Is the SL-C radiator outlet at an efficient location?

Quick introduction: 1st post, but I've browsed this forum for years! Have built a Cobra replica and am currently building a (very modified chassis) mid-engined 'kit car' out of Massachusetts. I actually bought that kit from Fran, someone had traded it in for an SL-C kit.
The SL-C bug is really biting, I love the concept and execution of the car. My gracious wife says I can pursue an SL-C.
One thing I notice about the design is that the radiator exit is close to the base of the windshield near a high pressure region. I have seen posts about cooling issues, needing bigger fans, bigger radiators, etc. Is radiator efficiency maximized as-is?
Am I way off here? Not a concern? Has anyone modified the hood to move the radiator exit forward to a low pressure region? Or has this been wind-tunnel tested?
I know Fran is a great engineer and has the pedigree, so I may be overthinking this completely...
Thanks!
Dave
 
I've followed your build on the other forums- it's a remarkable car you are making!

Everyone will have an opinion about this, so here's mine: the SLC is designed to look like a Group C or IMSA GTP car. Those cars tended to have an outlet like the SLC. That outlet size worked for engines making more power than most street SLCs.

Much experience with street-driven SLCs has shown that the standard radiator is capable of around 600 HP, when combined with the stock fans and a properly-designed and implemented shroud and proper sealing in the radiator area.

I think that "overheating" complaints derive from two things: a misunderstanding of just how hot LS and LT engines are designed to operate, combined with poor or no shrouding and sealing. More fan will help to compensate for a poor shroud design, but adding better fans is unnecessary for SLCs under 600 HP. It's not the fans, it's the shroud and sealing.

For example, my car has around 550 HP, a well-designed shroud and marginal sealing, yet the car doesn't overheat, even in 95 degree weather with the AC on, while in a 60 minute traffic jam.

My car does have a CF Gurney flap at the rad outlet, and that helps at speed, but of course does nothing while idling.
 
I'd add that there has been CFD work on the factory 01 SLC race car, but those were largely to gain downforce, not improve cooling, which wasn't a problem on track.
 
There are a few hood modifications that folks have performed over the years. My opinion (that's not based on any data) is that the factory outlet is too small. I performed a mod similar to Howard's gray car and opened up the rear. I also fabricated a fan and ducting to direct 100% of the radiator air out through my enlarged exit. I'm planning to run a gurney flap to help with the exiting airflow.

Michael Fling has performed a mod where the outlet has been pushed forward, closer to the middle of the hood. He has 2 openings (I believe 1 for each radiator but I forget the details of his mod right now).

Allan's most recent cars have a hood mod similar to the LaFerrari where there are multiple openings just aft of the radiator. (see Youtube clips, I don't think any high res photos have been posted). Granville is the owner of one of these LT4 beasts.
 
Thanks for the replies! I am a fanatic about ducting and giving the air a specific path to follow. I believe that can make or break high speed stability, and make the difference between a car that runs cool vs one that runs hot. I think that enlarging openings in an attempt at more efficient cooling can be detrimental. Typically an opening should be about 1/3 of radiator area, and the exit about 1/4 of radiator area. If you are needing significantly more area for either, something is probably off with the airflow. Cam, you may have found better cooling by enlarging your outlet area if you opened it into an area of lower pressure.
The SL-C has a prototype-like windshield with quite a bit of curvature compared to a typical car, so I would think that it would have a smaller central area of high pressure compared to a car with a 'typical' windshield. It would be relatively simple to do a yarn tuft test to confirm good airflow from the outlet and confirm air is not focally stagnant (or even flowing INTO the outlet in areas).
 
welcome Dave!
good to see you over here. :)

Like Will said, the opening is fine if the rest of your cooling system is properly done.

John
 
Will is right about the fan mounting to the radiator. I live in Florida and regularly sit in snowbird traffic with 90+ and have no engine (LS7) overheating problems, it's me that overheat!

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Beeman - can you explain in a bit more detail your comments regarding radiator opening areas in front and behind of the radiator? There are some non-intuitive statements so some clarity would be good for those of us who haven’t studied heat exchangers. What is the principal behind blocking 2/3 or the radiator surface area? Is there an ideal distance between the 1/3 opening size and the face of the radiator? Does this area need to be smoothly transitioned or are you trying to incite turbulent expansion? Since these are sub-sonic flows I’m struggling a bit to understand how making the opening in front of the radiator smaller is beneficial.

