Board logo

Interior heat from exhaust and cat
Slimy38 - 21/11/23 at 08:23 PM

I've got the exhaust and cat as the next job. The cat is on the first part of the body, on the outside of where the passengers feet would be. Then as normal the exhaust is alongside the passenger. Everything seems to sit fine, but just looking at positioning I'm wondering about the passenger getting a bit warm.

I'm assuming that many hundreds of Haynes roadsters before me have shown that an exhaust being an inch or two from the body doesn't cause an issue, but cats get a lot hotter so is there anything I need to do to protect the passenger? Or am I overthinking how hot things will get outside.

For reference there is 1mm steel between the cat and the passenger. In terms of height the bodywork is limiting where they can go, so the bottom of the cat and the exhaust is in line with the bottom of the chassis. Some cars I've seen have the exhaust sitting slightly lower than the chassis floor.

Badger_McLetcher - 21/11/23 at 11:14 PM

Mine's no further away and I've not yet had an issue with it

adithorp - 22/11/23 at 07:33 AM

Is it external as on a seven or through a sill, given your profile says Healey rep'?

Slimy38 - 22/11/23 at 07:44 AM

quote:
Originally posted by adithorp
Is it external as on a seven or through a sill, given your profile says Healey rep'?


It's outside of the chassis, but inside the bodywork. Here it is mounted on the chassis with the steel plate in place;




The outrigger for the body can be seen just underneath the exhaust, the body curves out from there. So yes it is enclosed in a sill, but the bottom is open to airflow.

Here's a better angle;



[Edited on 22-11-23 by Slimy38]

Mr Whippy - 22/11/23 at 08:04 AM

I don't think cats actually generate heat but are hotter simply because they are located closer to the engine. So it's not going to heat your car up any more than the exhaust already would.

adithorp - 22/11/23 at 08:26 AM

So very similar to a Fury/Stylu/Phoenix.

I'd say line both the side panels and sill with heat mat (foil topped, glass fibre stuff). If you're really worried about the bit next to the cats (yes, they run hotter) then double line there. Also open the front and rear of the sill to allow air flow while moving and put vent holes in the bottom (allows better convection while stationary). Use woven mesh at least on the front to stop flammable debris getting in.

Furys, etc have experienced sill fires but to my knowledge, only when the exhausts have been wrapped; exhaust runs hotter and any gap in the wrap or blow has been blamed for the ignition. So don't wrap it!

And your passenger will still complain about being too hot in summer...

U2u me your phone number and I can WhatsApp you some pictures of mine if you want.

[Edited on 22-11-2023 by adithorp]

Slimy38 - 22/11/23 at 09:12 AM

quote:
Originally posted by adithorp
So very similar to a Fury/Stylu/Phoenix.

I'd say line both the side panels and sill with heat mat (foil topped, glass fibre stuff). If you're really worried about the bit next to the cats (yes, they run hotter) then double line there. Also open the front and rear of the sill to allow air flow while moving and put vent holes in the bottom (allows better convection while stationary). Use woven mesh at least on the front to stop flammable debris getting in.

Furys, etc have experienced sill fires but to my knowledge, only when the exhausts have been wrapped; exhaust runs hotter and any gap in the wrap or blow has been blamed for the ignition. So don't wrap it!

And your passenger will still complain about being too hot in summer...

U2u me your phone number and I can WhatsApp you some pictures of mine if you want.

[Edited on 22-11-2023 by adithorp]


Ah brilliant, thanks! Yes it's the same sort of design to those cars, I knew I'd seen it somewhere but couldn't get the right search term. I've been looking at 'haynes roadster exhaust' as a comparison but a quick search for Fury's and Stylus's has brought back what I'm aiming for. Even the exhaust tip exiting out the side will be what I'm aiming for.

The bottom of the bodywork doesn't reach the chassis, it stops at the outrigger so there's plenty of airflow there. The rear is basically the wheel arch but I like your idea of opening the front. At the moment it's the front of the body section but replacing that with a woven mesh sounds like a good idea.

And yeah, I'm staying away from exhaust wrap.

cliftyhanger - 22/11/23 at 09:24 AM

A metal heat shield would be handy, close to the exhaust /cat. OEM cats have a shield crimped around them for good reason.
Then some heat mat stuff on the bodywork.
I would be inclined to direct airflow into the area too.

But copying what others have done in similar situations is teh obvious place to start!

adithorp - 22/11/23 at 09:36 AM

Initially mine was only open at the ends and when stationary or in traffic it got very hot. Onvection was only working at the ends.Opening up the holes in the bottom allowed a better convection flow, starting at the middle.
Your open bottom will be good when moving but when stationary, only if the rising hot air has an escape route.

Slimy38 - 22/11/23 at 09:49 AM

quote:
Originally posted by cliftyhanger
A metal heat shield would be handy, close to the exhaust /cat. OEM cats have a shield crimped around them for good reason.
Then some heat mat stuff on the bodywork.
I would be inclined to direct airflow into the area too.

But copying what others have done in similar situations is teh obvious place to start!


I am using the OEM cat but the shield was beyond repair, it had come loose and there wasn't enough real metal to put it back in place. Thankfully it had done a great job in protecting the cat inside so I've got a really solid unit that I can reuse. The temperature sender is also FUBAR but that is often bypassed on MX5's anyway. Apparently it's not even a proper sender, it's basically a temperature fuse that just breaks when things get too hot.

I'll have to look at air flow, I suspect moving should be fine but things could get warm sat at the lights.

Slimy38 - 22/11/23 at 09:50 AM

quote:
Originally posted by adithorp
Initially mine was only open at the ends and when stationary or in traffic it got very hot. Onvection was only working at the ends.Opening up the holes in the bottom allowed a better convection flow, starting at the middle.
Your open bottom will be good when moving but when stationary, only if the rising hot air has an escape route.


Hmm, you have a point about an escape route for hot air when stationary, I'm not sure there will be one. I'll have to have a think about that.

adithorp - 22/11/23 at 12:12 PM

quote:
Originally posted by Slimy38
quote:
Originally posted by adithorp
Initially mine was only open at the ends and when stationary or in traffic it got very hot. Onvection was only working at the ends.Opening up the holes in the bottom allowed a better convection flow, starting at the middle.
Your open bottom will be good when moving but when stationary, only if the rising hot air has an escape route.


Hmm, you have a point about an escape route for hot air when stationary, I'm not sure there will be one. I'll have to have a think about that.


An open end will be enough.