3" all the way out the back as in "dual 3 inch pipes" or are they referring to [1] 3" pipe all the way out the back from the collectors?
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'72 K5 - Greg's @MightAsWellK5 Build
- Thread starter Greg72
- Start date
This is the build description. I will improve this later. (GB)
3" all the way out the back as in "dual 3 inch pipes" or are they referring to [1] 3" pipe all the way out the back from the collectors?
Definitely go with dual 3" all the way, you don't want any backpressure, you just want velocity, to help scavenge the cylinder. And the primary tube size and length is more important for that than the exhaust pipe size. Anything less than dual 3" exhaust would most likely result in a power loss. Heck, I lose 20 hp on my 388 smalll block when I uncork the dual 2.5 pipes and leave it 3" open collectors. Also, it does not kill my mid range either, actually adds to it because I on the transbrake it goes from about 3000 RPM up to almost 3400 RPM, so it makes more power there as well. And when I changed the small pipes and carb on my BB to 3" pipes and EFI it made a noticeable of difference in power. Now that was a combination of both I am sure but I had that carburetor tuned relatively well. And it didn't sound choked up any more either, too small or too long of tailpipe after the muffler can make it sound very barky and poppy.
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There is no such thing as not enough back pressure. That is an old wive's tale, and I wish it would die. There is something to be said for scavenging, but you can always tune for loss of back pressure.
Martin
Martin
Agree with this....the backpressure myth runs right up there with warping the valves from running open headers
That said my headers and 2-1/2" pipes are optimally on the small side for higher rpm but realistically how much will you be running above 5k?
I think you would only notice a significant difference on an engine dyno
Anything else would be absorbed by weight and drivetrain losses
In that case, here's the $64,000 question:
The handbuilt headers will end up maybe 12 - 18" from the inlet of the mufflers (when located in that torsion box area. I have already purchased "header extensions" which are nothing more than 18" sections of 3.5" tubing with a 3-hole collector flange on one end.
My objective was going to be to cut those the long way and then re-weld them so that the outlet side was reduced down to 3" from 3.5".....over an 18" span, that would be a nice gradual taper and shouldn't create any turbulence or major restrictions in flow.
Mtnman pointed out that I can easily modify my existing 2.5" Flowmasters to make them a 3" inlet.... so I can connect the header extensions directly to that modified inlet.
On the outlet side, I can leave it at 2.5" to try it out for sound and potentially open it up to a 3" if I feel like I'm leaving power on the table. Keep in mind that based on my limited space, the tube coming off the end of the muffler is only likely to be maybe 2 - 3 feet long (maximum) anyway.... so I'm not sure how much "quieter" the 2.5" section would be over a 3" exhaust pipe???
I have also heard that using a slightly thicker tubing can help get rid of some of the resonance and "echo" when you start getting into these larger exhaust diameters. I have no idea how easy (or hard) it will be to find thicker materials to work with and/or if they will end up being stupid-expensive vs. the more conventional choices.
-G
The handbuilt headers will end up maybe 12 - 18" from the inlet of the mufflers (when located in that torsion box area. I have already purchased "header extensions" which are nothing more than 18" sections of 3.5" tubing with a 3-hole collector flange on one end.
My objective was going to be to cut those the long way and then re-weld them so that the outlet side was reduced down to 3" from 3.5".....over an 18" span, that would be a nice gradual taper and shouldn't create any turbulence or major restrictions in flow.
Mtnman pointed out that I can easily modify my existing 2.5" Flowmasters to make them a 3" inlet.... so I can connect the header extensions directly to that modified inlet.
On the outlet side, I can leave it at 2.5" to try it out for sound and potentially open it up to a 3" if I feel like I'm leaving power on the table. Keep in mind that based on my limited space, the tube coming off the end of the muffler is only likely to be maybe 2 - 3 feet long (maximum) anyway.... so I'm not sure how much "quieter" the 2.5" section would be over a 3" exhaust pipe???
I have also heard that using a slightly thicker tubing can help get rid of some of the resonance and "echo" when you start getting into these larger exhaust diameters. I have no idea how easy (or hard) it will be to find thicker materials to work with and/or if they will end up being stupid-expensive vs. the more conventional choices.
-G
The closer the muffler is to the engine the more important it is to be larger, because the exhaust hasn't had any time to cool off and therefore reduce its volume. This is also just one of the reasons long tube headers make more torque and hp than shorties. When I uncork even my small block to 3" it is a noticeable difference in acceleration and throttle response, let alone a big block. I would without a doubt go dual 3" all the way out. Stick with 1 7/8" or 1 3/4" primary tubes of a long tube design and leave the exhaust pipes large. Anything less is just uncivilized. 
One more thing, you may be better off getting collector flanges that merge to 3" directly instead of stepping from 3.5 down to 3. But I doubt you'll feel the difference there. Expensive step headers actually do the opposite, start out small and then step larger before going into the collector, the idea is to increase velocity out of the cylinder head, although it's another thing you probably wouldn't feel.
One more thing, you may be better off getting collector flanges that merge to 3" directly instead of stepping from 3.5 down to 3. But I doubt you'll feel the difference there. Expensive step headers actually do the opposite, start out small and then step larger before going into the collector, the idea is to increase velocity out of the cylinder head, although it's another thing you probably wouldn't feel.
newbie79
1/2 ton status
I saw the pics in thst post. I was referring to the routing from the headers to the flow masters. I like the idea of them being tucked up and safe from rocks and stumps. As well as seeing where the pipe dumps out behind them
I cant remember?
How many CI is that BB of yours...any projected HPs or has it never ran before?
