Rob116

I'm running Dynomax 2.5" cat back on my 350 with a performance cat. The exhaust was originally intended for a 305, but I'm just curious as to exactly how restrictive it will be on a 350 with flywheel horsepower in the neighbourhood of 350-370 hp? I know that it's not optimum and that Dynomax would recommend the 3", but will it make that much of a difference given the relatively mild build of the engine?

Thanks.

Mark A Shields

It will be hard for the engine to make that much power with that restrictive exhaust. If the engine can really make that power with 2.5", you'd probably pick up another 25hp from a nice cat-back.

That's just an estimate, but that engine will choke with it.

83 MidniteZ

As Mark said, it will choke the 350 and not allow it to perform to its full potential.

Something to think about: what a lot of us would all like to run is a nice dual system with 2.5" piping. Now that is a 2.5" pipe for each engine bank. You are running 8 cylinders through 2.5" piping

Mark A Shields

Actually, most of us would want 2.25" duals, that is about the equivalent of single 3" but better.

Momar

Actually if you are going to make enough power that you need true duals, I dont think I would go with 2.25. Sure 2 pipes dont have the restriction from the y pipe, and with a x/h pipe you can have real good scavenging but you are not going to gain that much from going from single 3 to dual 2.25. If I just had a motor that needed that I think i would stay with single 3" and make sure to have a y pipe with a nice smooth transition off of your headers. I myself, have hooker 2210 long tubes and plan on using duals with an x pipe and it sure aint going to be 2.25. Ok, I guess he wasnt even asking about duals but I thought I would throw that in there.

Ben

AJ_92RS

It's not "but better"!!!!!!!

It's "but different"!!!!!

Different engines respond differently to different exhausts.

AJ

Momar

I think it is better. You dont have the y pipe causing problems. The main difference between different engines is how much air they flow/how much power they make. The more power you make the more air you need to flow, but if you wanted you could go for a 4" single exhaust which should acomodate flow for most, but you get better scavenging with a well designed dual than a well designed single. Single was designed only to make clearance, and to cut costs.

Ben

AJ_92RS

YOU DON'T KNOW THAT!!!!!!!!! YOU'RE JUST GUESSING!!!!!!!

I have my opinion on what flows better to...... but just to save face, I'm going to keep it to myself because I DON'T KNOW that it's better than the other. It's simply a guess. Besides, it's irrelevant because something that may not flow 'better' may not have better scavenging effects, or better pressure wave tuning, or preffered sound.

When one of you people forks over $25K for a flow bench to test your theory, and then spend another $50K for a chassis dyno to prove which one is "better" you need to STFU!!!!!! Because that's what it will take to prove which one is better FOR THE ENGINE IT'S ON!!!!!!!!!!!!!!!!!!!!!!!!!!!!

EVERY ENGINE IS DIFFERENT!!!!!!!! When are you people going to accept that FACT!?!?!?!? And that's what it is..... A FACT!!!!!! It's the only fact that's ever going to come from this ignorant 'DUAL vs. SINGLE' war.

AJ

deadbird

.... do you feel better now ?

Momar

Ok, if no one ever showed interest why would anyone that had the resources test stuff out. Duh. Anyway, I didnt say that I have proof, but when someone that does comes around I will be all ears. I have seen several peoples testemonies from going single to dual and gaining a lot. Also, a single 3 is only supposed to support reliably up to around 350 hp. After that you have to do something or you are loosing power. How can they just be different and not one better or worse? If they were just different you would make the exact same power, and run the same times. I also agree that every engine is different. It also is going to depend on what the conditions around you are such as altitude and so fourth, but you cant say that one will flow better on one engine and one will flow better on a different engine. They are going to flow the same on any motor, it just depends on what that motor needs. Obviously your 305 tbi is not going to need duals and probably wouldnt gain a ton but still may have a gain if you design it properly. I however have a 350 w/ cam, ported heads/intake, and long tube headers and around 10:1 compression. I believe I should see a decent gain from duals. Just a question. What makes you think that you would have to buy your own dyno to prove what works better. Just rent some time on one, or do testing at the strip for crying out loud.

Ben

AJ_92RS

And what makes you SO SURE that I think single is "better" than dual? That's what you're assuming, right? If you would like, I can point out in your post where it's obvious you're assuming that.

