I have a '91 F-150 2wd that I just put a 12" glasspack with dual 3" rear exit pipes on, and am a little confused about the whole backpressure thing. I was considering putting on a high flow cat or (if i can get away with it) just taking the guts out of the ones I have. However, Ive read several posts on here that tell me that the 300-6 needs some backpressure to maintain its torque, while others say that no backpressure is needed, and it would actually improve the trucks performance. A little clarification would be helpful on this.
Thanks
Josh

 

Here is the rule for a normal, mostly stock 300. The 300 like ALOT of fresh air, can't seem to get enough! The 300 likes to have a good amount of back pressure. The back pressure is absolutely vital in maintaining your amount of torque. Remember, this is a torque engine, that was it's MAIN purpose for being built, and that's why it stayed around for such a long time. The people who say that it needs NO back pressure either don't know for sure what they're talking about, or they're very used to V8 engines. Believe me, I hacked off my whole exhaust system and it couldn't even pull itself up on to our flat bed trailer. This was with the gas floored, while popping the clutch.

Hope this clarified some...

Chris
1982 F100 Flareside
300HD I-6
SROD 4speed
Blue & Silver

 

Hmm, so I should probably leave my cats on then.
Thanks for your help.

 

Steakman_usaf: I don't know whether you saw my other posts re exhaust on 300 I6 engines, but maybe you would be so kind as to give me some advice. I have a '95 F150 supercab with 5 speed and short bed and the only mod is a K&N filter. Your post makes sense to me. I was told by a Flowmaster/Gibson exhaust dealer that the highly-touted-on-this-website 3-inch cat back system for the 300 I6 would reduce torque under say about 2000 RPM but would add power above that. While I could use the extra 10 to 15 hp (26 to 30 ft lbs) they claim on these systems (and I could use it at about 2500 RPM, I am reluctant to do anything more than put a Flowmaster 70 series muffler and larger tailpipe on mine as it already is a bit sluggish below 2000. Last night I measured my pipes OD circumference and figured 1/16" wall thickness to come up with the following inside diameters: dual manifold pipes 1&7/8" each, after first cat 2&5/8", after second cat (or resonator?) 2&3/16", and after muffler a scrawny 2". What would you do with this truck to boost power? I am particularly interested in exhaust mods since they can be low extra cost since they normally need periodic replacement anyway.

 

With the 12" glasspack and cat-back duals that I put on, there was a noticable increase in power to me all throughout the powerband. I havent checked it, but I think my gas milage is a little better also. Next Im planning to add a K&N system to help with the intake.

 

Steakman USAF: Please provide a non-anecdotal explanation as to why "...backpressure is absolutely vital to torque production..." in an internal combustion engine, and what intrinsic relationship that ("vital necessity" backpressure) has to basic engine architecture, i.e., six cylinders inline or eight in vee?

Silly me--here all this time I thought low-RPM torque was generated by engines in which inline 6, 120-degree/V-8, 90-degree power impulse variables were only part of the equation, along with cubic inch displacement, low internal pumping losses, stroke to bore ratio, conservative cam timing, high intake mixture velocity, efficient exhaust scavenging, richer jetting to correct a lean condition created by hacking off an exhaust system--and all along it was the exhaust backpressure diluting the fuel charge that was making lbs./ft. at low RPM.

With your permission, I will e-mail your empiric, insightful response to automotive powertrain design engineer colleagues in Detroit so they will realize their folly in spending ever-more research money designing freer-flowing exhaust systems for fuel efficiency, when all they need do is create more backpressure.
Eddie

 

Jeez, Eddie, don't beat the kid up too bad. The "backpressure" thing is the conventional, if wholly incorrect, belief. Smith's Scientific Design of Exhaust and Intake Systems isn't exactly a best seller.

What amazes me is that none of the cat-back manufacturers have any dyno data that they're willing to show us. You'd think they'd be publishing it everywhere they could.

 

I just wanted to know if I should take off my cats or not, I didnt mean to get anybody killed!
Josh

 

OK. Leave the cats on or replace them with a high flow cat (your option). There are several good reasons for doing this:

1) it is technically illegal to remove them, and the laws in this area are going to get tighter not looser, especially if Tree-hugging Al wins the upcoming election.

2) the cats do not hurt performance significantly if at all, and a computer controlled engine may actually run worse without them.

3) they actually do help clean up emissions, which is not a bad thing for those of us who breathe.

The question with the 300 is how open a cat back can you run. Every diameter change, up or down, in the exhaust causes a pressure wave to be reflected back up the pipe. If you get the lengths exactly right, this can create more power at certain RPM ranges and cost power at others. This is the origin of the "backpressure" myth, and why devices like stepped headers exist. Pipe diameter is related to exhaust gas velocity which also affects at which point in the individual cylinder's cycle the pressure waves get reflected back.

