H22A: Putting a 2.5" collector on a stock or cheap header?

[deevergote]

In my searching, I've found 2.5" collectors for about $150-$300. I'm just wondering if it would make sense to put a larger collector on the stock piece, or even a decent (yet inexpensive) header like Megan Racing.

How much of the power is in the piping vs. the collector? Am I missing any issues that would make putting a new collector on a header impractical?

[aero]

I am no tuning specialist, but I think that putting a 2.5 collector on a 4-2-1 header would be a bit more promising than using a 2.5 4-1 (kamikaze). 4-1 is more high end, where the stepped one keeps more low end power.

Thats just an educated guess from me though. I noticed less low end when I went up to 2.5 exhaust, specifically the 4-1 header. For example, I have a hard time spinning tires in 2nd anymore. (I do have 205/60's which is 1/2 upsize and of course nice grippy tires, but think the header also contributed). High end the thing opens up and screams though.

[deevergote]

Yeah, I think I'm leaning more towards high end anyway... I'll be getting some NA cams, which will give me power over 5000. I also will eventually get it to rev to well over 8k, maybe even over 9k...

Either way, I've come across 2.5" collectors for 4-2-1 and 4-1... the 4-2-1 collectors are just a larger "1" The 4-2 part is stock.

[optimusprime]

I am looking to do the 2.5" collector mod w/ exhaust on my H22 also. I will be consolting my tuner soon about this. Cause I need to get my exhaust soon before my tune

[owequitit]

I have been looking at a lot of literature lately, as well as consulting some gas flow/aerodynamic equations. This is what I think I have learned (based mostly on the experience of tuners, and fabricators).

The equal length of the runners is most important. Even with a small difference in length, the scavenging effect will decrease substantially. Since scavenging is such a big part of the equation, most of the higher end header makers seem to focus on this first (SMSP, RMF, ETC.) If you look at those, they look to be much closer in length vs say DC Sports.

That prototype header from Atlanta that is on here (Type G posted it) has absolutely equal length runners.

I don't know if it is the case with most cheaper Honda headers, but some guys have measured aftermarket tubes that were "equal length" but actually varied by 12" in some extreme cases. Unfortunately, it is a marketing ploy for many companies.

The stepped primaries slow the gas velocity as it flows, which results in pressure waves that take longer to arrive back at the port for scavenging. This aids low end power, as shorter wave travel time (caused by higher gas flow, reaching the collector sooner, and sending the wave back sooner) or shorter tubes add to the high end power due to time it takes them to arrive back at the open exhaust port.

They may also create more pressure waves since the volume is "stepped" and would create a pressure wave at each change in volume. This would basically allow for many pressure waves, all occuring all the time, that do their thing at a very specific RPM, allowing for better scavenging throughout the power band, and more overall power.

The second part to focus on is the collector. Generally, our engines do seem to benefit from 2.5". I think with proper tuning, you could probably see total HP gain all the way from idle to redline on a stock H22. I think this because I have seen otherwise stock D16Y7s (106HP non-VTEC base model hatchback engine from 1996-2000) gain all the way through with 2.5". A stock H22 is hypothetically moving about as much air as a 5.0 Mustang, and they benefit from dual 2.5" exhaust, so why wouldn't ours benefit from 1? A lot of import guys undersize their exhaust IMO.

As far as collector design goes, there are two main considerations. Moving the volume of air required for the engine, and moving/merging it while creating as little flow restriction as possible.

In aerodynamic terms, the more rapidly we change the direction/volume/velocity of a flowing gas, the more flow restriction or "drag" we create. Apparently, a good size for a merge collector is AT LEAST 4-5", ideally the "merge of gasses" will occur over 5-6". Obviously, the more slowly you can merge the separate gas columns, the more successful you will be, but there is a compromise with how much room you have to deal with. Merge collectors or "Burns" collectors are really the only way to go here.

I am skeptical of the prototype Atlanta header because of the collector design. It will make for a very rapid gasflow change on the 2 outer runners, which may reduce efficiency slightly. I also don't know if it is a merge collector. Time will tell on that one though.

