.84 @ 115.29 pass. 60 foot times dropped across the board with a best of 1.56. 2º timing and minor jetting adjustments had no apparent effect on vehicle performance.

Sunday October 6, one more attempt was made to complete the testing without the headwind. Weather conditions were similar to the previous day, with the winds now more cross-track than a direct headwind, and gusting to nearly 30 mph. A total of six passes were made with a best of 11.76@115.4 mph. 60 foot times remained in the mid 1.5 range with a best of 1.54.

Chart 1

Chart 2 graphically compares the best pass mile-per-hour with the 2" headers to the 1 5/8" headers.

Chart 2

Conclusions and Recommendations
Test results were evaluated and allowances calculated for weather conditions. Results of calculations to negate the effects of the winds encountered during the final test phases indicate that the change to the 1 5/8" headers accounted for a performance penalty of only .2-.3 seconds and 2-3 mph in the quarter mile.

One unexpected change in vehicle behavior occurred after the header change, relative to vehicle driveability. The nature of the tight 8" converter results in the test car acting more like it has a 3000 rpm stall converter (than one that stalls at 5000 rpm at wide-open-throttle) in normal street driving. Tip-in response with the smaller headers during normal street driving is greatly improved through the entire range from off-idle to 4000+ rpm. In addition, cold starts and low temperature operation is much smoother than with the 2" headers.

There are a few things that could be done to help compensate for the effects of the smaller primary tubes when seeking maximum performance while retaining superior driveability. One thought would be to run the headers with only an extension, eliminating the mufflers and tailpipes. Another possibility might be to increase exhaust duration to improve cylinder purge, either by a camshaft change or in the alternative by using 1.6 ratio rocker arms. Certainly a camshaft design could be optimized by a competent cam grinder to make the most of the tri-y design.

Notes:
(1) The spark plugs were replaced as a result of two being broken during removal (see test discussion for details). The spark plugs removed showed no appreciable wear or degradation, and were replaced with plugs produced by the same manufacturer, tip design and heat range, with the only difference being the hex size of the plug.

(2) While the temperatures were somewhat cooler the day of the second phase of testing, the resultant Relative Air
Density was within .3% and as such is considered insignificant for the purposes of these tests.

Appendix A - Test Data

Appendix B - Calculations
All horsepower correction factors relative to environmental conditions were calculated as the inverse of the dyno correction factor given in SAE J1349 JUN90.

Rear wheel horsepower estimates were derived by the using the following simple formula:

hp = weight * (speed / 234)³

This formula is considered sufficiently accurate for the purposes of this study, as the study is intended more to show the relative differences between the two test subjects than absolute horsepower data.

Best MPH with 2" = 120.28 = 489 rwhp
Best MPH with 1 5/8" = 115.96 = 438 rwhp

Comparing 120.28 mph to 115.96 mph gives averages of 489 to 438 rwhp, for an observed decrease of 51 rwhp.

A 15 mph headwind requires approximately 36hp @ 115 mph (per Aerodynamic and Rolling Horsepower Calculations by Bowling & Grippo), using a coefficient of drag of .40, and a calculated frontal area of 18.72 ft, at 63ºF and 29.29 in hg barometric pressure.

Adding in the additional 36 horsepower requirement of the 15mph head wind results in a net horsepower decrease of 15 rwhp, or less than 3.5%.

The above calculations concur with those using other slightly different, but widely accepted formulas.