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Author Topic: Exhaust tuning....  (Read 2383 times)


  • Dirt Poor Hotrodder
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Exhaust tuning....
« on: November 18, 2006, 10:32:49 PM »
        EXHAUST Tuning  MY  5.4L bird

Well, to start out, this applies to my bird and its NPI heads, sissy blower and F-150 Intake.
The  Dynotech Cats with 2.5" downtubes added 24 RWHP(with compensating tune, 8 hp without)
The increase from 2, and 2.25 inch pipes to 2.5 inch pipes added another18 rwhp with
corresponding tune adjusst on the dyno, so I figgured adding 3inchers to the mix could only
add more power, right? Wrong! Adding 3" mandrel bends and hogging out the cast iron
collectors actually caused a drop of OVER 20 RWHP, and showed a richer AFR than previous
by a whole point . (12.3 vs 11.2)..... verry perplexing indeed , and hadda stop and think for
more than just a few  seconds to come up with some answers, rather than just empirical
adjustments... Knowing how and why is valuable info, especially to guys working on getting
better performance outa a 5.4., or any engine for that matter.

At first it was WTF? Are the Bobsey Twins and Goldburg right about that valve shrouding
:censor: they are allways laying on the 5.4?        NAaaah! No Way! Theres more rational
answers.....Think first about symptomatic changes, and see how they can adapt to a plausible
theory. Hmmmm heres a clue...2 lbs. less boost after the 3" install... another clue, the engine runs
muuuch smoother, no more cam lope....Runs cooler.... more vac (about 1 inch). Lets add that
to the dyno's less power , runs richer......

OK, lets look at some aspects of the physical engine. NPI heads , but late ones cleaned up
a bit.Should flow halfway decent. The cams  have higher duration , more lift than stock, plus
the Comp "infamous 6 degree advance" and some OVERLAP. Remember this as we will get back
to this, but all indications, they flow pretty damn good. What about the manifold? Ya, what
about it? Well,I did a by guess and by gosh emperical runner length adjustment to what I
thought would be optimum for the 5.4, Inversely scaled to the 4.6 displacement difference.
Aaaah! Maybe that length is actually a pretty damned good length to enhance the pulse
charge characteristics with my little blower huffing away in its power band (I am pretty darned
sure thats so with an extra 33 foot lbs and28 hp at 3800 rpm.per Mr. Ramseysdyno) ok , so
the thing is running a good pulse charge under some boost .

Now, lets readdress that cam in perspective to other things and go thru the exhaust /intake
part of the cycle;
The exhaust apparently is quite efficient in this configuration, so much so that it may be even
doing a bit of extraction (face it exhaust systems are tuned by crapshoot)now the whole
thing has 6 degrees of advance, plus some overlap, so the intake is actually open as the piston
is sweeping out exhaust, and will stay open till the pulse charge has filled the cylinder, but
theres also that overlap to contend with.that causes the exhaust to stay open a tad more,
hmmm? OK, the injector starts spraying some small interval after TDC, and during the intake
cycle. thing is , even under boost theres a pulse charge going in, and part of it has allready
passed thru the  cylinder due extra efficiency of the exhaust.
The injector cycle remains same and is someplace after TDC, so theres complete fuel injection,
but Less air in the cylinder. Gives you a richer intake charge with less air, Ya Think?? Now
you can add more boost, but it will not do much as the same disproportionate flow is occurring.
At least thats what I see.(okay, so lets go thru events one more time; ehxaust cycle , sweeping
out exhaust as the piston approaches TDC. Now sometime before TDC due overlap as a function
of duration, plus the 6 degrees of advance , the intake opens up and the pulse charge under
boost enters the cylinder, and exits as exhaust too. At someplace past TDC, the exhaust is closed,
and  the cylinder fill beginswith a small amount of the pulse charge allready sucked out with the exhaust.
As the cyl. filling continues, after TDC, the pulse charge is some what diminished, but the fuel injector/eec
doesnt  know this so the the cylinder fill ends up short of air for the tune, and long on fuel. (Now
thats a sure recipie for less power..)

Answer; Short of changing to a non overlap/no advance cam with higher lift, I need some small
amount of back pressure, or change of the pulse frequency /Duration in the exhaust. 
Theres two ways that should work ok... go back to 2.5 Inch pipe for some of the length of the
system, and as I need to have the twisty part from the H-pipe to the muffs reworked due
clearance issues this will be the minimal cost step, a :censor: shoot, and another$100 for dyno
work needed. more elegant answer would be to buy some Super Trapp mufflers, and tune for
optimised power on the dyno with them by changing the number of internal baffles (Lonnies
 Idea) and a tad more expensive than what I wanna do. I certainly am not gonna start another
cam project, and will live with what I have in that department. Another thought, I noticed that the
exhaust is quite a bit cooler with the big tubes, and part ot that is more surface to dissapate heat,
but I wonder if that small amount charge air exiting also has something to do with that?Hmmm?
Looks like a bit may not be bad as cooling is allways an issue. Trick is to find the exact optimum
back pressure/ boost level to get  decent HP with a cooling effect as well. Now what would that
new HPS intake do to the mix?.... good question...

I learned another truth (anudder trooth?) that smaller (2.5") twisty section towards the muffs, with
the 3" from cats to the twisty section didnt work for :censor:....putting a smaller section down stream
from a larger section seems to create a resonance that sounds ugly, and runs fer :censor:....Answer;
Make the exhaust uniform in dimension, full length, or progressively larger, as the flow goes
downstream, and expands....just my two cents... I am now running 2.5 inch true duals with H-pipe,
and it is doing a whole lot better, again... I also mounted a smaller blower pulley, so make no claims
re; horsepower............too many variables added at this point,LOL
'95 T-Bird 5.4L blower and sum stuff
13.6sec 1/4 mi, 300RW at 5250 RPM
GVW 4340 lbs, go figure!       note; this is a new blogsite