My opinion is this. If you can find a good collector that meets the criteria above, I would go for it. Especially for $100. A 2" collector is small on a stock H22, so imagine what a bottleneck it would be on a built one. This will at least get the volume flow up to a better number. You are most likely not going to see anywhere near an SMSP increase, but you might be able to get a good 3-4 whp out of it. Maybe a little more, depending on what you start with. I haven't seen any 2.5" DC Sports numbers either, but I am interested to see what they can do. Maybe you can be the Guinea Pig.

Now that I have probably confused the topic more... I am going to go watch Trucks!

[wed3k]

I am no tuning specialist, but I think that putting a 2.5 collector on a 4-2-1 header would be a bit more promising than using a 2.5 4-1 (kamikaze). 4-1 is more high end, where the stepped one keeps more low end power.

Thats just an educated guess from me though. I noticed less low end when I went up to 2.5 exhaust, specifically the 4-1 header. For example, I have a hard time spinning tires in 2nd anymore. (I do have 205/60's which is 1/2 upsize and of course nice grippy tires, but think the header also contributed). High end the thing opens up and screams though.

thought it was the other way around?

the 4-1 would create more backpressure because not as much flow.

kinda like the whole butterfly/runners idea.

[owequitit]

thought it was the other way around?

the 4-1 would create more backpressure because not as much flow.

kinda like the whole butterfly/runners idea.

The 4-1 design really has nothing to do with backpressure, and neither does the butterfly setup in the intake manifold.

Everytime an exhaust pulse passes a collector, a negative pressure wave is created. This wave travels in the opposite direction of the gas flow (back to the exhaust port) at the speed of sound. If it reaches the port while the exhaust valves are open, the low pressure of the negative wave helps "suck" the exhaust out of the cylinder. This is the effect known as scavenging.

If the initial exhaust pulse has to travel a farther distance (longer tubes) to reach a point were it will create a negative pulse, then it will also take longer for the negative pulse to get back to the exhaust port (it is traveling at the speed of sound, so a longer tube will take longer for it to travel) and will aid scavenging at lower RPMs where there is more time between exhaust cycles (the wave needs more time to travel).

That is why the Kamikaze header will aid high RPM power. It has 4 very short primary runners that basically dump into a downpipe. The negative pressure wave will be generated when the exhaust flows from the primary into the downpipe. Because the distance it has to travel is only a couple inches, the negative wave that would normally help scavenge the exhaust, will reach the ports too soon to aid in scavenging, which results in less HP. As the engine speeds up and the exhaust valve opening time becomes less and less and more frequent, the pulses will get to the exhaust port at the proper time and aid in scavenging at high RPMs.

The total flow in CFM is what will determine backpressure. If the engine flows more than the exhaust, there will be backpressure.

[deevergote]

Awesome. That's a lot of info I didn't know! Points me in the right direction, for sure...

So basically, if I open up the collector, it may be beneficial... but much of the gains found in a good header deal with the length and flow of the primary pipes?

[owequitit]

Sorry, Deev, I just saw this.

Yes opening up the collector will help, because a 2" is just too small.

To see something like an SMSP type gain will take optimization of the entire package, hence the price.

For the money, a collector may meet your needs, I suspect that you will see at least a moderate gain, because the total flow should increase substantially.

I am actually going to start experimenting with 3" exhaust when I get mine in and running in the near future.

Please let us know how it works out, since nobody seems to be posting about the 2.5" DC Sports (I may start with this header).

[deevergote]

3" exhaust? I would be afraid of not having enough velocity in the exhaust flow with that! Of course, if there's anyone who understands the concepts behind such things, it's you!

So, if I can get say 2hp from a cheapie header with a restrictive collector... I wonder what modifying the collector to 2.5" would yeild. I say if I get another 5-6hp, it'd be well worth it. Still about half the gains of an SMSP or similar, but also about 1/4 the price!

[owequitit]

3" exhaust? I would be afraid of not having enough velocity in the exhaust flow with that! Of course, if there's anyone who understands the concepts behind such things, it's you!

So, if I can get say 2hp from a cheapie header with a restrictive collector... I wonder what modifying the collector to 2.5" would yeild. I say if I get another 5-6hp, it'd be well worth it. Still about half the gains of an SMSP or similar, but also about 1/4 the price!

I agree with the second statement but am unsure about the first!