Anyway, I've seen HP results that showed average HP and average TQ go ^UP by going to a smaller diameter exhaust. That's called scavenging. And average is what matters, not PEAK HP or TQ. The reason is simply because the engine spends all of it's time at average HP and TQ than it does at PEAK. Hell, I bet on a 14 second run, PEAK HP and TQ are seen only three times in the 1/4 mile. Maybe only twice if the car doesn't hit 3rd gear before the lights.

Also, the thing that NOBODY here has yet to mention is..... the length of the exhaust pipe can have MORE of an effect on HP and TQ than the diameter of the pipe. That's called PULSE WAVE TUNING.

I guess nobody would "have" to buy a dyno, but if they're going to take all the time needed to try and prove one's "better" than the other, and test all the variables involved, and do it on numerous engines, then they may as well buy one.

AJ

Mark A Shields

How can single be as good as duals, look at the design, it's been discussed in the past. And I thought proven that duals were better. The design of a y pipe can not suffice to the design of 2 separate pipes.

Apeiron

Why not? Not saying I agree or disagree with it, but maybe you should summarize the points that were discussed in the past.

Momar

Well, I am not saying you think single is better, but you said that one is not better than the other they are just different. I dont agree with that. Yes, if your motor doesnt need it your power can go up by smaller diameter exhaust, but we are not even discussing diameter here. We are discussing single/dual. You can have small duals or large single. I believe you that you have seen power go up by smaller diameter pipe, but that still isnt single vs dual. If you have a properly designed dual vs single that equals about the same size(dual 2.25 and single 3 for instance) I think you will have a higher average power also. Here is a question for you. How many people with mustangs that came with dual exhaust from factory have you seen that needed an exhaust upgrade and wondered if they should go with a large diameter single instead of upgrading their duals? Or any other car that came with duals for that matter. Why don't drag cars run large single? You are argueing more the size of the pipe than dual vs single. I believe one is definately better than the other.

Apeiron, in a single system you have to have a y-pipe connecting the 2 pipes. In this case you have the exhaust from both sides of the engine colide and try to take up the same space at the same time. This is going to add a lot of extra friction, and turbulance to the exhaust and cause you to loose power. Another thing, is the crossflow mufflers don't usually flow as well as the normal in one side and out another. To get rid of that problem(crossflow muffler) you could only have a tail pipe on one side but many people don't want to do that because they dont think it looks as good. The size isnt usually a problem unless you are making insane horspower because you can fit 4" single pipe under the car but it is expensive because you either have to get it from mufflex and have lt headers or have it custom made. On a side note something I thought was interesting is steve spohns car has dual 4" exhaust.

five7kid

iTrader: (14)

Rather than get into the middle of this domestic dispute, I'll just go back to Rob's original question: Is 2.5" too small for a 350?

I wouldn't call that combo especially "mild". I'd say you need a 3" cat (you didn't say what size your "performance cat" is) and 3" cat-back. The dual-cat (2.25" each) flowed better from the factory than their 3" single did, was worth about 10 HP, and their engines were more mild than yours. So, for best results, go up in size.


Okay, I'll say this about single- vs. dual-exhaust. Back in the early 70's, my brother had a '66 Impala SS with 275 horse 327 & 4-speed. It had a single 2.5" exhaust. I picked up new 2.25" dual pipes for it, brazed some 2" glass packs to the ends of those pipes (which was back at the rear axle, the way they made those pipes) - that car just simply woke up. Finally had some "S" in it's "SS". BTW, a 275 horse 327 was fairly mild by today's performance standards. My Camaro would clean it's clock (even if the 305 was sitting in place of the 327).

AJ_92RS

Some of you are not HEARING WHAT I'M SAYING!!!!!

There's a reason I said "smaller diameter" and not "dual or single". It's because the flow area is what matters, not whether there are two pipes or one.

It doesn't matter whether there are two pipes, one pipe, four pipes, or eight pipes. It all depends on what the engine wants!!! And what the engine was built for!!!

What you're saying about the y-pipe is somewhat true IF you're talking about the factory one. I have no argument there. The factory y-pipe is a piece of dung.

But with a y-pipe that's made for headers, or even SLP's replacement that hooks to your stock manifolds, it's made to merge the gases together. Not "collide" into each other.

From the logic that you're using right now, you're saying that putting headers on your car would make it slower because then all the exhaust would "collide" into each other at the collector. So why doesn't everyone run straight pipes right off your heads like top fuel cars do? I know why..... because there would be no scavenging at all, and there'd be very little LOW RPM grunt.

