That "additional restriction" might have been making more horsepower _and_
torque. Good extraction requires a constriction of some sort. When a
positive pressure wave hits an increase in diameter it reflects back a
POSITIVE pressure wave. Likewise, when it hits a constriction it reflects
negative. Designing an exhaust system requires striking a balance between
that strange bit of physics, mass flow, and other sonic tuning
considerations. At the bottom half of the RPM band a really GREAT pipe
delivers a positive pressure at the exhaust port just before the exhaust
You also want a relatively constant pressure in the exhaust system so that
the tuning works consistently. The speed of sound varies with pressure. Big
exhaust pipes have little pressure control.
The most expensive exhaust systems around use a very precisely shaped
collector with a convergent-divergent nozzle after the collector to reflect
a low pressure wave that helps with extraction. The nozzle also acts as a
bleed to maintain pressure. In other words, whacking the restriction off a
pipe developed with any kind of dyno effort probably just cancelled out the
Simply put, it's not about restriction, it's tuning. If low restriction
meant horsepower you'd see drag cars with no exhaust pipe at all. Even then,
they are a special case--they have so much exhaust to get rid of that
restriction IS the biggest concern.
From: owner-fot@Autox.Team.Net [mailto:owner-fot@Autox.Team.Net] On Behalf
Sent: Wednesday, December 01, 2004 11:06 PM
Subject: TR Related, Final Project, Exhaust Headers
I finally managed (with a little grinding) to run 2 1/2 inch flexible
exhaust tubing through the frame of the TR-3, as outlined in Kas's book.
The dB's went up from 88 to 95, but I have some after thoughts. The four
tubes terminate into one 2 1/2" collector with a bolt on piece that tapers
down to two inches in diameter. I cut off the smaller diameter portion of
this piece before connecting the 2 1/2" exhaust tubing, since I thought that
this was an additional restriction.
Could this tapered piece have something to do with the tuning of the exhaust
header? I got into tuned exhausts in the early 60's while still auto
crossing and hill climbing the car. This other guy, who traveled with us,
had a brand new Alpha rated about 30 HP more than my old TR-3. He could
beat my times in autocrosses, but then we went to the Lowellville Hill
Climb. Prier to our runs, he was telling me that there was something
drastically wrong with my car.
It doesn't run very well and burns oil. I then beat his times in the hill
He took his car to Boffo Motors in Beaver Falls and told them that there was
something wrong with it. I beat his times in the hill climb. They told him
that his Alpha may have more rated power than my TR-3, but he has less
displacement. As a result, I probably have more torque for climbing hills.
He couldn't accept this explanation.
They also told him that he probably lost power if he took off his stock
exhaust system and added a straight pipe. This was supposed to be a tuned
I have another nearly identical exhaust header I can use. Someone was going
to put it on his street TR-3. He gave the header to me after I told him
what he had to do to install it. -- pull the transmission, install a TR-4
flywheel, and install the shorter starter motor.