I have a 1989 crx Si and it does poorly on gas. I rebuilt the engine because it was burning oil and only getting 17 MPG. Now it gets 26 mpg and 28 if i drive like a complete grandma or highway drive. I also have a AEM cold air intake so it definitely should be better.
My Father suspects that its because i have a cherry bomb instead of a catylitic converter, some think that free flow exhausts increase fuel economy, but he thinks the lack of back-pressure causes it.
Would i expect to see huge gains if i swapped in a civic hx motor? The crx weighs less than the civic HX so it would probably be insane. It seems to be a rare civic model, any idea where i could find an hx motor and tranny cheap?
Could i see similar gains by buying an hf tranny?
It's not the biggest deal, but i don't want to resort to buying a hyandai accent like my fathers (he gets 44 MPG after purchasing a K&N)

 

your dad is right that cherry bomb is hurting your single cam. if its the one im thinking of, its just a resonator...so u probably have no torque and are dumping a good amount of fuel to compensate ... that as well as the gas you're using could be the problem. u need a cat...on a lightly modified/stock car back pressure is your friend, especially on an NA single cam. hell even on a nicely modded dohc...u wouldn't believe the difference a cat can make... better gas mileage and better performance

 

Well the only reason its even on there is the old cat got plugged so i had bought a cheap cat off ebay....Unfortunately 3-4 years ago when I did this i was stupid and didnt bother calculating gas mileage. I'd be willing to spend the 160$ on a new cat if i knew for sure it would pay for itself in gas.

 

<TABLE WIDTH="90%" CELLSPACING=0 CELLPADDING=0 ALIGN=CENTER><TR><TD>Quote, originally posted by saikouEF8 &raquo;</TD></TR><TR><TD CLASS="quote">your dad is right that cherry bomb is hurting your single cam. if its the one im thinking of, its just a resonator...so u probably have no torque and are dumping a good amount of fuel to compensate ... that as well as the gas you're using could be the problem. u need a cat...on a lightly modified/stock car back pressure is your friend, especially on an NA single cam. hell even on a nicely modded dohc...u wouldn't believe the difference a cat can make... better gas mileage and better performance </TD></TR></TABLE>Wrong, simply wrong.

You are thinking of EGV NOT back pressure.

Do a leak down test first off, check your plugs.

Being the cherry bomb isn't stopping any smog either, get a high flow cat like magna flow wont hurt you at all and stop all the nastys going into the air.

 

The plugs are fine, but I'm not familiar with leakdown testing, the only reason i would believe that backpressure hurt the gas mileage is because it definitely lost low end power after putting the cherry bomb in.

 

<TABLE WIDTH="90%" CELLSPACING=0 CELLPADDING=0 ALIGN=CENTER><TR><TD>Quote, originally posted by saikouEF8 &raquo;</TD></TR><TR><TD CLASS="quote">your dad is right that cherry bomb is hurting your single cam. if its the one im thinking of, its just a resonator...so u probably have no torque and are dumping a good amount of fuel to compensate ... that as well as the gas you're using could be the problem. u need a cat...on a lightly modified/stock car back pressure is your friend, especially on an NA single cam. hell even on a nicely modded dohc...u wouldn't believe the difference a cat can make... better gas mileage and better performance </TD></TR></TABLE>

Wha?

Adding a cat isn't going to magically open up 35mpg just because it creates "back pressure" which by the way, it really doesn't. That, and a catalytic converter will store a lot of heat, creating a hotter and faster moving exhaust stream.

If you want better gas milage, do a tune up. Seafoam the engine, then do plugs/wires and take it easy driving around. If you WOT your call all day you're not going to get **** for mpg, duh.

Also I will mention for the sake of Max and I, you're Honda has no low end, nor did it ever have one and nor will it ever have one. Kthanx.

 

<TABLE WIDTH="90%" CELLSPACING=0 CELLPADDING=0 ALIGN=CENTER><TR><TD>Quote, originally posted by griz50cal &raquo;</TD></TR><TR><TD CLASS="quote">The plugs are fine, but I'm not familiar with leakdown testing, the only reason i would believe that backpressure hurt the gas mileage is because it definitely lost low end power after putting the cherry bomb in. </TD></TR></TABLE> Leakdown test is also called a compression test, give it a search.

What is the rest of your exhaust setup header back? Money says it's all sorts of messed up and mis matched.

