Intake and Air Filter Modifications for the Spree

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Air Filter and Intake mods are an inexpensive way to start performance modifications for the Spree.


Air Filter Modifications

The following is a summary of my experiments with pod filters on a 1985 Honda Spree.

I played around with a K&N cone filter (RC-2540, special order at any automotive shop) and my little pile of jets. The #65 with the stock airbox still gives me the best performance, around 33 MPH. With a #68 jet and the stock airbox in place, it would top out at 30-31; it was actually slower with the larger jet.

The bottom filter in the picture below is the oiled K&N RC-2540; it has no internal velocity stack (aka bell mouth.) The K&N filter, with a #78 jet, would barely get going. I couldn't get it above 20, it just made a loud "BOGGGG" sound when cracked open. The K&N filter with a #68 jet, wouldn't go over 10. At WOT it would literally kill the engine. All this is evidence of the need for a larger main jet required for this setup. The K&N filter looks cool though. Chrome cap. Shame.

I built a custom cone filter using a stock Spree carb-to-airbox duct, so that it had the bellmouth end and correct length. This filter is made from a baby food jar lid, part of a yogurt container for the cap, the unoiled paper element from a cylindrical lawn-mower filter, and a bunch of latex caulk as sealant. I had no intention of running this for long, just to see if it would work. You wouldn't oil this type of filter; it wouldn't breath at all. This worked fine, but was no improvement. I think that I actually need to make a shorter bellmouth, then stick the cone filter over that. A shorter length bell should complement the higher RPM that the engine runs at with a new pipe, like an expansion chamber.

I used this same K&N pod filter, later, on a 1987 SE50 with no issues.

Bottom line is that Spree are sensitive to changes in the intake tract. And, of course, any change in intake filters should be accompanied by a review of the bike's jetting

Spree Pod Filters.jpg

Intake Tuning

The intake has a "tuned" length just like an exhaust. If you put the pod filter on an extension of the same length as the intake tract, it works just fine.

My takeaway was that upjetting for a short pod filter is only compensating for a mis-designed intake system, and that it's a form of inefficiency. The stock intake seems to be tuned to about 4000 RPM. Shortening it up raises the tuned RPM, but let's face it, how much higher RPM does one want to set the peak intake at for a street bike?

Here's a simple formula: 42000/RPM = length in inches. Length from reeds to intake mouth. For 4000 RPM, that's 10.5" Guys at mopedarmy were discussing this; this equation in similar form is in at least one internal combustion engineering books I've read so I think it's right: ... 57/105532/

For 4-stroke, it's double the above (2 rotations per intake breath.)

Then you can argue about whether the carb works better in the center of the tract or off-center, and how much off center. More centered = more tuned to one RPM, as I recall, but don't quote me on that.

It's the length mainly; cross-section is more of a sizing thing, and then you can get into the dynamics of the airbox, etc. If you really want to get into it, look up "Helmholtz resonator" online. Here's a taste. Empirical work is really better for this kind of stuff.

Author: noiseguy

Intake Tuning - Filter restriction

Along with the prior intake tuning theoretical section, members have found restricting airflow input to the filter itself can improve performance. A good example is the thread at this link

In that thread, a filter restricted such as this performed better than without restriction. Please note the black electrical tape used to mask off free area of the filter.

Spree Aftermarket Filter restriction.jpg]

Author: mousewheels

Credit for mods: TrafficJamz

Elite 50 (SB50) Intake and Reeds Mod

Thanks to a tip, found a good bolt-up mod for the Spree.

Turns out the SB50 intake and reed valves are MUCH larger flow than the NQ50 (Spree) stock parts. The SB50 intake has a better transition at the carb end, and a larger opening at the reeds. The SB50 reeds are also larger cross-section. Best thing is that that part will bolt up to a stock Spree, and have been reported to help top end. The SB50 intake is painted red in the following pictures.

SB50 IntakeMod 1.jpg

Note the larger transition at the carb. If anyone was wondering how the intake mod should look, this is it.

SB50 IntakeMod 2.jpg

Here's the Spree for reference. You can see the choke point here. Note that the Spree intake interface is 17mm diam, whereas the SB50's is 18-19mm (oval shaped). The Spree decoupler will work in this arrangement, a new SB50 decoupler is only $4 from Honda, and shows available:

SB50 IntakeMod 4.jpg

Here's a top view of the reeds. Note that the insert to case is identical:

SB50 IntakeMod 5.jpg

Only problem I see is that there's no oil port for the Spree's oiler system. Here are your options: - You could oil through the vaccum port, and convert the petcock to always flowing. Many have done accidentally by breaking the diaphram with compressed air; this requires you to cap off the lower vacuum connection on the petcock. - You could also run pre-mix, removing the teeth from the oil pump. - You can run down a SB50 carb; having looked at these they don't appear to be any different from the Spree, except for the oil injection tube at the manifold. - You could add an oil injection port to the manifold and press it in ala the Spree manifold.

Spree Intake Port 1.jpg

Author: noiseguy

Porting the Intake


Spree Intake Port 2.jpg

Spree Intake Port 4.jpg

After about 20 minutes with the Dremal/die grinder:

Spree Intake Port 3.jpg

Spree Intake Port 5.jpg

Did speed runs and found something interesting. This is with the exhaust open and a 68 main jet. A 70 main is too rich. The main part of the curve is about the same but the top end it higher. After it hit the 27 top end like last time the speed kept creeping up very slowly. Ended up just over 30 at about 9500 rpm. It will do 30 but the top end isn't real useful. I'd really like to see more area under the curve. That's what will allow it to push a higher drive ratio.

Spree Intake Port Performance Graph.jpg

Author: keithw

Porting the Intake II - Porting Top & Base

A Spree intake may be ported at the bottom too. If converting from stock Spree reed valve to the SB50's reeds, you can modify your intake instead of tracking down an SB50 intake. See Elite 50 (SB50) Intake and Reeds Mod above for a comparitive photo of the intake and reeds.

Stock NQ50 intake - bottom

SpreeIntakeStock Bottom.jpg

Intake after porting

SpreeIntakeAfterPorting Bottom.jpg

Stock NQ50 intake - top

SpreeIntakeStock Top.jpg

Top after porting

SpreeIntakeAfterPorting Top.jpg

Stock Nq50 intake - top photo 2

SpreeIntakeStock Top2.jpg

Top after porting

SpreeIntakeAfterPorting Top3.jpg

Bottom view of porting detail

SpreeIntakeAfterPorting Bottom2.jpg

Note: There is discussion in the full thread over whether porting is wider than needed at the bottom.

[Full Thread]

Author: Spreetard


Authors: noiseguy, keithw, Spreetard

Editor: mousewheels

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