Similarly, what drives the goal to reduce the radiator discharge area even further than the opening? Is it to accelerate the airflow via area reduction? Or is it to raise the pressure at the outlet to better combat pressure outside the discharge area and thereby drive flow?

My ideas behind radiator airflow are pretty simplistic and based on fundamental low pressure compressible flow. Since the radiarir is basically a resistor in the circuit I would think he best radiator performance comes from maximizing delta-p across the radiator and getting as much mass flow as possible. The only way I see this happening is by maximizing radiator frontal exposure and capturing as much of the oncoming flow as possible via seals around the radiator. If you’re not using fancy accelerator ducts and extreme bodywork to enhance incoming flow it’s the easiest way to get maximum pressure to the radiator.

On the backside, it would seem to me the best way to promote flow would be to minimize the pressure behind the radiator, getting as high a delta-p as possible. Using a duct keeps the air moving as smoothly as possible as opposed to dumping it into the front body cavity and providing it with a few openings to escape, wherever they may be.

I’m not familiar with the 1/3 & 1/4 principal though most of my car based knowledge is from passsenger cars and not race cars/hyper cars.

I would agree that improved cooling efficiency may be possible by moving the discharge further forward but I didn’t think I had the fiberglass chops to make that happen and have it look good. My approach was definitely on the lower skill tree. However, packaging is a consideration and there’s not much room behind the radiator to turn air away from the footbox before you’re hitting the top of the clam. My duct turns the flow almost 90deg before exiting which will hopefully give it enough momentum to make it out the clam, in combination with a gurney flap just forward of the opening. Flow inversion is definitely a concern but I believe my configuration is an improvement over stock all things considered.

I haven’t driven my car with the bodywork on so at this point all my mods are purely based on incorporating basic physics.
 

Neil

Supporter
Jerry;

You might consider putting your battery in a battery box. Sulfuric acid electrolyte vapor will eventually corrode everything near an open battery; the enclosed box is vented to the air by a plastic tube that you place somewhere that exits the chassis. A polyethylene battery box also offers a little bit of protection in a crash.

Regards, Neil Tucson, AZ
 

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Thank you for your concern Neil. The battery is a Braille Lithium Ion so vapor problems do not exist. The photo containing the battery is an old one so here are two current that show more detail.
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Apologies for my typos above, phone fingers.

As I’ve been sitting here contemplating radiator flow the only thing I can come up with for necking down radiator opening area is due to ingestion concerns. Too big an opening and the radiator isn’t able to pass through all the flow it’s receiving, causing a sort of deadheading.

Plausible, but I think the opening size works really well for one speed/driving condition but fails elsewhere. Drive too slow and the radiator doesn’t receive enough flow - perhaps here the fans kicking in will be sufficient to compensate. Drive too fast and the radiator doesn’t receive enough cooling air.

I can’t speak to what the SLC design is as I haven’t measured the opening or radiator frontal area to see if Fran tried to optimize the two based on this 1/3 rule of thumb.

I’m not sure how valid it is to bring up the greeen race car. IIRC the green car had a pretty massive opening behind the radiator, larger than even I’ve incorporated.

Anyway, interested in getting more discussion out there regarding this 1/3 / 1/4 principal. If there’s a tech paper/ testing available that would be most interesting.
 
Hey Dallas! (I think that's you?)
I agree that efficient fan shrouding is essential at low/stopped speeds. Whether you're sucking air with a fan, or shoving it in at speed, air will travel along the path of least resistance and will prefer to go around the radiator rather than through it. I'm more concerned about the airflow at speed.
Cam, the thinking is that once you exceed a certain percentage of radiator area for your intake, you are making essentially no gains in efficiency, but are paying a toll of significantly increased drag. Oh boy, trying to remember my sources for little tidbits like radiator inlet/outlet areas is straining my memory abilities for sure! One of those things where you see or hear something and just remember it.
That being said, I just found a reliable-appearing blog by
Willem Toet
Professor, Motorsport, F1 and Aerodynamics Specialist
https://www.linkedin.com/pulse/air-...-willem-toet?trk=portfolio_article-card_title
A few points from above link:
-Don’t be tempted to go for a monster entry to try to “ram” air into something – it isn’t the way to go. The main penalty is likely to be more drag and the benefits trivial or negative.
-With a fixed point of blockage (such as a radiator) you can increase the rate of expansion beyond the normal 7 degree limit as you approach the core (aerodynamic blockage) itself.
-A radiator entry should be at least 20% of the area of the core for most motorsport applications. Of course this is influenced by the speed of the vehicle, the radiator area, heat rejection etc. but that’s a good starting point. If you go significantly bigger you typically pay the price of some drag.
-Allow at least a quarter of the radiator area for your exit(s).

