Historical tuning begs us to believe that a smaller diameter system is better for low down torque, whilst a larger diameter lowers low down torque, but increases flow and therfore BHP at higher revs.
Unless Im wrong?

So I was wondering. Why doesnt someone engineer a simple butterfly type valve in the exhaust that opens with the throttle.
So that you can have a large diameter system, that is artificially restricted at low RPM, and gradually opens to be open by say 4000 revs. Its only the same kind of technology as a throttle body surely.

Or am I missing the point big time!?

robski

 

Yeah. All revolves around the unburnt fuel that escapes at mid revs.
Yamaha overcame this very problem with its EXUP (Exhaust Ultimate Power) valve.
Does exactly what you say. At mid revs, it rotates round to partially close off the exhaust to ensure optimum burn, then as revs increase it opens up.
Basically the same as the power valves used on 2 stroke bikes for years, hence the brill power band.

 

Do some ferarris have the same design too?

 

I would imagine so. The system is only really effective on high reving engines

 

Look up the tail pipe of a 3 series BMW new shape with twin tail pipes it has a butterfly in one of the twin pipes witch opens above a certain RPM.

Darren.

 

Kind of misses the point for turbo cars though.
Less back pressure the better.

Martin

 

Yeah it doesnt really work with fuel injection either, so pointless really nowadays, especially as ECU's are such clever chappies.
Fuel injection is fairly new on bikes cos its not easy to get smooth flow at low revs. Early FI bikes surged like buggery.

 

Im sure it would be beneficial still on cars.
I still look at loads of dyno graphs where exhausts loose low down, to gain higher up.
Most mags ignore anything but the peak BHP, and dont care if at 3000 revs its lost some.

robski

 

Variable Valve Technology....thats the answer my old son.

 

My Legacy RS has just such a device on one of the pipes that go into the rear silencer. Always wondered what it was for .... Taking up Martin's point, though, I wonder whether opening up the butterfly permanently might get rid of the power/torque dip that I experience around 4000-4500 rpm when the first turbo runs out and before the second turbo has wound up. Anyone care to comment?

Brian

[This message has been edited by HunterB (edited 06 December 2000).]

 

Robski,
Yes you are correct, but ONLY for a N/A engine, the turbo already offers such a big restriction that butterflies in the exhaust of a car thus equipped would make **** all power difference. I would suspect the one in the Legacy is to get past drive-by noise regulations or something like that - or controls the gasses to the different turbos (is it sequential?).
Best regards
Mike R



[This message has been edited by Mike Rainbird (edited 06 December 2000).]

 

Twin exhaust rear boxes may have valves on one tailpipe. If the open area is too large there will be an unpleasant low rumbly sound that may induce vibration into the cabin at low gas flow. With a large engine the gas flow through just the one pipe will cause 'gas rush' sound at the upper region of the engine speeds. Hence open valve at some point in between, get the best of both worlds. Also decreases back pressure in that critical region!

Mick

 

Hi Guys,
Billy is right here. Yamaha used this system on their EXUP very effectively but it never really caught on. Maybe because very little bike riding is done in the low rev range . (Well very little of mine)
We have done a little experementation with this but the minor difference noticed was not worth the hassle. We fitted different sized baffles to the exhaust from 2" to open pipe at 4". At open pipe when the turbo came in the car took off. At 2" the car had less turbo lag but in my opinion this was not due to the extra low down torque but due to less boost when the turbo did come in. Not worthwhile IMHO.Maybe there is a more scientific way of doing it but thats my 2p worth.
Sorry Guys

 

Mike R

Yes, the Legacy's turbos are sequential.

Brian

 

One of the exhausts in my spare room has a spring based valve in the tip.

Basically it did help compared to the extremely free flowing systems i've used before. The only problem with this SARD one was that it opened up a bit too early for my liking (extremly loud when theres enough pressure) at about 3500rpm. If if opened up a bit later it would have been awesome!

I'll try and sick a picture up later when I get home.