Forces Posted December 28, 2006 Share Posted December 28, 2006 *STARTING AN ENGINE WITHOUT EXHAUST* I am in a bit of a pinch here. My engine is all done, wiring done, ready to go. But I don't have any exhaust yet. It's just got some block huggers on it. I was planning on getting it running and towing it to an exhaust shop. The problem is that I want to start the engine, set the timing, Air fuel ratio, and make sure everything is in good shape before getting it towed any where. Is it Okay to start it w/o exhaust, or do you think it would be way too loud to hear if there was a tick or clack in the engine?? The guy at the exhaust shop told me I could start it up with headers and just drive it down there like that.....but, huh, I want a second opinion, that sounds kinda risky. Seann Quote Link to comment Share on other sites More sharing options...
boodlefoof Posted December 28, 2006 Share Posted December 28, 2006 You can damage the engine if you run it with a short exhaust and then shut it down in cooler climates. The outside air will quickly go up your block hugger and cause the exhaust valve to cool very quickly. With a full exhaust, outside air is less likely to get to the valve and air is warmed by the time it reaches it. Rapid cooling may cause the exhaust valve to crack. Quote Link to comment Share on other sites More sharing options...
nscason Posted December 28, 2006 Share Posted December 28, 2006 ok. i rebuilt the whole top end of my motor. all i had was a header on it with no exaust. it is ok to start it like that. you just dont want to drive it around like that a whole lot cuz there isnt enough back pressure. you can drive it to the muffler shop after the timing and all that is set up. it will be very loud though. if there is any laws about exaust noise you might want to go to a shop close to where the car is. the only thing wrong with a straight header is the lack of back pressure. thats only if you dont have a race engine that dosent need it. hope i helped some. Quote Link to comment Share on other sites More sharing options...
Alf Posted December 29, 2006 Share Posted December 29, 2006 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. Quote Link to comment Share on other sites More sharing options...
Challenger Posted December 29, 2006 Share Posted December 29, 2006 The whole back-pressure deal is non-sense Im not sure about that, I think a motor has to have some back pressure. Im not sure if its that you have to tune it but I took my exhaust from the catylitic converter back to do some work and started it and it wouldnt run well after 3000 rpm. If you didnt need backpressure Im sure ricers wouldnt have to buy theyre own 4 inch exhaust for theyre cars, Itd come from the factory as a "superduperfastperformanceenhancer package". Quote Link to comment Share on other sites More sharing options...
blueovalz Posted December 29, 2006 Share Posted December 29, 2006 My understanding of this issued is you'll need enough exhaust tube length to prevent the outside ambient air from getting to the hot exhaust valve during reversion of the exhaust gas at the begining of the intake stroke. Depending on the cam profile, a certain amount of exhaust gas, at a certain RPM range, will be drawn backward from the exhaust port (or tube) toward the valve. What you don't want is a tube so short that the ambient air is drawn far enough into the exhaust that this ambient air hits the very hot exhaust valve. I think it is generally agreed that when you include the length of the exhaust port itself, the header tube only needs to be a few inches in length to effectively prevent this from happening as the engine is running. I believe the more radical the cam is, the greater risk of this happening with very short open tubes. Quote Link to comment Share on other sites More sharing options...
280zwitha383 Posted December 29, 2006 Share Posted December 29, 2006 I've always thought that as long as you've got the headers pointed downward you should be ok because of the little principal that hot air rises. And that air is pretty hot, so it will be hard for cold air to get into the header unless it's pretty windy. However FWIW, I started my engine the first time with one header down and one header up because of the steering shaft. I didn't let it run for a long time because of what we talked about; but, If you have to let the cam break in I wouldn't do it with the headers pointing up. Quote Link to comment Share on other sites More sharing options...
Challenger Posted December 29, 2006 Share Posted December 29, 2006 Depending on the cam profile, a certain amount of exhaust gas, at a certain RPM range, will be drawn backward from the exhaust port (or tube) toward the valve. Thats what I was thinking that the headers make the gas move quicker because it concentrates the heat? The motor would kinda be hyperventilating, in a sense. Quote Link to comment Share on other sites More sharing options...
sweetleaf Posted December 30, 2006 Share Posted December 30, 2006 Yes you need a certain degree of back pressure to get a proper scavaging effect. How much depends totally on the whole engine combo. You should be more than ok to drive it to an exhaust shop with only headers though. Tunnning for proper scavaging/exhaust pulse/backpressure helps your engine breath better, with adds to more torque/HP etc... Quote Link to comment Share on other sites More sharing options...
Forces Posted December 30, 2006 Author Share Posted December 30, 2006 I figured that It wouldn't be a problem, I don't think I'll be driving it anywhere though....It's the end of the year and the cops are out for blood right now. Open headers are extremely loud, I know this. My neighbors know this. But will I be able to hear things like a knocking bottom end, chattering lifters/ rockers? (without stethescoping everything on the block and heads) Thanks for the help Quote Link to comment Share on other sites More sharing options...