Anyone familiar with any agreeing or contradicting data?
 
Beeman - I checked that link out but I'm not sure it's all that applicable to a car like the SLC (or any street focused car). The radiator in an SLC is located a few inches from the opening of the car - not enough room to fit any kind of duct that would make sense. The principal being pushed in the linked article is really talking about the importance of ducting and requires a LOT of length in front of the radiator to minimize boundary layer formation and turbulence. I'm not familiar enough with the GTM layout so maybe these design guidelines works there - but for the SLC this isn't going to be an easy solution to incorporate. A 7-degree inlet angle is basically a straight tube when it's only 5 inches long.

I won't reference the hillclimb car since that intake duct feeds the engine and isn't quite as applicable for a radiator discussion. However, the next car uses one of the front inlets for driver cooling - so the duct they fabricated ran the length of the front end before being somehow directed at the driver. That's a lot of real estate for a duct!

Later in the article the author references a Bonneville F1 car that had a very small opening area - everyone thought it was going to overheat but it did just fine at speed. And that's the critical element right there - at speed. That design, and I think the 20% rule referenced, is likely more applicable for a race car at speed than a car that has to deal with everyday driving conditions.

For "slower" cars here's what he has to say:

"In most club level motorsport people building their own cars don’t duct the air all the way to the cores of the radiators and some don’t even seal the gaps around the radiator. If you have a car like this and it has cooling problems then first seal the radiator to the bodywork so virtually all the air has to pass through the core. If that doesn’t give you enough cooling then look at ducting the air using a gently increasing expander / diffuser."

I think the key there is "gently increasing expander" - again, packaging issues for the SLC make a gentle inlet duct very difficult if not impossible. There's some contour/shaping of the bottom of the splitter, but I'm not sure if that was just because a transition was needed for mounting or if it was designed specifically for aero purposes (I'm guessing the former). You basically go from opening to full radiator core area in the matter of a few inches (if retaining the factory radiator location).

As for exit area - the 1/4 area rule is the recommended minimum. I believe he also talks a little bit about ducting behind the radiator and he basically says unless you've got CFD, don't bother. Just dump air to wherever it's going to be least harmful (under or out the sides of the car I think). I have to think about this some more but again, I'm not sure how applicable this would be for a street car. Coincidentally my radiator discharge duct has a minimum exit area probably close to that 1/4-1/3 number but that was a packaging-driven constraint. I was more concerned about airflow management than anything else.

I'd say if you're planning to build your car for maximum cooling efficiency at top speed your link provides a recipe that's probably a good starting point if you don't have access to CFD and don't want to hack/test/hack/test/hack/test a solution together. I'm just not sure these are good guidelines for a street car that has to deal with a much wider range of operating conditions. Such long ducts would drive some real packaging issues if you want to keep the radiator up front. Maybe if you did as Rumbles did and split your radiator into two smaller cores and placed them in the side pods, then converted the side pods into functional air tunnels (instead of using the rear entry ducts), then you could get a higher efficiency cooling design. But given the amount of cost, effort, and experimentation, IMHO the more optimized solution would be to stick with the factory setup, make minor body modifications, and see how the car runs. If you need more cooling the next easiest solution is to incorporate higher flowing fans or a larger radiator core. It seems to be working for most of the SLCs out there that I know of.

So getting back to the main thrust of your post - I think some SLCs have cooling issues but it's likely if you reviewed those builds you'd see something fundamental was missing (shroud, sealing, good airflow management, etc). I don't believe the SLC has a cooling issue by design (for reasonable HP applications) - but I'm paranoid enough to do a few things that I *think* will make it better.

If you're not familiar with the SLC layout I have a few photos on my blog here at this link. You can get a sense for layout, scale, and packaging constraints in these photos.
https://superlitesite.wordpress.com/2018/03/04/28-full-of-hot-air/

Somewhere down in the middle of this post is a discussion of my hood modification and a before/after comparison:
https://superlitesite.wordpress.com/2018/02/25/27-the-first-cut-is-the-deepest/

And at the very bottom of this post you can see what my exit duct looks like with the body on:
https://superlitesite.wordpress.com/2018/05/04/xx-splitting-h-air-s/

Michael Fling's modifications can be seen starting with post 394 of his build log:
https://www.gt40s.com/threads/fling-slc-build-thread.39723/page-20#post-523328

Frankly - I'm amazed he was able to package that turn in such a tight space, but his fiberglass skills are way beyond what I've acquired to date.

