Has anyone ever installed Exhaust Cut Offs after their headers? How? Welded? Do they help a lot at the track? Where would they go exactly? Where can I get them? Any problems? Suggestions?

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sure they work, it's about the same as running open headers. jegs has them in their catalog. they can be clamped or welded in.

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ICON Motorsports

1st & 3rd

 

Cut outs work great. The best place to put them is right behind your headers, before the y-pipe thats where mine are. My car runs about .1-.2 sec faster in a 1/4 mile with them open.

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cutouts right after the headers will act like open headers. The result is massive reversion at lower RPMs and a dramatic decrease in torque. A TPI car will be a complete and total dog with this mod.

don't believe me? try it and see.

For a streetable car, the best place to run a cutout is behind the axle, and even then the reversion still affects low RPM torque and only pays off when the RPM's come up past the torque peak.

Open headers work on cars that leave the line at 4000+ RPM and have shift recoveries above 4000 RPM. This rules out most street cars and most cars running a 700R-4.

Running the cutout behind the axle will net some gain by the end of the quarter because the bottom end losses are eventually outweighed, but the losses are still there.

You would be luck to not run a full second slower trying to run open headers on a fairly stock TPI car.

ODB

 

I have done extensive testing using a vericom on the effects of uncorking & cutout placement.
An engineer friend of mine has developed the solution to get the best of both worlds. It is a driver controlled electronic exhaust cutout. This will allow you to launch corked up for maximum torque and then activate the cutout at will, or better yet via an RPM switch. With the RPM switch you could also have it close up for shift recovery if that's what the car likes.

the site that distributes these is at
www.empiremotorsports.net

the pic of the cutout
http://www.mccordcg.com/car_stuff/cutout_pic1.jpg



[This message has been edited by The ODB (edited February 21, 2001).]

 

ODB,

No disrespect, I've read other posts of yours and you know what you are talking about, but I have tried "it" and like it was mentioned above I picked up on average .15 sec and 1 - 2 mph with the headers uncorked on a pretty much stock L98(have timeslips of before and after runs and it is very consistent gain).

After going the 383 route, the results are pretty much the same. Though, maybe I'm not getting the reversion you are talking about because my dump is at the end of the 'Y' pipe where the Cat used to be.

My friends 468 '70 Chevelle picks up .3 tenths and 4 mph and his dumps are welded right on the collector flange of his headers.

Love the idea of the driver controlled electronic dump. Are you planning on marketing it?

Sparks a flyin.

 

As he already said, the power cut-out is FOR SALE at the above mentioned site.

 

I know cutouts work. I've used them for about 9 years myself and get similar gains.

I also know from using a vericom during testing that you get both losses and gains by running uncorked. In the end the gains are usually big enough to see by the end of the quarter. Surely you've noticed the lower RPMs feeling very 'mushy' while uncorked because of the loss in torque?

There is always reversion, but it's effects can be deminished by obstructions (as in flowmasters) or by distance from the exhaust valve (placeing the cutout further away).
Open headers have terrible reversion and give away large amounts of low RPM torque. A car needs to stay at high RPM's to take advantage of open headers.

A severely restricted exhaust will respond dramatically to uncorking, but you are still giving up power over an optimized exhaust. Understand what I'm saying?

An optimum exhaust would block reversion at all RPM's and maintain zero backpressure as well. I am working on a muffler to do this.
In the meantime, the electric cutout is the next best thing.
The cutouts are made by a friend of mine, and I have nothing to do with sales or production. He is an engineer and is helping me with the new muffler design.

ODB

 

Heres a crazy idea. Wouldnt it be cool to build an electronic module that slowly opened the cutout as the RPM increased? hehe I think I just invented a new rice burner toy...

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the power cutout uses a stepper motor and what you're saying can be done in steps by using RPM switches or an RPM window.

 

What is reversion and how is it different from back pressure? I always thought you had to have some back pressure to stay running?? Thanks. ~Arnold

 

when a high pressure pulse from the exhaust stroke exits out of an open header,
there is an equal & opposite wave that travels back into the exhaust.
All of the high pressure pulses are immediately followed by a low pressure area (how scavenging works). When this low pressure pulse is exposed to atmospheric pressure via the open header collector, then the atmosphere tries to rush in to equalize pressure. This is the origin of the reversion wave that travels up through your exhaust towards the head and disrupts both flow and the out-bound pulses.
The loss of power from open exhaust is caused by reversion waves and NOT a lack of pressure in the pipe.
Flowmaster mufflers effectively block reversion waves by using chambers and fixed metal shapes to reflect the in-bound waves. This does reduce reversion at lower RPM's but also hurts flow at higher RPM's by increased backpressure.
At higher RPM's the outbound waves come quick and many, and they overcome the effects of reversion in open exhausts.
True backpressure is the stacking up of exhaust gasses in the pipe which causes a rise in pressure that is seen by the upward moving piston on the exhaust stroke.

Both backpressure and reversion reduce the engine's output. Unfortunately when you open an exhaust to the atmosphere to reduce the pressure, you also increase reversion.

As I said, I am working on a solution to this problem for street cars.

ODB

 

Well, I understand reversion as you explained it, but in answer to the "mushy" question - No, the car doesn't feel any less torquier (if that is a word ). Actually, at the track, I can't get into the throttle as quick uncorked because it will simply roast the tires. Corked, I can roll out, then matt it without loosing traction. (Have all this on video tape somewhere and will try and post it when I'm able to get it on my computer).

I'm not trying to argue with you about this, just stating what I have experienced.

Blade, yeah I saw it after I responded, but didn't feel like editing my post.

Sparks a flyin'.

 

sparks,
I see you run a 383. If you have a small exhaust already then that is a different story. Are you saying you run straight open headers?

If your exhaust was restrictive corked up then that makes the engine run much richer. I have experienced the same thing while running TES headers & flowmaster on a 383. The rich A/F ratio will kill more torque than the reversion would have. Opening the exhaust up will lean the engine and make an engine snappier if it was too rich before.

To qualify my claims, this was my test situation. My exhaust was not very restrictive through the muffler (Borla) and I had already tuned the carb to the lean limit of power while corked. This is a similar situation for many street cars as they do not want to run rich all the time incase they decide to uncork.
A guy with a racecar will tune the carb for running open headers and could care less about reversion since he is rarely at low RPM's.
I completely believe you about your car and have experienced the same thing myself. I have also seen the racecars get really big gains in torque by running collector extensions or several feet of straight pipe to reduce reversion.
The situation is complex and I realize that there is no blanket statement that will cover all scenarios.
A rich car will gain more from uncorking as it brings the A/F ratio closer to max power, just as a rich car will gain more on a cold-air night than a car tuned dead-on.
so yeah it all depends, but for most street cars it is not optimal to run open headers or a cutout close to the headers.

thanks for your input.
ODB

 

ODB,

No, my dump is on the end of the Y pipe of my TES's. Thats why I said that I might not be getting the reversion.

From what you just described about the overly rich condition and the dump, I believe that might be the situation I'm in.

I was watching the tape from one of my runs and saw the tell tale black smoke on the 2/3 shift, and a little more close to when I was hitting the traps.

I'm currently running the 1 5/8 TES to the dump, stock piping from there to a Flowmaster, and stock pipes from the muffler. I have 24# SVOs (other mods in sig)and am still running the stock '89 MAF. I know all these are contributing factors to the way my car is running right now, but have not had time (new baby and trying to build a new restaurant) to compensate for any of these.

Last summer it ran a best of 13.2 @ 105.4 with the computer picking up 90 knock counts and pulling about 15 deg of timing. I fixed that, only to have the tranny go out (could only muster a 13.5 at 99mph in that condition). Since fixing the tranny I've not been able to get to IRP here in Indy to see what she will do. All my runs were on Bridgestone RE730s?? street tires.

Now, after giving you my life history , here's some ?s for you. I don't have Vericom, but have a package called EASE (gives everything the computers sees). What would be the best way to tune for my haphazard setup until I either get a custom chip or make one? Should I shoot for an optimum O2 reading/injector duty cycle range by adjusting the fuel pressure (was at 46psi and getting about 80% duty cycle at WOT when I was in the false knock stage mentioned earlier)Which leads to - in your experience what would that optimum be? Should I try playing with the heat range of the spark plugs (go hotter)?

These are all things I have been planning on trying, but haven't been able to get to. Am I on the right track?

Any suggestions/help is appreciated.

Thanks again,

Sparks a flyin'

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Sparks,
TES headers on a 383? that explains everything. Like I said, a car with severe exhaust restriction will turn into a monster uncorked.. reversion or not. No doubt your engine is rich.
Pull a sparkplug and rub your finger over the tip to check for black soot.

Do not use hotter sparkplugs in your 383. I run the coldest nonresistor plugs I can get which are Autolite #51.

I think optimum O2 readings are said to be in the 500-600 milivolt area. My car ran best at 250mv in high gear. Does that tell you anything?

Put your money into the exhaust side, because TES is nowhere near enough for a 383.

You have a lot of good parts already. I wouldn't worry about O2 readings or duty cycles. Borrow someones vericom or G-tech for testing & tuning.
First thing is determine your total timing. You will need to know initial (of course) and how many degrees advance your computer adds and at what RPM.

You could play with fuel pressure all day and not get anywhere. The first tuning steps I would do is to get a much better exhaust system (no TES, no Flowmaster), and get a good quality non-computer distributor so you can control the timing curve easily. I know all this costs money, but be patient and do it right.
Once you get your total timing to 35-37 degrees before 3000 RPM's, then you can play with the fuel delivery. Look at your trap-speeds... go richer or leaner (usually leaner) until trap speed stops increasing.

Hope this gets you on the right track.

ODB


[This message has been edited by The ODB (edited February 22, 2001).]

 

ODB, good discussion. I don't want to take anything away from this. But when you refer to your O2 sensor, are you referring to the stock O2 sensor?

If so, I can already read what you are saying and I completely agree: tuning by the stock O2 sensor is some more "disinformation" given by the Ford boys to slow down us Chevy guys.

Unless a person has an expensive WB O2 sensor, you are wasting your time anlayzing the O2 sensor. ODB's readings confirms that.

The stock O2 sensor is strictly a "switching" device designed to operate in a narrow range around .450 mV. As you go WOT (or lean for "Highway Mode") the accuracy of the O2 sensor is way off. FYI, I have been experimenting with "Highway Mode" in the eprom and I get O2 readings between 4-50 mV when I lean out to 17.2:1. Coversely, @ WOT an equal days I can get different readings from as low as 780 mV to almost 850 mV.

Forget the O2 sensor (unless its a Wide Band..this is one of the reasons GM disabled "Highway Mode" in it's eproms even though virtually all have the code there). Go with the plugs and "quanitative results" (track times) as ODB suggests.