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If you have a smaller engine (<1915 cc), you can run the stock diameter OEM heater boxes - a bilge blower assist helps flow.  A larger engine will take larger diameter heat exchangers which are costly.  Another alternative is to add a reconditioned or new gas heater - it provides heat that you'll have to turn off.  They are more costly.  In Ohio I'd think heater boxes would extend driving time and be more comfortable than NO heater.

Eberspächer was one of the original suppliers of gas heater for VW and Porsche.  They made them for bugs (BN2), Bus/Campers (BN4) and the 412 (BA4?).  They are available used but parts are getting hard to get.  Maybe $400.  New ones are also available for around $1-1.2k.

Image result for eberspacher VW gas heater bn2

Last edited by WOLFGANG

Depends on where you live and how much you would like to drive in cooler weather. On CMC's they routed the heater box flow through the L & R side 2 x4 box frame, issue with that is the heat passes through a lot of cold steel surface and by the time it exits by your feet of " defrost" vents it's just warm at best.  I have done heat routing through the vertical door jambs (roasts your elbows) Also through the vertical panel below the rear seat worked very well but no defrost (like it matters) and even did a 12v high amp forklift cab electric heater using a higher amp alternator and a deep cell golf cart Aux. battery (this worked but wasn't cheap to do and I have no longterm info as I sold that project.

The gas fired jobbies are a challenge to install, but it has been done to good effect.  Other Folks here can tell you all about how that goes.  And they will air fry your tootsies, or other parts too close to the source.  I have heater boxes on my 2332/Sidewinder lash up, and they work -- not great, but they work.  Warm to slightly hot air comes out.  Takes the chill off.  Trouble is, side curtains do not allow much of that warm air to stay around.  If you really drive in cold weather and want to be warm, then do three other things: dress appropriately, like you were going to go skiing; carry a blanket; and install heated seats.  I have heated seats and several really warm jackets, and gloves.

PS, about the standard way the heated air is routed to the front of the car, which is as mentioned, via steel tubes along the outside of the car.  Lots of time for some serious heat transfer loss.  I have taken rigid Styrofoam and made a series of slabs that cover that tube with a nice large 1/4 round radius on the outer surface.  Sprayed with undercoating, this insulates those pipes to a fair degree, upping the discharge temp at your feet and defroster.  hahaha, defroster.  Great idea, leaving a lot to be desired. .  I have the ducting at the base of the windshield (not all do) but the advice about Rain-X defogger is right on point. 

In my previous life I had good experience with heater boxes on air cooled VW beetles but that was after making sure that everything was airtight.  On my speedster I have heated seats and the bigger (1.5”) heater boxes on my 2017cc engine.   So far just using the engine blower, though you can add an electric fan from a Porsche 914 or a later VW Bus for more air flow at idle.  After listening to the experience on this site, I will likely skip the flow through the rocker panels and just have the heat dump into the passenger compartment behind the seats to minimize heat loss.  The rain-x product for the windshield sounds like a good solution.  

I ordered my VMC speedster with heated seats and heater boxes.  Heated seats are wonderful and really help when its cold, however only a relative small portion of your body is in contact with the seats, so you still need to dress appropriately.  The heater boxes work, but don't put out much until things heat up in the back, and even then I would not describe the output at your feet as hot.  Warm yes, not hot.  And if the top is down, the warm air blowing at your lower legs does help a little (and when I've driven in temps around 50F I was glad to have even a little extra warmth), but it does quickly exit the cabin.  I've not tried the heaters with the top up, because why drive with the top up.


OK, no one's mentioned this twist yet, so here goes.

The old Vintage Speedsters came up with a slightly better mousetrap for heat ducting. They routed the heat from the heater boxes into flexible 3" plastic hose that ran under the car, but just inside the rocker panels, and connected that to the usual vents in the front footwells. (They also connected small ducts up to the defroster vents, but you could probably skip that.)

This provides pretty good, uh, 'heat'. If you go to the trouble of putting the top up and side curtains in, it almost feels like a modern car inside — after a while.

With the top down, though, not so much.

Only you can decide if a little heat with the top down is worth bothering with. In the 50's and 40's, it might mean the difference between going out or not. Or, it might mean the difference between driving with a passenger or alone.

Here in sunny California, with (in most years) mild winters, it has greatly extended the usability of our car, and I wouldn't be without it. Ironically, we may use it most in summer. Then, to beat the mid-day heat, we often start a long trip in the wee hours, when it can be chillier than you'd guess for the first hour or so.

What will call for larger than standard heater boxes is the diameter of the exhaust headers, and most engines larger than stock will require that. Decent quality one-and-a-half inch boxes are not too exorbitant. Larger than that, and look out!


