Another exhaust tweak

[69427]

Was trying to figure out the next modification/update to the '69, but hadn't thought of any big type item so I decided to try something I've kicked around for a long time. Some of the published engine gurus say that an H pipe near the header collectors is the most efficient place for it, good for a few horses, so I thought I'd entertain myself doing some exhaust work while figuring out the next "somewhat bigger" project.
I tacked a couple of 2 1/2" pipe stubs on the collector reducers to allow the crossover pipe to be a slip-on setup. I didn't want the weight or hassle of clamps or flanges.



I cut a couple of elbow pieces to connect them "together". The pieces wont be welded at the center as I wanted to have a place for the pipes to move as they get hot and grow. I'll make a sleeve to fit over the center to cover the gap.



The hardest part so far has been getting the pipes to meet in the middle due to the collectors (and reducers) not being mirror images of each other. The rightside header pipes are bent differently so the collector flange is about an inch farther rearward of the leftside collector flange. This meant that the bends had to go in three dimensions. The up side of this was that I can remove the two pipes by rotating them so the first one points away from the other, and then slide both of them out. I also flattened one side of the center section to give more air space by the transmission case front edge. This extra room will let me add a slight heat shield and an air gap between the pipe and transmission case.

It's definitely not fancy, but if I can pick up a couple somewhat free horsepower for a couple hours work, I'll be happy. The extra weight is minimal, and it's down low near the vehicle center.

 

[BBShark]

I like the elbow and sleeve idea (rotate the elbows to get them out). What are you planning on using for a sleeve?

 

[69427]

I like the elbow and sleeve idea (rotate the elbows to get them out). What are you planning on using for a sleeve?

Well, got two options in mind, just depends on what eventually works out. With my limited fabricating talent I figure I've got about a 30% chance to make a D shaped slip joint or sleeve to connect the two pipes, and a 70% chance to make a stainless steel wraparound sleeve for the junction (similar to our teenage years when we would wrap and clamp an aluminum pop can around an exhaust pipe leak).

Just gotta see what works out with the limited movement that I can do with these pipes.

 

[69427]

Finished up welding the pipes and painted/connected everything up.



I fired it up to check for leaks. To my surprise the exhaust note was noticeably softer. The car has always had a somewhat prominent (and unwanted) "thump" sound in the pipes as the exhaust came out of the collectors, but this has been reduced significantly.

I kept the engine running as I put the lift up to look under the car. The new paint and greasy fingerprints on the crossover pipe were smoking, so that indicated that I was getting a decent crosstalk of exhaust flow. That explains the difference in exhaust note. I originally started this tweak to get a few more horsepower (5-8 hp as claimed in some respected literature) but the softer exhaust note was a welcome bonus.

The two pipes ("D" shaped for transmission clearance) butt up to each other, and then I used some thin aluminum as a "soft gasket" wrap, then two pieces of tubing to sandwich/contain the aluminum, and then some worm clamps to keep the tubing in place. Everything is allowed to slightly shift and grow with temperature.

I re-did the area just to the rear of the "tap" pipes. Looking through the pipes from the collector's view it looked like the exhaust flow might impact the rear side of the tap pipes, causing a slight restriction, so I cut out an area behind the taps and welded in an angled patch so that the normal flow wouldn't be impacted, and would also allow the crosstalk flow to exit the other side easier.

 

[JeffP1167]

More weight

 

[mrvette]

Dumb question, does it make much difference if the pipe is done to the rear of the trans, vs up front the way you have it?? what say the Xperts??

:crutches:

 

[69427]

More weight

The weight is well below the vehicle c/g, and if I get 2-3 hp per pound of this added weight I'm a happy guy.

 

[69427]

Dumb question, does it make much difference if the pipe is done to the rear of the trans, vs up front the way you have it?? what say the Xperts??

:crutches:

The purpose of H and X pipes is to balance out the pressure/volume in both pipes. The further upstream that the "shunt" connection is made, the more efficient it is. Remember that there is always a pressure drop through the system, with the pressure being highest upstream at the source (the header collector output). The double exhaust pulses (from cylinders 8&4 and 5&7) cause higher instantaneous pressure at the collectors and reducers, and adding the shunt path close to the collectors reduces the magnitude of these backpressure levels.

 

[JPhil]

Here is something I posted on DC way way back when I left CF years ago ( and before VM)about exhaust pipes.....I actually had to log back in to DC & search my posts to find and copy this, I don't want to try to re-cap it....



