Carb Ducts and Filters

[sagero]

I’m considering building a ducted airbox for the carb on my Vee. A friend who offered to help with design first asked about filters.
I mentioned that many Vees don’t run an air filter, just a screen over a velocity duct.
So I have a 2 questions:
Does anyone have real info about how restrictive a K&N Filter like this is? Universal Clamp-On Air Filter (knfilters.com)
How risky is it to run without a filter in environments like road courses in the northeast?

[FVRacer21]

You want a filter more like this:

https://www.knfilters.com/ra-0510-un...-on-air-filter

A filter "may" restrict HP, but other things like exhaust and intake manifold might have a larger effect.

The theory is you want no restrictions above the top of the carb.

If you stay on the track, probably no issues, but if you go off at Thompson or New Hampshire, you run the risk of stirring up some dirt and debris.

That said, the majority of drivers run velocity stack with a screen door (metal) screen. Just taking the bodywork off it is easy to have something drop down the carb!

When I have an engine rebuilt I hope to do some testing and this is one thing we need to quantify, unless someone already has the numbers.

ChrisZ

[sagero]

Thanks Chris.

I suppose that filter should benefit with more airspace available directly above the carb.
My intake and exhaust are both good parts from respected suppliers so I'm pretty sure I don't have any low hanging fruit there.

I'm planning to build a ducted airbox and have some ideas. The challenges I see are figuring how to provide enough air with the least amount of drag and having the box intersect with a duct on a removable engine cover.

Any design suggestions are welcome.

[Ed Womer]

You should have at least a fine screen over the opening to keep out debris. Just remember that your opening should be at least three times the size of the carb inlet so as not to restrict the air flow. I would not attach a screen to the top of the carb or velocity stack it could cause flow interference so keep it away from the edges.

Some say a box around the whole carb is the best but then you have openings for throttle and fuel lines which makes it harder to make. I did this but I don't think it was worth the hassle so stopped doing it that way. I just know have a very large opening on the scoop that is part of the engine cover and a plate that fits against the underside of the engine cover and seals up pretty good. I also feed the oil cooler from the scoop so I have a baffle separating the incoming air and split it between the two areas.

It is fun doing it but I think worth the time and once done it is better that way. The screen is attached to the inside of the opening into the scoop and when the engine cover is off everything is visible and mostly accessible.

Just keep in mind a decent size object down the carb could ruin your session and lead to a rebuild or serious engine issue.

Ed

[Steve Davis]

Racing without an air filter is just fine (within limits), but your engine REFRESH times will be coming sooner than they should. Even on 'clean' racetracks, there are MANY types of debris that can enter the carb.. probably the largest is 'tire bits'. I find them in my cockpit constantly .. from both my own tires and those of others.
Trying to do a 'ram air process' for an FV carb is a wasted effort.. Trust me.. I KNOW. I wasted quite a number of weekends back in my early years trying to make it all work. It changes the carb jetting requirements if you raise the air pressure at the inlet of the carb.. and there is really no way to compensate for it. If we had FUEL INJECTED carbs with ECU's.. maybe .. but we don't. Personally, my experience has shown that making sure you have ADEQUATE - FILTERED air going into the carb is your best bet for long term success. Ed and many other builders.. including Vortech (my current car) kinda 'sealed' the airbox around the carb... but it's just 'covered', and not really SEALED. I had a bottom rubber cover that 'sealed' (i.e.BLOCKED OFF) the area underneath the carb from the engine area. After a year or so fighting excess HEAT buildup in that engine area (making it near impossible to check the oil hot coming off the track - not to mention bubbling the surface of the bodywork), I decided to remove that rubber block off and it dropped the 'under engine cowl' temps by at least 50 degrees. All you NEED to the carb is clean, fresh air in ADEQUATE quantities to feed the engine at it's highest RPM - any more than that is a waste of time and effort.

Focus on making your car HANDLE (and learning the best way to get the most out of what you have) and having reasonable HP in the engine. If you build your own engine, that's fine to get started, but best is to PAY an 'expert' to take care of that aspect of your program. They've been doing it a LOT longer than newbies and KNOW what it takes to make an engine fast (reasonably) and reliable (WAY more important for a newbie).

There are no SILVER BULLETS (other than that bodywork ) that make cars FAST. It is a combination of a good driver in a 'good' car, with a good setup, with a good engine, on good tires.. and a few more 'less tangible' things.
FOCUS on the cockpit - that's where you can get the most gains in the shortest time period. At the track, seek knowledgeable guidance from some of the fast guys. Most are more than willing to help you improve your game. If the opportunity comes around (a test day, perhaps) let a fast guy drive your car and give you some feedback. They can probably give you some good pointers just from watching YOU drive and walking around your car and looking at it.

I'm offering this based on a guess that you have 'not much' history in FV.. if I'm wrong, just ignore my suggestions - I'm just trying to save you some TIME getting faster on the track.

Steve, FV80

[problemchild]

If the opportunity comes around (a test day, perhaps) let a fast guy drive your car and give you some feedback. They can probably give you some good pointers just from watching YOU drive and walking around your car and looking at it.

I'm offering this based on a guess that you have 'not much' history in FV.. if I'm wrong, just ignore my suggestions - I'm just trying to save you some TIME getting faster on the track.

Steve, FV80

Tim is a "fast guy". Very well prepped car with nice pieces on it.

Questions like this should be asked of your engine builder. No question that no filter gives the best performance, but it comes at a price. At worst, run a proper filter on test days and practice, then without it when it matters to you. Always have some type of baffle or screen to keep the pebbles and marbles out.

[Hardingfv32]

PM me with your Email and I can provide tech articles and advice on how to do it properly. It takes a lot of work and the results are not earth shaking.

One hint: No such thing as ram effect when it comes to carb intakes.

Brian

[sagero]

Tim is a "fast guy". Very well prepped car with nice pieces on it.

Questions like this should be asked of your engine builder. No question that no filter gives the best performance, but it comes at a price. At worst, run a proper filter on test days and practice, then without it when it matters to you. Always have some type of baffle or screen to keep the pebbles and marbles out.

Thanks for the praise and suggestions Greg. Both you and Ed have helped me a lot!