I found a reference to Kepich Exaust on the FTE Aftermarket Products bulletin board and called them about the 300 I6. The guy I talked to has been racing, engine building, and working with exhaust for over 30 years. His recommendation for the 300 I6 is (as best I can remember) a 2.25 pipe to the muffler and then dual tailpipes of smaller diameter. I noted having read that a larger pipe say from the cat back to muffler and then a smaller tail pipe is supposed to work well and suggested a 3" cat-back with 2.25 or so tail and he thought that would work well. He also agreed that just putting a better flowing muffler on would help, noting that the Ford truck mufflers are all blue on the front but cold at the back indicating a lot of backpressure. He said the 3" cat-back is so big that fuel economy will suffer (I don't understand this). Finally (and this is interesting), he said that a 3" diameter exhaust tip on a smaller diameter tail pipe, say 2.25" or so, would tone down the exhaust note. I am re-reading the book, "Scientific Design of Exhaust & Intake Systems" and it says a stepped pipe with the larger diameter located the correct distance down the pipe caused a reflective wave at the step location which combined with the reflective wave from the pipe end, resulted in a longer period of reduced pressure in the manifold and helped scavenge the cylinder at the end of the exhaust stroke. These tests, of course, were done without cats, mufflers, etc.

 

On Tuesday I am picking up the new exhaust system...I decided to go with a single HighFlo Cat 2.5 inch pipe and a Dynomax performance muffler... I will post with the "scientific seat of the pants" results when its all done.

WHaynes

93 F150 4.9L 4x4
4" ProComp StII Lift
BFG 33in 4.10's
Detroit TrueTrac's

 

Great! Can hardly wait for the results. You are going 2.5-inch. That probably is sufficient as you will have a 4.9 sq.in. XC vs the stock (what is it 2-inch?) 3.14 sq.in. XC. Some mechanics have told me that the diff between 2.5- and 3.0-inch would not be noticable. For your test results, you may want to try something relatively scientific like going 60 in top gear on the freeway and right under a particular bridge floor it (preferably a manual so it stays in top gear) and see what your speed is at the next bridge. Do this several times with the old exhaust and again several times with the new. If you do this, post all the data so we can see the variability and determine whether the results are significant. Only thing is we probably are testing a degraded stock exhaust vs new performance exhaust and a new stock exhaust would be a better comparison.

 

i've got a 76 f100 w/a 300 i-6. i took the ole
sawzall to the exhaust, and it sounds great
i talked myself out of a flowmaster, so the new plan
is to route pipe, (2.25) straight out the back and expand
it to quad tips of an undetermined size, probably whatever's
available. any ideas on this setup?

 

Well, I have had the new exhaust in for a week now.--Single Hiflow Cat(First cat was hollowed out and left in place), 2.5inch piping, Hiflow Muffler---. The total bill for the custom work done was 400 and some change. I am definitly happy with the new sound. I now have a vehicle that sounds like a truck!!! There is a bit of resonation that I believe is coming from the hollowed out cat that is noticeable in lower rpms. As for performance, The acceleration in the powerband has improved and the pull is much stronger in the upper rpms. I don't have to open the throttle as much as before to accelerate. Another problem I was having was when the truck was warm, the acceleration seemed to be hampered. This could have been due to a clogged cat. Anyways, It pulls strong now when warm. My Gas mileage has improved nominally but I have been somewhat more heavy footed. I will be doing a long road trip next week and will be able to get a good read on the numbers.
Would I do it again. Yes..If you have an older exhaust system it is great upgrade. I haven't noticed any appreciable loss in tourqe and the sound and improved performance are more than evident.

WHaynes

93 F150 4.9L 4x4
4" ProComp StII Lift
BFG 33in 4.10's
Detroit TrueTrac's

 

Thanks for posting the results. Glad to hear you're happy with the new system. I have to wait for my stock exhaust to go before I can upgrade. I can't believe it: 74,000 miles and the stock exhaust is still in great shape. My '84 F150 used to eat a muffler every two years. Still, I guess I can wait as this '95 300 I6 with EFI has pretty good power as it is.

 

I want my 300 to be louder but i still want the torque that i have and dont want to lose any. Did you lose torque with the straight pipes and what would be agood muffler to get.