"The entire induction/exhaust system must be looked at as a whole. They must compliment each other to work efficiently. Efficiency = power.
Don't think of 'air flow' so much as 'pulses' and 'pressure waves.' At the mouth of the carb you have 14# of static air pressure. You open a valve to a vacuum (bear with me here for the sake of illustration) and the air column is not sucked in: it is pushed in. It is not a smooth flow. It is start-stop-start-stop because of the valves opening and closing. Pulsing.
Same thing on the exhaust side. The exhaust pulses in the pipes are a high pressure 'slug' followed by a low pressure 'blank'. This causes a high speed reverberation of pressure waves moving back and forth in the exhaust pipe system.
The idea is to get the pressure wave pulses of both intake and exhaust timed so as to enhance each other. You want a negative wave from the exhaust to be at the cylinder at the moment when the intake and exhaust valves are open (the overlap) to compliment the high pressure pulse in the intake manifold to help 'pull' more charge into the cylinder. This is 'scavenging.' This reverberation in a long exhaust tract does push some exhaust gasses back into the cylinder, but this is acceptable for the benefits it provides.
Your carb, manifold, cam, heads, & exhaust must all work in harmony to compliment each other.
One key element many people overlook is that the velocity of the gasses for both induction and exhaust is very important. Smaller intake runners and pipes typically work better on smaller less radical lower RPM (street! RPM) engines than big ones (high RPM track engines) because a smaller mass moving faster can carry a stronger 'punch' than a larger mass moving slower. It also takes less energy and time to change it's movement: more efficient 'pulsing'.
However, when you are tuned for a full length exhaust system and you uncork it, the exhaust gas is straight out and you get less contamination of your intake charge due to the lack of back pressures during overlap. The contamination richens your mixture, when you remove it, you now are running lean. Long pipes give a wide torque band, but with that comes some contamination of the intake charge. (EGR: introduce exhaust gasses back into the cylinder for a richer mixture and you then lean out the carb for better emissions.)
There is an awful lot more than this to it, it is the subject of many volumes.



--------------------------------------------------------------------------------


Where the 'H' & 'X' pipes come in is to reduce the pressure & pressure waves, both positive and negative, in the pipe by effectively increasing the volume of the pipe. The high pressure wave on one side will typically correspond to a low pressure on the other. By connecting the two, you create an alternative route for some of the pressure waves thus reducing the effect they have upstream towards the valve, plus reducing pressure differentials downstream in the pipe. This is why H & X pipes sound quieter. If you hold your hand on the end of a single pipe while the engine is running, you notice it flaps back and forth: pushed out by the pulse and then sucked back in. This is the 14# atmospheric pressure pushing in to the negative pressure pulse. The crossovers reduce this also.
The main difference between an 'H' and an 'X' is that an H lets most of the actual exhaust gas pulse travel down it's original pipe, while relieving the pressure waves. The X actually takes one pipe and splits it into two. You can hear the difference between all three: individual pipes have a distinct alternating 'thump' from each pipe. An H system will still have an alternating 'thump' but not as distinct--some of the noise & gas will come out the other pipe, but the most noticable difference would be less 'back-sucking' if you hold your hand over one pipe. The X system splits all pulses into both pipes: instead of individual alternating 'full thumps', you get continuous 'half thumps' from both pipes. They sound almost like a 12 cylinder, a soft purring instead of a nasty rumping. That's one reason some guys don't like them. But they do the best job of smoothing out a pulsing exhaust flow."



There's a few things I think I understand.....computers ain't one of 'em, but gas/mass flow I can......I think....As I said, there are volumes written about this and I know I don't know it all....

 

[69427]

Another system tweak

Making a quick tweak to the exhaust. I was real happy with how the H pipe softened the exhaust noise and I wanted to see if I could make things even a touch quieter, and possibly get a few more somewhat free horses. The old Y pipe (you can see where I cut it) behind the trans crossmember had a modest volume (about 170 cubic inches) to it (I just enclosed the Y portion of the pipes) but a larger volume ought to make life easier on the mufflers by reducing the pressure pulse magnitudes. Also, a large volume behind the header collectors is supposed to operate as a pressure wave termination box to facilitate low-pressure waves to reflect back to the exhaust port to help evacuate the cylinders. It's a neat concept, but I've yet to see any formulas to be able to calculate the correct pipe length and volume sizing for whatever RPM point I was interested in. So, I'm just adding the additional volume that will package into the area behind the crossmember. Some quick measurements and calculations gives me a number of 570 cubic inches of box volume.
Here's where I'm at presently.

 

[bobs77vet]

nice work !! i noticed a difference in both sound and low end torque when i added the H pipe. i am not sure how i see that new Y pipe working with only one collector end and splitting into two....i guess i am missing a piece some where?

