The 355 'spider' idle stumbling problem is solved!!!!

Yesterday I installed the shield recommended by Gothspeed. I may not have purchased the same flexible rubber that Goth did, it is about 3/32" thick and relatively stiff. In stead of making the square boxes he shows, I felt I'd get a better result for my flap doors by contouring them to the triangular shape that occurs in the area where it covers the radiator. I found I had to slice along the hinge line of the flap doors leaving only 3/16" hinges at either end of the top of each flap door, to get the flexibility needed to get the "ram air" to open the door. Buy having thicker material "60DURO" commercial rubber sheet I bought at the local hardware store, the flap dors return predictably to their cutout location and by having the narrow but thick hinges I get the flexibility of moment required.

I haven't driven the car during extreme conditions, (Its 75 degrees this week) to be able to tell whether high speed cooling will be compromised but I suspect not. I am an Aerodynamicist by training and experience working at Boeing for 28 years on the engine programs on inlet, nozzle and reversrer designs prior to my current job. My specialty as a liscensed engineer was/is flow in ducts. What is interesting about the problem here is that Ferrari could have done such a much better job in the space available to capture engine are without pressure pulses and temperature distortions. It's like they got 80% of they way to understanding what was going on and had a design freeze.

I don't want to bore everybody but there are a couple of observations about the location and execution of the engine air inlet design that I would like to share:

1. At speed, the farther you move the effective inlet forward in the duct by extending the bifurcation forward, the lower the static pressure in the duct and thus reducing the "supercharging" you get by diffusing the flow and recovering the freestream total pressure back at the radiator face. (in other words - you have to make the engine air inlet much bigger the farther forward you move in the duct because the air is less dense.). I only mention this as a consideration for those who may be racing these things and they want to try some modification to improve performance at the top end speeds.

2. For normal street use, if I were Ferrari, I would have installed a 30% larger inlet for the engine air in the duct, but would have covered it with what we call a "cascade" to capture and difuse the flow to higher pressure. What this "cascade" is, is a collection of turning vanes that are pointed at a 30 degree incidence angle to the surface of the duct wall and they turn the flow into the engine air duct. (The flow vanes in your dash that direct the air conditioning air to the spot you want it are an example of what I'm talking about, except they become the "inlet" capture device). These would have provided the perfect temperature and pressure isolation device. Downstream of the radiator they should have put a 100% coverage shroud to house the fan like 99% of all auto manufacturers do.

3. If you racing guys want to enlarge the engine air entry duct and try the cascade approach, the cascade vanes need to have a chord length 50% longer than the spacing between the vanes. We use these cascades in thrust reversers and we spend millions optimising the chord length and spacing to optimum performance size and weight. Perhaps the next time you get a window seat by the engine on a Boeing plane watch the reversers deploy and you'll see these "cascades". If you have any questions - feel free to PM me on the subject.

 

OK, some interesting things today. Here's a picture of the right side of my 348 spider *without* Goth's shield installed. Notice that the paper is being sucked in, none blown out.


(yes, right side fan is on)
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So looking at the above, I didn't touch the right side fans today.

But the left side fan was blowing the paper out without Goth's shield. I installed the shields on the Left side today and made a nice test drive. Water temps ran a bit cooler on the gauge.

Notice that with the left side goth shields installed, the paper is sucked in on the left side now.
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To make Goth's shields, I bought a $9.99 black rubber set of floor mats from Advanced Auto Parts.

I cut out the rubber into the desired shapes, then cut flappable vents into each one. In this fashion the vents open up a little when you drive without the fan being on, but press back closed when the fan causes pressure to build up that would otherwise flow air forward and out the sides of the left radiator/strake area.

Cheap. Easy. Rubber. No harm, no foul.

Good theory. Good initial test results.

 

I did both left and right fan Goth shields and drove today with water temp averaging, at speed, 5-7 degrees cooler. Ambient temperature was 78 degrees F. Oil stayed right at 210 where it always is except in traffic it seemed a touch higher but maybe that just an anomalie. One observation is that the engine was extremely smooth sitting for 10 minutes at idle 1000 RPM - no occaisonal stumbles - horray! I still had a 250 rpm drop (although it was a smooth drop with no stumbles) when the left fan cuts on as I'm coming to a stop on a decel transient, but it recovered in 3-5 seconds - only had one occurance where I was in decel with the clutch depressed when the fan came on. At smooth idle there is only a 50 RPM drop when the fan cycled. By the way I have just installed a 30 AMP circuit breaker in stead of the 30 AMP fuse that keeps blowing on the left hand fan. So far so good.

