Flat Floor and managing heat

[Fisher]

I'm trying to fathom how I can modify a front engined production sportscar with the exhaust routed under the floorpan to a flat floored configuration.

The major obstacle is the heat generated by the exhaust system.

I am toying with the idea of constructing an aluminium floor which would be flat but I hope it could serve as a heatsink for the exhaust system.

Is this a very silly idea I have had late at night and how could I calculate the coling effect as the underside of the car will hopefully have laminar airflow over it.

Does anyone have any experiences how similar problems have been solved?

[Fisher]

I have used thermal transfer calculations to work out the cooling effect. I am more concerned about exacerbating the heat problem in the cockpit.

[Alex Hodgkinson]

Hello Fisher.

Nice topic/discussion point, and a problem I also have to find a solution to.

This is obviously a really common thing to come across as a high number of GT1/2/3 cars are front engined and utilise a flat underside. From what I've seen most of those teams have less to worry about as they terminate their exhaust system before the rear wheels. I don't know what car you're running, but maybe you could look at doing that as one option?

Failing, or supplementing that solution I've thought of two possible routes we may take outself.

The first is to add louvres to the underside, along the length of the exhaust system. You're obviously thinking about adding a flat bottom for aerodynamic reasons, and the louvres option could work well with that. My theory is that by allowing heat to be transferred to the air under the car you're lowering the pressure. That's of it according to the simple fact that higher temperatures = lower pressures. In doing this you will be reducing lift, or increasing downforce depending on how effective your car bodywork is.

The second idea I've come up with is to channel air along the tranmission/exhaust tunnel from the front of the car. On our car this could be easily done as the engine is tiny (Mazda RX-7.) If you make sure the air stays neatly ducted to the rear of the car you could be reducing aerodynamic drag by allowing it to escape into a low-pressure area.

Although I'd thought about these solutions before you started this topic I hadn't really planned to implement any of them, having given aerodynamic efficiency priority over what is mainly comfort. I will probably put these into practise if exhaust temperature built up proves to be a problem. Our motor is turbocharged so it could be a real possibility..

[Fisher]

Originally Posted by Alex Hodgkinson

Hello Fisher.

Nice topic/discussion point, and a problem I also have to find a solution to.

This is obviously a really common thing to come across as a high number of GT1/2/3 cars are front engined and utilise a flat underside. From what I've seen most of those teams have less to worry about as they terminate their exhaust system before the rear wheels. I don't know what car you're running, but maybe you could look at doing that as one option?

Failing, or supplementing that solution I've thought of two possible routes we may take outself.

The first is to add louvres to the underside, along the length of the exhaust system. You're obviously thinking about adding a flat bottom for aerodynamic reasons, and the louvres option could work well with that. My theory is that by allowing heat to be transferred to the air under the car you're lowering the pressure. That's of it according to the simple fact that higher temperatures = lower pressures. In doing this you will be reducing lift, or increasing downforce depending on how effective your car bodywork is.

The second idea I've come up with is to channel air along the tranmission/exhaust tunnel from the front of the car. On our car this could be easily done as the engine is tiny (Mazda RX-7.) If you make sure the air stays neatly ducted to the rear of the car you could be reducing aerodynamic drag by allowing it to escape into a low-pressure area.

Although I'd thought about these solutions before you started this topic I hadn't really planned to implement any of them, having given aerodynamic efficiency priority over what is mainly comfort. I will probably put these into practise if exhaust temperature built up proves to be a problem. Our motor is turbocharged so it could be a real possibility..

The car is an Rx7 too. I am thinking of using the underside of the car to cool the exhaust. I had considered ducting air but felt it upsets the aero somewhat. I am more curious about how well an alloy underside could dissipate heat. its area is much larger than that of the exhaust system and being aluminium is a much better conductor. If I looked at front diffuser design or incorporated a wicker on the LE of the underside I could ensure that the airflow is well stuck to the floor of the car.

[CWT]

My theory is that by allowing heat to be transferred to the air under the car you're lowering the pressure. That's of it according to the simple fact that higher temperatures = lower pressures. In doing this you will be reducing lift, or increasing downforce depending on how effective your car bodywork is.

