View Full Version : open vs closed end cap

12-18-2008, 12:56 PM
here's the build:

'08 screamin' eagle dyna 110"

r&r stage 4 heads 1.94/1.63 valves

10.5 compression...200ccp/4% leakdown

screamin' eagle 257 cams

horsepower incorporated 55mm throttle body, s.e. injectors

bob wood 3.25" air cleaner

supermeg with 28 discs closed

advance tuned with power commander

we tried various combinations with the closed end cap, and 28 closed made best power...I'm attaching the dyno chart

now I have bought the steel open end cap with 1-1/2" hole...how many discs should I use?...I am looking for horsepower, even at the cost of a little torque

12-19-2008, 07:09 AM
The open end cap has the equivalent flow of 10-12 discs. So, 28-10 or 12=18 or 20.

12-19-2008, 10:07 AM
thank you!

02-10-2009, 12:59 PM
I have tuned to 12 disc open-end. I would start with 20-18 total with open end.

I have a post and currently waiting for a reply from the GURU

I am most likely going to re-tune at 18 or 9 open end cap, depending on his advice

02-11-2009, 07:12 PM
I'm guessing you may loose HP but you will gain Torque.

From what I now understand and you saying you got the best power with 28 and closed correct? - You theoretically can use 38 or 40 open-end cap, this will give you the equivalent to your present 28 and not any change to HP or torque yet possibly get a sound your ears are happy with.

I am working with a project similar to yours, mine is a 103" SE....

Well, I hope to see a new dyno sheet with numbers that really show the better way to go (other than sound) between using open end or staying closed....

Check out this site...

02-12-2009, 10:03 PM
You have this mixed up. You use less discs with the open not more. ie 20 closed equal 8 or 10 open. I don't think it would run very well with 40 and open.

02-13-2009, 08:20 PM
Thanks jeff...

On my build I am looking to gain more Torque, were as skyhook was looking for more HP...As you can see on his post skyhook did a lot of dyno work with a closed-end cap and getting the best Power with 28 closed...

Now I am starting to wonder if he is referring to power as Torque. I was thinking he ment Horse Power - HP....I most likely will never find out - would need a reply from the hook himself....

I was trying to figure his combination of 28 closed and what would be the equivalent to that with a open-end cap..sac a little torque, and gain more HP, I belive that was Skyhooks goal....

Now it is my understanding that less discs give you more Torque, more discs creates more HP.

I have done one tune with a open-end cap and 12 discs..So theoretically, I need to take away for my goal of gaining more Torque. What do you think?

Take a look at my dyno tune sheet. It is somewhat different than a DynoJet read out. You can download a free trial of WinZip to open it up. That is all I use myself.

My tuner is Marc at FactoryPro

02-13-2009, 09:49 PM
If you use a close cap and want similar performance of 12 and open end cap you need 20-22 discs. You may get a little more torq with fewer than 12 and open but not much. You don't want to go too. 28 and closed would be 16 to 18 open.

02-13-2009, 11:47 PM
thanks again jeff,

Since I am already close to were I want to be with open-end cap testing. I most likely should test with the closed cap on my build configuration. The two bikes that I saw dyno sheets on was this one a 110" best power at 28-closed and another one at 103" and best power with 26 closed.

My build, HD SE 103", SE 202 heads, 10.5 pistons, SE 260 cams, SE 58mm TB, SE injectors, HD SuperTuner fuel mapping, K&N-Af, and of coarse a Super meg Pipe

So I will start with 28 and go down 2 at a time or start with 24 and go up 2 at a time. I can't remember which was the best to apply first adding or taking away. I will research FAQ before beginning....

Like I mentioned to ginap, we are getting some very wet weather here in Santa Rosa, CA at this time and I am not sure when I will be able to get back to the tuner, approx 35 miles south in San Rafael.

I just don't know about the closed-end cap sound, will see & hear I guess