Exhaust size & bhp.

[jetpilot]

Some time ago it was talked about that if your outlet from the turbo was 3" there was no point having a larger exhaust system, i.e 4", as your flow was already restricted by your turbo outlet.

There is a lot more talk about 3.5" or 4" systems these days, so in basic terms, will larger systems net you more hp even if your outlet is 3"?

[jetpilot]

Just found this, which seems interesting.

http://www.tercelreference.com/tercel_info/turbo_exhaust_theory/turbo_exhaust_theory.html

[davehg]

Great find

[jetpilot]

Great find

Did you sort your exhaust Dave?

[Grizzly]

Thats a good read, TBH i dont have a problem beliving you would get a gain by putting a larger bore exhaust on but the question is would the HP gain over a decent 3" be worth the cost of a Custom exhaust being made? Would you still be able to keep a bigger bore quiet whilst keeping the advantage? wouldn\'t a Bigger bore need more to keep it quiet?

The one that confuses me slightly is the downpipes that go from the Turbo to a 4" back to a 3" pipe? because the way i understand it is as soon as the gasses leave the Turbo it starts to cool rapidly / expand, isn\'t letting it fill a larger bore and start to expand then try to force it into a smaller bore the last thing you want?  I know you would gain a bit of Turbo responce the Instant you hit the throttle but would it not cause a bottle neck at peak power/flow? Unless i read it wrong shouldn\'t the bore get bigger as the Exhaust gasses cool and expand? (i.e. 3" down pipe slowly getting bigger until it exits the rear?)

I\'m sure i have this wrong and feel free to correct me but it doesn\'t make sence to me.

[jetpilot]

Thats a good read, TBH i dont have a problem beliving you would get a gain by putting a larger bore exhaust on but the question is would the HP gain over a decent 3" be worth the cost of a Custom exhaust being made? Would you still be able to keep a bigger bore quiet whilst keeping the advantage? wouldn\'t a Bigger bore need more to keep it quiet?

The one that confuses me slightly is the downpipes that go from the Turbo to a 4" back to a 3" pipe? because the way i understand it is as soon as the gasses leave the Turbo it starts to cool rapidly / expand, isn\'t letting it fill a larger bore and start to expand then try to force it into a smaller bore the last thing you want?  I know you would gain a bit of Turbo responce the Instant you hit the throttle but would it not cause a bottle neck at peak power/flow? Unless i read it wrong shouldn\'t the bore get bigger as the Exhaust gasses cool and expand? (i.e. 3" down pipe slowly getting bigger until it exits the rear?)

I\'m sure i have this wrong and feel free to correct me but it doesn\'t make sence to me.

From reading that article, i am not so sure you get a gain from going larger, but you maximise the turbos efficiency, i.e perhaps getting earlier spool up?

I have also "read", how true it is i dont know, a larger system will be quieter, for some technical reason to do with volume and sound!

[davehg]

Not yet I haven\'t, I\'m thinking of getting both silencers re packed to see if that does the trick. I don\'t really want to put a smaller system on if I can help it.

[jetpilot]

Not yet I haven\'t, I\'m thinking of getting both silencers re packed to see if that does the trick. I don\'t really want to put a smaller system on if I can help it.

I repacked my Racing Beat and it made a world of difference, i used Acoustafil, also managed to make the it repackable for the future.

[davehg]

Thanks any pics of your exhaust or how you made it repackable? Where did you get the packing from?

[lee mills]

Not yet I haven\'t, I\'m thinking of getting both silencers re packed to see if that does the trick. I don\'t really want to put a smaller system on if I can help it.

What size are you using now Dave?

[davehg]

I have 4 inch pipe bore lee from turbo all the way back with a 5 inch tail pipe outlet

[jetpilot]

Didnt take any pics. I had it cut at one end, then welded a small piece of sheet metal to the cut section so the main exhaust would slide back over it and bolt/rivet in place. Acoustafil is available on ebay

[lee mills]

Same as mine.
Im debating at the moment the best way to reduce noise by about 5 db so that i have no problems getting on castle coombe next year. Currently db,s are 96 @ idle and 102 @4500 rpm.

[davehg]

I was tempted to take it to bell exhausts in Swindon to get them to repack it etc but I\'m not sure. I thinks it\'s definately worth a try rather than new systems that may not make any difference at all

[lee mills]

What noise levels are you at now.
Must admit i\'m tempted to either drop pie size to 3.5" or try and replace my 14" oval midbox with an 18" but not sure if it will make much difference.