Backfire KA-T Question

[IchigoMae]

My KA-T backfires a lot. and im not talking about wussy putput backfires im talking about loud bangs that can scare pedestrians. My question is, what will happen if i remove my cat? wil flames come out?

[AceInHole]

Uhh... you might want to fix the backfiring. Yea you might get flames coming out if you remove the cat (if you've got an S14 that's a bit complicated), but at the expense of f-ing your engine up, is it worth it??

What are you using to control your fuel system anyways??

[IchigoMae]

i have the older safc with 370cc injectors . the setting on the afc is the lowest possible. backfireing is a problem? i never knew that. how does backfireing f up the engine?

[tnord]

potentially shoots exhaust gas back through the exhaust, potentially forcing the exhaust valves to open whey they don't want to be open.........or am i way off? i dunno, just a guess.

[Jeff240sx]

Ace: Why is removing the cat hard in an s14?... well, aside from the code that gets thrown. I did it "accidently". Also, how does the cat-removal fvck up an engine? On an NA it can, due to the loss of backpressure and burning out exhaust valves, but on a turbo setup, backpressure is the enemy of spool, and the turbo itself provides enough backpressure to keep the valves in check. I don't see a logic behind that.
Ichigo... what exhaust are you running? Are your plugs gapped correctly? Is your ignition system up to par? Actually.. what is your entire setup, from plugs (with gap) to wires to mileage? Backfires are a mass of not-completely burnt gas vapors ignition due to the heat of your exhaust. The incomplete burn has got to be fixed, and you'll free up power. I understand that you are running rich.. so either turn up the boost and use more gas, or get an awesome ignition system to burn the fuel as well as you can. Also.. there is a possibility that you don't need a system, as your wires/distributor may be bad, your plugs are probably fouled from running rich, and the turbo can blow out the spark in improperly gapped plugs.
Wow.. lotsa **** that can be wrong. Just post answers to my questions, and we can think from there.
-Jeff

[IchigoMae]

i dunno how to explain this clearly. im not too much of a car expert but i doubt its an ignition problem type of backfire. a backfire usually happens after a shift. mainly when the thottle body is sudenly shut after a bit of gas. i always thot it was like the rally car backfires, is this true? im very sure my car never backfires non turbo. does the backfires have to due with the MAFs located in front of the turbo inlet or the does it have to due with the boost dependant fuel regulator?

[Jeff240sx]

Here's your problem. Your blow off valve dumps out air from the intake tubing that has been metered, and compensated with fuel from the MAF. Now your engine has tons of fuel, and no air, so you will run very rich at those points. That isn't something that an ignition can fix, but if yours is bad, it can help. Your AFC is the older one, the black box, right? If so, you can't compensate for an atmospheric bov like the newer afc's can. About the only thing you can do is deal with it, or re-route your bov into the intake tubing, making it a recirculating bov. Some are setup to be recirculating, and others can be (HKS SSQV has optional recirc. adaptors), and yet others can't be run that way. Which bov do you have?
-Jeff

[tnord]

ahh........he's boosted, missed that. if it was an NA setup would i be correct jeff?

[AceInHole]

Originally posted by Jeff240sx
Ace: Why is removing the cat hard in an s14?... well, aside from the code that gets thrown. I did it "accidently". Also, how does the cat-removal fvck up an engine?

Well, mainly since to replace the cat he'd have to get a custom pipe made from the header down, and an O2 bung to fit the rear O2 sensor on. Although, now that I think about it, he could always just disconnect the O2 sensor since he won't be "needing" it, and he could just find an SR downpipe to replace the cat.

As far as messing up his engine, I was referring to letting the car backfire all the time. Guess I should have worded my post better.

[Jeff240sx]

Tnord: Yea, you'd be right to a point. I've never heard of exhaust valves getting forced open, but the back-draft could keep exhaust gasses in the cylinder, and not allow them to escape (simply cuz of no room). Also, the gasses comming back can burn the valves/valve seats.

Ace: Yea. I bought an sr20det downpipe, and now have no cat. The O2 sensor though... where did you mount yours? There isn't enough room in the line to move it very far... I had to put it in my custom turndown pipe... that goes to my sr downpipe. I also used the O2 bung off the header and welded it in place.
-Jeff

[DarkRaptor42]

Jeff and Ace. Did you use both the O2 sensors on your cars? Im trying to figure out where to put them. Do I need them both?

[d240t2]

I noticed that this slipped by in earlier posts

On an NA it can, due to the loss of backpressure and burning out exhaust valves

NO! NO! NO! You don't need backpressure. Backpressure is always the enemy. No backpressure burns out exhaust valves is a myth. Backpressure increases torque is a myth. Fight the convention (and the muffler shop "gurus"). Backpressure is always evil.

However...what you do need, in any engine system, is high flow velocity, which helps with scavenging. The only way to increase flow velocity (with the same engine size, boost level, and RPM) is to decrease piping size, which in turn, increases backpressure. Backpressure is a by-product of something good. But it doesn't work in reverse. You can add backpressure without changing flow velocity (by adding extra bends into the exhaust). This will only hurt torque/hp.

