Question on VY2 SS Gen111 Exhaust sizes.

[chappy]

Ok guy I got a question that I have ask quite a few mechanics and they can not give me a direct answer.

What is the advantage of a duel 3” exhaust compared to a 2 ½” duel exhaust.
This is assuming the vehicle in question is running extractors as well.

• IS there a torque lost down low by using the larger bore exhaust?
• Will tuning (VCM) for example rectify the lost of torque?
• Can a Gen111 have to much back pressure?
• Can a Gen111 not have enough back pressure?

I have a full 2 ½” system on my VY and I keep getting told that I should have gone 3” I ask why but the only answer is “the sound better”.

Cheers

 

[azzss]

Ok guy I got a question that I have ask quite a few mechanics and they can not give me a direct answer.

What is the advantage of a duel 3” exhaust compared to a 2 ½” duel exhaust.
This is assuming the vehicle in question is running extractors as well.

• IS there a torque lost down low by using the larger bore exhaust?
• Will tuning (VCM) for example rectify the lost of torque?
• Can a Gen111 have to much back pressure?
• Can a Gen111 not have enough back pressure?

I have a full 2 ½” system on my VY and I keep getting told that I should have gone 3” I ask why but the only answer is “the sound better”.

Cheers

G'day mate, I am getting a full 2.5 inch duel hm setup including cats and extractors fitted tommorow. I was told that this was ample for the ls1. Hope this helps.

 

[chappy]

G'day mate, I am getting a full 2.5 inch duel hm setup including cats and extractors fitted tommorow. I was told that this was ample for the ls1. Hope this helps.

No thats is what I was talking about I know what 90% of people fit as I have the same.

What I am trying to find out is what the acvantage or disavantage of the different exhause bore sizes.

Cheer and thanks

 

[EXCESSV]

if you go twin 3" exhaust you will lose back pressure and lose small power down low in the revs....sure it might sound better but you are heading slightly backwards in the performance ranks. dont forget that low down power and torque should be the aim for a daily driver....the amount of time a car lives at WOT on the road is minimal.....
so dont worry about building a dyno queen....make sure its got the low down grunt
this twin 3" sort of system that everyone talks about is what you want if you have a big cam and head package or going forced induction...

if your like most LS1ers where simple bolt on mods of extractors, exhaust, high flow cats, OTRCAI and tune a twin 2.5" or single 3.5" are more than adequate and can handle a mild cam and head package anyway....

so in the end unless your gonna turn your car into a tyre frying forced induction machine...keep the 2.5" exhaust and tell ppl to get stuffed.....sound is good but lose power for it is silly

 

[minux]

Yep, i'd stay with twin 2.5, i run dual 3.5 but i am also running head/cam package to suit. You don't need to go to big ona standard setup.

If you have your zorst done, give chipmaster a call for a mail order tune to suit, your car will become a different machine

 

[klaw81]

if you go twin 3" exhaust you will lose back pressure and lose small power down low in the revs....

I can't believe anyone in this day and age still taks about back-pressure. Back pressure is a myth, people! :bang: The phenonemon you are referring to is scavenging, and it's about exhaust gas velocity in different size pipes.

However, I do agree with what you said. Bigger pipes may give slightly higher top end power but at the expense of low-end torque - and low-end torque is where it's at.

 

[enhancedss]

I can't believe anyone in this day and age still taks about back-pressure. Back pressure is a myth, people! :bang: The phenonemon you are referring to is scavenging, and it's about exhaust gas velocity in different size pipes.

However, I do agree with what you said. Bigger pipes may give slightly higher top end power but at the expense of low-end torque - and low-end torque is where it's at.

So u have tested this myth out have we, show me your results to back up what your saying. There is no myth, is reality........:bang:

Scavenging is as gases flow past a port eg extractors in the collector area, as it pulses past it creates little vacuum pockets, pulling exhaust gases from the next port, this i why we call them extractors, as they extract exhaust gases in a much faster and prefered way.

Going back to backpressure, do your self a test klaw81, unscrew o2 sensor, fit bung with connector, and connect to a vac/boost guage, go for a drive up a hill under full noise full rpm, and note your highest reading, anything over 0.5psi is a restriction....its as simple as that, this is called backpressure, make sure u use, a correctly calibrated guage.

An engine is a pump, the quicker u get air in, the quicker u get the air out the better.

Gen 3's exhaust systems should only be twin 2.5" or a single 3.5"

i hope this info helps out some of the guys.

 

[Reaper]

So u have tested this myth out have we, show me your results to back up what your saying. There is no myth, is reality........:bang:

Going back to backpressure, do your self a test klaw81, unscrew o2 sensor, fit bung with connector, and connect to a vac/boost guage, go for a drive up a hill under full noise full rpm, and note your highest reading, anything over 0.5psi is a restriction....its as simple as that, this is called backpressure, make sure u use, a correctly calibrated guage.

The debate is weather backpressure is good or bad (ie restriction in your exhaust). Try this experiment - shove a cork up your arse and see if you fart any better???

Reaper

 

[enhancedss]

obviously youve tryed it...........

ok exhaust sizing, heres a formula that i use, have used for many years.

engine size x max rpm x VE / 3456 = CFM (of the engine)

so

348 x 6800 x .85 / 3456 = 582 CFM so we know after all of this, this engine can only pump this much air 582 CFM.

then we use 582 / 115 (cfm per 1") = 5.06" / 2 = twin 2.5" system required.

for a single system

582 / 170 = 3.42" so we use a single 3.5 " system

 

[klaw81]

So u have tested this myth out have we, show me your results to back up what your saying. There is no myth, is reality........:bang:

Ooops, bad wording on my part. Obviously back-pressure exists, it's simple physics. The myth I was referring to was "you need back-pressure to make power" and you've already done a great job of explaining why that's rubbish. Sorry for the confusion....