This is a long winded exhaust question. Currently I have a SI/DO flowmaster 40 on my truck. A lot of people on this site say that cutting the cats off and using O2 simulators will hurt the engine and the performance due to the loss of back pressure. What I want to now is how it can and if you can give me some technical data.

Can you give me some input on these mods combinations also:
1. Real duals with stock cats with a flowmastors or magnaflows
2. X or H pipe after cats back into duals
3. Dual straights after cats
4. No cats with O2 sims with comboes 1,2,or 3.
5. With or without shortyheaders on all comboes.

Which of these will give me the most performance and which one will sound the best with some performance gain. I don't mind the drone sound while crusing.

 

Forgot one other is with high flow cats on any of the previous comboes. Also I know that taking the cats off is illegal, but hay I leave Tennessee, need I say more.

 

Hi CEE21,

As you can imagine, we get *hundreds* of emails, posts, etc., every single day. Typing takes literally hundreds of times longer than simply talking on the phone, so as we have said here many times, we provide Tech Support to our customers in person or via telephone, only.

I suggest making use of the excellent SEARCH feature that is provided here on F-150 Online so anyone can look up & read anything that has been posted here - it's all been discussed many times before here over the years, so you can use the SEARCH feature's filters, etc., to retrieve & read any such previous posts (and there are plenty of them). The SEARCH icon in the upper right portion of your screen here - thanks in advance for your understanding on this, it's greatly appreciated!

Last - it doesn't matter where you live in America - it is *federal* law that makes it ILLEGAL IN ALL 50 STATES to run without catalytic converters on any vehicle that required them (which is all 1975 & newer cars & light trucks, in all 50 states) in order to meet the allowable emissions standards for that vehicle's make & model year. Remember, you can easily get BIG power while still having a clean exhaust, and *that* is the way to do things, my friend.

Have fun reading up on this via the SEARCH feature, & good luck!

 

OK I have been able to find some info on the subject.

But I guess I should have just asked the main question I really want to know. I want to understand what parts of the engine hurt it if there is too little back pressure and maybe if you can explain how. Also what back pressure would be on like say a 93 f150.

Since I read that these stock cats only produce 3.5 psi of back pressure I won't mess with them (really didn't plan on it anyway, consider I know very little about these moduler engines). Would this set up keep my low end the same and gain some overall power: high flow cats, into an x pipe into magnaflow, flowmaster, or dynomax. Will this be louder and better lowend and overall power gain than having a SI/DO flowmaster after stock cats (what I currently have).

 

 

i think some more info from you would be appropriate: what do you want to gain? IOW, is low end torque really important to you? what do you do with your truck, race, tow, daily driver? are you looking for gains at the track only or better overall performance? and equally important is what are your current mods and what are your planned future mods? and what is your budget? if you're not pulling the air in quicker, then there's not much point in letting it out quicker.

i've always heard too little backpressure can burn or warp your exhaust valves. i don't know if that's true but what happens is this. you have some valve overlap meaning your intake and exhaust valves are both open at the same time for a fraciton of a second. less backpressure allows the gasses to enter and then excape from the combustion chamber quicker than is needed for optimal performance during the overlap and lowered pressure. and at lower rpm there is more time for this to happen. also, unburnt fuel in the gas escapes too which can throw off your readings at your O2 sensers and degrade performance.

sound is very subjective, some people like different sounds. many factors influence the sound.

i say keep cats no matter what. if you don't have big plans then go with a nice 2.5 dual setup with h or x-pipe or a nice 3" cat-back. straights are just too loud IMHO (think redneck farm truck scaring dogs & waking neighbors). if you don't have performance mods to go with the exhaust (intake, underdrive pulleys, heads, cam, ported intakes, etc etc) I really don't think you're going to see much of a true performance difference between any of the options you present.

good luck

 

thanks for giving me some input.

I have a K&N cold air intake and with just a SI/DO flowmaster after y pipe. What I really want to know is how exactly it can hurt these engines, because on early to mid 90's v8s it doesn't hurt them at all and helps them gain some power and gas milage. A friend of mine put straights from manifolds out to his 95 f-250 351 and he went from 14mpg to 16mpg.

I looked on the search and just found so many different answers and theories. All I want is what it hurts and how it does.

If Mike has time would high flow cats, x pipe, into magnaflows help at least keep or improve my current low-end and what total gain can I expect. Also 2.25 or 2.5 inch pipes.

thanks Slick_Sammy for the reply