Paz8 Posted December 30, 2006 Share Posted December 30, 2006 If there is any noise in the engine you will hear it, crank the engine a few times with the distributor disconnected so you build up some oil pressure, then start it. You have the right idea about setting timing, idle speed etc. Stick a vac. gauge on it and set the carb idle mixture, once everything is set you can adj. the idle speed down and it's not that loud. You can check everything for leaks, antifreeze and oil. Much better to have it setup and leak checked before installing the exhaust, you will have to go over your idle and carb adj. after the pipes are installed. Your not going to hurt the engine, unless it was put together wrong. Quote Link to comment Share on other sites More sharing options...
Guest scotty! Posted December 30, 2006 Share Posted December 30, 2006 My first post, but there is some good info for you here. You will not hirt your engine, unless you rev the heck out of it. The returning, cold, air coming back up the exhuast can cause your valves to crack and fall to pieces. But like has been said a few times, as long as you are just breaking in the cam, setting timing, and carb you will be fine. Make sure if you are breaking in a new cam, to get it up to the break in RPM ASAP, even if timing, and carb are not right. Keep close eye on Oil pressure, if it does not come up quickly, with in 30 sec or less, then you need to shut it down and find oil problem. Better to break cam in, then adjust everything else. A flat cam lobe is no fun, espcially after installing it once already. SCotty! Quote Link to comment Share on other sites More sharing options...
strotter Posted December 30, 2006 Share Posted December 30, 2006 Something to be careful of when starting a motor with just headers - dirt & dust & filth. On your fist start, you'll probably not have the air filter on. When that puppy catches, the rush of exhaust will motivate every bit of filth under the car into the air & thus toward your intake. Happened to me - it's amazing what's down in the lawn yet still mobile - and I wouldn't count on a driveway being pristine, either. Hit the area with a leaf-blower before first start. Also, a suggestion: if you have a couple of old mufflers laying around, mount them directly to the collectors - they'll fit with some hose-claps and creative use of strips of sheetmetal (I had some mufflers from a pickup with a 3" inlet, then used sectioned cat-food cans to fill out the diameter of the collectors). If you want you can go down to Kragen or P-boys and get an pair of adapters so you can mount them neatly - but find a size that fits on the outside of the collector, you don't want to deform it pinching down the hose clamps. Doesn't need to be a perfect fit, but it doesn't have to be, and you'll be able to take your time and hear everything. Obviously this only works if at least the front-end of the car is up in the air. Quote Link to comment Share on other sites More sharing options...
HarrisonTX Posted December 31, 2006 Share Posted December 31, 2006 I wouldnt really reccommend it. I installed some double hump heads (closed chambers) and 7 out of 8 pistons hit the head. Which caused major bearing wear, and a spun rod bearing. had to get the crank turned and new rod. It least make sure everything is gonne work together! (domed pistons) Quote Link to comment Share on other sites More sharing options...
defrag010 Posted December 31, 2006 Share Posted December 31, 2006 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. Quote Link to comment Share on other sites More sharing options...
Guest scotty! Posted December 31, 2006 Share Posted December 31, 2006 I wouldnt really reccommend it. I installed some double hump heads (closed chambers) and 7 out of 8 pistons hit the head. Which caused major bearing wear, and a spun rod bearing. had to get the crank turned and new rod. It least make sure everything is gonne work together!(domed pistons) I can not see how having the exhuast off would cause this. Sounds to me like you had some clearance issues internally. If the psiton hit the head would not quite turning, leading me to ask, how in the world did that casue bearing failure? SCotty! Quote Link to comment Share on other sites More sharing options...
Guest scotty! Posted December 31, 2006 Share Posted December 31, 2006 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. 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! Quote Link to comment Share on other sites More sharing options...
HarrisonTX Posted December 31, 2006 Share Posted December 31, 2006 I can not see how having the exhuast off would cause this. Sounds to me like you had some clearance issues internally. If the psiton hit the head would not quite turning, leading me to ask, how in the world did that casue bearing failure? SCotty! I guess i wasnt too clear. open exhaust didnt cause the pistons to hit the head. The open exhaust was so loud i couldnt hear it happening. They just hit a little nothing major. Now, i think that it caused bearing failure because the piston hits the head, and still needs to travel up, and the crank is trying to push it up, the piston gave a little, and the bearing took the rest of the damage, it was kinda smashed. No problem, this weekend was spent slowly clearencing the heads, and waiting on a new rod. Quote Link to comment Share on other sites More sharing options...
bluedragon436 Posted December 31, 2006 Share Posted December 31, 2006 I know that I have built and tuned plenty of older Chevy Camaros and such, and they ran ok without exhaust to get the down to the exhaust shop, just don't get on it until you get the exhaust.... Now one other thing, is possibly be prepaired to retune some after having the exhaust installed, as it will change your back preasure and can also cause some issues that way, but other than that you should be ok to run it up with no exhaust for a short bit... Quote Link to comment Share on other sites More sharing options...
BillZ260 Posted December 31, 2006 Share Posted December 31, 2006 Just FYI We started my SBC for the first time yesterday w/ just the shorty headers on. It was about 60 deg and damp but no issues. Quote Link to comment Share on other sites More sharing options...
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