Hot engine bay-

[rickeolis]

After removing my padding from the back seat area (savings close to 35 lbs), I read a post claiming that I'll find that the seat backs will be hotter than he$$, which turned out to be true.

So upon a closer look, I found out why: This is not an issue on an early 911, but as they went to the mid-year body style, they (for whatever reasoning) went to a separated space between the body and the rear shock\torsion tube location. This long, thin space between the two allows hot air from the underside of the engine to travel up and enter the engine bay, which to me is a domino effect on the heat problem with the mid-year cars.

I'm thinking about putting something in this space to prevent that from happening. Has anyone else done this?

[Grady Clay]

rickeolis,

You noticed an important fact about air cooled 911s – they need cool air. ANY hot air re-circulation degrades the cooling and the performance. Of course you also found it heating the cockpit.

The only time re-circulation might be useful is mid-winter in Edmonton or Fairbanks.

Many racers leave the tin off the engine. IMHO, big mistake unless the cooling air is ducted from the grill to the fan. Even in that case, there are heat problems with electronics and fuel.

A 911 is a thermally limited engine. This is one reason why we must use significantly lower compression ratios and consequently lower performance compared to water cooling.

So, how do you get the best you can? Prevent ANY hot air re-circulation. Pump more air across the cylinders and heads. Provide better ducting of the cool air in and hot air out. Reduce radiation and conduction of heat to the “cool side” of the engine.

For your issue, you can block the re-circulation air leak. There is an insulating product that is about 1/8” foam with silver foil that works well on the hot side of the metal.

Best,
Grady

[Bill Verburg]

Rick, I'm trying to visualize what you are refering to, can it be seen here?

[CliffBrown]

Grady

I'm sure this has been tried but I thought this would be a good thread to ask the question.

What are the pros and cons of attaching a large lightweight fiberglass duct from the engine deck lid grille to the fan opening, as you suggested? Eliminating any chance of hot engine compartment air entering the fan. Build the duct in the same fashion as the engine shroud. Just attach it to the fan opening and fit a rubber sealing ring at the inside of the grille, allowing the lid to be opened normally.

[Grady Clay]

Cliff,

Porsche did this on several race cars, 906 and later.

I think the reason it wasn’t used on 911s is the cooling air also was needed to keep the electronics and fuel injection system in a relatively cool environment.

On some Turbos, they sealed the air intake to the intercooler but still allowed air circulation in the engine compartment.

If you look at the effort Porsche made to seal off the hot air (356, Spyders, 904s, and on through 935, 956, and 962) you get an idea how seriously the Factory took this issue.

If you look at a later Spyder or 904, almost the entire engine and transmission is sealed in the cool environment and the exhaust system and head/cylinder heat is ducted down, back, and out.

On a 911, I would choose to seal and insulate the engine cool side from the heat. The induction air intake probably should be near the grill.

Best,
Grady

[TimT]

Bill i found on my early 911 tubs (both a '69, and a '70) that there was a gasket between the suspension crossmember, and what would be the seat backs...

On both of these tubs it looked like pieces of maybe 3/4 in dia rubber(foam) rod, or tube was placed between the crossmember and rear firewall ( seatback).

On the tubs ive seen the wiring harness is run between the crossmember, and seatback.. I guess Porsche just decided to seal this gap, and prevent hot air from re-entering the engine compartment. Or as Rick says heating the seatbacks..

[TimT]

Oh LOL

missed this in my run on ramble before..... Rick, yes I think a good way to re seal this area would be just stuff some 1" dia backer rod in there. you can get at most home centers ( I despise Home Depots) its used for backing up caulk in joints that are to big to support the caulk...

[TimT]

What are the pros and cons of attaching a large lightweight fiberglass duct from the engine deck lid grille to the fan opening, as you suggested?

Early 930s come with something like this on the tail from the factory

[CliffBrown]

Grady

Are there any aftermarket products available to apply this technology for street cars? Is there enough positive pressure on the deck lid at speed to use NACA ducts, one for induction air and another for electronics with an electric fan?

