Road Star Clinic Ported Manifold FAQ

Print
User Rating: / 37
PoorBest 
Written by Steve Graham (GRAM)   
Friday, 11 June 2004

Road Star Clinic

Manifold Porting FAQ 

Edit Note:  The Road Star Clinic has ceased producing ported manifolds.  We have elected to leave this FAQ on the clinic due to its informational content. 

What is “porting”?

How is it done?

What are the benefits?

Does it affect carburetor jetting?

Are there differences in porting styles?

Why aren’t your manifolds polished?

How do you feel about used manifolds?

Will your manifolds work with larger carburetors?

I have heard that the mounting flanges are not always flat…

Should I consider oversized o-rings?

Do I need to replace my o-rings, or can I reuse them?

How can I optimize the manifold during installation?

How do I get rid of backfire on deceleration?

What is porting?

Porting is a term used to label the process whereby we decrease a manifold’s resistance to air/mix flow into the engines intake ports.  On a Road Star, this process generally addresses the “T” junction in the stock manifold where the input from the carburetor meets the output tubes (or runners) to the individual heads.   The stock Road Star manifold is literally the joining of two rubber tubes, including right angle junctions which can be radiused to smooth airflow and reduce turbulence.

How is it done?

Porting is generally done with grinding/sanding tools, attached to some form of die grinder (a dremel is a small die grinder).

Owing to the difficulty of reaching and radiussing the corners of the Road Star manifold, the Road Star Clinic found it necessary to invest in special tooling to address this process efficiently.  Although it certainly can be done without our tooling, we find that doing the best job possible, as consistently as we wanted to be able do it required this investment.  Our tools allow us to achieve a high degree of consistent radius at the corners of the manifold, and to surface the entire manifold in a manner consistent with accepted norms for achieving best airflow at typical Road Star flow rates and velocities.  It also allows us to process them faster, which provides us with the opportunity to offer them at prices below what manifolds ported this thoroughly have previously sold for.

What are the benefits?

A ported manifold will generally add from three to five horsepower to any given configuration of air kit/jetting/ pipes on a Road Star.  This increase is directly attributable to the engine being able to draw in a larger volume of air/fuel mix for each revolution.  The more open your aftermarket accessories are, and the more airflow they can deliver to the manifold, the higher the benefit.  You should also notice a marked improvement in smoothness of acceleration.

When we radius the corners of the manifold it provides a shorter and less restrictive path for the mix to flow along into the head.  This ultimately means the cylinders can pull more mix in for each combustion cycle.

The Road Star motor (or actually, any internal combustion motor) is in reality, a large air pump designed to “suck” the air/fuel mix required for combustion in (this is a misnomer, as the air is actually pushed in by atmospheric pressure – but the analogy works for this discussion).  Filling the cylinder with 100% of its capacity is theoretically impossible, due to the inevitable restrictions present in the carburetor, manifold and exhaust pipes, unless some form of boost is added to the mix.  With an unboosted carburetor like the one on the Road Star, the best practice forms around reducing restrictions wherever possible.  This allows the engine to draw more mix into the cylinder.

The path of least resistance rules here.  Putting a better air kit and pipes on your bike will net a marked improvement in response, but doing so without porting your manifold will restrict you to the limited potential of the stock manifold.  It is designed to complement the stock air kit, jetting and exhaust pipes, and will provide best performance with these components.  It is not designed to accommodate the higher flow potential of aftermarket performance accessories.

Does it effect carburetor jetting?

Yes.

For any given carburetor, when you increase the internal volume of the manifold that draws mix from it, you decrease the density of air going into the manifold.  Air, being a compressible medium, will expand to fill the increased volume in the manifold.  Gasoline, being non compressible (liquid – misted but still liquid at the atomic level), will not expand.  This means that for any given volume in the manifold, there is less air to mix with the same amount of gasoline.  Therefore, if your bike is set for optimum jetting, and you trade a stock manifold for a ported one, you will end up being slightly rich.  This will require some adjustment to compensate for.  Some of our customers have only needed to adjust their PMS (Pilot Mixture Screw); others have had to reduce main jet sizes by as much as a size step.

This jetting change is more pronounced with the Road Star Clinic’s manifolds than with many of the others out there.   Our manifolds are increased in volume throughout, and thus have a much larger volume than most of the others we see (except for some that are being produced by top of the line race/performance shops).

Are there differences in porting styles?

Yes. 

