Diffuser anyone?

[Mark]

As to downforce, is your proposal that this is needed for street cars, or is it intended more for track use? Regardless, is a diffuser without splitter and full length veins (thank you Mark) really a good idea for a GT40? Is a splitter envisioned as well? If so how well will that work with our current flat tubs? Or do we work in veins there as well? There is, after all 4+ inches to work with, not to mention adjustable shocks......


(BTW, heads up: being in Sales sucks sometimes).

If we were to talk about what is needed for a street car we would be on the SUV forum right now. Come on Alan.....can I call you Alan yet? (all in good fun....I know I'm the newbie :thumbsup most of these cars are owned because they look killer and like race cars. I could point out at least 100 things on a GT 40 that don't make good sense for street use. Will more downforce make a measurable difference on the street.......? Probably not for most. Will it make a big difference on the track? For sure. The fact is, if you like it, run it. The important thing is that it serves a functional purpose and it looks the part. Most all cars that utilize diffusers have smooth belly pans up to the point where the diffuser starts. The air veins or elements are to direct the air wash as it exits the back of the car to not mix with air coming off the wheels etc. If people are interested the powder coated aluminum ones will probably sell for around $750-$800 add $100 for titanium hardware. I can also do a carbon one as soon as I build a mold (given there is enough interest)

 

[Mark]

Okaaaaay, just from a VISUAL ASTHETICS standpoint ('have no idea as to the effect on 'functionality'), I'd like to see THREE vanes on each side vs. the present two. IMO that would look much better...'more balanced...'more purposeful...'more NASTY! (There's nothing really adverse about the way the unit looks now, I just think it would look better with 3 vanes per side.)

Be advised 'purists' are likely to put a bounty on you for this regardless!

Larry, I think you're right, a three element detail would be more aggressive for sure. Any number of changes could be done to change/improve the aesthetics.
oh, and about the purists...I am used to them giving me flack at first. I dealt with a lot of that stuff when I was building Porsche 356 outlaws back in the late 90's. Now it's commonplace.

 

[Mike]

I suspect the cooling advantage will turn into a disadvantage when not moving, since paths for convection would be blocked

I would suggest that if your cooling system suffers from inefficiencies and is taxed at 100% already so that convection is required to keep the system from overheating, this is probably not the right mod for you.

 

[Bill Musarra]

Some help for you guys is already on the forum. We had a great discussion of the diffuser and its principles in the thread "Difuser design and function"
http://www.gt40s.com/forum/gt40-tech-exterior-interior-ac-trim/36702-difuser-design-function.html
This should give you some answers and something more to talk about. Take a look at it and see if it helps or brings out more questions. I waited til some of you posted some comments before I brought this thread up. I think it will help out a little now that a few questions have arisen.
Mark I was wondering about your location of the diffuser with the tail part hanging out behind the body work and whether it creates more turbulance. Also the fact that the bottom of your fins look like they are in the 1-2 inch range from the pavement. That wouldn't last long on Georgia roads. And lastly, I don't seem to see how this creates any of the Bernouli affect except for the slight upward angle. Do the fins get further apart as it progresses to the rear(hard to tell from the pics)? The angle of the fins doesn't seem to have any way of difusing the air around the back of the tires, a very turbulent area(Not sure that is stated correctly).
Sorry some of the pics are missing. Must have deleted them from my photobucket page. You can find them on the web under "images of".

Bill

 

[Mark]

I personally like it. Gives it a modern yet retro look. Especially when its balanced out with a set of canards or maybe a front splitter. If only it would clear the luggage boxes on my MKII.....:cry:

I need to look at the MK II, I'm not that familiar with these cars yet so I'll have to study the architecture a bit.

 

[Mark]

Mark I was wondering about your location of the diffuser with the tail part hanging out behind the body work and whether it creates more turbulance. Also the fact that the bottom of your fins look like they are in the 1-2 inch range from the pavement. That wouldn't last long on Georgia roads. And lastly, I don't seem to see how this creates any of the Bernouli affect except for the slight upward angle. Do the fins get further apart as it progresses to the rear(hard to tell from the pics)? The angle of the fins doesn't seem to have any way of difusing the air around the back of the tires, a very turbulent area(Not sure that is stated correctly).
Sorry some of the pics are missing. Must have deleted them from my photobucket page. You can find them on the web under "images of".

Bill

Thanks Bill, I'll go check this thread out in a while. But for now I'll try to answer your questions. The diffuser on Mikes car actually extends only as far as the longest part of the rear of the car. So, I don't feel that there is extra turbulence created. I think we have at least 4" from the bottom of the fin to the ground. I'm only partly familiar with the Bernoulli's effect. I thought it only applied to fluid dynamics. That being said, if it also applies to air flow it states that the faster the air speed the lower the pressure which is exactly what the diffuser is doing. The veins extend straight back without any angle. Adding angle here would cause in increase in pressure whicv we don't want. The outer most fins (nearest to the wheels) prevent wheel turbulence and belly turbulence from co-mingling. Again, I am only using general tried and trued principles. To really dial things in and perfect a diffuser on a specific car you would have to utilize a wind tunnel or some modeling software at the very least.