@WOLFGANG posted:

@pkdfw A convertible D vs Speedster - with taller top, windshield, windup (power) windows pretty much solves cold/water leakage issues.  Many (IM) have multi-layer Haartz fabric tops too.  You're adding weight and costs. Side curtains are difficult to seal.


I saw the Cruzing windows, but does anyone know who can make taller windows that can help seal up farther to the convertible top?

Many of us did that ourselves as each car is a little different.  Heck, My first car was a little different from one side to the other.  Making a standard set that'll fit every car is not possible.  That's probably one reason @aircooled made sure to market his as Cruzin' windows.

Doing it yourself isn't that had and plenty of us can provide guidance, aka blatant opinions.

The main problem with air cooled cars and heat production, is that the amount of heated air coming into the cabin is solely dependent upon the engine speed.  The higher the revs, the more the engine pushes heat forward to keep you somewhat warm.

So, it should be true that smaller engines that need to rev higher to get, and keep, up to speed, should produce more cabin heat.  At least, that would be the theory.  But we all know it doesn't seem to work that way.

In my case, it is the opposite.  Higher displacement engines usually run a little more slowly, especially at cruising speeds; therefore, less 'hot' air is sent into the cabin.

And that's why I wish I would have had a fan driven heater system built into my car.

For some reason, I prefer being warm these days.

Rather than making the windows taller, you can remove the top, take it to an auto top/upholstery place and have them run a stitch line parallel to the existing one above the window, but one inch closer to the window.   Works like magic.

If you know someone who is a Quilter with HD or leather/vinyl needles, they could do it, too.

And @Bob: IM S6 you sound like a candidate for an Espar gas heater!  Nice and toasty inside - Leave your coat at home!

Last edited by Gordon Nichols
@pkdfw posted:


I saw the Cruzing windows, but does anyone know who can make taller windows that can help seal up farther to the convertible top?

As Stan pointed out, I make CruZin Windows.

Over the years, I have been asked many times to make full size clear plexiglass windows and I've tried.  Unfortunately, I can't make a one size fits all version because of the slight differences between all the different Speedster manufacturers over the years.

However, I am now offering a DIY hardware only kit.  It includes everything except the plexiglass, which is actually the least expensive part of the windows and readily available anywhere.  Here's the link.

Last edited by Troy Sloan
@DannyP posted:

Nah, he just needs a booster fan. He has heat exchangers and a 6 cylinder, and that puts out plenty of heat.

Yes, it's great for heat in the summer.  Not so great in cold weather, and forget about heat if it's raining heavily.  The engine temp drops right down in any good rain, and can't even get up to 180 degrees, which is when the oil thermostat opens up for the oil coolers.  Rain for some reason cools this engine off so much.

I've been looking into some sort of booster fan arrangement, as you say.

The seal of the side curtains around the windshield posts are the hardest to seal.  When you hit 60 mph even a good tight fit is threatened.  Maybe pool noodles would help?

I went to college in central VA from home in NJ.  I still remember the Christmas breaks - had a friend with a TR3 and another with a Morgan 4.  Both had side curtains.  On trips north, we'd tape them in with duct tape.  They claimed the only heat was from the transmission.  We'd generlly have snow the whole way. I had an '67 Opel Kadette Rallye - so no problem with heat (it had a 1.1L engine with dual Solex carbs - think it put out 67 hp!) I'd draft big trucks to save gas - seems 36 cents a gallon was big $ back then.

Last edited by WOLFGANG

"The seal of the side curtains around the windshield posts are the hardest to seal.  When you hit 60 mph even a good tight fit is threatened.  Maybe pool noodles would help?"

I wound up putting strap on the window that snapped to the top header.  It solved the problem nicely, but you have to remember to unsnap them before opening the door.  They also come in handy as a way top pull the top of the window in as you shut the door.  By the time I was done, the only remaining leak was the bottom of the windshield post.  Damn near impossible to stop some intrusion there without fixing something permanently to the windshield.

Last edited by Lane Anderson


@Bob: IM S6 posted:


...I've been looking into some sort of booster fan arrangement, as you say...

Bob, suppose you already know about the fans for venting marine bilges? Designed for 'in-line' installation in existing duct hoses (usually 3" or 4" diam), water resistant, about the right flow-rate for this application, low power draw, and 12V.

Search Amazon or West Marine for 'bilge blower'.


The bilge fans that Mitch mentioned work pretty well.  Also, be sure your heat exchanger hoses are connected directly to the hoses that run under the rocker panels to the interior vents and NOT routed through the box steel body frame.  Kirk routed them through the frame for many years until he realize that the frame was not large enough to handle the large volume of air that the fan is pumping out.  That alone will make a big difference in the amount of warm air delivered to your interior.   Here's a video I did 12 years ago.

