Exhaust System Length Hot vs. Cold??

Are there any guidelines for how much clearance to leave your mufflers/piping in expectation of the exhaust system "growing" when it heats up??

The body of my muffler is about 1/2" away from my trailer hitch. I've noticed that when the system is cold it never contacts the body of the vehicle. However, when it finally heats up, i can hear the muffler rattle an mildly hit the body of the car. Thoughts???

I can relocate the muffler forward a little bit, but how am i supposed to expect the exhaust system to expand???

Thanks!

I had to put about an inch between the muffler and the hitch, due to expansion.

Steel has a typical thermal expansion coefficient of around 12 E-6 (m/m). If your exhaust system was 3 m (120") long from header to tail pipe tip and the average temperature increase of the exhaust system was 75 C during operation the increase in length of the exhaust system would be about 0.3 cm / (0.1 "). Given the typical flex in exhaust hangers, I don't think expansion is going to be an issue.

75c seeeeems pretty low.
I was taking to a guy who road races an RX7. The entire exhaust is mounted on spring loops because it grows over 2in in length... the scratch marked along the sides of the exhaust seem to confirm 1in+ of movement.

But, a road racing RX7 that shoots flames out the exhaust is not a volvo 240

culberro said:

75c seeeeems pretty low.
I was taking to a guy who road races an RX7. The entire exhaust is mounted on spring loops because it grows over 2in in length... the scratch marked along the sides of the exhaust seem to confirm 1in+ of movement.

But, a road racing RX7 that shoots flames out the exhaust is not a volvo 240

Click to expand...

I agree that 75 C temperature rise is low if you are considering the peak temperatures in the exhaust system. I was talking average temperature over the complete length of the exhaust system.

Consider an exhaust system starting out at an ambient temperature of 25 C. In operation, with an ambient of 25 C, I expect that the tip of my tailpipe when the car is not in motion might reach 35 C so a 10C rise. I have measured my EGTs at low speed and they are about 375 C. However, that is 3 cm from the exhaust flange. Down at my collector where the linear part of the exhaust system starts the temperature is more in the 200 - 250 C range, so a 175 - 225 C rise above the starting temperature. Assuming a linear drop in temperature along the length of the exhaust system (which I admit is probably not valid because of the effect of mufflers and bends in the pipe), the average temperature rise of the pipe using the 225 C value is (225 + 10)/2 = 117.5 C, so my initial estimate on average might be light. However, the 117.5C average temperature would only change the linear expansion to 0.44 cm or 0.17 in - still pretty minimal.

My example exhaust temperatures are at idle. The header flange temperature gets a lot hotter under high load; but, presumably the car is moving fast under high load. That motion creates a phenomenal amount of convective cooling and I expect that the linear expansion of the exhaust system may not be that much greater than at idle. If the car is sitting on a dyno with no air flow doing repeated full throttle tests that could definitely create a different situation.

The 2 " expansion on the RX7 seems a bit of a stretch (no pun). The RX7 exhaust is probably less than 3 m in length; but, lets stick with that number since a shorter system just reduces the expansion. To increase the length of a 3 m long steel pipe by 5 cm (2 "), due to thermal effects you would have to raise the average temperature of the pipe by 1388 C. That is the average temperature! Plain jane carbon steels melts in the 1400 - 1550 C range. The whole exhaust system would be bright incandescent and the head pipes would likely have melted.

That's good info!

I think i'm just gonna lop off about 1/2" of pipe, which will allow my muffler to come forward a bit.

Just to add to the discussion.... I also think that the exhaust system length varies based on load, driving conditions, air cooling, and weather. My contact point is in the very rear of the system. I've found that when it's cold and i'm just cruising on the highway, it doesn't contact. However, when in the summer months and i'm in traffic, the system contacts more.

Also temperature related is the compliance in the exhaust hangers. The rubber will stretch more under load when things are nice and toasty which may result in body contacts which are not present when things are cool.

Sawing 1/2" off the exhaust pipe is a viable solution. However, if the contact noise occurs when driving, examine your motor and transmission mounts. They may have failed allowing for unplanned motion in both the drivetrain and the exhaust system. On my 1987 745 B230FT I was chasing a clunk from somewhere up front for a long time. Finally discovered that the right side engine mount had sheared. The engine was still sitting on the mount at rest and there was no obvious visible displacement of the mount at rest. The engine's weight was supported by the mount at rest; but, the engine was free to move back and forth, sideways and hop on the mount.

culberro said:

75c seeeeems pretty low.
I was taking to a guy who road races an RX7. The entire exhaust is mounted on spring loops because it grows over 2in in length... the scratch marked along the sides of the exhaust seem to confirm 1in+ of movement.

But, a road racing RX7 that shoots flames out the exhaust is not a volvo 240

Click to expand...

I've had many, and worked on many/drove... RX-7s.

We're talking street ported Turbo II rtek ecu, 1994 w/spirit R goodies (twin turbos, etc), 1991, 1985 12a w/goodies (but 115hp~)

I have not noticed my exhaust moving enough to need springs. I ran the 1985 hot enough to melt the air snorkel off the intake LOL.