 

OK, 5 to 7 degrees cooler water in both cars that have installed the Gothmods. Anything else? Less competition for air? Cooler air intake?

The 348 never had an idle stumble when the fans kicked on, so I won't benefit that way. Speaking of which, why is it that this is the first thread that I've even *read* or heard about 355's having an idle stumble problem when the left fan kicks on??

Good Lord! How many threads on "Valve Guides" and exhaust headers for 355's...yet where are the threads for the idle stumble?

 

One more thing, are stock 355 fans that much bigger than stock 348 fans?

Compare Goth's to mine!
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You went to both sides already !!! Awesome !! I am very happy to hear your results !! Thanks for trying this mod!!! My 355 ran alot smoother as well . It was a bit sureal at first..... ......

That is pretty damn cooooool, if I do say so my self !!

I think your temperture drop illustrates that warm/hot air was being 'recycled' through the radiator

+1

They appear to be bigger, the 355s fans are 16" diameter. I really like your pics of the air travel with your paper wind vanes !! You rock ND!!!



Thanks for the confidence on this mod guys. I will be trying to refine the design by moving the 'goth-shield' slightly away from the radiator and enclose the sides to allow air from the entire radiator area to be pulled through the radiator w/ the fan.

 

Yeah Baby!

Paper wind vanes?! Nothing but the best for ya, mate!


Speaking of which, the 348 left side radiator fan is a mere 12 inches in diameter compared to your 16 inch bad boy on your 355.








(You were clearly on to something good with your shroud idea - Nice Job)

 

Goth's quote:

Thanks for the confidence on this mod guys. I will be trying to refine the design by moving the 'goth-shield' slightly away from the radiator and enclose the sides to allow air from the entire radiator area to be pulled through the radiator w/ the fan. [/QUOTE]

If I wasn't so busy driving my car this summer in the next few weeks, I'd be building a fiberglass mold to knock out a 100% enclosure of the radiator that pushes the fan aft about 1 inch from its current location. The 1/2 inch deep zig-zag rubber gromet could still be used for support, but be surrounding the outside of a fiberglass shroud ahead of the fan (which is circular by the way lol!). With 1 inch you could put a very nice "bellmouth" entry into the fan tips and probably pick up another 10% flow because the current design tends to "stall" the fan tips which is why they make so much noise. I recognize you would have to build some relief in the area of the A/C lines so the shroud could tuck under that area and not provide the full depth, but it would be a significan improvement over what we are currently doing. Around the outside of the shroud I would build in a semi-circular 1/8" diameter half pipe to install "o" ring material for a good kiss seal. I would use all the current mounting lugs with "turnbuckle" thread extenders to get the extra inch.

I've thought about the increased downstream pressure bump from the wheel well caused by moving the fan aft 1 inch and concluded the gains in fan efficiency by getting the "dirty-air" coming from the radiator fins farther away from the leading edges of the fan blades is enormous in comparison, not to mention the noise reduction. (The wake off of these verticle and horizontal cooling fins create a local "hole in the flow". When the leading edge in the fan blade strikes this "hole" the fan blade "stalls" at a localized area (very small area on the leading edge of the blade with the current design - but there are thousands of them for every revolution) What you are listening to, is the flow trying to redistribute itself across the blade. By moving the fan 1" aft these "holes" mix out so that the fan sees more homogenious velocity of air it is trying to compress.

If anybody wants to step up and make some of these shrouds as I've outlined above - I'd be a taker at a reasonable price.

 

Exactly! I was also thinking of tapering the shroud starting at 1/4" (shroud to raditor gap) at the furthest extremities to 1" when it reaches the fans outer mounting points. This should be enough to fit under the AC lines at the upper RH corner. If not the just drop it a bit lower at that corner only.

Yes, by integrating the existing metal mounting parts the shroud could be made light enough to handle the air only, and not be burdened with having to structurally hold the fan in position.

Now your talking my language LOL, I too have an aerospace background 15+ years. Now its Product Developement for a major defense contractor (thats about all I could say). But enough of that, its making our cars the best they can be that interests me here . You are correct the fan will see a more consistent air density a tad further away and with the shrouding there will be no blade efficiency lost. Rather it will accelerate the air when it gets to the fan, not only because of the fan but because it is drawing air from a larger area and forcing it through a smaller aperture (the fan zone).

That is the goal, this is just one of a few 355 improvements that 'gothspeed' has been pondering . The mold wil be made and the shroud will be of fiberglass then tested for effectiveness. So............. might as well make few.........

