Engine shroud

Moderator: Larry Jowdy

Re: Engine shroud

Postby RBP » Tue Apr 30, 2019 1:10 pm

I used the DTM from the type IV store and an inlet bell. Use a cone shaped section from a round air diffuser or something similar. You can use anything of metal or fibreglass that will give the desired look. Use fibreglass to mould the inlet bell to the DTM so they would become one so to speak. After the fibreglass cured use body filler so smooth everything out and make it look like there are no joints. Then have the fan sand blasted primed and painted silver. Next paint the inside of the fan inlet bell silver. After dry, tape up to prevent overspray and paint the DTM a nice high gloss black. That's what the picture I posted on page 1 of this thread shows.
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Re: Engine shroud

Postby RBP » Tue Apr 30, 2019 3:42 pm

You just order the DTM unit as per the link I sent you, 900 or so I believe. I don't have a centre cone as they discuss. Look at my picture and you will note you can see the fan clearly. I might add a centre cone if can find one the right depth and diameter as per Ed's post on the original engine. The only changes I made to the DTM was the addition of the funnel shaped inlet bell where the air enters the fan area, see the picture. This you have to look for and buy something that will work and look they way you like. Then glass it in, smooth it out and paint it. Again in the photo you can see the inner area of the fan housing is silver and the outer housing is gloss black.
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Re: Engine shroud

Postby RVosari » Tue Apr 30, 2019 5:48 pm

If you use the spinner cone…be mindful that small is better…or you will most decidedly affect airflow into the fan !

Much like jamming a fan that in free air flows 2000-3000 cu. Ft/min (depending on diameter) into the firewall; you reduce overall fan efficiency.

It may cool your motor…but it will most assuredly make lots of unnecessary noise!

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Re: Engine shroud

Postby Larry Jowdy » Thu May 02, 2019 7:07 am

In my experience, the CB or similar style center mount fan shroud (without baffles) create a lot of air but it's not directed "down" towards the cylinders but rather in a circle. Some time ago, a baffle was designed that increased the performance of the center mount shroud. Not sure if it's still around but here a link.
https://shoptalkforums.com/viewtopic.ph ... an+shrouds
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Re: Engine shroud

Postby Ralphc » Thu May 02, 2019 11:53 am

I was following with interest; unfortunately the latest info seems to be that it is going nowhere
https://shoptalkforums.com/viewtopic.ph ... &start=105
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Re: Engine shroud

Postby albaran » Fri May 03, 2019 11:35 am

There was some sort of baffle at the bottom of my center mount but I don't know if that's the one they were talking about.
I'm not an engineer but in all of this discussion about which way the air goes, all I can say is that I have never had a cooling problem and you can really feel the hot air shooting out of the bottom of the engine. I Have the larger doghouse fan without the doghouse which forces more air into the engine. The 911 style fans don't rely on internal redirection of air, they also just force a lot of air under pressure into the engine. (axial vs radial flow?) Attached is a picture of when I was building the car so you can see the front of the shroud.
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Re: Engine shroud

Postby RBP » Fri May 03, 2019 3:38 pm

Al, I don't understand, to me the "dog house" is the old style oil coolers built into the side of the fan housing. Do you have that?
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Re: Engine shroud

Postby Larry Jowdy » Sat May 04, 2019 4:29 am

Well technically, the Dog House isn't the old style shroud but rather the new style. The cooler area or "dog House" was on the side of the shroud that faced the flywheel. Those are actually very good shrouds for stock and mildly modified engines.
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Re: Engine shroud

Postby egrant5329 » Sat May 04, 2019 4:46 am

How did the 911/912 regulate engine temperatures? I have not been around them enough to know.
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Re: Engine shroud

Postby albaran » Sat May 04, 2019 5:02 am

To expand on what Larry said, originally the oil coolers were inside the shrouds. The drawback was that it blocked some of the airflow to some cylinders and kept the heat from the cooler inside the shroud. The doghouse moved the cooler out of the shroud which fixed that problem but to allow for the extra air now bleeding out of the doghouse opening, they increased the size of the fan.
The center mount has no provision for cooling the oil so you have to cap the oil cooler holes on the top of the engine and take the oil from the galleries at the bottom of the engine to a remote oil cooler. The larger fan now pushes more air into the engine. I had to do some fabrication to the center mount to allow for the larger fan.
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Re: Engine shroud

Postby RBP » Sat May 04, 2019 5:42 am

Funny, I've never actually seen the "dog house" style oil cooling fan housing. I thought the rectangular opening, on the distributor side, in the fan housing wasn't a good idea since the hot air from the oil cooler would just short circuit back into the fan inlet. This would reduce the air flow to the cylinders and increase the temperature of the air cooling the cylinders. The higher the entering air temp to the fan the higher the cylinder head temperatures will be.

So Al, I take it you don't have a dog house then. To me that would be the opening in the fan housing. I picture your fan housing to be one piece with no opening.
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Re: Engine shroud

Postby egrant5329 » Sat May 04, 2019 6:08 am

If you have all of the engine tin on the dog house style shroud the hot air is directed out of the engine. The attached pic isn't great, but it shows the tin in place. I always liked the VW Thing shroud and the way the dog house directs the air out.

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Re: Engine shroud

Postby DannyP » Sun May 05, 2019 5:50 am

Ed, great pictures of the doghouse oil cooler hot air exit, which dumps the hot air out the bottom where it belongs. On a Speedster or a Bug, the intake air is divided from the hot exhaust by tins. We don't get that option on a Spyder unless you fabricate/duplicate the undertray.

Al Baran has an aftermarket center-mount shroud. These are not known for equal cylinder cooling. Neither are the 911 shrouds.

Yes the 911 operates on sheer volume of air. I have taken great pains to seal the shroud to my case, thus forcing all flow over the cylinders. Also, I installed a later 911SC part with 2 vanes and a cone on the back of the alternator to direct flow towards #3 and #4. Mine cools fine, as evidenced by pretty close CHT on all 4. I also fabricated "sled tins" from aluminum which makes the hot air go rearward away from the fan inlet. At idle or in traffic, the hot air used to bounce off the ground and get sucked back into the fan intake. Now the engine remains cool whether driving fast or slow or stopped.

Ed, there are no thermostats or flaps on a 911, I think the engineers designed it to give adequate flow for all 6 cylinders. Overkill on a 4 cylinder, which is why mine runs too cool in fall. The engine warms up nicely in the summer, but stays just right.

The 911 temp is controlled by the oil thermostat as far as I can tell. I have a 190F thermostat on my cooler but there is always some flow through the thermostat. I'm actually thinking of making an adjustable flap to block my oil cooler to get more heat in my oil in cooler temps.
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