Seems like a rather simple and low-cost modification but then again I wouldn't like to alter the stock deck lid with NACA ducts or any other external alterations.

[Randy Webb]

Use a duck tail. It increase air flow into the engine comp.

[Zeke]

Originally posted by CliffBrown
Grady

Are there any aftermarket products available to apply this technology for street cars? Is there enough positive pressure on the deck lid at speed to use NACA ducts, one for induction air and another for electronics with an electric fan?

Seems like a rather simple and low-cost modification but then again I wouldn't like to alter the stock deck lid with NACA ducts or any other external alterations.

Yoy didn't ask me, but I think it's universally assumed that there is seldom any positive pressure on the deck lid. A duck tail would help with the air flow to the grille.

IMO, letting the fan draw air from the well sealed cool side on the engine is to allow less heat build up in the engine bay. To have the engine induction isolated from competing with the fan for air, it would seem that ducting cool air from an outside source would be the answer.

In fact, my '89 Chevy truck draws intake air from the under the front fender. That's a factory setup. So, engineers have at least had this in mind for sometime now.

To do this in a 911would necessitate some serious panel cutting and I think most of us would avoid that. There were a lot of very creative ideas on this subject in the 914 group. A favorite was to supply massive amounts of cool air to the fan intake. I had planned on building a plenum from the air cleaners up to and flush with the underside of the grille to ensure tha carbs breathed only outside air. The negative pressure at the 914 grille was definately a major design flaw. Some run a type of reverse wickerbale at the rear edge of the roof to try to supply some non turbulent air to the grille area in general (or quiet the turbulance, as it were). Certainly the 911 is way better in that aspect.

[47silver]

since hot air rises it would have made sense to have the fan pull air from below the car and the natural convection would help pull the air over the cylinders (chimney effect) that way you could duct the air directly out the grille and put a separate intake for combusion air,

just a thought

gary

[Randy Webb]

Gary, free convection will be overwhlemed by by forced at speed. one would need to work on the ways to duct air from a high pressure area while the car is underway.

[addictionMS]

the reason for the gap was there was a lot more stuff that needed to go from the tunel to the engine bay. So a bigger gap was created to allow the harness and fuel lines, etc.... to be routed to the top of the motor without having to interfere with the suspension.

If you put a later motor into an earlier car, you can sit for quite a while trying to figure out how how to get that stuff routed without the gap.

Jim

[Bill Verburg]

Here?



I can't imagine it being an issue while the car is moving forward.

If it is just make an air dam like Ruf did, bolt to trans mounts

[widebody911]

A friend of mine's 914 has a piece going across the bottom, like in Bill's picture. He told me is was for cooling, but I thought he was FOS.

What's the theory behind this?

[rickeolis]

Bill, yes that is the place, thanks for the pics-

Unless you guys had constant overheating problems like I do, you may not understand why this is an issue.

During drives in traffic, there is not enough airflow to get rid of the heat there, really.

So I have put an insulator into that space now. We'll see how it feelsafter a couple of drives.

Tim T seems to understand where I'm coming from though... I do run a turbo tail which should be even better than a duck for positive pressure in the engine bay, but again, that's only at speeds above 40 or so.

[smestas]

My '78 has what looks like blocks of foam placed in that space. I once tried to move them and they pretty much disentigrated upon contact so I just let them be.

[rickeolis]

smestas: Now that you mention that, this car did not have the original unsulating pad. I'm wondering if maybe it helps out with the issue?

[Jim Sims]

OEM part used to seal these areas: 911.504.930.00 available for $18.39 each (2 required). I've seen it listed as a strap, bitumen block and plastic block. The part is a tar soaked piece of plastic/rubber foam that is stuffed into place. Many of the original blocks are now gone or ready to crumble to dust at the touch. On the later years, on the driver's side, so many hoses and lines passed through this space that the block used on that side was trimmed back. Cheers, Jim