Modification Test Runs

Due to the number of changes Rob was making, plus the problems we were having getting consistent data, Rob made a total of fourteen runs over the three days we got together to play with his Impala. The first pre-modification write-up is set up separately as a baseline; this write-up discusses the following test runs:

Test Run Set #1: Intake only (also new plugs) - three runs, performed on 4/11.
Test Run Set #2: Intake and exhaust - two runs, performed on 4/11 (after exhaust install.)
Test Run Set #3: Exhaust only - three runs, performed on 5/1 (after intake was removed.)

What we found was that while there were some patterns from run to run, we were getting seeing some "flyer" runs; runs that just did not match the other data. After looking at the data for entirely too many hours and replaying the test results in my head several times, eventually the conclusion I drew is that due to the extremely rough Test Site North road, several times we moved to a different starting point on one direction's runs to avoid heaved pavement. However, there is a slight slope in certain spots and this was enough to skew a few of the runs.

The supporting data for this conclusion comes from variance found within the trap speed of Rob's runs. While ET can vary based on how well you launched the car, the trap speed is entirely a factor of engine power and aerodynamics. Simply put, whether you lug the engine, light up the tires, or hit the launch perfectly your trap speed should not vary, although your ET will. Check out the acceleration comparison from the first set of test runs. You can clearly see a significant difference on one run due to where we staged that particular run. Accordingly, I weeded out these flyers and used the 90% of the runs that were consistent within their groups. If you look at this comparison you can see how the data coelesces even across all four test sets.

Acceleration:

In this case, what we're trying to accomplish is compare Rob's changes from one run to another and see what the mods actually did to his performance. Since Rob has an automatic transmission, we weren't able to perform power runs since we can't lock the tranny in one gear. Instead, we performed lots of acceleration runs so we could compare and contrast them from one run to another. Since some of these runs were performed during different times and days, the atmospheric and temperature varied enough that I created three correction spreadsheets (below) to accurately compare the various changes.

Power:

Since we couldn't perform accurate power runs due to the auto tranny, we attempted to eliminate down to one variable and then figure out how much each modification may have changed. For example, to see how much difference the intake made, we compared results from test run #2 and #3, where #2 included the intake but #3 did not. The acceleration runs yielded a clear, if slight, improvement in trap speed for test group #2 with both intake and exhaust. Using the corrected results from below, the best estimate for the intake Rob installed was approximately three horsepower (225 bhp versus 222 bhp) and one MPH of trap speed. This is actually a pretty significant improvement for an intake filter system...

When evaluating the exhaust system changes -- which included two changs, a less restrictive downpipe plus a full cat-back exhaust system -- we can compare test group #1 (intake only) with test group #2 (intake + exhaust). After corrections are performed, the best estimate for the exhaust changes are two horsepower "at the crank" (225 bhp versus 223 bhp). Put together, that means both sets of mods increased the Impala's horsepower by about five bhp "at the crank."

Technique:

As before, Rob found that rolling onto the throttle gently for the first half-second and then pasting it to the floorboards resulted in the fastest ETs. Anything too hasty meant massive wheelspin due to FWD and an open differential.

Corrections and Other Factors:

Once again I've corrected the quarter mile results to the weight of a single person, quarter tank of fuel, and SAE J1349 standards. Each set of runs has a separate corrections spreadsheet, so click on the run descriptions below for each copy:

	Recorded Data	Corrected Data
Pre-modifications (4/3):	0-1/4 mile in 15.4 secs @ 90 mph	0-1/4 mile in 15.3 secs @ 91 mph
Intake only (4/11):	0-1/4 mile in 15.1 secs @ 93 mph	0-1/4 mile in 14.9 secs @ 94 mph
Intake and Exhaust (4/11):	0-1/4 mile in 15.1 secs @ 94 mph	0-1/4 mile in 14.9 secs @ 95 mph
Exhaust only (5/1):	0-1/4 mile in 15.1 secs @ 93 mph	0-1/4 mile in 14.9 secs @ 94 mph

So, after both mods Rob was able to cut a new personal best for the Impala SS, faster than the original runs when it was stock and new! Not bad. When we test it after the shift kit he should knock off another tenth or two for another new best... :)

Pictures:

Stock Intake	New Intake Parts	New Intake Installed
New Exhaust Parts	Old Exhaust	New Exhaust
Both Exhausts	An Installer, with Dawn.	The Results: Less Rubber!

Videos:

Rob's Intake and Exhaust Low resolution (WMV, 5MB) High resolution (WMV, 10MB) Streaming video (faster)