If you increase pipe size, you lower velocity and the reversion waves will arrive at a different point in the cycle. It will change the RPM range at which the exhaust is most efficient. However, it will also reduce back pressure so the engine has to work less to pump exhaust gasses out at all RPMs less. This only works up to a point though. If you increase diameter too much then the gasses lose too much velocity tend to stay put in the exhaust system and the engine has to do MORE work to pump them out. The trick is to find the balance for each engine in it's specific application.

All of this is WAY over simplified, but you get the idea. The manufacturers put the best compromise exhaust they can at the lowest price. For 99% of owners it's OK. Those of us on this board are the weird 1% who are looking for something more or at least different from what the factory provided. We still have to make compromises, there's no way around the physics, but we look for different benefits and create different weaknesses than the factory exhaust.
It pisses me off that tha aftermarket exhaust manufacturers won't talk about gains AND losses, because BOTH must occur with any change.

End of rant.

 

ok, thanks for the explaination, that answers a lot of questions I had. Any suggestions on the diameter of the pipe I should put on?

 

That is EXACTLY what we're all trying to figure out.

If you go by Clifford's recommendations, a stock to mildly modified carbureted engine with headers takes a 2" head pipe and tailpipe and a built up engine takes a 2.5" head pipe with a 2.25" tail. They recommend 2 sizes larger for multi-point injected engines.

That would mean a 2.5" system for a mild EFI engine and a 3.5" head with a 3" tail for a built engine.

If you throw a converter or two into the stream, the numbers may change. By the time the exhaust gasses have passed through the cat(s) there may not be much tuning going on and a slightly larger exhaust may help or, because of lost velocity in the cats, it may hurt.

None of us on this list knows for sure, but we all have an idea of what ought to work. The aftermarket exhaust makers may know, but it seems they would rather shovel their BS than give out real info.

But diameter is not the only issue, the size, location and type of muffler all come into play. Otherwise identical 2.5" systems with different mufflers can have radically different performance characteristics. One may be a jewel for towing but suck as a street racer.

There is almost certainly no one "best" exhaust for all purposes, but it would be nice to be able to compare DATA and make decisions based on facts rather than hopes and opinions.

You got me ranting again, didn't you.

 

Eddie: You seem to know the most about exhaust modifications. Let me run this by you. I have gotten all kinds of responses to the 3" cat back on the 300 cid six. Some said I would get loss of low end power below 2000 rpm. Edelbrock said the larger the pipe the less low end torque. One company selling Gibson and Flowmaster backed off from 3" to 2.5" recommendation when I told them I was concerned about low end power below 2000 rpm. Yet Jardine said the 3" on that engine improves power from 800 rpm on up. You also reported improved power from idle on up. Your truck is the only actual example we have that it works, though I am not sure how it compares to my bone stock truck since you also have headers. I suspect that the long EFI intake runners combined with restricted size intake (and exhaust) valves/ports on this engine is the overriding factor for low end torque production and that when good tech people tell me I am going to lose low end power with a 3" system, they are not understanding the design of this engine. Make sense?

 

If you are completely stock, as my 81 F100, do you not need some backpressue to keep your EGR valve responsive? Or is the amount of backpressure required for the EGR valve so low that it is not a factor?
Thanks.

 

Well guys from reading all of the posts regarding the reconstruction of the exhaust on an inline 6 I would have to say you don't want an exhaust pipe bigger than 2.5". To reinforce this statement, over the weekend I called several exhaust specialists and all of them agreed this engine does require some backpressure to maintain torque. Not a whole lot but more than a common V8 from what I was told in my discussions with them. This engine is made for torque and d**n near torque alone. Also these "specialists" I talked to said opening the exhaust beyond the stock exhaust would give a noticeable difference in power for this engine. A real by the seat of your pants feel. Most claimed to have worked on this engine at least three times or more. They also mutually agreed on combining a dual outlet header into a single exhaust pipe system. One didn't want to give me a dual exhaust not only for the lack of backpressure but for the prevention of overheating some type of switch for the back breaks. This switch apparently detects loads in the bed of the truck and as this weight increases, it increases the amount of work the rear brakes do. I don't know if this is true haven't researched it yet, but it sounds good.
For your info I have finally decided to install a set of dual outlet Clifford headers, a single 2.5" inlet/outlet cat conv, a single inlet/outlet 2.5" Flowmaster 50 series muffler and straight out the back. You probably didn't care to read this but I thought I would just add my 2 cents.

 

paulg: I talked to a number of performance exhaust companies too and got some results like you did, but call Jardine. They will tell you that they never heard of needing backpressure and that the 3" will bive you added power on the 300 six off idle up to 4000. Also go see the recent stuff on the next thread titled Gibson exhaust (the other flaming thread). I talked to my neighbor Ford-engineer about all this. He said back pressure NEVER helps power and that the 3" would not hurt power on my truck, but may not help either. My '95 300 I6 has a 2" tail pipe which is about 3.14" cross section. A 2.5" cat back raises the cross section 60 percent, whereas a 3" raises the cross section 120%. One thing for sure, if backpressure never helps, then I might as well go with the 3". I'd like to know how your truck does with the 2.5-inch, especially right off idle.