@MusbJim posted:

From one that follows the beat of a different drum...

1. Install a fused Auxiliary Power take-off (many configurations available).

12v Aux

2. Purchase a 12v Car blanket. Typically $15-$20 through Amazon, WalMart.

12v Blanket

Easy andInexpensive  way for staying warm Top-up or Top-down = Done!

That was one of the first "improvements" I made to my car so nancy wouldn't get too chilly.

@Gotno356 posted:

About the bilge fan. I’m intrigued. But would the fan cool the air too much before it gets to me. Not completely but alittle too much. I had an oil cooler installed and have notice how long it takes to get some really decent heat in the car. It take a bit to warm the engine up now. Not complaining, because if love the lower temps in the summer.

An Atwell bilge fan moves the cu. in. of air in high volume it really doesn't cool the existing air temp it's moving. I had two Atwell's in a Bradley GT many years back and kept me warm even in the winter.

Last edited by Alan Merklin

I mentioned this on another thread. If you take the time to seal you cabin the Vintage Mod is cheap and easy to do. Bypass the frame and put ducts from the heater boxes directly to the tubes under the frame. It increase heat and air glow significantly. I also fitted the OEM flaps for faster warm-ups. If you have an external oil cooler you can add a gateway that blocks flow to the cooler in temps lower than 80c. On my 1915cc it still never gets warm enough to heat the cabin if the ambient temp is below 35f. As far as gas heaters, I had a type 2 bus I restored many years ago and because l camped in the NW I added a BA 6 heater. That is the hottest heater ever made. With the timer you could set it to fire up 15 minutes before you needed to use the bus and it was tropical in there. I have seen some fantastic gas heater installs on this site over the years. I may still go that way at some point.


Just to be clear, the time it takes for the oil to come up to full operating temperature doesn't affect how long it takes for the cabin heat to start working at its maximum.

If anything, an exhaust-driven heater box starts putting out almost maximum heat sooner than a liquid coolant-based system. Exhaust gas is pretty much the same temp no matter what the engine oil temp is. And the thin sheet metal in the header and heater box heat up pretty quick, although better heater boxes have somewhat heavier internal vanes.

I don't have any experience with forced-air systems in our cars, but in a modern car, turning up the ventilation fan doesn't seem to make air from the vent get any cooler.


Bob's problem is the engine fan doesn't move enough air at low speeds to give meaningful heat. I can assure you, if you touched his heat exchangers you will burn your hand. Bob just needs a little air flow assistance.

And about those oil coolers: I would never install a remote oil cooler without an oil thermostat(like the one Wolfgang put up). My engine doesn't have a cooler in the factory location. I only have a remote cooler, and it is thermostat-controlled at 190 F. I also have a thermostatically-controlled fan on the cooler, the sensor is attached directly to the cooler, and operates at 190 F as well.

I have a Webasto Airtop 2000 gasoline heater hooked to two under-dash outlets and two defroster vents(I built a flap to turn defrost on or off). The Webasto was several hundred dollars less than a modern Eberspacher/Espar unit. It has been 100% reliable and trouble-free for 6 years so far, and gives me 8000 btU/hr. and burns a few ounces of gas each hour it's running.

My wife and I also installed the electric seat heaters and they work darn well too. Comfortable top-down to about 45 degrees(and 55 mph) with both heaters on. If it's colder and/or higher speed you're gonna need to defrost when you get to your destination.

Keep in mind I drive a topless Spyder. There really isn't much room for heat exchangers in the exhaust area. I've seen it done, but it's not for me. The whole heating unit and hoses is way under 20 pounds, the unit itself weighs 12 pounds.

Last edited by DannyP
@WOLFGANG posted:

@Gotno356 You can add an oil cooler thermostat to bypass the remote oil cooler until oil gets hot.  It opens when oil gets to 180 degrees (Empi one).

Image result for VW Bug Oil Cooler Thermostat. Size: 174 x 185. Source:

As I understand it, this bypass does not open when the oil gets to 180 degrees, it closes.

The oil is always allowed to travel to the cooler, but when it's cold, it's also open for oil to do a U turn and return to the engine. Most of the oil is going to choose to make the U turn and return to the engine, because this is the path of least resistance. Allowing some oil to go to the cooler, will allow all of the oil to slowly come up to temperature. When the oil gets to 180 degrees, the bypass will close off the shortcut and force all of the oil to go through the cooler. This prevents the shock of cold oil all of a sudden being forced into the mix.

Jus sayin.

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