MotorTrend have done some dyno testing comparing a twin 2.5inch exhaust system vs a twin 3inch exhaust system on a 618bhp & 580lbs/ft engine. How do their exhaust test results compare with the numbers from the StrikeEngine calculator? The StrikeEngine exhaust calculator is here.
MotorTrend Exhaust Test – The Results
These are the power and torque numbers MotorTrend got for each of the exhaust setups.
With open header the engine made 618bhp @ 6600 rpm (guesstimated from screenshot) and 582lbs/ft of torque @ 4800rpm
2.5 Inch Exhaust
With the 2.5inch exhaust system the engine made 601bhp @ 6100rpm and 554lbs/ft @ 4600rpm
3 Inch Exhaust
With the 3 inch exhaust system the engine made 614bhp @ 6200rpm and 569lbs/ft @ 4600rpm
|2.5inch||3inch||Open Headers||2.5 Difference vs Open|
Actual & %age
|3.0 Difference vs Open|
Actual & %age
|2.5 vs 3.0|
|Torque lbs/ft||554||569||582||-28, -5%||-13, -2%||-15, -3%|
|Power bhp||601||614||618||-17, -3%||-4, -1%||-13, -2%|
What Does the StrikeEngine Calculator Say?
If we assume we are dealing with 2 engines each producing half the power of the V8 engine in the MotorTrend test.
With open headers, half the power of the engine is 309bhp.
Bolting 309bhp into the StrikeEngine calculator, we get a diameter of 2.25 inches as being optimum.
However, the calculator also states that if the result shows an exhaust gas velocity of over 250ft/sec, we should use the numbers for an exhaust with a 20% bigger diameter.
How big is the 20% bigger exhaust?
We get a number of 2.72 inches and that gives us an exhaust flow speed of 190ft/sec, well below our threshold of 250ft/second.
In short, the StrikeEngine exhaust calculator recommends we go for the 20% bigger diameter exhaust of 2.75 inch (nearest standard size).
Exhaust Test Conclusion
In the video they mention a difference in air flow ratio between the exhausts and the open headers. The dyno operator stated that he thinks this is more down to the length of exhaust rather than the diameter of the exhaust.
It is possible that the power of the 3 inch exhaust is down vs the open headers due to the exhaust system length and not exhaust diameter.
In other words, it’s possible that the 2.75 inch system would also give the same power as the 3 inch.
Unfortunately, MotorTrend didn’t test a 2.75 inch exhaust.
Changes To Our Calculator
On the results of this exhaust test, the StrikeEngine calculator’s initial recommendation of a 2.25inch exhaust looks to be too small. The 20%+ diameter looks close to optimal.
I will offer an alternative calculator on the same page as the original calculator. It will use the same formulas as the original calculator but with some small tweaks. It will use the MotorTrend dyno results as a guide. This alternative calculator will also use a different maximum exhaust gas speed threshold of 190ft/sec instead of the current 250ft/sec.
Something to Note
The MotorTrend results are not definitive for every engine and exhaust combination, as I mentioned above, the engine dyno operator stated that exhaust system length can also play a small part.
Some links to forums where people are comparing 2.75inch exhaust systems (StrikeEngine calculator recommended size for the MotorTrend engine) to 3inch exhaust systems (what MotorTrend said is the optimum size, from the diameters they tested in their video).