Pressure Drop is Pressure Drop

I want to take a perfectly good 550 xj and modify it for no good reason. One route is replacing the air box with pods and changing the pipes. If I do that I will need to play with the jets and may or may not ever get them right. An alternative route is to make an air box and go with straight though pipes after my 4 into 2 joint.

For an air box I can manifold all of the carbs intakes together and attach one pod. I would put a restrictor plate between the manifold and the pod. I would start the motor with the original set up and set the rpm at 4000. I would then swap out my air box and play with the restrictor until the rpm was 4000. Both air boxes would have the same pressure drop.

I could play the same game with the pipes using an internal muffler. More restriction longer internal muffler, less restriction less internal muffler.

Why will that not work? And if it does not work, how can I make it work?

 

So instead of adjusting the jetting, you think it might be easier to modify the intake AND exhaust set-ups to suit the standard jets? Hmmmm.....

Far easier just changing the jetting IMO....

 

ya i'm with hillsy

 

You need to know the Cubic Feet per Minute Rate of Air drawn into the Stock Airbox. The stock Airbox opening to atmosphere is the "Restriction" you need to duplicate in order to have the Carbs function without extensive re-jetting.

Creating a manifold or Plenum will be a good start and might even work!
Although the Rubber Boots that connect the Airbox to the Carb Intake Sides also double as Velocity Stacks.

Therein, is the problem which arises for those who modify their Bikes installing Pod Filters in place of the Airbox.

1. The Cubic Feet of Air per Minute is drastically altered by the individual Pod Filters.
2. Turbulence at the Carb Intake Opening rather than a directed flow of Air into the Venturi.
3. Main Air contribution unchanged when an enriched flow of Main Jet Supply is needed to compensate for the Increased Volume of Fresh Air into the Individual Carb Intake.
4. Needle Heights altered to attempt to resolve the Main Air Issue inadequate causing Mid-range or WOT response to suffer.

The situation is handled by an acceptance of a "Least affect on performance" leaving an imbalance in the Carbs to supply an Optimum Air~Fuel ratio delivery ... settling for "It's running OK" and leaving the performance difficulties unsolved.

 

As far as pressure drop being pressure drop, well, maybe. I think the best way to think of it is what Rick said: it's actually the air mass flow rate you care about most.

However, the method of tuning everything so that you're still running at 4000rpm should be a quick and dirty method of setting the air mass flow rate. The primary effect of changing the amount of turbulence in the flow is to create more blockage (this is why performance tuners polish ports and stuff like that--the flow will be more smooth, or laminar). So, you just might be able to balance things out pretty well by doing it this way. I'd say go for it.

OTOH, changing the jets doesn't sound that bad to me, although I've never done it. I mean, it's not like it runs "perfect" from the factory anyway. What does perfect mean? There are always tradeoffs.

 

I think you'd have issues once you changed the flow of air going past whatever set up you have. ie: going from idle to 60 mph/100 K.

The factory set up controls this problem.

Many years ago people would turn over the breather on their carbed vehicle motors. The thought was that it would get more air. Problem was that air going by the filter ( as it was open on all four sides) created a bit of a vacum which lowered the air pressure into the carb. It certainly made the engine sound mean but didn't really add any performance.

If you can go to a ram air set up with the factory airbox & filter being able to get cool combustion air you'd have a real good set up. That would set up a positive pressure on the breather side of the carbs. I would think this would help the engine breath a little easier but shouldn't really cause any jet issues. Most vehicles today use a similar set up to this to get cool air from the front of the grill.

Good luck on whatever you try.

 

lets see, you want to use a pod air filter and plenum and restrict it to act like the stock airfilter/box, then use a different muffler setup to get the same characteristics as a stock muffler
so now everything is different but the same
i think i would just go drink some beer

 

Polock – You hit the nail on the head. I want to do this “for no good reason”.

Hillsy / 07spacker – I might still end up going the jet route. I have never changed jets but from what I have read on this forum it is not a simple change out. First you have to take out the jets which involves drilling and hoping that you can free up the screws. Then you have to guess the new jet sizes. Then you have to change clips and do addition drilling. Then you have to somehow set the pilot depth. Then you have to determine if that is the right setup and repeat until you find the best setup and even if you do it right, you may still get flat spots in the performance curve. I have the skills to hack and weld and trying for “about” as good as stock is fine with me.

RickCoMatic / Pygmy – I believe that you are right when you focus on mass air flow. It all comes down to duplicating the performance at a fixed condition. If I hit one point on the curve, I am hoping that the performance curves will match up closely.

Cycleman – No doubt a ram air set up may give better performance, but I haven’t a clue how to set the carbs on a bike that has it added.

Thank you for all of you input. While I have no good reason for modifying a perfectly good bike I would like to end up with something like the picture below. I have yet built up the courage to start the chop.

 

If you haven't destroyed the Airbox ... DON'T!

Save it. Even if youhave to unbutton the mil to get it out. You might find that the Project is getting out-of-hand and want to go back to Stock.

You'll never find another Airbox if you ruin the one you have. So, I'd take it out and save it for a rainy day. Then, you'll be able to rethink the project and go back to a stock arrangement if need be.

With us being right on cusp of a Summer Season ... why not try fine-tuning that bike Stock and getting as much out of it as you can.
You can't be that far off.
Right?

 

It seems like it should work, but I'm sure it's more complicated then it sounds. Anything is possible if you have the time and talent.
I'm still interested in changing the carb setup to two-into-one manifolds and using two carbs instead. It's been done on 750 Hondas and works great. Or even a single, two barrel carb setup with a 4-into-one manifold like on my car. Why not?
I'm not sure about having enough room for a single carb and manifold setup, but I know there is for 2-into-one setup.
Just another one of my thoughts. PD

 

Does anyone have a picture of a 6" stretch hard tail frame on a 550XJ?

 

I run straight pipes and pods with stock jets with no problems. You might want to wait to see if you even NEED to do any of this. I tweeked on mine for a little while and got it dialed in without all the headaches that people here warn of. Patience is the key.


edit--- let me also state that I *DID* have a major problem getting everything correct right after I made the changes to pods and straight pipes. I toyed with other solutions such as restrictor plates. I eventually found that my ignitor/TCI was faulty/failing and after getting a replacement, everything was smooth sailing.

 

You know PainterD, if you were thinking about going through all that trouble to switch out the carbs, why not take it a step further and come up with a fuel injection system with a changeable performance curve based on what you want to do. Fuel Economy or performance at the click of a switch. Now that would be cool.

 

That kind of Ignition costs a bunch of greenbacks.

I was just browsing the DYNATECH Site, looking for a set of performance Coils and saw the Ignitor Box they producue for Metric Cruisers.

Four hundred buck-a-roo's ... and, ... that's just for the Part!