I don't know about the velocity issues yet, I am still learning, and I just found a very useful book full of gasflow equations that my Dad bought me. He is a used book whore LOL!

I just have a theory that most people undersize their exhaust on imports.

If a 400HP small block benefits from dual 3" exhausts, then a 200HP four banger should too. After all, HP is based on total airflow, NOT displacement. Obviously, time will tell if I am a quack or not. But based on the fact that I have seen STOCK NON-VTEC D16s see gains from idle to redline using 2.5" exhaust, I am relatively convinced that it can be done.

We'll see. I am not afraid to get laughed at if it doesn't work.

Usually even the DC Sports will give you about 3-5HP (not much considering the price, and it does seem questionable on many pulls). I also am not totally sold on "less than the OEM header" statements either though. I don't think it makes a noticeable difference, but at the same time, it isn't really inferior to the OEM design either.

I can tell you this much. The secondary collector is the same size (no additional benefit), but the collector designs on both the primaries and secondaries are superior to the OEM manifold (even the JDM one) which probably accounts for incrementally less flow restriction and incrementally more power. That is one reason why I want to see a pull on the 2.5" DC. It would stand to reason that it is THE major flow restriction, and may be good for an additional 5-7 HP on top of what the regular one does. An 8-10HP gain would give something like the SMSP a run for its money in terms of cost per HP.

[optimusprime]

so from this a 2.5 collector mod on a DC will benifet. My engine tunning will include. Skunk2 cam gears, VAFC2, Fuel pressure regulator, and possilby type S cams. I think with the cams alone and the specs they will put down some good gains. With the modded cams the exhaust should be??? 2.5 or up to 2.75" It is a minimal difference but with the cams and mods I want to squeeze as much HP as I can.

owequitit: You have some good stuff to say. Very informative. I am trying to plan my exhaust. thanks

[owequitit]

Thanks optimus. It is a VERY simplified explanation, but it sorta gets the gist of it.

I expect that the DC Sports with a 2.5" collector will net a noticable gain over the stock manifold. It is speculation based on a number of factors.

I have no idea what that gain may be though. Based on increases from the Kamikaze and the Kteller, worse and better respectively, I would expect from 5-8 WHP. Again, that is just a ballpark guess. I hypothesize that the DC tubing is probably also undersized on a built engine, so as you add stuff, your gain may taper off, and the header will again be a flow restriction.

That is one of the main reasons I want to see what kind of gains someone gets off of it. I would be the guinea pig, but I don't have a stock manifold to start with, so I will have to make the jump directly to header, and will probably just go for broke and get an SMSP or RMF to start out with.

[wrichards2005]

If a 400HP small block benefits from dual 3" exhausts, then a 200HP four banger should too. After all, HP is based on total airflow, NOT displacement. Obviously, time will tell if I am a quack or not. But based on the fact that I have seen STOCK NON-VTEC D16s see gains from idle to redline using 2.5" exhaust, I am relatively convinced that it can be done.

If you put dual 3 inch exhausts on a 4 cylinder accord, it will perform worse then if you put single 2.5 inch exhaust. Sure it sounds stupid, more air flow, more HP, right? Not necessarily because if you just LET the exhaust air out of the engine, it will not move as quickly as if it were forced out of the engine. If you put a 2.5 inch exhaust pipe on the car, the air is going to be slightly constricted (to the point where it is able to be FORCED out), yet it will be able to move the optimum amount of exhaust air so it can flow nicely.

A good header will do wonders, but for example if you connect a 2.25" 4-2-1 header to a 2" downpipe, the gains will be minimal, if any. Air flow has to be smooth, the less bends/steps, the better. Thats why port and polishing can do wonders for your car, while a bad PnP job can actually reduce air flow/HP.

[owequitit]

If you put dual 3 inch exhausts on a 4 cylinder accord, it will perform worse then if you put single 2.5 inch exhaust. Sure it sounds stupid, more air flow, more HP, right? Not necessarily because if you just LET the exhaust air out of the engine, it will not move as quickly as if it were forced out of the engine. If you put a 2.5 inch exhaust pipe on the car, the air is going to be slightly constricted (to the point where it is able to be FORCED out), yet it will be able to move the optimum amount of exhaust air so it can flow nicely.

