this is the second message related to the same question.
I asked about the cherry bomb because I wanted to know if it was taking too much back pressure off my engine. I know if that happens the engine will be slower, but the pipe isnt that big.
I just wanna know what could possibly be the reason my second gear all of a sudden has no guts.

 

Backpressure never helps power. Reducing backpressure never reduces power. However, often someone will chop off the exhaust system and the truck runs like crap with no low end power. But A does not cause B, rather A causes X which then causes B. Simply put, they have destroyed the scavenging effect of the stock exhaust by cutting off the exhaust and the loss of scavenging has reduced power. An aftermarket exhaust specifically designed for the particular motor is better because it will be designed to reduce back pressure without destroying the scavenging (and hopefully improving scavenging). Incidently, my neighbor, a Ford engineer, told me that with the stock 300 I6 all the tuning effect (which produces the scavenging) is happening between the ports and the cat. Accordingly, anything you do to open it up behind the cat should not affect power.

 

Great, please define scavenging...

Thanks!
-Nick.

 

Yeah, What's Scavenging?

 

Scavenging is removal of exhaust gasses from the cylinder. It occurs where pressure waves from the exhaust pulses are timed (called tuning because it is sound waves) to create a negative pressure in the exhaust manifold in the latter part of the exhaust stroke. (The pressure waves bounce back and fourth or more precisely are reflected back, even from the end of an open pipe.) The negative pressure then essentially sucks (scavenges) the remaining spent exhaust gasses out of the cylinder so that the next intake charge is not contaminated. The more exhaust left in the cylinder for the intake stroke the less fresh air gets in and the less power. Some tuned exhaust systems are so good that the negative pressure is present through most of the exhaust stroke. This improves power by reducing the work of the piston pushing the gasses out. It is possible to get the pressure wave to increase pressure in the exhaust manifold when the piston is trying to expel exhaust gasses. This fights against the piston and reduces power. The other thing to remember is that one piston can be near the end of the exhaust stroke while another pistion on the same manifold is just beginning the exhaust stroke, backing up the exhaust process in that piston. This can be overcome by dual manifolds, tri-Y manifolds, longer manifold runners before pipes join, etc. All these things affect tuning as well as pipe diameter and total length. Now the last thing to point out (before I write a book) is that I have never build an engine and never modified an exhaust (I have put beer can patches on exhaust pipes in my younger days ), but have studied the exhaust process a great deal.

 

Talk about scavegine effects. The dual outlet headers I bought from CP do some good scavenging. Right after I had put them one I fired up my truck in the garage and noticed a faint blue hue coming off of the dual outlet header. Man was that awesome, after closing the garage door and holding my breath, the exhaust really showed its true color. A nice blue jet like flame coming out of the headers. Woo hoo!!!


1994 Ford F150
300-6
Highly Modified and more to come!

 

 

You got the blue flame out the open header, right? Just like the dragsters. I understand (if I remember correctly) they get that because the nitromethane burns slower and that combined with the radical cam (extensive valve overlap) causes the flames to extend out the pipes while idling. In your case, I would guess you have a pretty radical camshaft.

 

I guess so if that is the cause. The truck idles kind of loppy, but the head shows no signs of ever being removed. Yes the small blue flame could be found at the open header. Remember to that I have CP headers and that these headers aren't very long either. I was wondering whether or not someone might have replaced the camshaft before I got it do to the loppy idle and the blue flames. Anyway I plan on replacing that cam with another one.


1994 Ford F150
300-6
Highly Modified and more to come!

 

I found out what was making my truck way too slow. I had always thought my truck was slow because it was a low end torque pulling truck (which made me confused when I couldnt come off the line very quick). I also wondered why my truck was so quiet. All of a sudden after I put on a cherry bomb exhaust and had the exhaust come out in front of the rear tire I noticed my truck had no power at all. That is when I posted this message. One (#####) mechaninc told me it was the carburator. I took it to a different mechanic and told me it was my Catylitic converter. Turns out years ago before I started driving my truck Some mechanic replaced the converter with a converter for a four cylinder. finally it just blew out. I had the converter replaced with a much larger one and now it is hella fast and super loud. I love it.

 

heh heh he said hella.


87 f-250
" if you are going to have a bear, it might as well be a grizzly"
4.9 granny tranny 4.11 gears