 

I've sea foamed the engine, the plugs are good and i have these oversized, stupid, nology plug wires(with their own individual grounds, lol) because a guy on ebay included them with a rebuilt cylinder head i bought. It's just because of this ridiculous Si tranny, The car sits around 2900 rpms going 60 in 5th gear. Many people claim 4 cylinder cars need catalytic converters to do well on gas, I'm not sure myself

 

mismatched.... lol I have an ebay cat (looks like a cherry bomb inside), and a pace setter cat-back because it was the cheapest and didnt weigh a ton like Greddy set ups I've seen.

 

<TABLE WIDTH="90%" CELLSPACING=0 CELLPADDING=0 ALIGN=CENTER><TR><TD>Quote, originally posted by griz50cal &raquo;</TD></TR><TR><TD CLASS="quote">I've sea foamed the engine, the plugs are good and i have these oversized, stupid, nology plug wires(with their own individual grounds, lol) because a guy on ebay included them with a rebuilt cylinder head i bought. It's just because of this ridiculous Si tranny, The car sits around 2900 rpms going 60 in 5th gear. Many people claim 4 cylinder cars need catalytic converters to do well on gas, I'm not sure myself</TD></TR></TABLE> Throw out those junk wires get some good old NGK blues first off.

the si tranny is shorter geared but trust me it could be worse lol. If it's really a problem and you are willing to give up some get up and go, go with DX tranny.

Many people are retarded, 4 6 8 10 12 piston motors are all air pumps really, they all follow the same rules. Yes there are some things that differ but over all all the same. No way does a 4 banger suffer with out a cat, heck hondas cats are great 3 way cats stock, really unless they died no reason to mess with them.

 

I have a compression tester and could give it a look but ive changed the piston rings and cylinder head, i seriously doubt it has any compression issues.First time ive seen a honda not burn any oil....

 

<TABLE WIDTH="90%" CELLSPACING=0 CELLPADDING=0 ALIGN=CENTER><TR><TD>Quote, originally posted by griz50cal &raquo;</TD></TR><TR><TD CLASS="quote">mismatched.... lol I have an ebay cat (looks like a cherry bomb inside), and a pace setter cat-back because it was the cheapest and didnt weigh a ton like Greddy set ups I've seen.</TD></TR></TABLE> So it's just a test pipe, not even a real cat?

Pace setters exhaust for imports is junk it's quite odd the muscle car stuff and truck stuff they do is pretty darn nice.

Any leaks in the exhaust anywhere?

 

I have 3 sets of wires and they all test out with a multi meter, my other set is blue NGKs, honestly i dont see any point to expensive wires unless i had an after market ignition set up. The old cat did die and when i gutted it it was EVEN louder than this set up.... As far as get up and go is concerned I'm past that point, the car handles amazingly and has beaten many other ricer cars.... but now im 21 and get pissed at a honda that cant get 30 mpg

 

The exhaust does leak now, thats the worse part of it, I lessened the leak by changing the front cat gasket, I'm going to change the rear gasket next but need new bolts b4 proceeding (old ones are rusted to ****) hopefully it isnt leaking from the manifold(has new felpro gasket) anywhere...

 

<TABLE WIDTH="90%" CELLSPACING=0 CELLPADDING=0 ALIGN=CENTER><TR><TD>Quote, originally posted by griz50cal &raquo;</TD></TR><TR><TD CLASS="quote">I have 3 sets of wires and they all test out with a multi meter, my other set is blue NGKs, honestly i dont see any point to expensive wires unless i had an after market ignition set up. The old cat did die and when i gutted it it was EVEN louder than this set up.... As far as get up and go is concerned I'm past that point, the car handles amazingly and has beaten many other ricer cars.... but now im 21 and get pissed at a honda that cant get 30 mpg</TD></TR></TABLE>After market ignitions are worthless on these cars. I can't say it easier stock NGK.

gutted cats are bad news all around, exhaust is science and flow matters alot as well a smooth inside randomly hacking away messes with this. I've seen people lose power with this and they arn't smog legal anyway, so gutted cats are just silly.

how hard are you driving this car? Any check engine lights? What about other factors like wheels tires aligened? what is the weather like? stop go non stop car warmed up or left sitting ?

I'm 22, and my daily gets 21 in town if I behave, so really I don't see where this is going. Oddly enough my camaro does better on the highway then my honda. Plus leathers oh so soft

 

When i run the hell out of it it gets 26 mpg, when i grandma drive it it gets 28 mpg, I generally grandma drive it. No check engine lights, The weather is cold, i start it up and go slow til its warm, but dont let it sit.
The aligment is hopeless, idn if the steering rack is shot or what, but i have play in the steering, im going to change the rack end bushing tomorow, but seriously think theres more to it than that. The car is definitely not worth a 300$ steering rack.....
The car runs its best when its cold, when its warmed up it appears hesitant somewhere between 2000 and 3000 rpms but above 3000 the car is smooth and while cold its smooth throughout. I originally thought it had a warped head, but it has an entirely different rebuilt head on it and hesitates just the same.

