How to Judge Back Pressure
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Tourer
Joined: Aug 2008
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How to Judge Back Pressure
So im lucky enough to have sets of baffles for my reinharts at my desposel so I took one and decided to try this back pressure deal . I used bolts in the baffles for starters. I know i know your going to say order this or that for back pressure but many of those baffles don't work. It was easy for me to do it that way and it made it quick and easy to install more or pull one out withen 15 min. So i started with one and noticed I got a little more bottom end tq. I installed two and got a lot of low end tq. I installed three and got a LOT of low end tq.
Now while doing this, I noticed this, I got more bottom end yet was loosing top end. The more bottom end, the less top end.
So my question is this. How much is to much back pressure ? How do you measure when its enough back pressure ? Why do you have to forfit top end for low end torque ?
Now while doing this, I noticed this, I got more bottom end yet was loosing top end. The more bottom end, the less top end.
So my question is this. How much is to much back pressure ? How do you measure when its enough back pressure ? Why do you have to forfit top end for low end torque ?
Last edited by FLHXXHLF; Dec 30, 2008 at 07:57 AM.
Cruiser
Joined: Oct 2008
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There is a physical relationship between TQ and HP. TQ acceerates your bike, and HP gives you top end speed. Think of it as a gas engine versus a diesel. The diesel has tons of low end torque but not a great deal of rpms, and a high performance gas motor does not come on strong until the upper rpms. These statements assume all else is equal as it is possible to maximize a combination with add-on parts and tuning.
To maximize both TQ and HP, you will need to understand the components of the engine, such as engine design, compression, cam specs, cylinder head flow, etc and then decide where in the rpm band you want the maximum TQ and or HP. Typical Harley V-twins are known for their low end TQ by their design, so you may want to optimize the rpm band up to about 3500-4000 rpm. This is one of the reasons the SE 255 cams work well for Touring models (heavy bikes with two people). The design of the 255's allows for very good breathing in the lower rpm, but most say these cams run out of breath above 4000-4500rpm.
Now look at a V-Rod. It also uses a V-Twin, but with a very different design for more HP and rpm.
Measuring backpressure is easy. You will need to put a low range pressure gauge on the exhaust system before the baffles.
To maximize both TQ and HP, you will need to understand the components of the engine, such as engine design, compression, cam specs, cylinder head flow, etc and then decide where in the rpm band you want the maximum TQ and or HP. Typical Harley V-twins are known for their low end TQ by their design, so you may want to optimize the rpm band up to about 3500-4000 rpm. This is one of the reasons the SE 255 cams work well for Touring models (heavy bikes with two people). The design of the 255's allows for very good breathing in the lower rpm, but most say these cams run out of breath above 4000-4500rpm.
Now look at a V-Rod. It also uses a V-Twin, but with a very different design for more HP and rpm.
Measuring backpressure is easy. You will need to put a low range pressure gauge on the exhaust system before the baffles.
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Tourer
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Very informative. Im trying to decide what is enough back pressure to get max torque. You made mention of operating rpm's. Most I hear like to "LUG" their motorcycles and stay withen the 2000 to 3200 range. I was told by Harley they suggest you run in the 2600 to 4500 range. This is a big change from the old days for sure.
Outstanding HDF Member
Joined: Jan 2008
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From: Pacific Northwest
You'll know it's right when it's the most enjoyable for you to ride. Remember, though, you're only going to get so much torque no matter where it is in the rev range and having some "rev out" is a good thing.
Cruiser
Joined: Jul 2006
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I also am running Rinehart TD's and was thinking of doing something like this. I was going to tack weld two 5/16 or 3/8 bolts in a cross in the baffle. What size bolts did you use, and how did you space them?
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Road Captain
Joined: Oct 2007
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From: Near Raleigh N.C.
I once found a formula to calculate back pressure...had to do with diameter and length..
a Goggle search would probably turn it up for you.
a Goggle search would probably turn it up for you.
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Tourer
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You first take off the muffler. Then take off the end cap . Take a hammer and put the wood end in the input end of the muffler. Tap it till the baffle falls out. Now notice at the beginning of the baffle where it shrinks. I installed two 5/8 bolts right there making a cross. I drilled two holes in mine. Not I would suggest trying one bolt first. Then if you want more put another. I get plenty of low end toeque with two in. I can't believe the difference. Let me know how it goes.
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