Exhaust question...

[1986F150six]

Good morning,

When replacing the head pipe, beginning at the 4.9L stock log exhaust manifold, can a larger diameter pipe be used? Can the end of the larger pipe be flared such that it will mate with the exhaust manifold ball shaped exit?

Thank you in advance!


1986 F150 w/ 4.9L engine.

[Harte3]

A muffler shop would be a good place for your question.

[Max_Effort]

Good morning,

When replacing the head pipe, beginning at the 4.9L stock log exhaust manifold, can a larger diameter pipe be used? Can the end of the larger pipe be flared such that it will mate with the exhaust manifold ball shaped exit?

Thank you in advance!


1986 F150 w/ 4.9L engine.

Usually you cannot use a pipe much larger or the flare won’t mate. It will be too large. The pipe also has to fit through the flange. There might be some fudge room, you'd have to mock it up see.

What you can do is step the pipe up an inch or two after the flare.

[sdiesel]

furthermore, do u really want a larger pipe?.
what are u hoping to gain by going up in size?

is ur log manifold a HD or a light duty manifold.

[1986F150six]

My overall quest is for fuel efficiency. I was hoping to create a more free flowing exhaust. It is an original log manifold, not HD.

[bubba22349]

I made a 2 1/4 inch exhaust pipe with a Turbo Muffler, work with the orginal log exhaust manifold on my 1965 F350 300 engine. Was a good free flowing straight pipe system with a total of only 2, 30 degree or less bends, good luck.

[pmuller9]

What carb did you end up using and what is your present gas mileage?

[Mdixon300f100]

I went efi manifolds, walker downpipe, 30 degree exit behind passenger door. Woke it up right to the 3800rpm wall from the carter carb. Got there a lot faster though

[1986F150six]

What carb did you end up using and what is your present gas mileage?

Carter YF 4901 S [from a 1970 F350].

The following is a report from last year:

Well, I am back from the 2018 Garagemahal Ford Truck Show [what a great time to be had by all who attended!!!].

I attended in 2017, but due to mechanical problems with my truck, was unable to drive it.

I decided to update this older thread by including the gas mileage for this year in the same format as before. Since the previous listings, the distributor has been replaced with a NOS Motorcraft Duraspark distributor which was recurved by a man I have come to know through another forum. He asked if I might be interested in slightly better performance and gas mileage, but would be required to use 89 octane gasoline. I agreed and the static timing is @ 16* BTDC.

This year, I decided to drive to get there as quickly as possible and drove the speed limit or with the traffic flow [faster].

Instead of my "normal" tank-by-tank description, I am going to provide the overall average for the approximately 1200 mile trip as compared to the previous 3 years.

2014 55-62 mph with no aerodynamic modifications = 21.93 mpg [highest tank = 23.26 mpg]

2015 55-62 mph with aerodynamic modifications = 24.13 mpg [highest tank = 25.72 mpg]

2016 55-70 mph [weighted towards 65-70] with aerodynamic modifications = 22.05 mpg [highest tank = 23.46 mpg]

2017 Missing data due to having driven another vehicle.

2018 65-75 mph [closer to 75] with no aerodynamic modifications] = 19.87 mpg [highest tank = 21.3 mpg]




Thank you for your interest!

[sdiesel]

in general terms, smaller initial pipe close to the exhaust valve should be small on a vehicle that does not see a lot of wide open throttle.
after a certain length the pipe can be opened up.
that certain length is black art. I keep the small, even on the 6.9 diesel, which has similar pipe to the 300. to urge the gas out as rapidly as possible.
I then employ a reversion tube in the down stream collector to reduce...reversion.
engineers and F1 guys spend how much? trying to figure out exhaust. and there are mountains of engineering papers on design
my simple mind approach rely on luck and the logic thst if I follow basic institutional knowledge, then luck will smile,.
or better than not having it. which aint always true.
but small pipes and reversion tubes are generally accepted as useful.
.
small pipes=fast exhaust
big pipes=gasses lolly gag and and stall in the pipe.

[Lazy JW]

small pipes=fast exhaust
big pipes=gasses lolly gag and and stall in the pipe.

Which makes me think that the pipe behind the muffler should be smaller than the inlet maybe?

[sdiesel]

just the opposite.
as gases cool they slow down, and a larger pipe is better after the muffler. there the gases lollygag, and are swept along by momentum from the smaller pipes push.
but there is room for more gases in the larger pipe and the gases are contracting so I can shove more in.
sorta, like that.
my tail pipes are 4 inches sometimes . from my hen scratching research this is the best for street driven rigs.

[1986F150six]

Thank you, gentlemen. Please keep the information flowing!