Size of holes at plenum / dash interface

Mitch Krause

Silver Supporter

Northern climate (Minnesota) so definitely wast to have windshield defrosting available through the demister vent. The question is with the holes that go through the dash from the blower being much closer to the demister vent (through the interface between the dash and plenum) what size of holes in total square inches should one have as compared to the vent holes ? It would seem that maybe those should be kept much smaller because of proximity as welll as being able to throttle the side vents to divert more air to defrosting if necessary? Anyone have thoughts or recommendations?

Randy V

Most of the holes I’ve seen in the dash recess of the demister are roughly a handfull of 1/2” holes. You can increase the flow of air through the demister by closing off the cabin air vents. I had planned on making air valves from a piece of 1.5” PVC (I think thats the size) and fabricating a butterfly to fit inside - much like a throttle body would look.
Additionally, you have your fresh air vents to work out - but that’s another battle...
Welcome to the fun, but sometimes tedious job of building a car...
Reality....the stock HVAC system provided with the car does not push enough air to make it effective, even at the high setting. That squirrel cage fan also makes a racket at that setting.

Consider using high-speed computer server fans (12v) as boost fans that will fit inside your duct hose to give you the cfm air flow you need. I use the single and dual-stage brushless fans and flow very well. They can handle a 100% duty cycle and operate at low-amperage (1-3amps each).
Last edited:
Apologies if this repeats - just had a glitch!
Be careful and carefully check your windscreen profile if you go the heated screen route. I got the heated screen for my RCR Mk1 from Ricky Evans Motorsport here in UK (Heated Windscreens | Ricky Evans Motorsport). They carefully worked through the variations and we ended up with a good fit in the aperture. The variations are not great but enough to make a decent fit impossible so it's important to get it right with a series of accurate measurements.
Roger Allen