There are as many differences in porting styles as there are shops that port manifolds.  We adhere to the porting style most prevalent in the community of race shops that provide performance parts for Road Stars.  These typically have increased cross sections over the stock manifold, not just radiussed corners.

The Road Star’s head intake ports are 41mm in size.  The stock manifold is 40mm throughout.  There is a realizable benefit to be had from increasing the stock manifold to match the size of the intake ports.  In a stock configuration, the manifold is not delivering the full potential mix that the heads are able to accept.

To be clear, there is a very real benefit to be gained from even a simple porting job, which only addresses the radius of the corners.  Benefits include smoother acceleration and slightly higher horsepower.  However, increasing the cross section increases the realizable horsepower even more.

Why aren’t your manifolds polished?

Polishing of intake manifolds is a very popular modification in high RPM performance circles.  Benefits tend to start around 6000 RPM and higher.  Below this RPM, the polishing actually hurts performance, as the air flow velocities are not sufficient to strip the interior walls of their boundary layer.  The boundary layer is a layer of air that actually “sticks” to the walls and does not flow with the rest of the air/fuel mix, thereby reducing the effective cross section or area of the manifold.  By surfacing the interior surfaces of our manifolds, we actually end up reducing the thickness of this boundary layer by giving it a less than optimal surface to stick to.  The surface of our manifolds also promote “tumble” which helps to keep the air/fuel mix in suspension.

How do you feel about used manifolds?

The Road Star manifold is prone to cracking around the joint/edges of the mounting flanges.  This area covers a metal insert, and in most cases the aluminum insert maintains the seal.  We do leak test used manifolds we receive for porting, and fill any existing cracks with a silicone designed to stay flexible and maintain its properties up to 400 degrees.  When tested and sealed, we are confident that these used manifolds will last as long as a new one will. So far, we have been able to utilize the majority of the used manifolds we have received for porting.

We do not port manifolds which have cracks in other areas of the rubber, or that have a poor external appearance.

Will your manifolds work with larger carburetors?

Yes.

Our manifolds will work with any carburetor that the stock manifold will work with, only better.  We generally recommend our manifolds for use with carburetors up to 42 mm in size.  The Mikuni flatslide will mount right up (if you change your throttle cables).

I have heard that the mounting flanges are not always flat….

This is true.  In fact, they never are.  Yamaha addressed this with the use of an o-ring to serve as gasket.  To optimize this seal (manifold to head), the Road Star Clinic surfaces the mounting flanges.  When we are done, they are flat and will provide the best possible seal with the stock o-rings.

Should I consider using oversized o-rings?

We have found that the stock o-rings provided by Yamaha work very well.  In order to use larger o-rings, it is necessary to remove the o-ring holding tabs in the stock manifold.  This is a good idea, given that the stock manifolds do not have flat (or surfaced) mounting flanges. 

The surfacing we perform on the flanges of our manifolds will reduce or eliminate this problem altogether.  We have not had a problem using the stock o-rings on any of our installations.

Do I need to replace my o-rings, or can I reuse them?

The majority of our customer’s have reused their old o-rings.  However, there have been some failures due to bad o-rings.  We suggest the following if you choose to reuse your o-rings:

Inspect your o-rings for any inconsistency in shape, are they stretched or nicked?  If so, do not reuse them

If your o-rings look good, and fit well into the detent groove on the manifold, take the time to wash them off and remove any contamination present on them.  You can do this in the kitchen sink with dishwashing soap.  Run the o-ring through your fingers, being careful not to stretch them out.  Most of the time, your o-rings will come out looking new.  If they still fit the o-ring detent groove in the manifold, they will most likely be just fine.

For ultimate assurance of a quality seal, have a new set on hand.

How can I optimize the manifold during installation?

The Road Star Clinic’s ported manifolds are already optimized for the average Road Star heads.  However, there is an additional step you can take to optimize your performance, should you desire to do it.  We recommend that this be done by a professional, and can not take responsibility for any mistakes you make during this process.  If you proceed, we assume that you have done this before, know what you are doing, are a professional, and know how to protect your engine from potential damage.  We will not be responsible for any mistakes you make, or damage you do to your engine.  Proceed at your own risk.