And to add to how your logic doesn't make sense, why is it that when using a dual system, adding an h-pipe or an x-pipe BOOSTS low RPM torque WITHOUT giving up any upper RPM HP????? From your thinking, the gases would again "collide" into each other and the engine would loose power.

One more question for you... why is it when they show flow numbers on the exhaust side of a cylinder head, by adding headers INCREASES the flow numbers? Isn't that odd since "the exhaust from both sides of the engine collide", but in this case both sides would be each exhaust port, but that's the same logic you're using.

And once more..... you're assuming. You assume way too much. I never said one is better than the other, nor will I ever say that. You're assuming that I think single is better than dual, and your also assuming that dual is better than single.

You use the words "I think" and "I believe" too much. And you will NEVER be able to prove that one exhaust is "better than the other"!!! NEVER!!!! It's not gonna happen.

And as far as Spohn's car is concerned, that's not what I would call a "street" car. It's main goal is 1/4 mile time. Therefore it's called a "drag car". It's main goal is UPPER RPM HP. Therefore it has an exhaust system that's best for it. Why don't you put dual 4" exhaust on a stock LG4 and see what happens?

AJ

Momar

First, I didnt say you thought single was better. I pointed out that you said neither is better. Ok, as far as the x-pipe goes at one point on each bank of cylinders you have 2 fire on the same side. When this happens there is a large amount of pressure to flow down the pipes on one side and none on the other. The x-pipe allows for the pressure to divide and go whichever way it has less resistance so it doesnt slow down the exhaust flow as much. Also, as far as going to headers they colide farther from the engine for one thing, and also have a smoother transition than with manifolds. I also agree that the aftermarket y pipes are better than the stock one. Next, I would like you to demonstrate to me how you design a motor to work better with single exhaust than dual. You said its all in what it was designed for. Part of the reason headers work better than the y pipe is that different cylindars fire at different times causing there to be a steady flow of exhaust through the pipes except when the 2 fire from the oposite side which is where the crossover pipe comes in. In a y pipe you always have pulses at the same time and therefore the gasses are being forced into the same spot. I will not say that you like single better because I honestly believe you that you don't necessarily, but why do you keep making arguments for single and against duals. Assuming you would rather have duals or just dont care as long as its designed well you are arguing just for the sake of arguing. I see no point in that but go ahead and lets see what you have to say about my logic this time. Oh, and I was not using spohns car as proof of anything, I just thought it was cool.

Ben

Momar

Also, about bolting the 4" duals on a stock lg4 I just have to say that thats retarded. That has nothing to do with single or dual either that is simply overboard on size. There was no point to that statement whatsoever because I dont think anyone would even try that on a stock motor let alone a stock lg4.

Ben

AJ_92RS

By you stating that, I think you are finally starting to understand what it is I'm saying.

You have to fit the exhaust system to the application. Whether it's single, dual, with a y-pipe, with an x-pipe, etc. That's what I mean by they're all different. Besides, you're the one who brought up Spohn's car, not me.

And I'm sorry, but you're incorrect about the y-pipe causing exhaust gases to collide BECAUSE no two cylinders release exhaust at the same time. The only way that could happen is due to the different lengths of the pipes coming off the headers. But even then, as long as the y-pipe is of sufficient size, the two pulses arriving at the "Y" at the same time 'could' help each other, similar to how an x-pipe works. That's part of what "pulse wave tuning" is.

If you really think about it, a y-pipe (if designed right) is the same concept as an x-pipe, but just has one leg cut off. As long as the internal area of the single pipe is at least as large (if not larger) as both pipes going into it, then there would be no restriction.

Is it possible that you're thinking if you run two 2.5" pipes into a y-pipe that goes down to only one 2.5" pipe? If so, then you are correct that it will cause restriction. But if they're joined (i.e. designed) in a way that increases scavenging, then they may actually help low-mid RPM torque.

But again, this is all relative to the engine being built. If you're looking to build a tire shredding monster, while still using the stock TQ, and keeping the gears below 3.23, then big pipes (or dual. However you want to say it) that flow 1,000,000 cfm is NOT what you want.

That's why there's no such thing as a "better" exhaust system. That's all I'm saying. Every engine has a specific job that it was built for. If you want to increase it's ability to perform it's job, then you put the kind of exhaust system on it that IT likes.

