Sarting and engine without exhaust

[Forces]

I see threads like this all the time, how many of you have ever had an exhaust valve burn up or crack from ambient air getting into the exhaust port? I've started up alot of brand new motors with open manifolds, and some even without manifolds.. and never had a valve break or crack. I wouldn't worry about it in the least bit at all. It will just look like this. There's no way air is going to make its way in the exhaust port with the gas flowing out like that.. even on decel. The only time ambient air will make it to the exhaust valve is on engine shut down, and it's not going to do anything.

 

That's what I've heard.

I am less concerned about breaking a valve than other issues. It isn't THAT cold where I live during this time of year. On a bad day it gets down to 30 degrees F during the middle of the night. It is most often about 45 degrees during mid day. I am more concerned about noise and shooting flames all over the ground that are coming right off the collectors.

So I'm thinkin I gonna put the final touches on her, and fire it up today....maybe tomorrow if I run out of time. That'd be a good way to kick off the New Year. Thanks for all you help guys.

 

Seann

[boodlefoof]

I have never had it happen, but have seen it happen on dirt track cars running open headers, and nothing more. Have help repair a few of these heads after the fact. One particular guy could not figure out how he could alway be runnig so well then down the back strech he would run into lap traffic and have to slow down. When he slowed down the exhuast was no longer pushing Hot air past the valves, instead it started to suck in cold night air. This caused the valves to become brittle and crack, then when you rev it up again, they would start falling to pieces, and the enigne would lock up tight.

 

HE never could figure it out either, then he asked the guy that I was helping rebuild heads, what is causing this?? He told him to put on a longer exhaust, or a header muffler, and the problem would go away. And guess what? It sure did. SO yes it can happen, but not just from starting it and running it for 10-15 minutes at 2000 rpm. What will cause it is running it real hard, 5000+ rpm, with no exhuast, then slowing down to 2000 rpm or less in a short amount of time. The reason it does not happen with full exhaust or even a slightly longer exhaust has no thing to do with back pressure, but everything to do with incoming COLD air.

 

SCotty!

 

 

I've seen it happen too. At a shop I used to bum around at, a guy (customer) had run his car with open manifolds for a half hour break in and then shut it down. When the head came off, one of the exhaust valves had a big crack in it.

[Teekass]

I've also had exhaust valves crack when running uncapped headers....mostly the zoomie style (Schoenfeld) http://www.schoenfeldheaders.com/zoomie.htm .....but since you are using a header with a collector, there is an easy fix......PLUG the collectors when you shut down the engine......something as crude as a welders glove will work fine. Is it a must that you do it? Probaly not, but it's pretty cheap insurance.

[Forces]

Shouldn't be a problem, except for the noise. If this is the first start on a brand new engine, or an engine with a new camshaft it will need to be broken in, which means at least 20 minutes of stationary running. I've never heard about exhaust valves cracking due to running open headers, but it sounds possible. The whole back-pressure deal is non-sense, all back-pressure dose is push down on the piston while it's trying to push exhaust gasses out. Zero back-pressure is what your shooting for, less negative force on the engine, the more power reaches the wheels, that's why most racecars don't run mufflers. It's even possible to have positive pressure sucking ehaust gasses out of the engine if you place the exit of your exhasut in a low pressure area.

 

The back pressure will change once exhaust is added into the mix. If I set the timing with zero back pressure, and then ad exhaust onto the enigine, would that manipulate the previously set timing?

[Alf]

It shouldn't affect the timing, unless you have severe back-pressure that the engine pulls significantly less vacuum at idle, and then maybe your vacuum advance may not work the same as before, but I don't really see that happening. It may change your fuel mixture, so some carb adjustments may be needed.