Exhaust tuning thoughts from wayne

[wayne petty]

 back in 1979 or 1980... i had a 66 GTO.. needed headers... had issues with crossmember sag.. happened to work near cyclone headers in north hollywood.. i stopped in on a friday afternoon and sat in their lobby.. across the room was a conversation i was not in but overheard clearly..

they were talking about Anti reversion headers for the 440-3 motor homes...  the prototype increased the fuel economy to 33 MPG... but it melted the  pistons as it went too lean.   they could never sell them as some customers would install them without doing the proper carb mods...

this thought bounced around my head for about 30 years...   some of you know i worked at a non sanctioned drag strip in the 83 and 84... most of my daytime hours was as race starter. i sent thousands of cars down the 1/8th mile.. including top fuel funny cars... top fuel dragsters... racers on foot... racers running backwards on foot... racers naked in their cars.. just a helmet..

some of the open header cars just annoyed me no end... sounded like mad men with hammers slapping the exhaust pulses..

mid 2000's... the mechanic across the street who i have known since we were 9 years old... built a high powered honda drag car..  no turbo.. but like 14 to 1 compression..  ported heads.. insane amount of money... but man that exhaust raspy noise just twisted my brain...  i looked under his car... behind the flex joint is the collector flange...  i grabbed a paper towel and sketched out a pipe with a tapered lead in to a straight section.. so he could just unbolt the collector and slip it in...   a few months later he called me... he had it on the bolt on dyno making tuning runs... he hated the noise also..  he slipped in the tuning pipe..  he went from 265 HP to 275HP with just that one change in 5 minutes... the car no longer was raspy.. it sounded like a healthy big block chevy now...

about 5 years ago.. i was visiting a friends shop.. the next door neighbor ran winged 360 and  410 sprint cars on dirt..

i mentioned about what i found out about the tuning tube...  he and i looked at his exhaust and discussed it... no place to really install it...  he then told me why he was so interested in it... this is over an hour in conversation in..  he said that he had dumped a bunch of money in as many brands of headers that would fit his car as possible...  they don't race any more...  son/driver got a job working on the new nasa telescope build..

he showed me a pile of headers... said one pair had stepped primaries.. but stepped in a different way.. they were stepped down... about 2/3rds of the way down the primary..  they made a lot more power at high end.. but they were already putting out over 800HP. and it was just too much for too short a time during the short straights..

so what am i talking about.. think of a header flange reducer... nice tapered inlet.. but it has a foot to 18 inches of straight section.. smaller than the exhaust pipe in that area but only slightly larger than the primary tubes...

this has a complex function.. the exhaust pulses coming out of the primary tubes into the collector scavenge off the other tubes.. but then the pulse leading edge moves into the tapered area and accelerates thru the straight tube... this creates a second scavenging effect..  lowering the pressure in the collector and the primary tubes..  so when the exhaust valve opens.. the pressure in the primary tubes is even lower... pulling out more exhaust gasses from the cylinder and more during overlap from the intake valve just closing..  imagine having a vacuum cleaner lowering the pressure in the collector and primaries...

beyond the tuning tube... any pulses that expand into the rest of the system slow down..  any reflections that bounce off the muffler or open pipes.. cannot get thru the tuning tube as the sped up exhaust flow is constant.. but the reversion from the muffler goes around the tuning tube into the tapered end and is squashed..  can't reflect back...

you will really want to have an wide band air fuel ratio display if you attempt this...

fittings in exhaust that look like an hourglass do part of this... but they don't have the extended tube to speed up the exhaust flow to create more scavenging..

can somebody create custom tuning tube inserts for their already finished exhaust... more than likely...

if you think.. old wayne has had a brain fart... next time you go to an air show.. look at the stub exhaust on a P51 mustang.. you will see an inner sleeve the same size as the exhaust port... and an outer sleeve around it.. they call it Anti Reversion .. i have seen Harleys with versions.. but again this is right at the exhaust port... the tuning tube after the collector lowers the entire pressure in the collector not just the exhaust port..

[Bruce Dorsi]

Very interesting, Wayne!  ....Thanks for posting this.

[Pete]

There was a show on Engine Masters awhile back about all of that and what they played with definitely helped.

I have only ever done flathead Fords for vintage circle track racing and over the years found a combination by trial that works very well.

[wayne petty]

was that the baseball bats to the primary tubes ... yep saw that...

i used to be a moderator on a bunch of TEN forums...   CC, CHP  automotive.com  the pontiac and mopar forums. the mustang forums...  the blue 55 pontiac with the broken headlight and the Fuel injected 421.. i spotted in a craigs list ad..  posted it to the pontiac forums.. a staffter from HRM contacted me.. i gave him the info and it became an article and a cover car..