 

[Seymour Snerd]

I would suggest that if your cooling system suffers from inefficiencies and is taxed at 100% already so that convection is required to keep the system from overheating, this is probably not the right mod for you.

It's not a cooling system issue. It's an exhaust manifold stored heat and low speed heat-soak issue. You may have noticed other GT40s owners (including FoMoCo in the 60s) have gone to great lengths to insulate the rear clip from the engine, the transaxle from the exhaust, etc., and to add fans somewhere in the rear to keep air moving. And it's much worse with big blocks due both to engine thermal mass and the much larger exhaust header diameter.

 

[Seymour Snerd]

Come on Alan........ most of these cars are owned because they look killer and like race cars. I could point out at least 100 things on a GT 40 that don't make good sense for street use.

I was asking in light of the fact that Michael initiated the project and yet does not use his car on the track. I think it was a reasonable question.

 

[Mark]

I was asking in light of the fact that Michael initiated the project and yet does not use his car on the track. I think it was a reasonable question.

Agreed.....

 

[Larry L.]

What effect, if any, might/does sealing the area under the engine have on venting hot air from the engine compartment?

Regarding heat, if anything I believe it would help or at least have no adverse effects. Without it, I believe there is a lot of turbulence back there that probably reduces airflow through the side vents and out the rear vents.

"We ran witbout a flat floor under the engine for most of the season.....engine bay temps rise very quickly when you seal the floor of the engine bay." - Fran Hall (regarding the SL-C specifically...not the GT40) (taken from the "Diffuser design and function" link)



So, maybe it'd be a crap shoot??? :shrug:

Driving around in your own '40 for a 'short-short' with the diffuser installed and an eye on the temp gauge ought to answer the question...especially in traffic.

(I'll just refer to you a 'Mr. Beta' from now on!)

 

[Mike]

Ha I'm okay testing for you guys. FYI this diffuser does not seal the bottom of the engine bay. It seals from the crossmember back. The engine bay and more specifically the oil pan is still fully exposed to airflow.

 

[Mike]

As a side note my Ford GT had a fully sealed underside and was far more prone to heatsoak with that big supercharger sitting on top and two big cats on either side below. I think you guys may be overthinking things? I will either work to redesign or remove if there turn out to be detriments to mechanical systems. Other than that it really does look good and really rounds out the back of the car nicely.

 

[Larry L.]

If you get the chance, Mike, how 'bout lifting the rear of the car and taking an underside shot with the diffuser in place? (I'm having some trouble visualizing the whole picture from your discription. 'I be thinkin' the air flow under the car would [should?] be badly disrupted if the area under the engine were left completely 'open'. But then, you know how dense I am...)

 

[Mike]

Will do. Keep in mind that the oil pan fills most of the opening below the engine. The bottom of the pan on my Roush is on the same plane as the belly pan of the car.

 

[Larry L.]

Thankee -

 

[René]

what a great build that car, also that diffuser, more a R option

maybe for road use the fins better not be that high especially at the end?

 

[Mike]

Thanks! My conclusion was that dragging of a fin would not be an issue. They are not that low nor so far behind the axle as to cause ground clearance issues. I never once drug the diffuser on the Ford GT which was quite a bit lower than this one. The risk rather is backing into a curb as the diffuser will make first contact. Think of it as a crumple zone I guess

 

[René]

lol at crumple zone , the rest sounds okay then, then your only "worry" is that you stay alert when parking backwards at places like this>>

cars with lower spoilers/bumpers on the front have to be careful too

But this counts for most sport cars.

Canards on the front clip sounds almost like a must when you do very high speeds occasionally me thinks.

 

[Jimmy P]

Michael, as usual I'll stay on the fence. Liking the idea and your move toward improving stabilty.....but I love the look of your car without it.

FWIW I had a CF rear diffuser on my 3rd Gen RX-7 and I can't say I noticed any vast improvement in handling while driving on the street. It did cause some concern when reversing and hitting any dips in the road at speed. On that particular car it looked wicked on a 40....I'm a purist.....but will continue to monitor your progress. The good news is its removable so if your going to a "purist" show take it off if not leave it on.

 

[Larry L.]

If I've understood Michael's intentions with regard to this diffuser correctly, Jimmy, he's not really looking to create any aero 'bennies'...he's just going for looks. If the car's aero ends up being improved, then so much the better.:thumbsup: