Normally, exhaust gas temps are handled in three ways: heat shield, exhaust/header wrap, or ceramic coating.  This assumes that increasing the distance between the heated muffler and car body is not possible.

Your problem is appearance.  You want the muffler to contain the heat, send it out through the exhaust pipes, not radiate it to the car body.  Since you have no room for a heat shield and a exhaust wrap tape is unsightly, you seem limited to ceramic coating.

It would help to know what the surface temp of the muffler in question is at shutdown after a high rpm run.  Inexpensive laser temp gauges are available to do just that.  You would measure the muffler temp as it is, then compare temp after the same run with the ceramic coating.  Then you could measure coating effectiveness.

You could try a halfway measure first with VHT or similar high temp paint.  Check temp with no coating, then with high temp paint. 

Of course, as in many automotive subjects, you are engineering a test with lots of unknowns.  What temp will harm fiberglass? blister paint? Can you lower the muffler 1/4", 1/2", etc?  How much exposure at what temp will cause damage?  The paint is likely more prone to cosmetic damage/blistering than the fiberglass.

As replica owners, we are often faced with engineering challenges. My advice is to mount the muffler, drive it hard, measure surface temp.  Let everything cool, mask, coat with high temp paint, drive, measure temp again.  If you see a sufficient drop in temp, then consider having the muffler ceramic coated, which should lower temps further.  Some coatings look great, some not so great.  All you have lost with high temp paint is some labor and the cost of the paint, since the coater will strip it first anyway.

I can't imagine high temperature paint would provide any insulating value to the muffler. My understanding is that high temp paint is called that because it doesn't flake off under high temperatures. I do agree that ceramic coating the muffler would be your best solution.

There are claims by a number of manufacturers that their paint insulates but those claims are in stark contrast to the results of testing by a number of different universities.

First, I have zero experience in dealing with exhaust temp on a fiberglass Speedster body, but I have a LOT of experience dealing with exhaust heat in Snowmobiles.  So that said and assuming that you do not wish to paint or otherwise wrap your muffler/exhaust pipes, here goes:

So the fiberglass fibers begin to melt around 1,200º F so no worries there - your muffler will never get to 250º F.  Don't know what you have for resin, but the cheapest stuff out there begins to melt around 380º F.  

Your head temps are usually under 350º F but the exhaust temps (coming out of the head) can be 500-600º F on a highway.  Out at the muffler, with lots of air flow around, it will be much less, under 275º F range. 

So now we know that the fiberglass will be OK, so the concern here would be the type of paint used and at what temp it begins to "cook" and discolor.  That, because we simply don't know, is the issue, so I'll assume nothing there is gonna change.  

If you spot probe your mufflers after a hard run you'll have some idea of what you're dealing with.  One of my local Autozones has an IR spot probe temp probe - maybe one of your local ones has one, too, to rent.  Come tearing into their parking lot, shoot the muffler outer housing right at the body and there you'll know.  Please bear in mind that you have no idea if (a.) it is a good quality probe or (b.) it has been recently calibrated or, (c.) it is anywhere near accurate (many are not), so you're winging it.  I have noticed wide variations using my Fluke IR temp probe, and Stan the Refrigeration guy confirms that they're all notoriously inaccurate because of reflection of the surface being probed.  

Personally, I would shotgun the situation - if you have 1/4" or more of space between the muffler/pipe and the body, you're probably OK, but to be safe you could place a piece of header/muffler wrap or wood stove braided fiberglass door gasket between the muffler and the body and be done with it.  

Another alternative would be a piece of silicone heat shield, commonly used as kitchen trivets and found at Bed Bath and Beyond  

https://www.bedbathandbeyond.c...ord=silicone-pot-pad

That stuff will protect the paint from muffler heat up to around 400F, but you'll have to design something to keep it in place (maybe silicon caulk?)   I would attach it somehow to the body, if the muffler sits in a hemispherical cut-out, but use your imagination.  It should easily protect your paint from excess (>350F ) heat.  Just figure out a good-looking way to get it between the muffler and body.