A good header will do wonders, but for example if you connect a 2.25" 4-2-1 header to a 2" downpipe, the gains will be minimal, if any. Air flow has to be smooth, the less bends/steps, the better. Thats why port and polishing can do wonders for your car, while a bad PnP job can actually reduce air flow/HP.

Before you attempt to talk down, I thought that I IMPLIED a single 3" exhaust, as 4 cylinders are 1/2 of 8 and 200HP is 1/2 of 400. I guess I should not assume, and looking back, I wasn't perfectly clear on that. How dare I try to save space. It is because of replies like this that my posts end up being 99999999999999 pages long. I get to the point where I type EVERYTHING to save the effort in the first place. Then, nobody reads because they are too long, so I end up back at square one. I am not sure if that makes me the dumb one or the reader.

Don't be lazy. Read and understand my entire post if you are going to attempt to make me look stupid. At least do me the favor of coming at me after having read the stuff.

I also thought that I specified that it was a very simplified explanation.

Your argument is flawed.

it will not move as quickly as if it were forced out of the engine. If you put a 2.5 inch exhaust pipe on the car, the air is going to be slightly constricted (to the point where it is able to be FORCED out),

You have just described a "pumping loss." This is the antithesis of HP. The engine expends more energy to FORCE the air out of the engine as you say, thus resulting in less wheel HP. Why? With each combustion event we produce a finite amount of energy. The more we have to spend forcefully sucking air in, or pushing air out, the less that is available to turn the crankshaft.

The trick isn't to force the air out. The trick is to maintain the gas velocity without expending any additional energy to do it. This is where carefull tuning comes in. In fact, if we do it right, we can use the existing energy in the exhaust stream to help pull the next exhaust pulse out, resulting in an even lower pumping loss (scavenging). Unfortunately, like anything relying on resonant tuning, we will see success over a very narrow RPM range with one given length.

As previously stated, this is where collector location, stepped diameters, system length diamter etc come in. EVERYTHING must be optimized to work together. Also, maybe I could set the cams up to produce a little more bottom end, since I will be seeing a nice top end gain with the exhaust. Everything is about compromise.

You will generally see a loss at low RPM's with a larger exhaust system, for several different reasons, other than "backpressure". Also, there is a RANGE of velocity where the exhaust stream should be at. Therefore, there is a RANGE of sizes that will be beneficial on the car. Some will net more top end, some will net more bottom end. Again, it is my belief that a low end loss can be at least partially offset by tuning.

I have seen STOCK NON-VTEC D16Y7s MAKE MORE POWER FROM IDLE TO REDLINE USING 2.5" EXHAUST! IT REV LIMITS AT 6500, AND HAD NO OTHER REAL MODS. IF A 1.6 WITH NO VTEC WILL BENEFIT ALL THE WAY FROM IDLE TO REDLINE FROM 2.5" WITH PROPER TUNING, WHY SHOULD I THINK THAT A 2.2 LITER, WITH VTEC, AN 8,000 RPM REDLINE, AND POSSIBLY SOME HEAVY BOLT ONS WOULDN'T BENEFIT FROM A 3" EXHAUST?

The flow area would be approximately 55% larger on the 3". I would have to calculate what that turns into in terms of volume. The displacement of the 2.2 is 38% larger than the 1.6, but the total airflow is almost double as evidenced by the HP figure. A 55% increase in flow area doesn't seem too off base to me vs a 200% airflow difference.

As I said before: (I really shouldn't have to repeat myself) HP is based on airflow through an engine, NOT displacement. If the system flows enough air to justify the use of a 3" exhaust, then by God, it should have one.

1) "Not enough back pressure" is a fallacy.

2)The most often sited argument for small exhaust sizing: "The OEMs don't use big exhaust," or "I don't want to go too big." How do you know what too big is until you get there? The OEM's number one priority when building exhaust: Quiet. Number 2: Cheap. 3. Everything else.

So using the OEM size as a baseline is a mistake. Afterall, why would they still be using almost the same size exhaust they used on the CB7 on a 240HP V6? That is why there is almost 30whp to be had in a modest exhaust upgrade on those engines.

Thanks though, I guess I will start typing my findings with exhaust flow and add it to the technical section. The only problem with that is balancing complexity with being able to understand it, and my time availability.