Backpressure

[ltrhdevo]

When I first got the bike it came with a dyno read out and said it had python exhaust, guess they were wrong on that one,
thanks again for your info, still learn'n

[ltrhdevo]

by the looks of the pythons they are not true headers, being both pipes are not the same length

[Kainam]

Harleys are a little tricky in that regard cause of their uneven firing sequence....

Unequal length headers are meant to time the exhaust pulses inorder to maximize the scavenging effect.

[Hunt/fish 365]

Interesting...I wasn't aware of the odd firing sequence. I was thinking they should be equal length to have even pulses of the exhaust. I was wrong.

[67Duce]

Performance headers on V8 engines can be tuned , or should I say come in different configurations for a RPM range. The primary's have different lengths and diameters depending on what RPM range you want the most effect. The collector length and diameter are configured to work in the same range as the primary's.

The key is to have the whole motor built for a certain RPM range , that's having the induction system , heads, valve size , cam, lift , duration & timing and exhaust to be designed to work in the same RPM range or power band. That brings you to volumetric efficiency and the ability to flow efficiently is the key to power. An engine is really just a air pump.

Back pressure is a restriction , however you also need to maintain flow so if the stock pipe is only 1.75" then that's what it probaley flows best with for the stock engine configuration and the power band it was built for.


Take your garden hose 5/8" with no nozzle , it pours out with some pressure behind it. If we went to a 2" hose it would still pour out but with little or no pressure behind it thus not sized correctly for the volume of water flowing from the pipe. This would be like putting a 2.25" header pipe in place of a 1.75" pipe on a stock engine because you haven't increased the flow of the rest of the engine.

Now put the nozzle on and you have back pressure , the 2" hose will built up pressure once it fills ,as you have plugged the exit , but your not flowing freely either.

[98DARKHORSE]

ive read most all of this and back pressure is a loose term used by people who dont understand it
as stated before you want the scavanging effect and velocity not "back pressure"

most would say a 3.5 inch pipe would have less back pressure then a 2 inch pipe...well that really isnt the case, everything goes back to RPM, Displacement, and Efficiency

an 80 inch motor at 6k will need a larger exhaust then an 80 inch motor at 4k

if any motor picks up power from adding some sort of "restriction" to the exhaust then its not because it needed back pressure, its because it needed velocity

thats like puting the largest intake valve you can into a motor and expect good results, doesnt happen...you need velocity over volume

and when the exhaust is being pushed down the pipe, any size change will effect how fast the exhaust flows, same thing with a river, and this makes it real simple to think about...when a big body of water moving slow is forced to go through a tighter channel it picks up speed....to an extent...to tight and it jams up, but if its to open, it becomes slow and lazy...this is what happens when you throw an exhaust on a motor that cant utilize the size of the pipe, this is why a motor will gain power by puting somthing up it like a torque cone or baffel

[Billyram]

Try tuning a single cylinder engine. The exhaust plays a more critical part in tuning than a multi cylinder engine. With out a pulse from another cylinder you have to have a baffle to reduce reversion in the low to mid RPMs or you'll only have power in a very narrow band usually in the upper RPMs. Some times it's much better to have a broader power band than to have a bit more hp in the upper RPMs that you would rarely be in. In mini dragsters using lawn mower motors running at a constant RPM have no use for baffles and are tuned to make peak power in a very narrow RPM range. The torque converter they use is tuned to keep the motor in the peak torque. Dirt bikes can use a bit broader torque curve in the midrange. Super Trapp has been doing tunable pipes for singles for years.
Billy

[mrmetal]

Originally Posted by ltrhdevo

With just a ev27 cam, S&S shorty/with thunderjet carb tuner ,dyna signle fire and "Python" thunder headers, it takes for ever to warm up, and you cant dog the bike at all. 3&4 gear are a pain, its where its needs the the back pressure the most, and cold start ups.
If you run it like down the strip when riding it runs great. The best way to ride it is to keep the rpm's up. Python thunder headers are tuned exhaust and a good street/drag every day use. Still wish there was $$ to put stock exhaust back on , even trade so some one else can make noise & play with horse power

It sounds like your bike is not tuned properly. The EV 27 cam works from 2000 to 5500 RPMs. The thunder jet card tuner adds more fuel at about 4500 RPMs. I think if it was put on the dyno with someone that is good at turning they could fix the problem without change in the exhaust.

[Kainam]

Yeah, sounds like they only tuned your bike for Wide Open Throttle, and not all Throttle openings/RPM ranges.

[ltrhdevo]

I tuned it. The dyno was just to see what it was going. The guy running the dyno had been doing it for ten years and said he hadnt seen one flat line g/a mix with a carb.
Still if some one would trade me for stock exhaust for the thunderheaders I'd be happy too.