From what i know from Highschool physics is that higher temperature increases pressure... T1/P1=T2/P2

So having hot gases under the car would increase the pressure under the car decreasing the effect of the diffuser.

Am i right?

[Mr.Jingles]

In a known volume (sealed system), yes. In this case? Who knows! Like you I've only got school physics to fall back on, so I can't decide if the heating of the air will cause it to expand, 'spill' out the sides and thus lower pressure or if the aerodynamic effects will trap it under the car, increasing pressure. Any aero guys around?

In any case, I'm guessing - purely guessing - the effect will be minor. I mean, it will hardly heat the air at all due to the shear volumes involved.

[JohnD]

My experience is with a front engined car that I modified to have a rear radiator. These cars are notorious for excess in-cabin heat, often attributed to the exhaust, but without the front radiator it became quite chilly.

Ultimately, the rear-rad experiment was not a success, but I'd suggest striving to vent the hot air from a front radiator somewhere other than under the car. Then the remaining under-car airflow will be adequate to deal with exhaust pipe heat alone.

John

[AU N EGL]

Originally Posted by JohnD

My experience is with a front engined car that I modified to have a rear radiator. These cars are notorious for excess in-cabin heat, often attributed to the exhaust, but without the front radiator it became quite chilly.

Ultimately, the rear-rad experiment was not a success, but I'd suggest striving to vent the hot air from a front radiator somewhere other than under the car. Then the remaining under-car airflow will be adequate to deal with exhaust pipe heat alone.

John

an Extractor hood to vent the hot air going though the radiator to exit out of the hood. Or buy some plastic or Carbon fiber vents and cut holes in the hood behind the radiator area and put the vents in the hood too allow the hot air to escape.

also there is some very good heat shielding matting that you can put under your car or up against your fire wall and transmission tunnel to shield heat.

The other thing could be to put new ceramic coated headers on. This helps move the heat out the exhaust pipe vs radiate out from the pipe.

Old headers should really not be ceramic coated so new headers would be needed.

There are many ways to cut heat in the cabin down.

[dtype38]

Originally Posted by Fisher

I have used thermal transfer calculations to work out the cooling effect. I am more concerned about exacerbating the heat problem in the cockpit.

Just a suggestion. Duct the exhaust over the flat floor from the engine bay to the back of the car but leave the ends of the duct open to allow (natural or ducted) airflow from the radiator air into the front of the duct. Then heat-wrap the exhaust tubing for the length of the duct and coat the upper section of the duct in foil coated heat shield matting to minimise the heat transfer to the cabin. Think that should keep the temperatures under control.

[S14GJC]

NO! Don't heat wrap the exhaust horrible, useless stuff! I have found recently this coating from Zircotec through Thorney Motorsport which is ceramic and is brilliant! It is only 350 microns thick so for packaging it's brilliant as well. Check out their site to fill you in, don't wrap though you'll only end up doing it in another 6 months or so.

Lots of benefits.

[dtype38]

Originally Posted by S14GJC

NO! Don't heat wrap the exhaust horrible, useless stuff! I have found recently this coating from Zircotec through Thorney Motorsport which is ceramic and is brilliant! It is only 350 microns thick so for packaging it's brilliant as well. Check out their site to fill you in, don't wrap though you'll only end up doing it in another 6 months or so.

Lots of benefits.

Would agree if you know that your exhaust system is optimum and you won't be adjusting it in any way. With mine I seem to be regularly modifying the downpipes to fit other changes on the car, which is much easier if its wrapped.

[S14GJC]

Originally Posted by dtype38

Would agree if you know that your exhaust system is optimum and you won't be adjusting it in any way. With mine I seem to be regularly modifying the downpipes to fit other changes on the car, which is much easier if its wrapped.

dtype38 - How do you modify your downpipes, do you mean change them for a different configuration?

[dtype38]

Not for different configuration as such. More stuff like (in my case) altering the downpipe angle to miss a revised passenger footwell, adding two extra joints to allow easier assembly, changing the length/angle of the final section before the silencer to accommodate a different silencer box diameter, adding on a boss to fit a lambda sensor. That sort of stuff. But I guess it only really applies if you constantly fiddle and modify like I do, many will get a setup and keep it that way, so ceramic coating would be a better way to go :-))