Air flow Under Rear Clip

Ian Anderson · 05-19-08, 04:47 PM

Has anyone built in some type of fan in the engine compartment to get air moving whilst plugging along in traffic?

If so what did you do?

I was considering one of these per side to be activated by a manual switch when the going gets slow.
http://cgi.ebay.com/ebaymotors/Marin...spagenameZWDVW

1) Would 250cfm per side be enough to make a difference?
2) If not what cfm would be good?
3) Has anyone used these units (or similar) elsewhere and can confirm if they are a "toy" or a good idea?

cheers
Ian

Big-Foot · 05-19-08, 07:47 PM

Ian,

I've used Bilge Blowers such as the one pictured for Brake Blowers (placed in the ducts) and also for fresh air to the driver. I don't think that they would be sufficient to really evacuate the kind of heat you want to rid your car of..I would take a look at mounting a pair of 1000+ CFM fans in the rear bodywork on either side of the exhaust outlets..
I recall seeing pictures of a member's car in one of the galleries or in his build thread that had something similar but I believe he had oil coolers mounted back there along with the fans..

Ian Anderson · 05-20-08, 01:35 AM

Wow 1000cfm per side - from my reading that would be at least a 14 inch rad fan per side and I don't think there is enough space for that!

Is it better to draw air out or force cool air in and over the motor?

Fans in the rear would move air out through the drills but sould surely draw most from under the car and not from around the engine and exhaust - again just my take on it

Cheers
Ian

Pat Buckley · 05-20-08, 01:57 AM

I was talking to a fellow GT40 owner about this just last weekend at the Cobra Bash in Reno.

I was thinking a fan mounted under the rear deck vent which is 11" X 17"....of course it would need to be insulated from the exhaust pipes.....

Cliff Beer · 05-20-08, 02:32 AM

Ian, my guess is that even at slow speeds you'll have some flow through the clip by way of the side vents, so, best to draw air in there to accentuate this flow and push the drawn in air over the hot engine and out the back and bottom - perhaps some smallish SPAL fans could accomplish this. And, you could add two similar fans drawing air out the grate on either side of the exhaust as well for extra flow.

My sense is that you don't need sufficient flow to evacuate the entire engine bay every 30 seconds or so to make a meaningful difference in operating temps. Just enough flow to steadily move some cool air through the area should work well.

Pat Buckley · 05-20-08, 03:07 AM

I think the whole reason for this is when the car is at rest and vapor lock is a possibility.

Ian Anderson · 05-20-08, 03:29 AM

Pat

I'm not too worried about vapour lock as I'm running a looping Efi Swirlpot tank system.

But that would also happen with big heat build up

I liked the sizing of the bilge blowers as they could be made to fit close to the side ducts to "assist" when moving slow and take over when stationary.

Just looking at fan sizes - anyone have any experience with Bike Radiator fans? They will be smaller and possibly easier to locate but are they any good in terms of cfm numbers? I cannot find any info so please point to any sites that have it - again a good reference for people going the same route in the future

Cheers
Ian

RichardH · 05-20-08, 05:28 AM

These should do the job for you - might be a bit of a tight fit, but the air could be ducted to various sections of the engine bay?

Jac Mac · 05-20-08, 06:33 AM

Why not use a cowl flap actuator as used on a Porsche or VW. Just a simple bellows that could be used to open the grill/flap above the collectors/muffler or similar to allow hot air to escape. Would be fully automatic. Guys with Homebuilt/kit A/C use them to control temps while taxying etc.

RichardH · 05-20-08, 07:02 AM

no reason why a second radiator couldnt be mounted at the rear with a thermostatic valve. Something like this?

Bill Musarra · 05-20-08, 08:23 AM

Ian,
I have the fans mounted at the rear of mine. Yes they are for the radiators. One for the engine oil, that comes its way when the proportioning thermostat opens. The other is for the trans. I have been told that even the rally Porshes don't use them, but I already had one and I think it keeps the proportions even. These are Spal 9" units. Have them keyed up to run either on thermostats or override with a switch.
You might take a note from the Lotus guys. My 85 turbo Esprit had a fan mounted over the engine that bew out the louvers behind the rear window. Was not a big fan, but then it was only a 4 banger.
I inverted my coolers thinking that the coolers would not fill. What I forgot was that the resistance to flow is in the cross fins. The filling area is low pressure and would fill before the resistance to the fins would allow flow. The oil coolers in the original cars are mounted that way.
I had a few extra shots of some other cooler users, so threw them in.

Bill

Big-Foot · 05-20-08, 08:48 AM

Some fans for you Ian.. I noted that they also have a single HP fan that moves 3,000 CFM but is probably too large for your application.