 

im sticking with what i said earlier... you think a cat doesnt create any back pressure? ok. but if u wanna say egv then go ahead... but if u have a cat and we're talking about egv then id use the term back pressure for the scavenging effect of having some sort of back flow. i didnt wanna confuse the guy with jargon.

<TABLE WIDTH="90%" CELLSPACING=0 CELLPADDING=0 ALIGN=CENTER><TR><TD>Quote, originally posted by griz50cal &raquo;</TD></TR><TR><TD CLASS="quote">gutted cats are bad news all around, exhaust is science and flow matters alot as well a smooth inside randomly hacking away messes with this. I've seen people lose power with this and they arn't smog legal anyway, so gutted cats are just silly.</TD></TR></TABLE>

 

<TABLE WIDTH="90%" CELLSPACING=0 CELLPADDING=0 ALIGN=CENTER><TR><TD>Quote, originally posted by saikouEF8 &raquo;</TD></TR><TR><TD CLASS="quote">im sticking with what i said earlier... you think a cat doesnt create any back pressure? ok. but if u wanna say egv then go ahead... but if u have a cat and we're talking about egv then id use the term back pressure for the scavenging effect of having some sort of back flow. i didnt wanna confuse the guy with jargon.



</TD></TR></TABLE>


You have a lot to learn about back pressure....search it and do some reading...LOL..

This thread is incredible.

My Si gets 35-38 miles per gallon bone stock. OP you seem to have some kind of ignition problem with that hesitation you are talking about..or maybe a leak in the vaccuum lines or intake gaskets..get honda oem wires and some regualr ol ngk plugs and do the cap and rotor with honda oem as well.

If there is one lesson I have learned about Honda, always use OEM on anything that is ignition related, unless you are ditching the ENTIRE system and going crank triggered or something.

 

u guys should seriously just stop talking at/to me. not here to argue about the definition of "back pressure" or how much is good/bad/whatever. i already agreed egv is the correct terminology.

 

One of the most misunderstood concepts in exhaust theory is backpressure. People love to talk about backpressure on message boards with no real understanding of what it is and what its consequences are. I'm sure many of you have heard or read the phrase "engines need some backpressure" when discussing exhaust upgrades. That phrase is in fact completely inaccurate and a wholly misguided notion.

How the myth came about:

It is easy to see how this misunderstanding arises. Lets’ say that Max puts a 3-inch system on his normally aspirated car. He soon realizes that he has lost power right through the power band. The connection is made in his throbbing brain….

Put on 3" pipe = loss of backpressure = loss of power.

Max erroneously concludes that you need backpressure to retain performance. He has ignored the need for exhaust gas velocity to get that scavenge effect.

The other myth: “engines can get burned valves from not enough backpressure”


How this myth came about:

The other reason why people say "backpressure is good" is because they hear that cars (or motorcycles) that have had performance exhaust work done to them would then go on to burn exhaust valves. Now, it is true that such valve burning has occurred as a result of the exhaust mods, but it isn't due merely to a lack of backpressure.

The internal combustion engine is a complex, dynamic collection of different systems working together to convert the stored power in gasoline into mechanical energy to push a car down the road. Anytime one of these systems are modified, that mod will also indirectly affect the other systems, as well.

Now, valve burning occurs as a result of a very lean-burning engine. In order to achieve a theoretical optimal combustion, an engine needs 14.7 parts of oxygen by mass to 1 part of gasoline (again, by mass). This is referred to as a stochiometric (chemically correct) mixture, and is commonly referred to as a 14.7:1 mix. If an engine burns with less oxygen present (13:1, 12:1, etc...), it is said to run rich. Conversely, if the engine runs with more oxygen present (16:1, 17:1, etc...), it is said to run lean. Today's engines are designed to run at 14.7:1 for normally cruising, with rich mixtures on acceleration or warm-up, and lean mixtures while decelerating.

Getting back to the discussion, the reason that exhaust valves burn is because the engine is burning lean. Normal engines will tolerate lean burning for a little bit, but not for sustained periods of time. The reason why the engine is burning lean to begin with is that the reduction in backpressure is causing more air to be drawn into the combustion chamber than before. Earlier cars (and motorcycles) with carburetion often could not adjust for his.

Once these vehicles received performance mods that reduced backpressure, they tended to burn valves because of the resulting over-lean condition. This, incidentally, also provides a basis for the "torque increase" seen if backpressure is maintained. As the fuel/air mixture becomes leaner, the resultant combustion will produce progressively less and less of the force needed to produce torque.

