Graves Garage-Why use Smog Block off Plates? What do they do?
This seems to be a popular set of questions over here at Graves Motorsports. Most people tell me “they don’t make any horsepower so why should I spend the $35 and take the time to install them”.
Here is a quick but complete explanation on the benefits of using the pair valve block off plates on just about every model motorcycle.
*This is for off road racing applications and not meant to be used on the street.
If you have something to add please feel free.
From the factory most motorcycles will come with a pair valve system on them. This is basically an air pump that pulls fresh air from your air box and through a bunch of hoses and fittings it pumps that fresh air into your exhaust ports on your cylinder head and sends it through your exhaust
There is unburned fuel that makes it through the combustion stroke, exits the cylinder head, and makes its way down the exhaust system to finally exit into the atmosphere. The pair valve system adds fresh O2 to the exhaust so that as the unburned fuel come into contact with the extremely hot catalytic converter, the additional fresh air will cool the exhaust and aerate the fuel for a more efficient burn. This equals less emissions being released. Plus the additional O2 in the exhaust the lower the emissions reading. I have heard a number of reasons this is done but I will stick with the basic theories that I am aware of.
(1) This requires a catalytic converter to really complete its job. Once you have installed a full race system and have removed the cat, all this fresh air will do it
create the popping and back firing that throws those incredibly cool but damaging flames out of your exhaust .Great for night riding, but terrible for tail sections and leg hairs. To eliminate this add the smog block off plates.
(2) If you decide to have your bike dyno tuned then you cannot run the pair valve system because as it adds oxygen to the exhaust. The dyno “sniffer” is picking up false readings from the additional O2 added after combustion. When it reads the extra O2 it thinks the bike is running lean and will compensate by adding fuel to the fuel map. Now you have a map that is much fat (too much fuel) and the bike will run terrible. The person that is tuning the bike will have to pinch these off every time you dyno the bike. To eliminate this add the smog block off plates.
(3) The tubes, pump, and wires that are included in the pair valve smog system add unnecessary clutter and weight to the motorcycle. To eliminate this add the smog block off plates.
(4) This is the last reason I have found to run the plates but it is NOT A FACT. It seems only I have noticed this and nobody has proven it right or wrong so do not take this as absolute. It could very well be just by brain acting up.
I believe I felt it help my engine breaking a little. Before I installed them I felt like the bike (08 R6) had a really strong engine breaking and would pop and back fire a lot on
de-acceleration. When I installed the plates I would swear that the engine breaking became more suitable. It was not as abrupt and not nearly as strong. Again this was only a guess and has no proof to it, however I did talk amongst a few tech savvy buddies and we worked out a somewhat scientific theory. Possibly as the hot engine gases are pouring smoothly out of the cylinder head like melted butter this cold fresh oxygen is forced into the exhaust ports. This could very well slow those gases down and create more back pressure which in theory could possibly cause stronger engine breaking. So…..maybe Im not crazy.