Best regards,
Cam
 

Howard Jones

Supporter
Well...……….here's what I know. Everything I talk about here is in reference to my car...……..only...…….as it is the only SLC I have intimate factual knowledge of. My car has a 350 mk1 Small block Chevy in it that makes about 500hp. When I built it and the fiberglass was unpainted I took the opportunity to open up the nose radiator exhaust outlet hole an additional 4 inches . Why? a few reasons, 1st, pictures of the green racecar clearly showed that it had it's similar exhaust hole opened up quite a bit. It had won a national championship and thus I copied it. 2nd, I wanted to contain airflow through the radiator and its intake/exhaust track completely and force all the air that came in the inlet through the radiator and contain all the exhausted air in ducting and flow it out of the top of the car into the face of the windscreen and over the top of the car in an attempt to improve downforce.

I added covers over the small triangular openings at the lower rear of the radiator enclosure to prevent air from entering the area from the wheel wells. I then made my own ducting out of fiberglass and attached it to the radiator exhaust side with its exit placed so that its flow would leave the car as described in the above. Now all the air went in the front and out the top. ALL of it.

At the point when I got the car running I found that coolant temps were high, not too high, but high (215F-240F) at speed on track. At that point I began to investigate the cooling issue and finally came to the conclusion that the water pump was cavitating and possibly the radiator itself was marginal. I should add that I have a GT40 and I worked very hard to get it to cool properly on track. Without going into that too much it came down to pump speed, airflow in and out of the radiator, and pump impeller slippage on the pump shaft. I promised myself that if I had cooling issues on the SLC I would very quickly give up on belt driven water pumps and try a electric pump. I removed the belt driven pump and installed a Davies Craig electric water pump. It took awhile to finalize its installation but it ultimately clearly worked much better that the belt driven pump did however I still was unhappy with the reserve cooling capacity of the system. It didn't get as hot as before and didn't spike temps like before but now even though the coolant ran a constant temp (200-220F) and never pumped any water out the radiator cap I just wasn't happy with that high coolant temp on track at speed. I will add here that I had already found that a restrictor plate with a 3/4 inch hole was just about right as a replacement for the thermostat. Without it the system seams to take longer to burp out air. I think the pump likes to see some restriction to build some pressure inside the engine. Just a little but some. The pump doesn't seam to run more duty cycle or longer so I am not certain just removing the thermostat as suggested by the Davis Craig install instructions would be a problem. When I called them about this issue they suggested gutting out a thermostat and using the remaining disk to "give the pump something to pump against". Whatever the technical reason it seams to work.

So I called C&R racing and after some discussion sent them my radiator and had them make me a best of technology radiator to replace the OEM one that came with the car. Absolutely dramatic improvement. Solid 191F (default coolant temp as controlled by the Davies Craig electronic flow controller), and if I selected a lower temp, 180F for example the pump system would immediately come on and pull the coolant temp right down to 180 and hold it there rock solidly. This was a clear indication that the radiator I how had installed and ample reserve capacity.

At this point I should add that I always have had the OEM fans installed inside the ducting as they are necessary for very slow movement around the paddock. Other than that I never run them. I also should note that it is VERY important to vent the coolant system of ALL the air back to the expansion tank. My system vents both sides (top) of the radiator, the rear (both sides) of the intake manifold to the top of the thermostat housing and on to the expansion tank. There is no air in my system.

I also have a 8 quart oil system with a large oil cooler installed. Oil runs a constant 220-230F once hot.

I agree with Cam in the use of a small gurney at the forward exit of the hood radiator exhaust exit hole.
 
