Muffler Mod

[Jim]

I just had my bike dynoed tuned at your shop this week. I was a little dissapointed in the 86 H.P.! I have made a couple of post here, and I have the 97 FLHT with the 107 c.i. S&S motor that was turbocharged and now has 8.5:1 compression. I did take out the S&S 600 cam (255?/260? duration x .600" lift, Int = 20?/55? & Exh = 20?/60? timing). and replace it with a Red Shift 559V2 cam (242? duration x .560" lift, Int = 16/105/46 & Exh = 48/107/14 timing @ .053"). My torque seems fairly good and flat from low to mid rpm. I am making 100+ ft./lbs. right at the start of the pull at about 2200 - 2300 rpm, and peaks at 106 ft. /lbs at about 3700 rpm, and at 4000 rpm it drops off pretty sharp at a rate about 8 - 9 ft./lbs. per 500 rpm.
The Horsepower starts at about 43 HP at 2200 rpm and climbs steady at about 20 HP per 1000 rpm until 4000 rpm where it is making 80 HP, then its rate of climb slows to a crawl and from 4000 rpm to the peak HP rpm of 5500 it gains only 6 HP. Also note that I ended up with a 160 main jet. Which seems very small to me, considering I had a 230 main with the Turbo.
So that tells me one thing with that small main jet! At 4000 rpm this motor is becoming air restrictive and isn't flowing much air thru it. I know the stock 40 mm Carb isn't helping. I do have a big sucker air filter kit from Arlen Ness on it. But I can't help to think that these screaming eagle mufflers are too restrictive. I noticed on the highway at about 60, if I open the throttle, I hear more noise coming from the airfilter than the exhaust pipes. I noticed after buying these they look like a stock muffler, except a little bigger baffle ID. But they are not a straight thru design. What would it do if I was to ram a bar thru it and opened up the deflector plate inside about half way made it a partial straight thru? Would that help me, or hurt me? What I would like to see is at leat 95 HP, and I would like to move my power band up another 500 rpm. I am happy with the low speed throttle response of the motor. I just wished it had a little more on top, without taking too much away on the bottom. It could be the cam I guess, but I feel it's more carb and mufflers. I have a friend that has a 88" evo with the same came and carb and is making about 95 H.P.. The only difference (besides heads) is he has 1 full point of compression more than me. But 19 less cubic inches. So what are your thoughts? Should I try to open the barn doors and see if I can get more air through the motor?

[Mike]

Jim, I've reviewed the dyno charts from your tune and I would agree that the exhaust is most likely the cause of the HP loss. I base this on how the torque curve becomes virtually a flat line (progressing down) from 4k on. This is commonly a function of a loss of air flow through the motor (I.E., restriction). Opening the mufllers, as you suggested, will help the top end power (guess would be 5-10). You can also drill the holes in the baffle sides larger and add more of them. As you also indicated, the small jet size would reflect a restriction. If you open the mufflers, you may have to go up 1-2 sizes on the main. All this may however cause a loss of low end torque, typically around 2-3K, the more restriction you take out, the greater this torque dip. With the torque your motor is making at 2200 (100ft/lbs), I would suspect a minor loss would not be a problem and you may actually gain from 3.5 on.
Last, a 44 CV or Mikuni 42 would also help pretty much everywhere.
Mike

[Jim]

Have you seen any mufflers called "Rolling Thunder" mufflers? I can get a pair for $200. And they have removable baffles and the baffles come in 1-3/4", 2", and 2-1/4" dia. I was thinking of maybe selling the SE set instead of modifying them to where they're worthless, and just buying a set of straight thru mufflers. And I thought the changable baffles would give me a little tunability with the back pressure. I am curious of the quality in workmanship though. Which is why I was wondering if you have seen these and used them before.

[Mike]

I have heard of the Rolling Thunder mufflers but, as yet, have not tested them nor have I seen any independant dyno charts. I'm also looking real hard at the new S&S stainless mufflers and a new model by Samson. Both of these offer changable baffles but with the stock baffles offer pretty good performance with less noise. We have yet to test these but hope to soon.