Some basic exhaust theory

Your exhaust system is designed to evacuate gases from the combustion chamber quickly and efficiently. Exhaust gases are not produced in a smooth stream; exhaust gases originate in pulses. A 4 cylinder motor will have 4 distinct pulses per complete engine cycle; a 6 cylinder has 6 pulses and so on. The more pulses that are produced, the more continuous the exhaust flow. Backpressure can be loosely defined as the resistance to positive flow - in this case, the resistance to positive flow of the exhaust stream.

Backpressure and velocity.

Some people operate under the misguided notion that wider pipes are more effective at clearing the combustion chamber than narrower pipes. It's not hard to see how this misconception is appealing - wider pipes have the capability to flow more than narrower pipes. So if they have the ability to flow more, why isn't "wider is better" a good rule of thumb for exhaust upgrading? In a word - VELOCITY. I'm sure that all of you have at one time used a garden hose w/o a spray nozzle on it. If you let the water just run unrestricted out of the house it flows at a rather slow rate. However, if you take your finger and cover part of the opening, the water will flow out at a much, much, faster rate.

The astute exhaust designer knows that you must balance flow capacity with velocity. You want the exhaust gases to exit the chamber and speed along at the highest velocity possible - you want a FAST exhaust stream. If you have two exhaust pulses of equal volume, one in a 2" pipe and one in a 3" pipe, the pulse in the 2" pipe will be traveling considerably FASTER than the pulse in the 3" pipe. While it is true that the narrower the pipe, the higher the velocity of the exiting gases, you want make sure the pipe is wide enough so that there is as little backpressure as possible while maintaining suitable exhaust gas velocity.

Backpressure in its most extreme form can lead to reversion of the exhaust stream - that is to say the exhaust flows backwards, which is not good. The trick is to have a pipe that that is as narrow as possible while having as close to zero backpressure as possible at the RPM range you want your power band to be located at. Exhaust pipe diameters are best suited to a particular RPM range. A smaller pipe diameter will produce higher exhaust velocities at a lower RPM but create unacceptably high amounts of backpressure at high rpm. Thus if your power band is located 2-3000 RPM you'd want a narrower pipe than if your power band is located at 8-9000RPM.

Many engineers try to work around the RPM specific nature of pipe diameters by using setups that are capable of creating a similar effect as a change in pipe diameter on the fly. The most advanced is Ferrari's which consists of two exhaust paths after the header - at low RPM only one path is open to maintain exhaust velocity, but as RPM climbs and exhaust volume increases, the second path is opened to curb backpressure - since there is greater exhaust volume there is no loss in flow velocity. BMW and Nissan use a simpler and less effective method - there is a single exhaust path to the muffler; the muffler has two paths; one path is closed at low RPM but both are open at high RPM.

So why is exhaust velocity so important?

The faster an exhaust pulse moves, the better it can scavenge out all of the spent gasses during valve overlap. The guiding principles of exhaust pulse scavenging are a bit beyond the scope of this doc but the general idea is a fast moving pulse creates a low pressure area behind it. This low pressure area acts as a vacuum and draws along the air behind it. A similar example would be a vehicle traveling at a high rate of speed on a dusty road. There is a low pressure area immediately behind the moving vehicle - dust particles get sucked into this low pressure area causing it to collect on the back of the vehicle. This effect is most noticeable on vans and hatchbacks which tend to create large trailing low pressure areas - giving rise to the numerous "wash me please" messages written in the thickly collected dust on the rear door(s).

Conclusion.

SO it turns out that engines don't need backpressure, they need as high a flow velocity as possible with as little backpressure as possible.

 

<TABLE WIDTH="90%" CELLSPACING=0 CELLPADDING=0 ALIGN=CENTER><TR><TD>Quote, originally posted by saikouEF8 &raquo;</TD></TR><TR><TD CLASS="quote">u guys should seriously just stop talking at/to me. not here to argue about the definition of "back pressure" or how much is good/bad/whatever. i already agreed egv is the correct terminology.</TD></TR></TABLE> Then shut your mouth when grown ups are talking, you are spreading grossly wrong misinformation.

There is such a thing as back pressure it is NOTHING like EGV, do you even know what that stands for?

It's not jargon, it's you don't know what you are talking about, you might get past 99% of the tards on HT but sometimes a few of us still know a car from a hole in the ground and we'll do what we can to teach someone real tech, not silly net myths.

Infact explain to me how a myth like your back pressure isn't jargon but EGV a known science when designing exhausts is jargon?