Now since you keep assuming (I never accused you of saying it , but it's obvious you're assuming it) that I like single exhaust better than dual, you're wrong. I at least like to see two tailpipes coming out of the back of a car/truck. IMO, it just looks better. But I'm never going to say a dual exhaust is "better" than a single exhaust. That's because a small, crappy designed, dual exhaust is not "better" than a well thought out, properly designed, and quality built single exhaust. There are too many other variables involved that dictates which one is "better".

If you haven't figured out what it is I'm trying to say by now, then forget it. Just keep living in your "dual" world. (Just a joke, don't get flamed )

AJ

Rob116

I've just gotten back from purchasing a full 3" catback system (no cat yet ) I'm going to try and install it myself (Headers, no big deal, cat back... I'm not a very experienced welder/might not weld at all) Any tips or pointers on installing the cat back?

Momar

I have understood all along that you need to pick size and parts to match your engine all along. Here is a quote from the dr gas web site(www.drgas.com):

"The firing order imbalance is a necessary sacrifice for dynamic balance of the rotating and reciprocating crank, rods and pistons. Each time the firing order is completed (two full revolutional), two cylinders within each bank will fire and exhaust within 90 degrees of each other. These two cylinders will be exhausting into the header collector or exhaust manifold almost simultaneously. This overlapping condition creates a lot of back pressure and a pop sound. Meanwhile, the opposite exhaust manifold has no activity (or pressure) in it at all. This is where the characteristic, low harsh popping V8 sound comes from.

The most common way of reducing this out of sync imbalance is with rebalancing crossovers such as H pipes and over-and-under X style crossovers. These provide both noise reductions and efficiency (power) improvements. H pipes are effective only at low and mid range. The over-and-under X crossover works a little better but still does not completely rebalance the exhaust at high RPM.

Dr. Gas, Inc. has developed a new generation of crossovers that uses a tangentially siamsed junction, in place of the H or over-and-under X. It evenly splits, smoothes, and synchronizes exhaust pressures at all engine speeds. The high and low pressure pulses in the two pipes are evenly imposed on each other, leveling out the pulse time history pressure differences between right and left cylinder banks. Re-synchronizing the exhaust pulses reduces back pressure, cancels sound, and provides additional scavaging."

This is what I was trying to explain. When these pulses are so close to each other in time they are going to try to take up the same space at the same time. With the single it does not allow the excess pressure to go to the other side it causes it to flow together which causes "backpressure" As far as it the pulses not being in the same place unless the pipes were different lengths, you have cylinders from each side fire close to the same time all the time and like mentioned in the quote above at one point two on one side fire close together causing extra pressure. If you run a y pipe the pressure travels together all the way out. Also, if you havent noticed the primaries arent usually the same length unless you have long tubes, and even if they were there is different distances of pipe between the header and y in the y pipe because the single pipe runs down the passenger side instead of right in the middle.

Rob116, about the catback, which brand did you buy? Many of them have flanges that will bolt right to the cat if you get the right size cat. That would make a really easy install. I think you might have to lower the rear axle, but am not sure. I guess that depends on how the pieces are divided up. Good luck.

Ben

AJ_92RS

I already told you I'm done.

AJ

Momar

ok

Mark A Shields

lmao

Rob116

I actually had an exhaust shop custom bend the tube for me...it's actually two separate pipes, so I shouldn't (I don't think) have to drop the axle. I'm not actually going to run a cat, so it should bolt right up to the collectors. I'm mostly wondering whether you guys feel that it is actually necessary to weld it up or if the clamps will do the trick on their own?

Sorry,this is obviously not perhaps the most difficult question, however, I've been unable to find anything specific about it using the search function.

Thanks

Momar

I know when I got rid of my cat when I was running a single exhaust I just cut it out and clamped a pipe int there. I was able to get it to seal almost all of the way, but not quite. I have heard that there is a better type of clamp out there than just the u shaped ones, but I dont know what they are called. I think I remeber someone saying that they were like metal straps or something that tightened down more evenly. If you could get it welded I would, but I think it can be done well with clamps.

Ben

babyIroc87

You can pick up those clamps he's talking about from summit for about $8 bucks a piece. They're made by Dynomax and are steel clamps or something like that. Call them up and ask, my brother put these on his car when he put his long tubes on. They work really well and they help from messing up the exhaust as well. If you don't want to weld but welding is going to be a better idea.