The Stovebolt Site Forums The Shop Area Engine Shop Running an engine without exhaust....Risky?

I read where you shouldn't run an engine without exhaust due to possibility of cold air being pulled back in the block and bending/cracking something. Seems like a newly rebuilt motor would be more susceptible to this. I'm talking about in the shop, with good ventilation, not running down the road like this. What do you guys think, an old wives tale? Has anyone ever actually had it happen to them?

I've heard the story [possibility of warping valves] but never had any problems, what I do when running an engine with no exhaust [just manifold] is stuff rags in the outlet as soon as it's shut down .... I doubt it's a real serious problem

Bill

When I finished the restoration on my '59 I drove it about 20 miles to the muffler shop with only the Fenton headers on it, no pipes at all.
That was a few years ago, no problems yet!

Yes sir, when I was 16 I thought it was so cool to run open headers on my 71 rally sport and laughed at my mechanic whom had been an army mechanic 100 years prior to knowing me. Well, guess what, 2 years later he was rebuilding my heads...lol Yep most of the exhaust valves had been "frosted" as he put it. darn know it all teens...lol

Tim

The real draw back is you can not hear if there is something wrong with the engine. Never run a rebuilt engine without a muffler, it could be tearing itself apart and you would never hear it.

For short drives on a worn engine, you shouldn't have any problems. Add a foot of pipe if you really worried about it. Exhaust pulses don't travel to far up the pipe, and even if they get to the valves, the valves are not going to be hot enough to get damaged. Pulling a full load up a hill is a different story.

Joe

I have looked that subject up many times, and have really seen no hard evidence that that is anything other than a myth. I have no actual experience, other than driving to the muffler shop after I put the headers on. Mostly because it is simply too loud, and I don't need the possibility of being asphyxiated from the exhaust gas.

When there's no hard evidence either way, I like to act on the side of caution. You'd probably be OK, but it would be a shame to donate your rebuilt engine for research and have it become hard evidence

I am not sure how that is accomplished when you consider valve action and fuel flow.

If there is any cold air getting sucked in through the exhaust, then you have a more serious condition than just a lack of pipes going on.
Carl

the [reputed] problem isn't while the engine is running but after it's shut off hot, and then cold [ambient] air washes across open valves [hot air draws it up the manifold]

Bill

Lots of people run open header, their engines are still running. Think drag racers. They run balls out then shut the engine off.

10 minutes of operation on a drag engine means it's ancient. Some of them hand grenade on the first run. Who cares if the valves warp a little?
Jerry

Do you have first hand exp Lincoln? I'm not trying to sound like more of an a-hole than usual, I'm actually curious. I drove 25 miles open headers to my exhaust shop. Also broke in my cam open header. Valves aren't bent or burned. Is it with OE equipment that it becomes an issue?

What about the old piston engine air planes? Isn't a lot colder at altitude? Wouldn't they have been severely suseptibale?

they shut the engine down at altitude??

Bill

Have never ran an engine without at least the exhaust manifold bolted to the head but I've ran a many of them with just the headers.
...my two cents is some one, years ago, maybe on the artic circle, fired up and engine without even headers bolted to and when it shut off the cold air tighten up the valves

yes Alvin, that is about where I came out in my search. But the way I heard it, it wasn't the artic circle, but a really cold winter day in Michigan...

Being an old fart almost 80 I have lots of tales to recall. I owned a circle track and a couple of the hobby stock cars ran no exhaust manifold at all, just straight out of the heads for the whole season. An interesting bit was that when standing in the infield on the dark side of the cars you could see the exhaust valves glow bright red while the driver had his foot in it and darken instantly when he backed off. Tale # 2: a fellow that owned a motorcycle shop across from my shop had a Nash Metropolitan with a 327 in it and the exhaust came straight out of the heads. I would go to work at about 3AM when we had lots of parts to vat (hot tank) as the heat was unbearable after about ten in the morning. I could hear him fire up the Metro at about 7:30 and drive it the dozen or so blocks to his shop. I guess the cops were more mellow back then.

I had a friend who owned a Taylor Craft airplane, a two seater fabric plane, tail dragger. I flew with him one weekend scouting roads over my deer hunting area in southern Arizona. The exhaust manifold bolted to the block was the only support for the manifold and short exhaust pipe. The next time he flew it, he was in final approach at Tucson International Airport, and when he throttled back, the plane started to vibrate and shake so severely, he felt it necessary to shut off the engine to keep the thing from shaking apart. He landed safely without power.

Upon inspection, they found a crack in the exhaust manifold from the stress of the unsupported exhaust pipe and an exhaust valve had cracked. They speculated that cold air hit the valve when suddenly throttled back. I believe they fabricated a support for the exhaust pipe to keep it from happening again.
Kent

I'm with Alvin and others on this one, myth busted. Aircraft engines aside. My Dad ran dirt track cars in the late 70's and like everyone else ran with only headers...no exhaust. Depending on how we ran, if we lasted late in the season, we were racing in late October and I remember sitting in the stands freezing my butt off. Never an issue. I'm thinking the myth got started when someone was cleaning up the shop with compressed air and shot a blast up a hot manifold and maybe bent a valve. Unless the Professor comes up with some good Physics/thermodynamics here, one less thing to worry about. Keep calm on bolt on !

