Jump to content


Question for those with individual air filters on round tops


Recommended Posts

I bought a set of K&N filters to install on my round tops. I know I’ll see no performance gain or gain any credits amongst the purists but I just kinda like the way they look, along with the valve cover breather. I’m keeping all the original parts in a box when/if I want them in the future. 
My question is what do I do with the float bowl overflow lines that are routed into the factory breather box?


Link to comment
Share on other sites

Well that’s an easy solution!  Why didn’t I think of that...  sometimes I tend to overthink things. Thanks @Terrapin Z

what about the line that comes from the device, I can’t recall the name off the top of my head, fuel guide maybe?  Not sure of its purpose but it also is connected to the back of the breather box. Do you know what it’s purpose is and an alternate way to connect it?

Link to comment
Share on other sites

I just run a separate small k&N off the valve cover breather. I suppose if you have a lot of pressure in the engine it could get messy. Mine does not. 

It is quite a bit bigger than the carb overflows. I am not sure if there is enough room to stuff it in the back of the K&N plates. 

My carb overflows circle around under the carbs, back to the k&N's


  • Like 1
Link to comment
Share on other sites

How about some of these to keep the “look” clean!?



I vexed over this one for many years as I too like the look and sound. However I discounted it on the basis of heat and performance loss due to sucking in hot air. Do yourself a favour and improve the heat shielding while you are at it.

If your state allows it, pipe the crank case vent to a catch tank! That way you aren’t contaminating your intake charge or carbs with the crud / sulphurous juices that collect in mine!

Also, not putting you off, this is a great read for info. You may wish to duct some cool air to them - I’m going to do the same experiment on mine once I get my arm out of the sling (quite a dramatic fall off the bicycle on the way to my Covid jab! Oh the irony LOL ).

“Recently I have been measuring under bonnet temperatures. My car is a 1985 V12 coupe with the cruise bellows removed, good aircon which is on all the time, and with the cooling system in top condition. Additionally it has a slot in the front bumper to admit air straight into the radiator stack, with the horns also moved out of the way. I have NO foam round the radiator and a separate transmission cooler. Therefore the airflow into my engine bay is far better than standard.

I have recently purchased a digital measuring device that runs two channels to a unit in the car. One sensor is strapped to the rear of the engine just aft of the capstan, one to the front, just behind the aircon compressor, both are suspended in the air about 2 inches below the bonnet.

These are the results so far at ambient 27 degrees C, aircon on, 12 inch aftermarket electric auxiliary fan on (replacing OEM auxiliary fan). Water temps remained stable and well below 90 C at all times in these tests:
On the move above 40 MPH the front temp is about 56 C; the rear is about 52 C
On the move below 40 MPH front temp rises to about 60 C and rear to about 56 C
Once stopped, engine off, bonnet closed, both temps rise to 80 to 82 C immediately. These temperatures do NOT fall significantly after ONE HOUR if the bonnet stays closed. An open bonnet immediately reduces air temps to about 50 C. Even with the bonnet left open for an hour, air temps at the sensor are still 46 C
On the move again temperatures drop to under 60 within a mile or two. If the main electric fan is manually switched on (it does not trigger on my car until a 90 C water pump inlet temp), temperatures drop slightly faster.
At rest, aircon on, engine on, auxiliary and main electric fan on: At rest after a run, simulating a traffic hold up, front air temp climbs very fast to 70 - 72 C. Rear 4 C lower. Water temps remain stable, possibly dropping a touch. The extra air temp I attribute to the main fan pulling large volumes of air through the rad to keep the water temps stable, and thus extracting substantially more heat from the coolant, while air is not being extracted from the engine bay as well as it is when on the move. In fact, at rest air temps climb when the main fan is activated, though water temps fall.
The front of the engine bay is normally hotter than the rear.
The air temperatures under the bonnet on the move are, at about 60 C, not excessive. Therefore louvres are not necessary to preserve reasonable on the move under bonnet temperatures
Cold air intakes are definitely worth it as they reduce intake temps to ambient (ie 33 C lower than under bonnet air temps).
At rest, engine off, under bonnet temps rise very fast to a high, loom-cooking and rubber component-cooking level (80 to 83 C).
Opening the bonnet after you stop WILL definitely substantially help preserve the wiring loom and all rubber components from heat damage.
Louvres would greatly reduce under bonnet temps once the vehicle is stopped.
I will repeat these tests when (if) we get a day with ambient temps over 30 C.

I attached temperature strips to the oil sump, oil filter, oil cooler in, oil cooler out pipes and A bank cam cover at the front top. Oil temps showed a maximum of 85 C throughout the oil system, including the oil cooler inlet, except for the oil cooler outlet which was 77 C. My car has the bypass oil cooler system. From these temps I conclude that the bypass system does indeed cool the oil efficiently and that there is plenty of flow though it - if not the oil temps would not be the same as the sump and oil filter.
The can cover temperature maximum showed 95C. All these temperatures are well within the oil's correct working temperature range (Shell Helix 100% synthetic 5W 40).”

  • Like 2
Link to comment
Share on other sites

I didn't like the idea of running hoses from the float bowl vent to the air filters. As long as it's just fumes coming out that's fine but in some cases fuel can come out. Especially bad if you have headers. So I ran some clear yellow Tygon fuel line from both vents to the nozzles of couple of Sriracha bottles taped together where the windshield wiper fluid used to be. The caps with the nozzles need to be loose on the bottles or it won't work. So if there's overflow while driving or adjusting the floats no big deal. I've never gotten more than about 1/2 ounce of gas in the sriracha bottles.

If you car has been sitting awhile like mine was until this week it makes it easy to prime the bowls. It's way easier to pour a little gas down the Tygon lines than it is to try to get it down the vents.

For the cam cover vent I previously ran a hose from it with the open end between the K&N's. Using an ITB now. Haven't decided about that vent. Hope no bugs go in.

Link to comment
Share on other sites

Create an account or sign in to comment

You need to be a member in order to leave a comment

Create an account

Sign up for a new account in our community. It's easy!

Register a new account

Sign in

Already have an account? Sign in here.

Sign In Now
  • Create New...

Important Information

By using this site, you agree to our Privacy Policy and Guidelines. We have placed cookies on your device to help make this website better. You can adjust your cookie settings, otherwise we'll assume you're okay to continue.