To Snorkel or Not to Snorkel

by Brian Law

vfr

This article was written by Brian Law and it was originally on his site. Brian is very active on the VFR List and also very knowledgeable about the technical side of bikes IMHO. Anyway, Brain has graciously allowed me to list his article on this site. This is a very interesting and well written article and it has answered one of those "forever VFR questions" for me.

Many Riders often ask about the benefits of removing the snorkel on their VFR. I can remember all kinds of arguments on both sides of the debate, but none offered the thoroughness of Brian's report. While I don't have the graphs, here is what he has to say about this topic.

Introduction

I did have a web page up for a while briefly detailing why and what I chose to do when installing the TBR slip on. The truth of the matter (snorkel wise) is there is no definitive answer. Removing it can help increase power but it *does* have some negative effects. The general tune of the bike also greatly effects the results. Altitude is also a factor.

In a nutshell ...
Stock exhaust - leave it in place,
Slip-on is a 50/50 decision - remove it if you are confident you can correctly re-jet otherwise leave it in
or if you have a full system - remove it and re-jet.

The Technical Presentation

A '94 VFR was used to compare the stock exhaust and air filter to a K&N and a Two Brothers CM slip-on without any carburetor changes. All runs are on a Dynojet model 150 and the results are SAE corrected.

The VFR was in for service at 35,557 km.

Before and After Tune-Up

The stock air filter was interchanged with a K&N and more air/less air runs were done. More air is achieved by using an air box lid with a larger than stock opening (leaner). Restricting the air intake-less air (richer). Then the intake snorkel was removed and all combinations were tested again. Fourth gear and all gear roll ons were done for every combination (17 runs).

With the stock exhaust the K&N made virtually no difference. The best gain was made by removing the intake snorkel with a stock air filter. The stock exhaust seems too restrictive to realize any significant gains.

A Two Brothers slip-on was installed and all the combinations were tried again (16 runs). This time with a less restrictive exhaust, the best combination was the K&N and a larger than stock intake. The main jets had previously been a little rich.

We started with a bike rich on top, lean on the bottom and mid-range. Adding the slip-on, K&N, and enlarging the air intake hole has made the bottom end and mid-range worse. There are two basic solutions for this.

  1. Shim the stock needles and open the air screws
  2. Install a jet kit
I left the stock main jets in place. The slide lift holes were drilled using the Dynojet supplied drill bit. Stock springs and needles (clip in the 4th groove) were also replaced by Dynojet ones. The air screws were set at three turns out.

Findings

  1. Except for a small dip below stock at 4000 rpm both the horsepower and torque graphs show a nice gain over stock.
  2. The bike starts well hot or cold and accelerates strongly in all gears.
  3. The most noticeable difference is improved throttle response.

I hope you have enjoyed this article. I highly recommend Brian's site and it is certainly worth a visit.

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