Hope this helps.  gn

I agree that ceramic coating is a great way to keep a lot of the heat inside the exhaust. I would definitely do that.

The next thing I would do is attach a metal shield of some type around the muffler on the side near the fiberglass you want to protect. But leave an airspace, even 1/4" would help. Airspace is an excellent insulator.

Silicone smells when it gets hot. 

Thanks guys!  The headers on the car are ceramic coated and everything is wrapped right to where the muffler bolts on.  The engine and trans are coming out soon and I will take your advice and adhere some insulation to the underside of the fiberglass.  I've used high temp silicone before with success so will go that route.  I'll also see if I can get just a bit more clearance.

Contact Pete Weber. 

http://www.quiethorsepower.com/mufflers.html

He has custom mufflers for 911

On my IM6, Henry had to custom make a muffler as one was not available commercially in the time span we needed.  He did a good job on this.

My point is that the muffler actually touches (just barely) the rear valence that we installed on the car.  So far, the fibreglass is fine, and that is with the muffler making direct contact to the fibreglass.  There is a little discolouring, but is not that noticeable, and it's under the rear bumper, so who sees it?

So, if your muffler does not actually touch the car body, all should be fine.  

I also asked Henry for some 'insulation' for above the oil tank, which gets hot, but the fibreglass there is just fine so far.

I need to get to NOTL someday and see that car of yours.  

Thanks Bob.  That car will be coming apart soon, as soon as my hand heals.  Don't try to hold a metal piece with your bare hand while drilling a hole in it with a drill press.  And the glove were right there.

I came across this and just for piece of mind I think I will fit them on the muffler.

https://paceperformance.com/i-...-4-thick-2-pack.html

This sport muffler is 2 in and 2 out and here is a picture of one.  It is a tight fit, has to be angled up and then turned.  You couldn't remove or replace the engine with the muffler on.  It has a real nice sound and, with a couple of chrome tips, looks very much like a stock 356 unit.

https://www.google.com/url?sa=...ust=1541540363467954

Hopefully the links work.

Inter911 posted:

Thanks Bob.  That car will be coming apart soon, as soon as my hand heals.  Don't try to hold a metal piece with your bare hand while drilling a hole in it with a drill press.  And the glove were right there.

I came across this and just for piece of mind I think I will fit them on the muffler.

https://paceperformance.com/i-...-4-thick-2-pack.html

This sport muffler is 2 in and 2 out and here is a picture of one.  It is a tight fit, has to be angled up and then turned.  You couldn't remove or replace the engine with the muffler on.  It has a real nice sound and, with a couple of chrome tips, looks very much like a stock 356 unit.

https://www.google.com/url?sa=...ust=1541540363467954

Hopefully the links work.

 

The Google link doesn't work.

Funny, both links in your message work for me.   I can never figure out how these computer things work or don't work.  Give me a car any day.

Robert M posted:

Inter911 posted:

Thanks Bob.  That car will be coming apart soon, as soon as my hand heals.  Don't try to hold a metal piece with your bare hand while drilling a hole in it with a drill press.  And the glove were right there.

I came across this and just for piece of mind I think I will fit them on the muffler.

https://paceperformance.com/i-...-4-thick-2-pack.html

This sport muffler is 2 in and 2 out and here is a picture of one.  It is a tight fit, has to be angled up and then turned.  You couldn't remove or replace the engine with the muffler on.  It has a real nice sound and, with a couple of chrome tips, looks very much like a stock 356 unit.

https://www.google.com/url?sa=...ust=1541540363467954

Hopefully the links work.

 

The Google link doesn't work.

Inter911 posted:

Funny, both links in your message work for me.   I can never figure out how these computer things work or don't work.  Give me a car any day.

 

Yes it is working. I didn't realize it was a "redirect" message. I saw the white page and message and assumed it was a 404 type error. The only error was a keyboard-user interface problem.

Carry on.

Here's a product that new Miata owners use to protect the plastic back bumper from heat - called Lava Shield.  I've not tried it.

https://www.heatshieldproducts.com/lava-shield