By John Wilhoit
We performed four comparisons using our latest 1925TR 356 engine with a stock Dansk muffler, the NLA Dansk Supersound muffler, the Bursch 1.5" header (with street muffler and race stinger), and the WR 1925TR Sport muffler. These tests were performed at Carobu Engineering in Costa Mesa, California, on their DTS engine dyno, in February 2007.
The 1925TR is very streetable but is by no means a mild engine. The peak power is at 6000 rpm and peak torque is around 5000 rpm. These are recorded during a full throttle sweep from 3200-6500 rpm in which the performance is recorded and calculated by the dyno at 100 rpm intervals.
The dyno also plots the fuel curve at 100 rpm intervals across the entire run using a lambda sensor. In the 356 engine there is always a dip (richness) in this curve just below peak torque, which is made worse by the use of a sport cam. The tuning and back pressure of the exhaust can help this (as you'll see in the tests), but it can't be eliminated with carburetors, which depend on the intake vacuum signal to atomize the fuel. EFI can eliminate the fuel curve issues because it can be tuned based on throttle position and rpm, not vacuum, but because of the increased overlap with a sport cam, there will always be some power loss at lower rpm (below peak torque). On these tests, carb jetting was only changed with the Bursch Racing Header. Both the Stock and Supersound mufflers backfired on deceleration after the full power sweep on the dyno. One possible explanation is that these exhausts have too much restiction for the 1925cc engine with the sport camshaft.
Keep in mind that these test were performed with an engine that produces over 50hp more than the most powerful stock 356. The fact that the stock muffler did so well says a lot for the guys at the factory. It is possible that the comparisons could be different on a completely stock engine.
All runs on the DTS dyno are corrected for ambient temperature and air density. The 1925TR engine has been extensively tested and the performance figures shown here are very repeatable and consistent.
Test #1: Stock Muffler vs. NLA Supersound
This shows the horsepower and torque curves of the Stock Muffler vs. the NLA Supersound. The Supersound has a different sound than stock, much deeper, but with the added chrome resonator tips sounds too much like a VW. The Stock and Supersound both make good low end torque. They both tended to lean out the richness in the fuel curve between 3500 rpm and 4500rpm and this helped the lower end torque. The Supersound made more top end power but for no really obvious reason (probably because of slightly less back pressure but it's difficult to say for sure). If you run a Supersound vs. a Stock muffler you might be able to tell the difference in performance...you'll definitely hear the difference.
View dyno test graph #1
Test #2: Stock vs. the Bursch "Street Quiet"
This test shows the Stock vs. the Bursch Header with a street muffler attachment. Low end torque is way down but the tuning of the individual runners seems to help under 3500 rpm. The top end suffers as well and we don't have any real explanation, except that the fuel curve has gone very lean overall. This could actually be the reason for the improvement under 3500 rpm. My guess is that the street muffler attachment on the Bursch is too restrictive. Without question, the Bursch exhaust dramatically affects the fuel curve and shouldn't be used without testing for a lean condition, either on an engine or chassis dyno.
View dyno test graph #2
Test #3 Stock vs. Bursch with "Stinger"
Since many Bursch systems are used on the track, we thought it would be fair to try the Bursch with a straight pipe "stinger." We also removed the air cleaners for maximum air flow into the carbs. The main jets were increased form 135 to 145 to correct the lean condition encountered on the last run. The improvement in the fuel curve brought a dramatic change in performance compared to the last run, but the results of the Bursch Race Header were disappointing, since it couldn't duplicate the performance of the stock exhaust. With more time on the dyno, it might be possible to optimize the fuel curve and improve the top end, since the curve with the 145 main jets was a little too rich.
This test does show that sound can definitely make you feel like you're going faster.
View dyno test graph #3
Test #4: Stock muffler vs. WR 1925TR Sport Exhaust
The last test compares the Stock muffler to our WR 1925TR Sport Exhaust. This is a hollow can-type muffler with equal length primary pipes entering the can. The inside of the can is empty but the stainless exit pipes have internal resonators. It is the loudest of the mufflers and has a very different sound, with very low back pressure. It will not work with the stock J tube heaters and is specially designed to clear the lower plug wires on the twin plug engine. The equal length pipes boost the very low rpm torque and the low restriction design carries the power into the higher rpm.
The WR 1925TR Sport Exhaust is a good balance of performance and looks on a 356...it is also about 3.5 times the cost of the other exhausts.
View dyno test graph #4