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Heat Exchanger Ducting Advice

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Hi all,

After watching the episode on the ducting of the cooling system of project sr86 I am motivated to attempt to fix the overheating issues I have with my sr20 turbo Silvia, and was hoping I could get some guidance.

The car is registered and street driven, as well as being used for track duties so unfortunately a full race car cooling setup is not an option for me. Having said that, I would like to incorporate some race car inspired ducting to help with my issues.


The car currently has an intercooler mounted directly behind the front bumper, with an oil cooler mounted behind that - directly in front of the radiator, which is in the stock location.

The radiator has twin 12” thermo fans (from an AU Ford Falcon) that are shrouded and sealed to the back side of the core.

Power is fairly modest at ~230kw atw using a GTX2860 and all supporting mods you'd expect on a typical early 2000's street car build.

I have done some measuring of the car, and also drawn up some concepts (refer attachment) of what I believe to be my options.

Option 1

This involves leaving all of the heat exchangers in their current locations & adding ducting only.

Option 2

Like to Option 1, all heat exchangers remain un-moved. The only difference being where the upper portion of the ducting picks up on the intercooler.

Option 3

This involves moving both the intercooler & oil cooler (most likely replacing both) in an attempt to lengthen the duct in front of the intercooler. It also enables a shallower angle on the upper portion of the ducting.

Although not shown on the drawing, this option would require the intercooler piping to come out towards the front of the car to connect back up to the existing piping – some of this piping would be inside the duct work unfortunately.

Due to the car being fairly clean and registered. I have some limitations on what I am willing to do, which include:

1. No cutting of structural supports, of which there are 3x~40mm wide struts tying the radiator support to the chassis (indicated on the drawing)

2. No cutting of bumper bar

3. No cutting of bonnet

Are you able to review the proposed options and advise which one you believe would give the best result, or whether there may be a completely different option that I haven’t considered that would yield an even better result.

While I understand the importance of what happens behind the heat exchangers, I am unsure as to what I can do in this space without ducting the air flow out of the bonnet. Any additional advice on this would be much appreciated.

Thanks in advance,


Attached Files

Of those options, personally, I'd go for option 1, and I'd also see if I could find a reasonably priced front bumper that could be trimmed to improve the feed to the charge cooler.Most vehicles have also got air inlets above and/or behind the bumper, if you have that option run the top of the ducting from the upper edge of that opening - even use a split duct if required.

Don't forget, getting air to the coolers is only half the problem, you also need to pass the air from the back of the radiator to the outside of the vehicle. Can you post a pic' of the fan and shroud design, there are many people who have very restrictive shrouds with insufficient clearance and/or lack of 'pass through' vents - even some commercially produced ones are sorely lacking. Most modern vehicles also have lower engine covers/belly pans/splash guards, largely for aero' reasons, and these can also compromise the exit of underbonnet (hood) air - you may find some gain removing part, or all of, it.

Thanks for the response Gord. I'd prefer not to go down the path of a second bumper. I'd like to at least try and get something to work with what I've got. I understand it'll never be ideal, but that's just the way it goes when you try and use a street car as a race car 😝 Getting additional air from above the bumper (through the grille) could be an option worth exploring, although there is a fair bit of structure there that houses the bonnet latch.

Interesting that you mention the factory splash tray, as it is a recent addition as I was of the understanding that without it there was a high(er) pressure zone behind the radiator. The tray extends about 200mm past the front of the engine, behind that I believe the openings would be large enough to let the air escape, whether they are and whether it does, I have no idea 😂

I've attached a couple of photos of the thermo fans.

Attached Files

Fan setup looks about as good as any I've seen, so not likely an issue. If it's just the front part, the under-tray shouldn't be hurting the evacuation of the under-bonnet air - one of those things to try with, and without, to see what affect, if any, it has.

I'd be inclined to go with option 1 myself as it's the easiest and cheapest solution. you'll be surprised how much benefit you see form proper ducting to these coolers. Personally I'm not a huge fan of oil coolers being mounted in front of radiators where possible. It's easy for the oil temp to reach 120-130 + deg C so of course the air flowing off that cooler onto your radiator is then going to be super hot. Of course this is really dependent on how much excess capacity you have with your radiator in the first place but since we're having this conversation I'll assume it's not enough.

Thanks for the response Andre. I am glad you're also advising to go with Option 1, as that's what I have been working on for the last week.

I understand what you're saying about the oil cooler. I not long ago increased the size the oil cooler in an attempt to control oil temperature - which it did, but it just transferred the problem to the coolant.

I guess once I am confident I am getting as much air to the area as possible I can fiddle around with the size of the oil cooler.

It is certainly a slow process, especially with so many obstacles in the way. Once done I will post some photos, but for now the best I've got to share is a rough CAD model to help get my head around the angles and connections.

Attached Files

Get a vented bonnet you sook.

Might have better off with twin oil coolers at the sides like the FD. I do have a S5 rx7 one with internal thermostat new in a box I was going to use on the old rx7 if you could fit that in, maybe a modest "v-mount" with that over the radiator slightly tilted back?

I'm going to make a bold statement and say the stock viscous fan would solve all your overheating problems :p

Many have tried going to thermofans to ditch the ugly viscous and ended up going back, we tried this on my bro's s15.. similar power level 330hp at the hubs and it became common for it to reach 100c temps. His tuner at the time who was familiar with sr's advised to swap back as he'd seen it many times before. It sits around thermostat temp now under all conditions.

If you want to stick with the thermo's it might help to revise your temperature presets and consider running it more like the stock fan does, ie lots of constant airflow... trying to cool it down after it starts getting hot just doesn't seem to work that well on sr's for some reason. Not at all saying ducting or bonnet vents are a waste of time though

I am certainly not against using the factory clutch fan, and it actually looks quite neat when combined with the factory shroud, even if it does hinder access to the front of the engine a little (a lot)

I would've thought the best scenario on a racetrack where the car is going at speed would be to have no fans at all. What obviously is important, as mentioned by Gord, is the design of the shrouding on the back side of the radiator, maybe this is where the Nissan engineers got it right, and where the main advantage lies over most thermo fan setups. The design certainly appears to follow a lot of the correct principles.

I'll certainly keep it in mind if the current plan doesn't solve my problem.

Attached Files

This is just my two cents. But I’m my opinion you probably do need ducting. But have you also considered moving the oil cooler to behind the radiator? That way the cooler radiator air (especially at the bottom of the rad) is flowing over the hotter oil cooler. Rather then the problem you have of the reverse.

also the biggest issue I see with your fans over stock is flow. Perhaps added some venting to it would help. Like the cosworth rubber shroud vents that open under air from from driving but are sucked closed when the fans activate.

the way I see it is the current shrouding is restricting air flow over the radiator. The stock shroud and fan would also probably fix that and grant you room to relocate the oil cooler. Unless you have your mind set on the thermo fans then it might be something to consider.

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