Exhaust Back pressure

[HamaTime™]

Well I just got my pacemakers installed today (in 44 degree heat, not soo good haha) and I was talking to a mate and an apprentice mechanic and they said my exhaust system would make me lose power because there is not enough back pressure.

All it is is the extractors, stock cat and a 2.5 inch pipe all the way back to the tip. No muffler or resonator (as far as I can remember). I've searched a couple of old threads, couldn't find a specific answer. So all I'm asking for is whether that sort of system is fine, both EPA legally and performance wise. If not then I'll most likely go for a Lukey Ultra-Flow muffler and chuck that on. Suggestions?

Not interested in a full on debate or criticism about exhausts, I've read and contributed to my fair share of those threads. Straight answers please.

[Stressball]

He's wrong. You don't want back pressure. You want the correct flow for the gasses to retain their heat and therefore velocity. Too small creates backpressure and as such, restrictions on the engine. Too large allows the gasses to cool and therefore lose velocity, creating another restriction. You want the right size for the right engine. General consensus on here seems to be that 2.25" or 2.5" are good for an NA 6.

As long as the exhaust is quieter than whatever the ADR is, 93dB or something...? It meets that regulation; EPA-wise, I'm not sure. They can be pretty strict, you may be better off talking to them yourself.

[HARVZ-6]

He's wrong. You don't want back pressure. You want the correct flow for the gasses to retain their heat and therefore velocity. Too small creates backpressure and as such, restrictions on the engine. Too large allows the gasses to cool and therefore lose velocity, creating another restriction. You want the right size for the right engine. General consensus on here seems to be that 2.25" or 2.5" are good for an NA 6.

As long as the exhaust is quieter than whatever the ADR is, 93dB or something...? It meets that regulation; EPA-wise, I'm not sure. They can be pretty strict, you may be better off talking to them yourself.

A key point missed there is too little backpressure you wont get exhaust "flow" and the heat wont be removed from important places like valves resulting in burnt valves...

but in saying that you should be able to run extractors on their own and not get that problem..

you might lose some low end power but less likely to lose in the higher rev range

[HamaTime™]

A key point missed there is too little backpressure you wont get exhaust "flow" and the heat wont be removed from important places like valves resulting in burnt valves...

but in saying that you should be able to run extractors on their own and not get that problem..

you might lose some low end power but less likely to lose in the higher rev range

That was my original thought, too little backpressure, which is also what my mates were saying. I'm still just a little concered about the noise level (it is barely that loud at the moment) but mostly concerned about making sure there is enough back pressure, not too much, but not too little as to ensure maximum performance.

If that is the case, (I'm going to ring an exhaust shop to be doubly sure, be good to get you guys' opinions as well) I was considering as I said before, adding a Lukey Ultra-Flow muffler, and maybe a Magnaflow Cat as well, or maybe a medium 5" resonator instead of the muffler.

Any thoughts?

[Ado]

Dont know about back pressure but i can tell you that if you have no mufflers fitted it will fail an epa test.

[Haydz]

get a muffler on a 6 or a resonator if it were an 8.


and you should at least have one muffler. legally you should have both a muffler and a resonator.

[Not_An_Abba_Fan]

Back pressure is bad in any situation. It just loads up the engine and makes it work harder.

Exhaust velocity is what you need. Extractors achieve this better than any exhaust system on it's own can. The reason they are called extractors is they 'extract' exhaust gas from the cylinders. As each exhaust pulse travels down the header tube it creates a low pressure point behind it, as this pulse passes through the collector it draws the next pulse in the firing order out of it's header tube. All these pulses need to maintain a certain velocity as they travel along the exhaust system to help draw the next pulse along behind it.

Once the exhaust gas cools it takes up less space, so to keep the velocity going, you will need to reduce the size of the exhaust pipe the further you go from the engine. Too small and it creates a restriction thus causing back pressure. Too large and the cooler exhaust loses velocity and the pulses behind it bank up and end up pushing the exhaust out, this is back pressure too.

A 2 1/2" cat back system really needs a smaller tail pipe to maintain good velocity and reduce back pressure. People confuse back pressure with velocity, thinking that they are adding back pressure and increasing engine performance, but what they are doing (sometimes) is keeping velocity.

Restricting the exhaust can give the impression of more power, but all you are doing is throwing in more fuel so it feels like there is an increase in performance.

[HamaTime™]

Back pressure is bad in any situation. It just loads up the engine and makes it work harder.