I think that the biggest risk of running an engine without any exhaust is not being able to hear something going wrong mechanically. I can't say I've ever ran and engine without an exhaust manifold or a header on it, but I think the idea is just plain stupid, as in just don't do it. I can't offer up any physics to the why not but the flip side, why would you? We ran a dirt late model in the 80's and early 90's and in those days all we ran were "zoomies" one pipe for each cylinder that went straight down toward the ground, they were about 12" long. We didn't worry about bending valves even running in the fall with temps in the 40's. Then in the 90's the engine guys realized how much horsepower those darn zoomies were robbing and we went to chassis headers, equal length tubes with a collector that was 3-4" and usually around 18" if we could make it fit. The drivers all could tell the increased throttle response with the change.
I guess it all comes down to, if it sounds stupid it usually is stupid.

A "friend" of mine ruined my Brand-New 350 LT-1 (WITH DUAL EXHAUST).when he tried to get my '73 Monte Carlo up to 130 M.P.H.! Three miles later, The P.S. belt blew,which took out the alternator/fan belt! Between the two belts breaking,both upper and lower radiator hoses were slashed! When we got it pulled over,and got the hood open,the exhaust manifolds were glowing Yellow! I came back the next day,and when I got the belts,hoses,and anti-freeze back in it,I started it,and all I heard was POP-POP-POP,from the warped valves! The exhaust "donuts" disintegrated,allowing cold air to warp the valves. If you're going to drive it at any speed or distance-GET SOME PIPES ON IT!!

Wetwilly, from your explanation it sounds like you had dual exhaust out the back but the donut disintegrating cause the problem. I still can't understand this. Could the engine's excessive heat have been the culprit rather than the disintegrated donut? Other than the valves was there other damage? I hung around in my brothers shop where he built racing engines for over 30years (doesn't mean I'm any smarter though) and we seen a lot of strange things, and some not so strange, on the dyno and on the track. Just curious of all the other damage.

Are you people serious? 130mph and cold air got in your manifolds? That's like 6000+ rpm. Cold air got in and warped some valves? I need another beer...

I think that you would lose performance without a properly tuned exhaust. Stock systems are to make a car quiet for the consumer, not to help the engine perform.

Tuned systems are not required to be loud.. proper cylinder savaging means you need to do the math, not just unbolt stuff.

Well as I understand it running without exhaust is not good. the reason I think is because of vacuum both in the intake of fuel and the exhaust of hot burnt gas. The exhaust manifold is a vacuum chamber. Anytime one cylinder fires and is exhausted it creates a vacuum on the other cylinders. this helps exhaust the hot gas. Where the warped valve comes into play is not when the engine is running but when the engine is shut down. At any given time either an intake or exhaust valve is open or both valves to some extend. Not to say it could not happen running if their is no cooling your headed for a melt down, but what happens is when you shut down the hot exhaust heats the air in the cylinder causing is to expand. the expended air flows thru the partial open intake valve causing a vacuum drawing colder air into the cylinder, cooler air warps the valve. Ok that said, running no manifold at all not very good, running with manifold good, low rpm short time 3 to 5 minutes no not temperatures, manifold with short tail pipe 2 to 4 foot Ok I guess keep eye on block temperature, manifold tail pipe muffler best, I thing back pressure comes into play here also, manifold tail pipe muffler get hot so if cold air gets sucked backed into engine it's from the hot exhaust not outside air. Yes rebuild fresh engine run hotter than use or broke in engine, Best way to break in new rebuild is with every hooked up, this way you did everything you could to insure it will run without problems, peace of mind.
When I did rebuild on a engine I run 3 to 5 minutes with exhaust manifold hooked up no hot temp just warm to see if any weird noise, then install with full hookup everything,do break in rpm 15 minutes rpm and 15 minutes higher rpm, shut down cool engine startup engine drive moderate. Well could be wrong but safe way is full hookup Jbennett

Wouldn't the valve have to cool very rapidly to warp? Manifolds get hotter than heck. I think the time frame just doesn't work for me. I personally have never seen an expample of this. Im also in sunny San Diego where we don't have seasons. Maybe Ive just been lucky thus far.

Where did this 'cold air will warp the valves' thing come from?

A tube header holds no heat. Cast iron was not chosen for exhaust manifolds because of some mythical 'keep the valves warm longer' reason, but because cast iron is easy to pour into complex forms (like an exhaust manifold) and machine.

I'd like to see some sort of verified information (SAE or something) that talks about this issue.. most of what I see is rumour and 'I hear that'...

Like you should not lay a crankshaft on its side or it will warp and you need to add ZDDP to an old motor.

From what I have read so far there are two origins for this.

Back in the 1920's there is a lot of 'talk' that rapidly cooling an engine (running full speed then shutting the motor off) will warp just about everything. Evidently back in the day cool downs were uber important. But also in the 1900-1920's people were tinkering with motors and the ICE was very new.... so most of what is documented can be attributed to either poor material quality and poor design. I know of no aircraft motor that required a cool down, most had no exhaust other than a short tube (if that.. rotary engines were common in AC) and most, if not all were shut down immediately after landing. This is a Sopwith Camel Clerget motor.. open exhaust Over 5000 Camels were made, so if there was an issue with open exhaust valve warping someone would have said something.

Second, taking off headers will reduce back pressure and lean out your mixture. Very lean mixture can lead to burned valves and very high cylinder temps. I'm gonna say THIS is the main reason why (since the 40's) that people think they need headers. Take a motor with a carb mix set for X back pressure it will run fine. Remove the header and now the motor moves MUCH more air, and the carb is not jetted for to account for the increase, leans out the mixture and burns the exhaust valve/warps it due to high temps. But since the 'only thing done' was removing the manifold TA DA! A new myth is born!

I think we have killed the horse and no need to continue beating it ....although I like to read.