I think this is a great discussion, and these issues are applicable to any mid-engine car. Thanks for sharing the recipes that work. Having a way to make sure all air is being continuously bled from the system, ie steam vents to a header tank, radiator bleed point/bleed tube are critical. Efficient ducting (telling the air where it can and cannot go) is critical as well.
Cam, thanks for those links, I'll take a look at them. I'm not making any arguments about the air intake size on the SL-C. My concern is only the location of the air outlet. And ducting after the radiator in my mind is to dictate where the air exits the body of the car, ideally to a low pressure region. The 25% number is, again in my mind, a way to evaluate the efficiency of the system. If you are having to exceed that area, and all other inefficiencies have been resolved, something is probably off- I would suggest the location of the exit (too high pressure). Primarily if these cooling issues are occuring at speed of course. The discussion of ducting in the above blog, as you stated, isn't applicable in most cases to pre-radiator ducting, I again have no concerns about anything pre-radiator in the SL-C. The intake area just has to be large enough not to be a restrictor of flow at idle and at speed. But air that enters the intake pretty much immediate hits a major restrictor of flow, the radiator. That's going to be the rate limiting component, unless something is ridiculously too small before or after.
Air flow through the radiator, ignoring air momentum at speed, is simply the result of a pressure difference in front of and behind the radiator. If pressure is higher in front (at speed), or lower behind (ducted fan), air will flow through the radiator. The greater the pressure difference, the better the flow. As you increase speed, pressure builds up in front of the radiator, but pressure also build up at the back of the hood near the windshield. Flow through the radiator is based on the difference between these pressure values if you are exiting air in this region. Look at some CFD models of cars to give you an idea, they all look very similar when looking at hood and windshield cowl region pressures.

CFD models all look about like this. If you look at the Static Pressure, the pressure at the base of the windshield at least approaches the pressure at the front fascia, ie the pressure in front of your radiator. I can't comment on logarithmics, etc, but if you can get into the green zones, there will be markedly increased flow through your radiator. The SL-C opening appears to be at the intersection of the yellow/orange/red zones:
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Here's an actual pressure pattern:
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Thank you for your concern Neil. The battery is a Braille Lithium Ion so vapor problems do not exist. The photo containing the battery is an old one so here are two current that show more detail.
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You did an amazing job with that. Where did you source that battery enclosure? It looks nicely finished although a bit unnecessary.

Every thread I look in around here I see people that have skills, that I simply don't possess.
 
Dave - I don’t know why you’re holding onto the 25% value so strongly. Even the article you linked states that’s a minimum, not a maximum - and also is a starting point, not an end point.

The pressure data you posted is interesting but again, I’m not sure how applicable it is to an SLC other than to support your statement that placing the radiator exit directly in front of the center of the windshield is not ideal - I think everyone would agree with you there.

Moving the discharge forward would be beneificial - no one arguing that either. However, I think what you’re suggesting is that the SLC is incorrectly/poorly/(put your word in here) designed because the factory radiator discharge location puts it in a high pressure zone - do I have that correct?

Here’s a photo showing my car, I’ve drawn a line where the factory exit used to sit. It’s about 6” forward of the base of the windshield and situated below the contour of the hood. Width is about 24”. The actual opening height is adjustable, just trim it to whatever you want (up to the upper plane of the hood of course).

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You can draw what conclusions you like about the SLC design based on the data you’ve referenced and the base SLC information that’s been provided so far.

I’m not at all sure what’s been accomplished other than agreeing that the center of the windshield is a high pressure zone and that placing the radiator exit here isn’t ideal. I don’t think we can make any conclusions (positive or negative) about the SLC design based on what you’ve brought up. Can it be made better? Sure. Every design is an optimization of compromise based on the designer’s value system. Yours may be different than Fran’s.

I suspect if Fran had located the exit in the middle of the hood and didn’t provide the required ducting to mate with it (or a builder decided they didn’t want to use it) you’d see reports of bloody murder and overheating and flawed design because the builder deviated from the requirement. Placing the exit at the back is the easiest lowest common denominator position ensuring the best possible exit assuming the builder does NOTHING to properly design their cooling system - you know there’s folks like that out there.

All that said, I wouldn’t push the 1/3 opening / 1/4 exit philosophy to others without something more concrete/applicable than what’s been presented so far. Reading what you initially posted could drive a builder down the wrong path without due diligence on their part to really understand what’s going on with their cooling system design.

Builder-designed/fabricated radiator enclosure, shrouding, sealing, ducting, etc is going to have a far more dramatic impact on cooling efficiency than the exact size or location of that exit.

Regardless, I wouldn’t let that feature dictate whether you purchase an SLC or not. Just move it if you really feel it needs to be moved. There are plenty of examples out there (by way of silence or otherwise) where the factory exit was retained and there are no reports of overheating - but who knows how accurate that statement is that I just made.

Make yours 25% and located at the middle of the hood and tell us how it performs :)
 
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