Exhaust velocity is what you need. Extractors achieve this better than any exhaust system on it's own can. The reason they are called extractors is they 'extract' exhaust gas from the cylinders. As each exhaust pulse travels down the header tube it creates a low pressure point behind it, as this pulse passes through the collector it draws the next pulse in the firing order out of it's header tube. All these pulses need to maintain a certain velocity as they travel along the exhaust system to help draw the next pulse along behind it.

Once the exhaust gas cools it takes up less space, so to keep the velocity going, you will need to reduce the size of the exhaust pipe the further you go from the engine. Too small and it creates a restriction thus causing back pressure. Too large and the cooler exhaust loses velocity and the pulses behind it bank up and end up pushing the exhaust out, this is back pressure too.

A 2 1/2" cat back system really needs a smaller tail pipe to maintain good velocity and reduce back pressure. People confuse back pressure with velocity, thinking that they are adding back pressure and increasing engine performance, but what they are doing (sometimes) is keeping velocity.

Restricting the exhaust can give the impression of more power, but all you are doing is throwing in more fuel so it feels like there is an increase in performance.

You answered everything I wanted to know mate. Much appreciated

[CommVTS]

Who installed if for you haym? I wouldn't worry too much about the EPA man, unless it's ridiculously loud. I'd just chuck on a muffler to be safe though.

[HamaTime™]

Daalder Exhausts in Box hill mate, on corner of Station st and Albion rd.

It's not ridiculously loud, it isn't really that much louder than it was before, it doesn't drone, it just sounds deeper and it warbles. I'm actually quite impressed with the note. It was more the issue of back pressure I was interested in...

However, down the track I might chuck on a new Magnaflow Cat and a Lukey Ulta Flow muffler. If needed. Or I might just wait until I'm off my P's, stop spending money on this car and wait for my VS Clubsport.

[CommVTS]

Sounds good man, all i'm doing to mine now is a chip then saving for LS1.

[HamaTime™]

What chip are you getting mate? I'm going for a Delcowizzid tune (guy off this forum). What were you thinking of?

[feistl]

Back pressure is bad in any situation. It just loads up the engine and makes it work harder.

Exhaust velocity is what you need. Extractors achieve this better than any exhaust system on it's own can. The reason they are called extractors is they 'extract' exhaust gas from the cylinders. As each exhaust pulse travels down the header tube it creates a low pressure point behind it, as this pulse passes through the collector it draws the next pulse in the firing order out of it's header tube. All these pulses need to maintain a certain velocity as they travel along the exhaust system to help draw the next pulse along behind it.

Once the exhaust gas cools it takes up less space, so to keep the velocity going, you will need to reduce the size of the exhaust pipe the further you go from the engine. Too small and it creates a restriction thus causing back pressure. Too large and the cooler exhaust loses velocity and the pulses behind it bank up and end up pushing the exhaust out, this is back pressure too.

A 2 1/2" cat back system really needs a smaller tail pipe to maintain good velocity and reduce back pressure. People confuse back pressure with velocity, thinking that they are adding back pressure and increasing engine performance, but what they are doing (sometimes) is keeping velocity.

Restricting the exhaust can give the impression of more power, but all you are doing is throwing in more fuel so it feels like there is an increase in performance.

Wow great post mate.

Little off topic, but would my exhaust be causing a restriction?

Its a 6.3L LS1 with Pacemaker Extractors 4 into 1s (3") with a 3" section into dual high flow cats (4.5" body, 100cpsi) then going back into a 2.5 twin system (full stainless steel). I was worried that it was 2.5" after the cats, but by this post would that be an advantage?

Engine is making around 330rwkw. Is it worth upgrading to dual 3" system?

Cheers

(Sorry for hijacking)

[Not_An_Abba_Fan]

Wow great post mate.

Little off topic, but would my exhaust be causing a restriction?

Its a 6.3L LS1 with Pacemaker Extractors 4 into 1s (3") with a 3" section into dual high flow cats (4.5" body, 100cpsi) then going back into a 2.5 twin system (full stainless steel). I was worried that it was 2.5" after the cats, but by this post would that be an advantage?

Engine is making around 330rwkw. Is it worth upgrading to dual 3" system?

Cheers

(Sorry for hijacking)

If you want good dyno figures put the 3" twin on as dynos like a big exhaust. For street drivability and punch off the line, keep your 2 1/2".

You will find your top end improve a bit with the 3" but you will lose low end torque as what you are doing is losing a bit of exhaust velocity. I would keep what you have.