After fitting your manifold to the heads, thread all the mounting bolts into the threads a turn or two.  Reach into the mouth of the manifold, and feel the intake ports on the heads with your finger.  In any location where the intake port on the head is smaller than the manifold, light sanding can be used to reduce the difference.  This can be done by placing a small rag into the intake port (to catch filings – make sure it blocks the entire port) and sanding the high edges of the head port with 120 grit sandpaper.  Work slowly and frequently replace the manifold to check your progress.  Taking off more than needed to match low spots on the manifold will not benefit you.  When removing the rags, do so slowly and take care to drag them along the entire internal circumference.  Any missed filings have the potential to get into your cylinder and ruin your engine.

Wash your intake ports with a clean damp rag.    Proceed with installation of your manifold.

How do I get rid of backfire on deceleration?

You should not be having this problem if you have replaced a stock manifold with a ported one.  Backfire is indicative of a lean mix condition, and ported manifolds will richen your mix, not make it leaner.  If you are experiencing backfire after installation of your ported manifold, and you did not change your jetting, you most likely have an air leak.  This is most prevalent at the head mounting flanges, the rubber unions of the manifolds to the mounting flanges, and at the carburetor mounting joint to the manifold.  You can check for this by taking a can of wd-40, and spraying it slowly around each of these suspect areas.  We suggest you start with the head/manifold mounting area, where the o-rings should be sealing the union.  Next try the rubber union on the manifold side of the mounting flanges, where cracks tend to develop on the stock manifolds.  Finally, try the area where the carburetor mounts to the manifold.

With your bike idling on its stand, spray each of these areas slowly, while listening for any changes in idle or smoothness of operation.  If spraying any of these areas causes a change in the idle, you have most likely found a leak.  If the leak is at the union of the manifold and the head, you may not have tightened the manifold enough to seal the o-ring.  You also may need to replace your o-rings if they are used.  If you choose to retighten that side of the manifold, we suggest backing both bolts off, hand tightening, and then alternatively tightening each bolt in ¼ turn increments until tight.

If the change occurs when spraying the infamous “crack” area on the manifold side of the mounting flanges, you may have an air leak caused by a crack in the manifold’s rubber.  While this is rare, it does happen, and your manifold is warranted for this type of problem.  Please contact your retailer to arrange a cross shipment (with deposit), or a return for replacement.  The Road Star Clinic does vacuum test our used manifolds, but the process of mounting the manifold to your bike can cause a new crack to emerge, or an existing one to enlarge.

If your idle is effected by spraying around the manifold/carburetor joint, tighten up your carburetor hose clamp.



Questions should be asked in our forum (Use discuss link below). The forum is very active and you stand a good chance of getting your questions answered there. If you would like to leave feedback for the author, or have additional information you think will benefit others, please use the comment section at the bottom of this page.

Discuss this article on the forums. (4 posts)


DISCLAIMER: This information and procedure is provided as a courtesy and is for informational purposes only.  Neither the publishers nor the authors accept any responsibility for the accuracy, applicability, or suitability of this procedure.  You assume all risks associated with the use of this information.  NEITHER THE PUBLISHERs NOR THE AUTHORs SHALL IN ANY EVENT BE LIABLE FOR ANY DIRECT, INDIRECT, PUNITIVE, SPECIAL, INCIDENTAL, OR CONSEQUENTIAL DAMAGES, OF ANY NATURE ARISING OUT OF OR IN ANY WAY CONNECTED WITH THE USE OR MISUSE OF THIS INFORMATION OR LACK OF INFORMATION.  Any type of modification or service work on your motorcycle should always be performed by a professional mechanic. If performed incorrectly, this procedure may endanger the safety of you and others on your motorcycle and possibly invalidate your manufacturer’s warranty.


Quote this article on your site | Views: 38897

  Be first to comment this article

Only registered users can write comments.
Please login or register.

Powered by AkoComment Tweaked Special Edition v.1.4.6
AkoComment © Copyright 2004 by Arthur Konze - www.mamboportal.com
All right reserved

 
...................................................................... ...................................................................... ...................................................................... ...................................................................... ...................................................................... ...................................................................... ...................................................................... ...................................................................... ...................................................................... -->



The Road Star Clinic is a collaborative community of riders who archive and publish user contributed technical data about Yamaha Road Star motorcycles.

We also sponsor the creation and support of other community websites similar to our own. Inquiries about availability of a website for your community can be submitted to us via any "Contact Us" option on the Clinic.

Copyright 2003-2007 Road Star Clinic and its respective authors. Road Star Clinic is sponsored by the folks at MLSHomeQuest.com.