BTW - That ebay auction is using bilge blowers that were made by Atwood as far as I can tell - they are nothing any more or less special than the Detmars. I've used them both as brake coolers and they work fine. They also seem to tolerate ingesting water well but don't do so well when going off track and gobbling up grass / gravel.. ((sigh))...

Yes Bill - yours was the car I was thinking of!

So how was your logic faulty on the positioning of the oil coolers?

10" Fans
1115 CFM
SPAL USA Webstore - High Performance Fans

1115 CFM
SPAL USA Webstore - High Performance Fans

11" Fans
1363 CFM
SPAL USA Webstore - High Performance Fans
1363 CFM
SPAL USA Webstore - High Performance Fans

Bill Musarra · 05-20-08, 10:59 AM

The faulty thinking is that there is increased resistance in the thin cooling fin runs compared to the open filling tubes which are a larger diameter. The oil or whatever will seek the lowered resistance to flow, thus filling. I am at work right now, so I can't show you the pictures of the cooler in the 66Road car elsewhere on this forum. But the true test is to put your hand next to the cooler, and feel the temp at different parts of the cooler. They will be the same. Look at any race car that employs coolers above the engine and unless space is a problem, they will fill and empty from the bottom. Remember that most have a pumping mechanism or pressure high enough that the fluid will go to the least resistance first and the highest last. there may be air "bubbles" but not vast sections of the cooler not filled.
I have been advised also by more than one race mechanic that there is no need for an air bleed from the back of the heads when an electric water pump is used just for the same reason.

Let the flames begin!!!!

Bill

ragrod · 05-20-08, 09:18 PM

On the street oil temp never got above 200, But I was seeing oil temps around 250-270 at my first trip to Thunderhill so I added a 19 row Mocal cooler .I mounted in the back like Bill has shown with no fan in my CAV. Went to Spring mountin and could only run 15 min. and oil was at 270. So I have moved the cooler to passenger side air scoop and built a shroud to duct the air in. Hope this works better! So I question the rear as a good location, as I am not sure witch way air is moving , in or out. Panteras have a rear mounted A.C. condenser and the fan pulls air into engine compartment.So are we sure if air is being sucked in or out, of the rear??
Any one else out there with track car experence? I could have tied short streemers to the rear grills an had a chase car see if streemers are giong in or out. But I chose to move to use side scoop as per original .
Randy

chuck smith · 05-20-08, 10:23 PM

SPF Daytona coupes have a 11 inch fan mounted under the front of the engine that blows cool air up through the motor and out the hood ducts when in slow traffic. They really do seem to reduce under hood temps

Bill Musarra · 05-21-08, 06:51 AM

The fan(s) in the rear of my car are setup as pullers. I think that is more efficient than a pusher set up??? I would think that the area at the rear of the car would be a low pressure area due to the little bit of spoiler effect?? I am thinking of closing in the rear lower segment of the car(under the trans) and adding a little down force area under the rear of the car. That should negate any negative pressure in the engine bay. If I can get my ignition people on the ball, the car should be on the road in a week or two. Will post more then.

Bill

EGLITOM · 05-21-08, 07:35 AM

Bill

Are you saying it matters in terms of cooling if one had mount the cooler your way or with the openings down the other way ?

My thinking is there is no difference in cooling nor in filling ( like you expressed) . it is just a preference and desicion how to do the best and most convinient installation ( the way the oil lines run to the cooler). Can you confirm this ? ( want to do it right from the biginning)

Thanks
TOM

Bill Musarra · 05-21-08, 11:15 AM

Tom,
If you go back to my build site(DRB#5) you will see I had them originally mounted with the entry/exit in the down position. After some discussion on the forum, I decided to make them top fill/exit. After talking to the engine builder, I found out that it doesn't matter which way you mount them. I guess with the up mount, you will expel all air bubbles. It seems that the only place you would have the bubbles would be on the exit leg. The cross fins would be filled due to the pressure head and the resistance to flow. With a few heat cycles I would think the air bubbles that might accumulate on the down mount would be done away with. I could change mine back, but then I would have to cut my tubing to fit, and I have gone through enough and my wife has me back on a budget.
Speaking of which, I got almost all of my hoses through ebay. Some at a great price, some at marginal price. I always stuck to the principle of not bidding more than 50% of cost of new. Some of it I got for 10%. What I have found out from my builder was, that if you buy fuel tubing or tubing that has seen race gas and has been allowed to dry out for as little as a few months, it will essentually dry out, crack, and leak. If the line is teflon coated, it will probably be O K. The only way to tell if the lines are dry rotted is to flex them 180 degrees in a bend equal to their rating. If you hear crunching, the lines are no good and should be thrown away. I have already lost 4 or 5 pieces. Some as large as 10 AN. If they flex without crunching they are probably O K. The braided lines are not cheap to do the job right. Almost wish I had used the Teflon wrapped hoses. They look pretty good and are fairly easy to work with.

Bill