Dennis

[AceInHole]

Originally posted by Jeff240sx
Ace: Yea. I bought an sr20det downpipe, and now have no cat. The O2 sensor though... where did you mount yours? There isn't enough room in the line to move it very far... I had to put it in my custom turndown pipe... that goes to my sr downpipe. I also used the O2 bung off the header and welded it in place.
-Jeff

My 2nd O2 sensor wouldn't fit into the bung from the front O2 sensor.... I have an early OBD II setup with the $30 front single wire O2 sensor and 3 wire rear O2 sensor. I lengthened the 2nd O2 sensor wires to mount it past the random tech cat that I 've had in place of the stock hollow cat (ran with 2 cats for a while). For my downpipe I ended up paying $120 for 2.5" piping from the turbo to the cat, with the stock O2 sensor mounted and such. Anyways, since my rear O2 sensor stripped itself coming out, and I didn't have a bung for it, I had it welded in place right after the 2nd cat. When I replace the cat-back section of the exhaust I'll have to buy another 2nd O2 sensor or dremel it off and reweld it on.....

[Jeff240sx]

Dennis... how can you be so sure? I've been told forever now that you'll need a certain ammount of backpressure... but you seem very certain about this.

Also.. I used the bung out of the stock header for the main O2 sensor.. and then realized that I had a rear one. I tapped a hole into my sr20det downpipe with a drill and tap, and threaded it in there, with a bit of JB Weld for extra measure. So yea, I've got both, byt my rear O2 sensor keeps throwing a f'n code!
-Jeff

[240racer]

I agree with dennis, the tuning affect that you are looking for on a NA engine is purely due to scavenging and has nothing to do with any amount of backpressure. The scavanging has to do with the pulses reflecting back to the exhaust valves from each joint or change in area of the pipe in the exhaust system. This is the reason for 4-1 and 4-2-1 headers, they allow different pulse lengths due to different timing and runner lengths. This tunes the engine for a different speed. The size of the pipe determines how fast the pulses travel. what you want is the pulse to reflect back from any joint or change in area of the pipe to the valve and then relfect back away from the engine right before the valve opens so it creates a low pressure zone on the back side of the pulse and this larger pressure difference means more air is let out of the cylinder. The smaller the pipe the faster the air and the pulses travel at lower engine speeds and comes back to the valve sooner. This is the reason that backpressure is developed. With turbo exhaust you want as low of velocity (smallest pressure) in the exhaust after the turbine, that's why turbo exhaust is larger in diameter. 300hp semi truck have two 6" diameter pipes, they don't need to be that big, but why not.

[d240t2]

This is the reason that backpressure is developed. With turbo exhaust you want as low of velocity (smallest pressure) in the exhaust after the turbine, that's why turbo exhaust is larger in diameter. 300hp semi truck have two 6" diameter pipes, they don't need to be that big, but why not.

Not true...you still want high flow velocity in a turbo setup.

And as for the 300hp semi truck...I dunno about that. If it is true, then the reason is simple. The 300hp semi truck doesn't have comparable power to a 300hp 4 cylinder. The 300hp semi truck revs really low...so to get 300hp, they have a ton more torque than we ever have. And you need to tune for torque as well.

You heard wrong, Jeff. Backpressure is needed is a myth, or an urban legend. You need high flow velocity, not backpressure.

In fact, it is so important, it is one of Corky's Rules (from Maximum boost): Backpressure in an exhaust system is evil. Go back to the people who taught you that backpressure is good and demand a refund!

If you don't believe me, go to www.google.com and do a search for backpressure myth. Articles like these are good ones, and come up at the top of the list:

http://www.hondalife.com/articles/backpressure.htm
http://www.uucmotorwerks.com/html_pr...torquemyth.htm

Dennis

[240racer]

Not true...you still want high flow velocity in a turbo setup.

You want the lowest pressure possible on the exit side of the turbine so you can have the largest pressure drop across the turbine. This means you need a large pipe, or no pipe which is pretty common on non-street legal turbo cars. Whenever you have a large pipe flowing the same amount of air, it flows slower, simple fuild mechanics. So maybe I should have said: you want less pressure in the exhaust and to achieve that you use larger diameter pipe and that means you have slower flow as well (this is mass flow rate)

300hp is 300hp, I don't care how you get it. Also, they do not use the exhaust to tune for torque at all, they use boost. Diesel engines may use a different air/fuel ratio and may use more or less air (mass) per HP, I'm not sure about that and that would affect the size of the pipe as well. In my book I think you would be just fine in a 300hp I-4 engine in a 85,000lb semi as long as the torque curve was wide enough to support the gearbox that is chosen. It would just get considerable worse gas milage due to the brake specific fuel comsumption being higher then a diesel engine.

[98s14inaz]

I am new around here but I stumbled on this thread and I have a question. I have a 98 s14 NA ka24de. I have a custom 2.5" catback with high flow cat, cai, and hs header. I relocated my second o2 sensor behind the new cat and the engine light hasn't been on since. I heard you mention that back fires are bad. I have really mild ones while deaccelerating, same amount and sound everytime. Am I inadvertantly doing damage to my car? Or is this a normal thing that comes with ditching the stock exhaust? Thanks

[tnord]

i get mild backfires every once in a while, i'm guessing yours are fairly similar to mine. i doubt ours are serious enough to cause any problems.

[98s14inaz]

Thanks