 

Just as a datapoint, driving along today I noticed that my coolant temp dropped dramatically from 195 down to 160 when the left fan kicked on (while car was moving).

This was visibly faster than the needle would move prior to adding the Gothshroud to the left side radiator.

Left fan also turned off much faster (i.e. wasn't on as long) as before.

 

Thanks for the update ND ........ BTW that is the coolest name !!!! I love it !!!!! Mind if I use it???

 

Hey just had to bring back an old but very good thread....

Newer 355's (post 1995 as I understand it) all draw the intake air from the lower door scoop, just on top of the radiator. The upper C-pillar inlet is just open, passing air into the engine bay (applies to GTB's and GTS's only). Is there any reason not to reroute the intake to the shorter and more direct upper inlet? This seems like a big improvement, my OEM tube is partially crushed in snaking over the radiator, especially on the driver side. I've heard that there may be some issue with air flow at high speeds with windows open, explaining why the 360 inlet is further back away from the windows. Is this really a big issue?

Plan is to do some air flow studies with yarn once the new intake setup is installed...

Thanks!

 

Need to find the thread, but I believe Aeroengineman found that most of his "idle draw down" issue seemed to be related to the start up of his radiator fan motors. As they age, they seem to draw an excessive amount of current..he was even blowing fuses and installed 30 amp self resetting circuit breakers. Installing new Spal radiator fans seemed to eliminate the fuse blowing and greatly reduce the idle issue.

 

Nice work, goth.

So to sum this up... the electric fans need shrouds on them like most other cars

--Nick

 

Do you have an image of what that looks like?

 

After reading this post, I deciede to take a look at my car's ('98 Spyder) intake scoop. The fins inside my air scoop are different in that the top most fin is dedicated to the air intake... no break between the fin and the intake duct. To explain this better, when looking through the air scoop all I see is intake air duct when looking through the upper-most fin. Looking through the other fins below, I see radiator.

Is this different from your car, Goth? I don't have this stumbling issue when the left fan comes on.

--Nick

 

I guess the spiders use the same 'side ducts' for all years .............. mine 'had' that stumbling problem ......... it was corrected with the pseudo shroud I made to isolate the intake from the reverting hot air from the radiator ....... Dave Helms top fin deflector 'isolates' the intake air very well also .........

According to others experience apparently there are several causes of this idle 'fan on' stumbling ...... which include the current draw of 'old fans' ........

 

My 1997 F 355 Spider stubles badly at fan-on, LH side. Down to 400rpm and oil pressure light flickering....massive chugging and sputtering at traffic lights. At this point I am wondering which mod is easiest? I like the Goth-shroud idea...so I am guessing I have to take the rear wheel off, as well as the wheel-well plastic, then start fitting the rubber Gothshrouds. Do I have to unscrew a lot of items on the radiator to get these on? How are they staying on?

 

Why make the mod? These cars should not behave as you say yours is behaving. Making such a mod is treating the symptom not the fixing the cause.

 

normally I would agree ...... except in this case the factory, for this very purpose actually redesigned the side vent slats on later 355s to isolate the engine intake from the portion leading to the radiator .....

Edit: Here are pics of the 'early vs late' side duct design. The early version .. the 'engine inlet' port shares the duct cavity with the radiator inlet ....... whereas the later factory redesign gives the 'engine inlet' its own dedicated and isolated port .....

Note: first 2 pics are 'early' (4 slats) vs 3rd pic is 'later' (5 slats)
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I would have to ask, why then do not all cars with the earlier duct exhibit stumbling when the fans kick in? My car is an early car with early style duct but does not stumble. Ready321now is saying in his other thread that his RPM drops by 400. There has to be more to this than possibly ingesting some hotter air do to possible recirculation through the outer edges of the radiators. If the MAP and air temp sensor are working correctly the correction to the fuel trim should not result in such behavior.

 

John, this is 'one' cause of that idle stumble and 'one' solution ..... there are other factors which may also be at play .... like a weak alternator, bad ground(s) or fans which draw too much current for whatever reason etc. .....what year is your 355? this would not happen on 1995 models as they have a different intake/temp sensor configuration.

There were many known reports of this 'fan on' idle stumble, even when these cars were brand new (1996-1997) ..... with customers were bringing their cars into the dealers with this very complaint.

I have seen this 'hot air recirculation' drop the idle by 400 - 500 RPM on numerous 1996-1997 355s and each was vastly improved if not 100% corrected, with the installation of the shrouds I described in the thread.

.

 

I really don't want to be argumentative about this. So I'll just drop it. I don't see the problem with my 95. If it's a 96-97 problem so be it.