Need some Mr. Rohrich instinct

ochster

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• Apr 14, 2003

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Getting ready to spend a day with the welder (little Jeff), to finalize mounting locations, for the muffler and airbox. This is on the "long in tooth", project XR300, I just put a custom subframe on.

In locating the airbox, there will be a need for a nearly 25mm spacer, in the intake tract. The XR250/300, has a fairly short intake compared to many other machines. I'm wondering if it would be worth while, to fabricate a longer intake manifold, instead of simply putting a coupler on the back of the carb. I could use a slight deviation in the mounting angle of the carb., and this would also allow me to incorporate that into the new manifold. Is there something to be gained here?

I also am going to be aligning and mounting the muffler. I have alway's like the megaphone design muffler on the small bores. Spark arrestability, is important, and the supertrapp is easy to deal with. Should I be broadening my thinking, being the new subframe can be made to fit anything right now?

I realize these question are very general in nature, as well as being subjective to testing. Would just like to hear from others, and get some input, while things are still in the "soft" stage.

Thank's
Mark

 

Ol'89r

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• Apr 14, 2003

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Originally posted by ochster
I'm wondering if it would be worth while, to fabricate a longer intake manifold, instead of simply putting a coupler on the back of the carb. I could use a slight deviation in the mounting angle of the carb., and this would also allow me to incorporate that into the new manifold. Is there something to be gained here?


Would just like to hear from others, and get some input, while things are still in the "soft" stage.

Thank's
Mark

Click to expand...

Mark.

IMO, I would be careful changing the length of the intake tract. If you do, you should do it on a dyno. Minor changes here can greatly affect the way your engine will run. It could help, but it could also hurt and without doing a before and after on a dyno, you won't know for sure. If it were my engine I think I would add the spacer behind the carb.

Just my $ .02

 

Rich Rohrich

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• Apr 14, 2003

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ochster - That's good advice from Ol89r :thumb:

Keep in mind, what you do on the intake side has repercussions on the exhaust side as well, and cam timing (especially at overlap) is also pretty sensitive to tuned lengths .

 

ochster

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Thanks, gentlemen.

Nice to have a site, that is deep in "real world" resources, such as DRN. I knew someone would grab me by the ear, and pull me back onto the sidewalk.

How about some opinions on the magaphone vs. straight muffler design?

A little off topic, but how do you highlight someones quote, like OL89r did, in a reply?

 

Last edited: Apr 14, 2003

SFO

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On our old half mile xs-650 motors we would use pcv pipe for our intake manifold tuning and we could cut it on the spot to get different intake lengths and use a stop watch to see how it would improve or not the performance of the motor. You might consider this, pcv is easy to work with and once you get the right lenth you can duplicate it in aluminum.

 

Rich Rohrich

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• Apr 14, 2003

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Originally posted by ochster
How about some opinions on the magaphone vs. straight muffler design?

Click to expand...

What type of power are you looking to build?

 

ochster

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The bike will see woods use, the majority of the time. So naturally a healthy bottom to mid, or broad midrange would be prefered. I say that, but I tend to ride in the mid range and up.

Good idea SFO, Thanx's.

 

Vic

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• Apr 15, 2003

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Originally posted by ochster
A little off topic, but how do you highlight someones quote, like OL89r did, in a reply?

Click to expand...

Press the "quote" button next to anyone's reply and their post will appear in a reply box with the required code already added.

 

ochster

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Press the "quote" button next to anyone's reply and their post will appear in a reply box with the required code already added.

Click to expand...

I "copied and pasted" Vic's quote, into the field of the quote box (that pops up when you hit quote), but mine does not say "quoted by Vic?"

 

Last edited: Apr 15, 2003

B

biglou

• Apr 15, 2003

• #10

Originally posted by ochster



I "copied and pasted" Vic's quote, into the field of the quote box (that pops up when you hit quote), but mine does not say "quoted by Vic?"

Click to expand...

Click the button shown, then the "Originnaly Posted By" will show up. It will quote the entire post, but then you can edit out what you don't want by backspacing over it or highlighting and backspacing if it's just a portion of a post you are wanting to quote.

 

Ol'89r

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Originally posted by ochster



I "copied and pasted" Vic's quote, into the field of the quote box (that pops up when you hit quote), but mine does not say "quoted by Vic?"

Click to expand...

Mark.

Just hit the quote button and then delete everything except what you want to be quoted. Then add your text to that. (Never mind, Big Lou just beat me to it. :confused: )

When we were dyno tuning the 750 twin factory dirttrack engines we used somthing similiar to what SFO suggested. On the intake manifolds we had telescoping aluminum tubes. You could move the carbs back and forth while the engine was running on the dyno. Once you found the 'sweet spot' it didn't take much movement in either direction for the power to drop off drastically.

Another method of determining the correct intake length is by watching the fuel vapor blow-back through the carb. When you are close to the correct lengh, you will have minimal blow-back. The further you move the carb back from the correct length, the more blow-back you will see.

In regard to the megaphone. I agree with Rich, it depends on what kind of power you are looking for. I prefer the SuperTrapp set up mostly for the ease of tuning. It's nice to be able to add or remove discs to suit a particular engine and they are a spark arrester for the woods.

Just my $ .02

 

Last edited: Apr 15, 2003

ochster

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Originally posted by Vic


Press the "quote" button next to anyone's reply and their post will appear in a reply box with the required code already added.

Click to expand...

See if this works!

Thank you gentlemen, you just made my day. I have been trying to figure that out since I joined DRN.