As always Bryan great work

OP, freshen up the ignition and your poor aligment is probably hurting you a good deal, what kind of wheels and tires do you have and are they inflated correctly etc? Extra weight in the car? Timing ok?

 

ok ... so you wana be a typical ht ******* and be a rude ***** fine but read this then get off my ****:

he uses the term egv, but read the context...

from yawpower.com


"If the exhaust gas velocity is low, (Such as at low rpm) the vacuum created by the increasing chamber volume can easily reverse the flow and pull the gasses back into the chamber. If, on the other hand, the exhaust gas velocity is high, it will take a great deal more energy to reverse their flow, and the result will be less exhaust gas dilution. This is why large exhaust ports, and large diameter exhaust tubing reduce low speed power."

so youre saying that adding a cat wouldnt slow down the exhaust gasses before exiting the system and that it wouldnt allow for some reversion of the gasses and give u more torque thus producing a back pressure type effect and allowing for more low end power? just tell me why thats not back pressure? sure as hell sounds like it to me... whatever acronym u wanna use to decribe a PART of the whole process is your own business.

btw, you dont know me, so dont use my name like you do ***...snyper.

 

<TABLE WIDTH="90%" CELLSPACING=0 CELLPADDING=0 ALIGN=CENTER><TR><TD>Quote, originally posted by saikouEF8 &raquo;</TD></TR><TR><TD CLASS="quote">ok fine... you wana be an ******* and insult me fine but read this then get off my ****:

he uses the term egv, but read the context...

from yawpower.com


"If the exhaust gas velocity is low, (Such as at low rpm) the vacuum created by the increasing chamber volume can easily reverse the flow and pull the gasses back into the chamber. If, on the other hand, the exhaust gas velocity is high, it will take a great deal more energy to reverse their flow, and the result will be less exhaust gas dilution. This is why large exhaust ports, and large diameter exhaust tubing reduce low speed power."

so youre saying that adding a cat wouldnt slow down the exhaust gasses before exiting the system and that it wouldnt allow for some reversion of the gasses and give u more torque thus producing a back pressure type effect and allowing for more low end power? just tell me why thats not back pressure? sure as hell sounds like it to me... whatever acronym u wanna use is your own business.</TD></TR></TABLE> Firstly, child, you wish i was on your ****, i'm sure you think about it a good deal.

What part of you know nothing, and are spouting bullshit is the part you are missing? I'm guessing it's all of it, do you have leperorcy of the brain?

You are the *** hole if anyone, I'm here helping someone, you are lying online to look smart, so you are a **** bag, i'm not only helping him i'm helping you, sadly you are proving to be hopless, only cure for people like you is a toaster and a nice warm bath.

You don't understand what you just posted do you? It's saying something Bryan and I have said, this has NOTHING to do with your stupid myth of back pressure, it's like saying "but the loch ness monster could beat a whales ***!"

A good cat like the OEM 3 way cats will not slow the exhaust down esp on a stock setup, even on quite so, because it doesn't stock and the OP has put in something worse in shocking it' not helping. Cheap test pipes and gutted cats are known to lose power even more so in the low end because of the way they are made/cobbled together.

Now lets play with your moronic logic, if I take a potato stuff it in your muffler but cut a few slits in it you'll have more low end. The op has leaks in his exhaust it's mis matched and other problems with is car. It's not because it's got no cat, well that's not helping.

The cats such as mangaflows new metrix or what ever it's called is lossless to 250 HP same as a test pipe for many even turbo setups on this site. Hondas 3 way cat's really don't do anything as far as distupt flow, so why would going back to something that really doesn't make any back pressure to start help? shocking it won't.

I told him to get a good cat because the junky resonated vented test pipe is hurting more then an OEM cat or after market.

If you understood that snipet you posted like a parrot you'd see he talks about both back pressure and EGV not them as the same thing as you claimed before.

My god, were you shaken as a child?

 

You need to go back and read my post very carefully. It expains what you googled in greater detail than I think you understand.

 

<TABLE WIDTH="90%" CELLSPACING=0 CELLPADDING=0 ALIGN=CENTER><TR><TD>Quote, originally posted by saikouEF8 &raquo;</TD></TR><TR><TD CLASS="quote">
btw, you dont know me, so dont use my name like you do ***...snyper.</TD></TR></TABLE>
Umm, what? I was talking to SIRED, but maybe you think you are the only person with that name.

I'll call you what you are to me, puke.

how's that?

You acting hard when making things up online is pretty sad. What's even sadder you used my own SN to mock me great work, why not try even being smarter my real first name is in my freaking sig.