Is a bigger exhaust likely to improve my fuel consumtion?

[jungle_surfer]

Hi all!

I've damaged my standard exhaust, and I'm trying to work out if it is worthwhile upgrading.

I've a prado 3.4L V6.

Do you think a bigger (dunno how much bigger) exhaust will improve power / economy?

[-Scott-]

Typically, factory exhaust systems compromise power, economy, noise and emissions.

When starting from cold, emissions need to reach a target level in a certain time. Many manufacturers (I'm not saying Toyota does this) achieve this goal by forcing the engine to heat faster - by constricting the exhaust. The constriction may be required to pass the emissions test, but once the engine is warm, it's not helping.

If there is such a constriction, finding it and removing it should improve your economy.

Good luck,

Scott

[tna racing]

i would go 2 and 3 quater zorst. much better flow wich will mean go better 2

[KiwiBacon]

It'll help top end power. But not fuel consumption.

At cruise, your exhaust volumes are pretty low, so your exhaust has no problem coping.

[PGS 4WD]

In theory yes as by reducing backpressure reduces pumping losses, the exhaust gas escapes more easily without the piston having to push out the exhaust gas past a restriction. Unfortunatly the tune of the vehicle will change, potentially beyond the scope of the factoy tune and the benefits may not be realised, as the full performance benefits wouldn't be realised either due to this limitation. A piggy back chip is an excellent means to rectify this and improve performance and economy.
Joel

[jungle_surfer]

Ok, thanks for the advice! So it sounds like it isn't going to make a huge difference. I don't redline it very often!

If it isn't going to cost a lot more to upgrade than to repair, I'll consider it.

[tweak'e]

Ok, thanks for the advice! So it sounds like it isn't going to make a huge difference. I don't redline it very often!

If it isn't going to cost a lot more to upgrade than to repair, I'll consider it.

it depends a lot on how bad the factory exhaust is.

eg an old car of mine got a wopping 30+% increase in fuel economy with bigbore and extractors.
i was in the same sitution, i had damaged the muffler on some rocks and the rest of the eexhaust was faitly stuffed. to fit the big bore was only $50 more than the standard. well worth the cost.

my old addage..... no such thing as too big downstream of turbo or tuned extractors.

i would get a bigger exaust while you can. even if there is no economy advantage it will be a bit more powerfull and sound nice

[Mrs Ludacris]

If you go a bigger exhuast arnt you going to lose torque down low in the rev range because of the loss of back pressure.

ludaCris

[zagan]

The exhuast shop people would already have an idea of what will be a better size.

usually it does help all the exhuast shops say the same thing increase power reduce fuel usage, more power is only going to help turns the wheels quicker/faster so once your up to speed you can then back off.

Anyway, big exhuast doesn't do much for a petrol engine though it'll help but for a diesel does heaps of good all around.

[PGS 4WD]

If you go a bigger exhuast arnt you going to lose torque down low in the rev range because of the loss of back pressure.

ludaCris

I agree in principle but due to the incresed exhaust flow and potentially air flow it can upset the tune enough to cause a loss of power or low down torque. Reduced back pressure should reduce intake reversion as the residual pressure in the combustion chamber is less after combustion when the exhaust valve is closing as the intake valve opens to overlap(improved scavenging of the combustion chamber), this can on a carby car increase the efficiency of the diffuser and cause richer mixtures and in an EFI MAP based car leaner mixtures potentially due to the increase air flow at the same average vacuum. I have seen this occur on the dyno many times and it is why sometimes exhausts shops hesitate at big exhausts. Ive seen late model cars fail to go into closed loop due to cooler exhaust temps and the relocation of o2 sensors downstream in extractors, it can cause idle and light throttle driveability problems, the V8 AU is an example.

Joel

[tweak'e]

If you go a bigger exhuast arnt you going to lose torque down low in the rev range because of the loss of back pressure.

ludaCris

i've heard that many times but i've never had it or know of anyone who has had that.

the only time that it would happen is if you change the exhaust on a V motor from the headers. that upsets the tuning of the headers as the pipes from header to the join where the two sides join together is actually part of the header system. a big bore on a V motor is really only AFTER where the two sides join up (that also goes for those with crossover tubes).

downstream of the tuning section of the exhaust (ie headers) there is no such thing as to big especially for you guys that have to run cats, the cat makes most of the back pressure anyway, so a cat back big bore shouldn't make it any worse.

with extractors i would have thought the later heated o2 sensors would be fine in a different location. i know the non heated ones where a problem if they got shifted.
i think the on later model vehicles the thing that changes most with modding the exhaust system is the EGR. later model vehicles are usually fairly well setup to use EGR but dropping the backpessure lowers how much EGR goes in which changes what position the throttle sits at. also a poor egr location means the exhaust is to rough (big pulses) which upsets the throttle.

[KiwiBacon]

downstream of the tuning section of the exhaust (ie headers) there is no such thing as to big

Until mr plod pulls you over for excess noise.

[j-top paj]

wheres the exhaust guy that had the question thread?

[zagan]

If you go a bigger exhuast arnt you going to lose torque down low in the rev range because of the loss of back pressure.

ludaCris

downstream of the tuning section of the exhaust (ie headers) there is no such thing as to big especially for you guys that have to run cats, the cat makes most of the back pressure anyway, so a cat back big bore shouldn't make it any worse.

Yeah there is, the bigger the exhuast the bigger the intake has to be.

Also a larger exhuast can make the engine run too crap due to the pulses not tune properly for the size and length of the exhuast.

