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View Full Version : TIP: Fine tuning your DRAG PIPES/backpressure.



Jethro
02-02-2010, 1:54 PM
I'm running a 71 T120 650cc with twin VM-32 carbs. This is a quick tip if you have ever found yourself at a loss about a "stumble" or hesitation from the start of acceleration from a stop on your Triumph, with Mikunis or Amals (remember neither have an accelerator pump). If your bike is properly tuned, both in timing AND in your carb... and that is a big if... then what you may want to experiment with is backpressure in your unbaffled exhaust. It seems as if just about every chopper nut like myself runs open exhaust. NOTHING wrong there, and keeping in mind that a motorcycle engine is basically an air pump, it seems logical that the engine would make more power with unbaffled exhaust. It DOES, usually, in limited areas, primarily WIDE OPEN throttle, which it seems like many of my riding buddies spend most of their time doing.

BUT, for many of us, you "drive torque", and surprisingly, open exhausts don't usually put out the best numbers in low end torque without SOME back pressure. IF, and I say IF, your bike is properly tuned, and your running open exhaust, try sometime putting in a little back pressure in the exhaust by drilling a hole about an inch and a half from the end, and putting a bolt in there, with a nut or two, depending on what you find. I've posted a couple of pics of how I do it, so that I can adjust the back pressure by moving the wingnut... but hell, that just is for looks... you can stick JUST the bolt in there and you might be surprised. My bike runs poorly without a little pressure back there.

BTW I love those brass tips from Lowbrow. Nice product.

http://i11.photobucket.com/albums/a172/jettoes1/IMG_3211.jpg

http://i11.photobucket.com/albums/a172/jettoes1/IMG_3208.jpg

ridemore16
02-02-2010, 2:04 PM
thanks for this info Jethro...I'm going to keep this in mind when I get my scoot back together and running.

bhead1968
02-02-2010, 6:25 PM
[QUOTE=Jethro;37034]I'm running a 71 T120 650cc with twin VM-32 carbs. This is a quick tip if you have ever found yourself at a loss about a "stumble" or hesitation from the start of acceleration from a stop on your Triumph

though my bike is running really good at this point, i do get that hesitation, stumbling off the line. I got those 4" baffles you get from JP cycles. I heard you can bend those tabs inside the baffles, down to tune it better. hmmmm. You make a great point "IF" your bike is tuned properly.

SMChewy
02-02-2010, 6:32 PM
Ever tried torque cones? Was always wondering if they really help that much?

Jethro
02-02-2010, 9:11 PM
I run open, unbaffled drag pipes... which means that for ME, my bike runs better in most of the type of driving I do when having SOME back pressure. I like to run the set up I do because it is unobtrusive, and I can do some adjusting of that restriction. Usually I run the wingnut about 45 degrees to the exhaust flow. I have it perpendicular in the photo just to show it better. Hell, you could even just run the bolt alone. I've done that in the past.

Pete
02-05-2010, 7:03 AM
Great tip, thanks Jethro!

ARBY
02-05-2010, 8:34 PM
Ever tried torque cones? Was always wondering if they really help that much?

Snake oil. Don't waste yer money on torque cones.

nobattfxr
02-05-2010, 8:53 PM
Tuned length ends at a 17% change in inside diameter

Torque cones eliminate any type of pulse tuning

this bolt trick works very well, often it takes just a thin nut

TRUMPNUT
02-08-2010, 10:33 PM
Triumphs Cams are made for a 1 1/2" pipe . There is no low end velocity in a big pipe that is why the factorey made the ports smaller later on . The stock ex port on 650 doesn't have a great flow rate until the high rpms Because they are to big for low end velocity .

Back presure helps alot in getting large pipes to run well in the lower RPM range . I think the best part of this trick is the cops night stick test . As long as they can't get there night stick down the pipes then yo don'tge a ticket . ( At least here )

Jethro
02-09-2010, 9:34 AM
Man Trumpnut, I have NEVER even thought of that. ALso, I didn't post it but I am running the 1.75's

TRUMPNUT
02-10-2010, 11:41 AM
Man Trumpnut, I have NEVER even thought of that. ALso, I didn't post it but I am running the 1.75'sFor open racing a 1.75 pipe with a collector is great in the rpm's over 6000 but under they lack that is why you have to make the bike rev up quick to get to the power range . But I have ran TT pipes lots and they are fine for the street just hard to tune to keep them from changing color . If you don't mind some color then 1.75 is fine with back preasure .