exhaust question

[Islander]

If I have a 360 V8 with great low end torque.
then modify with throttle body/intake/headers/larger exhaust pipe..

and end up with less low end torque and more high end torque....

could I get my low end torque back simply by putting a plate in the exhaust pipe downstream of the muffler?
and do you think the low end torque would be higher than it was before the mods.

 

[cycle]

Do you feel as though you have lost low end torque after your modifications?? All indications are you should have made more torque,possibly at a slightly elavated r.p.m...If you feel a loss ,it is possible one or more tuning parameters are "off".. Engine modifications need to work as a complete package,although you made the , what I believe, the right choices to a mild hop-up.. As far as a plate in the exhaust tract , the problem would be what is the correct angle. A fixed plate would only work at a very narrow r.p.m. range. Likewise, the plate would hurt torque/horsepower in all others..I suppose a baffle of some type(not a plate)could be used in the exhaust. We use these on motorcycles(different inside diameters)to change engine characteristics. Your best bet would be to use an adjustable system like Supertrapp.This uses a series of disc"s in the end of there muffler. Quite simply the more disc"s the free-er the exhaust flow.Almost infinitely adjustable with the purchase of additional disc"s. Craig

 

When you get things like these (intake, headers) they are always tuned to work best in one environment, and relativley good at others, for example, in a header, if it has a longer thinner primary tube, over a shortef fatter one it will produce more low end torque , the other one would be for high end torque. Intake runners have to be tuned to a specific RPM also. If all you want is low end torque then you should have gotten mods that were designsed for that, i would suggest this - by sacrificing low end torque which is plentiful - you can get a big increase in higher in the powerband, making the entire powerband better

 

[Islander]

putting a plate in the exhaust is not something I plan on doing.

Rather..for the sake of the knowledge...I kinda tend to think of an engine as an air pump.

if I did go and figure the correct amount to restrict the exhaust pipe downstream of the muffler..do you think I would end up with more low end torque than before I added the mods?(disregarding what ever happens to the rest of the torque curve).

 

[ivymike]

Perhaps someone can answer a slightly different question:

Is there a "correct" amount of restriction to have in the exhaust pipe downstream of the muffler? I would have guessed that the less restriction the better...

 

[Islander]

that's right, you would not want restriction downstream of the muffler, I could have said..between the header and the cat...however the main issue is what will happen to the torque with the pipe restricted under the conditions specified.

 

[ivymike]

well, assuming that your exhaust system is "too free-flowing" for good low-rpm torque production, then restricting the flow slightly should improve low-end torque with a penalty at the high end. This would, of course, undo any benefit you may have received at the high end.

If you could install a valve that would allow you to vary the restriction, you could conceivably get the best of both worlds... Almost like having variable valve timing.

Correct me if I'm wrong, anyone, but isn't the main reason that you'd see lower torque @ low rpm with the reduced restriction due to thru-flow in the cylinder? I'm guessing that some of the intake charge is heading out the exhaust valve, thus reducing vol. eff.?

 

[ivymike]

Wouldn't a cam with shorter duration and/or less overlap provide better low-rpm performance (with or without headers)?

 

[evelrod]

Yes to both questions, Isaac.

Rod

 

[hpfreak]

Most likely the loss of low end torque is due to the reduction of velocity, not reduction of backpressure. This is a consequence of the larger headers.
DW

 

[ivymike]

I'm going to have to change my mind about my previous statement "isn't the main reason that you'd see lower torque @ low rpm with the reduced restriction due to thru-flow in the cylinder? I'm guessing that some of the intake charge is heading out the exhaust valve, thus reducing vol. eff"

It seems more likely that the flow is going the other way, some exhaust heading up the intake, but with the same net result (reduced vol eff).

That doesn't help explain how a valve in the exhaust could improve low-end torque (and such devices ARE used successfully in motorcycle applications)..

 

[hpfreak]

Valves are used in the exhaust of motorcycles to change the phase of the exhaust pulses. (Im thinking CBR 900RR, you may be talking about 2 strokes) The valve switches between 1&4, 2&3 collection to 2&4, 1&3 combination to get the best exhaust pressure pulse combination for the rpm range. These pulses are additive in different rpm ranges. In all cases, you try to minimize the pressure at the exhaust port when the ex valve is open.

With larger headers, the exhaust flow at low rpm is at a lower velocity so it can change direction easier. This leads to the situation Ivymike mentioned, with the exhaust backfilling the cylinder, and even mixing into the intake charge.

 

[ivymike]

I thought that they were throttle valves in the exhaust stream? Here is all the info that I could find on them in a brief online search:

http://www.automotiveforums.com/vbulletin/t13199.html

"We see these in the Mercedes McLaren F1 car. If you think this is somebody's Frankenstein pipe dream then guess again. The new Suzuki GSXR1000, Honda Fireblade, and Yamaha R1 already have these."

http://biketransplant.tripod.com/exup_replacement.htm

"The Yamaha R1 is fitted with an EXUP valve. This is a Yamaha specific invention designed to improve torque at low to medium revs. It works by placing a small "flap" in the exhaust collector. This flap can be rotated to reduce/increase flow through the collector. When reduced the back pressure is increased and the wavefront at lower speeds can be reversed to improve extraction. "

http://www.power-sports.org/new/bikes/gsxr1000.html

"as well as Suzuki's Exhaust Tuning system that optimises back pressure in the pipes for optimum power throughout the rev range. "

http://www.suzukisa.co.za/ModelDisplay.asp?ID=90

"The new GSX R1000 features the new digitally controlled Suzuki Exhaust Tuning (SET) system. The new exhaust control system uses a butterfly valve to modify back pressure and thus tulle the pipe to suit running conditions. The butterfly valve is opened and closed by a servo motor, via cables, depending upon engine rpm and transmission gear selection.
At low rpm, die valve closes, increasing hack pressure and improving low-end torque while also reducing exhaust noise. As engine speed increases, the valve progressively opens to reduce back pressure and improve mid-range and high-rpm output. The four-into-two-into-one exhaust system itself uses a combination of titanium, stainless steel and aluminium parts to reduce weight. The header pipes, collector and internal muffler core pipes and core-pipe mounting plates are made of titanium"



(good typos in that one, eh?)

I think that these bikes are all 4-strokes, but I have to admit that I'm not entirely certain about that, as I don't really know much about motorcycles...

 

[ivymike]

another article on the subject:

http://www.motorcyclenews.com/news/detail?sectionID=50678&documentID=22528&pageNum=1