You can buy a top secret 10 inch straight through + cannon but you can't put that onto a stock car in the hope that it'll go better, if anything it probably won't kick over.

There's a balance to it, also a turbo changes the size of the exhuast as well as you want the pulses to be hiting it at the right times, other wise it will spool up too quick thus comes on quick but you could surge it, or it spools up too slow thus you get lag and it'll never hit top RPM for highest air flow.

Intake volume also matters, because if it's too small and the exhuast too large it can also make the turbo surge.

I have the maths to work out the airflow voumes and the exhuast pulses but it's all algebra plus you need the correct sizes, diamaters and lengths for the intake, bore, exhuast sizes to work it out, besides working out the fuel/air ratios, valve flow rates etc.

If only there was a quick way of working it out, lol. I'm sure there is

[tweak'e]

If you go a bigger exhuast arnt you going to lose torque down low in the rev range because of the loss of back pressure.

ludaCris

downstream of the tuning section of the exhaust (ie headers) there is no such thing as to big especially for you guys that have to run cats, the cat makes most of the back pressure anyway, so a cat back big bore shouldn't make it any worse.

Yeah there is, the bigger the exhuast the bigger the intake has to be.

Also a larger exhuast can make the engine run too crap due to the pulses not tune properly for the size and length of the exhuast.

You can buy a top secret 10 inch straight through + cannon but you can't put that onto a stock car in the hope that it'll go better, if anything it probably won't kick over.

There's a balance to it, also a turbo changes the size of the exhuast as well as you want the pulses to be hiting it at the right times, other wise it will spool up too quick thus comes on quick but you could surge it, or it spools up too slow thus you get lag and it'll never hit top RPM for highest air flow.

Intake volume also matters, because if it's too small and the exhuast too large it can also make the turbo surge.

I have the maths to work out the airflow voumes and the exhuast pulses but it's all algebra plus you need the correct sizes, diamaters and lengths for the intake, bore, exhuast sizes to work it out, besides working out the fuel/air ratios, valve flow rates etc.

If only there was a quick way of working it out, lol. I'm sure there is

ROFLMAO

intake can be snaller or bigger if you want. all depends on what your doing with it. exhaust is a reflection of intake. to get the best result you need to tune the exhaust (headers) to match the intake.

simply have a look what race cars run, big fat short exhaust (unless class rules dictate otherwise). you don't get pulses down an exhaust thats only 1m long and has nothing to reflect pulses back.
for those who have to run full length exhaust system and have size/weight restriction you can get the system to run better by tuning certain mufflers and getting location right to match wave reflection etc but your humble local garage generally dosn't have dynos etc (and the $$)to do all that. its cheaper and more effective just to go up a size. the drop in pressure makes up for any lack of tuning.

leave the race tech for the race car

also don't forget the cat, it basicly screws up the pulses anyway so your never going to get much tuning effect affter its gone through one.

[KiwiBacon]

Yeah there is, the bigger the exhuast the bigger the intake has to be.

Also a larger exhuast can make the engine run too crap due to the pulses not tune properly for the size and length of the exhuast.

I'm with Tweake on this one.
You don't need a bigger intake to match your bigger exhaust.

All pulse tuning is taken care of in the headers. The exhaust after that just needs to get rid of gas as fast as it can. For this bigger is better.

[PGS 4WD]

It is true to all I've ever read that once the exhaust passes the headers the exhaust should offer the path of least resistance. In my experience though that for high RPM power a long header tube is used and for lower power a shorter header tube, such as the difference between tri-y headers and 4 into 1's. Race cars that rely on exhaust scavenging (not forced induction) have long primary tubes (I beleive it has to do with exhaust velocity relative to rpm)
In regards to intakes the reverse applies, short intake runners for high RPM use and long for low down torque, this is why many new vehicle use duel length manifold with internal butterflies (such as th BA BF I6 non turbo)
That dosen't change the fact that a significant change to the exhaust usually requires a retune to ensure the benefits are realized as both mixture and timing can be changed by improved scavenging/engine efficiency.

[me3@neuralfibre.com]

Don't forget the 6" chrome tip to make it go faster

[tweak'e]

Don't forget the 6" chrome tip to make it go faster

lol

also don't forget th "speed tip" which accelerates the exhaust gasses puling them out of the exhaust. guaranteed to make your car go faster.....and we will give you a set of free steak knives.

[smurf182]

All pulse tuning is taken care of in the headers. The exhaust after that just needs to get rid of gas as fast as it can.

Larger volume (diameter) pipe = more expansion/cooling of gas within = slower gas flow (but reduced losses due to pumping).

So perhaps you meant "The exhaust after that just needs to get rid of gas with as little restriction as it can (minimizing pumping losses).?

I agree that after the tuned section (headers/manifold etc) there is no such thing as too big. Unless it's a 4" system on an otherwise stock Honda Civic.

[/b]

[jungle_surfer]

...That dosen't change the fact that a significant change to the exhaust usually requires a retune to ensure the benefits are realized as both mixture and timing can be changed by improved scavenging/engine efficiency.

Pardon my ignorance, but what / where / how is a car tuned, in particular my car (petrol prado)? Does it all happen in the computer, ie replacement or add-on chip, or are there hardware settings to tune? (Timing... that has something to do with the... ummm... spark-thingy-distributor-cap-turning-thingo methinks And I think somehow that doesn't apply to my car.)

And does an LPG conversion add any things to tune?