 

[69427]

nice work !! i noticed a difference in both sound and low end torque when i added the H pipe. i am not sure how i see that new Y pipe working with only one collector end and splitting into two....i guess i am missing a piece some where?

The two pipe ends are the pieces that slide (from the rear) into the crossmember. The "S" pipes from the header collectors feed those two pipes. The large pipe is the output section that goes to another Y (under the differential) that feeds the two mufflers. Here's an ancient picture of the system a couple iterations ago.

 

[Twin_Turbo]

Why not try a terminator box?

Sent from my GT-I9000 using Tapatalk

 

[69427]

Why not try a terminator box?

Sent from my GT-I9000 using Tapatalk

You'll have to pardon my confusion, I guess I'm not following your question. According to my understanding of things, this is a terminator/resonator box in function. The trouble I have is that the packaging location options in a Corvette are limited, and I'm still trying to find credible info (ie:formulas, wave velocity) to tell me what location (pipe length downstream) will resonate at what RPM.

Am I misunderstanding your question?

 

[Twin_Turbo]

I was merely pointing out another possibility for your mad quest to modify this car

I typed that on my phone and it looks like there were more posts in this thread than tapatalk was showing me when I posted the above


good read here
http://www.thirdgen.org/techboard/exhaust/489631-exhaust-termination-boxes-3.html

 

[bobs77vet]

ok i got it now thats a sweet set up

 

[69427]

I was merely pointing out another possibility for your mad quest to modify this car

I typed that on my phone and it looks like there were more posts in this thread than tapatalk was showing me when I posted the above


good read here
http://www.thirdgen.org/techboard/exhaust/489631-exhaust-termination-boxes-3.html

I appreciate your input. :thumbs: Looking over my previous post I am concerned that it could be misinterpreted as a discourteous or smartass reply. It was a sincere request for clarification, and I apologize if it came across in any other way. My English composition skills are not always as proficient as I wish they were.

I'll take a look at the link you provided.
Thanks again!

 

[69427]

Made some progress. Got the basic structure done, and put a small stiffener in the middle to keep the upper and lower walls from vibrating excessively.

Welded a lid on it and sprayed on a coat of flat black oven paint.



Still have some aluminum sheet work to finish up as I had to take some out to fit this larger exhaust section in.
Haven't started the engine yet to see what changes, if any, have happened to the exhaust note. It's in the thirties temp-wise here, with 4-8 inches of snow expected this weekend and I'm just not in the mood to start the engine on cold days if it's not necessary. I did pick up some European blend Mobil 1 oil the other day. It's a bit thinner than the oil I have in the engine at the moment, but still has a somewhat decent amount of ZDDP in it. I'm going to give it a try. I've got more oil pressure than I need, so if this stuff flows decently and takes some load off the pump shaft I'll be happy.

 

[69427]

Got some warm weather recently so I changed the oil to that Mobil 1 0W-40 European formula. Started the engine up and the exhaust note wasn't noticeably different than I remember. I hadn't run the engine in a few months, so it's hard to quantify any small changes. The H pipe I installed last fall seemed to be the big improvement in the exhaust note/harshness. Like a lot of things on this car I wish I had done that change years ago.
The exhaust termination box/volume is supposed to be a slight horsepower improvement, so I suppose I can't be too disappointed if it didn't do much for the exhaust tone. I'm anxious to get the car out for a track day soon to see how it runs after the tweaks (carb, exhaust, and bellypan) I did over the winter. (I don't have plates on it yet 'cause I've got to get it inspected before I can register the car here in Missouri, and the horn has stopped working, and I have to debug that before the inspection.)
Even with the 0W-40 oil I'm pegging the oil pressure gauge during a short run time. I'm hoping that the pressure will settle down once the engine gets up to full temperature.

 

[69427]

Hooking up a couple pressure gauges to the car before the next track day. Here's last week's picture when I was mocking up things. The bracket bolts to the passenger seat frame.



The top gauge is a 40"H2O differential pressure gauge. I'm using this to measure the pressure drop across the air filter (AC Delco vs K&N) and the drop across the carb at WOT. I also have a "pressure tap" under the bellypan (near the front axle C/L) to see what the undercar pressure looks like there.
The second gauge will be attached to the exhaust termination box (at the O2 sensor fitting) to measure the internal pressure (backpressure) at WOT. If the reading is low enough I can swap the lines and use the differential pressure gauge to get a more accurate measurement.
I put a stiffening rib on the mount and painted it, but the flash obviously washed out the gauge faces.



I've got about 30' of small hose running under the hood (hopefully things don't melt) and out the right side louvers, and up through the passenger window to the gauges. Hopefully some painters tape will hold the hoses down sufficiently to keep from beating up the door paint.