Did Scott build that one too or was that just Mikes?
How many CI is that BB of yours...any projected HPs or has it never ran before?
Did Scott build that one too or was that just Mikes?
Scotty built them both....at just about the same time too. I was keeping him busy! Mikes is the stroker 496, he bought that from me a couple of years ago. It was originally destined for my '72 Burb, but when I sold the truck I didn't have much use for a spare, fresh BBC in my workshop.
The one I kept is a 502.
It is set up to be EFI friendly, using a cam with modest LSA specs. The objective was to maximize low RPM torque, so I think it will be in the 400HP range with over 600LB/FT of torque... Torque peak is at something like 2500rpm too......
And it's never burned a single drop of fuel....better economy than a Prius.
EDIT:
I found a CompCams simulation printout from back in 2007.... not sure if this is the final set of numbers or not??
Scott may have saved something newer on his side, but this is all I have here.
-G
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That sounds like a very nice engine for your blazer...should pull all that weight around just fine!
This thread was at risk of falling to the third page!!! I could not allow that to happen 
Pretty pics of steel fab work please! 
Pretty pics of steel fab work please!
This thread was at risk of falling to the third page!!! I could not allow that to happen
That's a great way to get lynched around here....
About 700 subscribers get notified that the thread was updated, and when it ends up being you asking for an update they will want to grab pitchforks and shovels and chase you...
As you might have seen, I was in Texas all last week. Checked out a sweet 1st Gen restoration (Bigcatt's).... It was great to be reminded of the simple pleasures of driving a completed truck project. It felt great to cruise around on a sunny day.
I should be able to get into the garage today, the trick will be remembering what I was supposed to do next.... After a week away, it will probably take me an hour to get back in "the zone".
-G
bp71k5
3/4 ton status
Greg,
Just to help you back into the zone, it was rock sliders and exhaust routing.
Just to help you back into the zone, it was rock sliders and exhaust routing.
2014.10.28 - UPDATE! - SLOW & STEADY PROGRESS.... !!!
I didn't bother posting up yesterday, but I did get out into the shop the last couple of nights so I've got a few updates and photos to share.
The first is an idea that I thought would work out better than it did.... It's a large diameter pipe cutter (up to 2.5" diameter). Whenever I want to cut a nice, square end on a piece of tubing it's always a struggle to either wrap it with tape or just some other time-wasting process. So I bought this from Amazon.
It worked great for exactly ONE tube. It scored the line I needed and I was able to then follow that score line with my cutoff wheel and get a good result. Unfortunately, I think the cutting wheel isn't designed for harder metals (like steel) and now it "walks" all over the place instead of cutting straight...... if I ever need a barber-pole mark on my tubing, this will end up being really useful.
I'm not sure if there is such a thing as a hardened cutting wheel replacement, but if I can find one this tool would be an awesome time saver.As is usually the case with "new" ideas in the build. It takes just about forever to get things mocked into place. The rockslider structure was no exception... there are so many simultaneous dimensions to check, that just getting the part under the truck and secured on jackstands with shims and c-clamps is a real time waster. Eventually though, I got what I needed and was able to burn-in the two outrigger tubes that connect the rockslider to the outer frame rail area.
The front support gets welded underneath the factory cab support hanger (centered directly under that large bolt), and the rear hanger will end up welded to those massive upper link mount brackets that I'd built many years ago. The nice thing about the way this is going together now is that instead of having a TON of small parts all bolting to the boxed framerails... I am actually creating a second "perimeter style" frame structure that will include both rocksliders, the new 2"x3" cross sill, the upper link mount brackets and the front cab supports. Fewer individual parts should translate to an even stiffer frame structure, and less worry about bolts working loose over time.
When I break this project all the way down for final painting, etc. The parts should still be a manageable size to lift and work with... and that's important too.
With the supports fully welded to the slider, I started fitting them up to the frame and cross sill area. Lots of clamping and squaring-up was required. It's hard to get a good visual reference when the tubing has such a pronounced radius at the corners... so I find myself using small pieces of plate steel to make sure I'm landing the parts against each other perfectly.
Here is the shot that shows the rockslider in final position.....MUCH further inboard than the last time I posted a photo of it (when I believe Eric had a small panic attack about the aesthetics
). As I mentioned before, the target was to get it about 3/4" behind the vertical inner rocker sheetmetal so that there would be ample room to position the body back down on top of all this stuff and not scratch up the fresh paint, etc.
You might also notice that the "inner rocker" that I just spoke about is actually mostly cut off now. The challenge with leaving that sheetmetal in place is that I need to get access to the 5 index holes to install my rocksliders. I could drill a series of 2" holes to let the tubes pass through the rocker, but there is also going to be a large flange that connects all those tubes together and allows me to run bolts from the outer rock slider assembly into the inner rocker support area to clamp them together strongly.
Here is another image... sorry it's not very well lit or focused.
Ultimately what will happen is that I will take the piece that was cut off and bend it a couple of times (lengthwise) so that when it is rewelded the panel will come down to the top of the rockslider (within maybe 1/4" then travel horizontally toward the center of the truck for a few inches, then bend again to that it is aiming down toward the ground again.
This will allow any debris in the area to rinse away, and will prevent me from having a large gaping hole visible when the rocksliders are installed and the door is open. Being able to preserve the factory weatherstripping across the bottom of the door is going to be tricky when the factory outer rocker is removed... so I'm trying to keep as much metal intact in that area as possible to attach to.
In other news, I've pretty much decided to pull the trigger on the Miller Dynasty 200DX TIG welder in the next few days. I've wanted to learn TIG for a long time now, and it would be great to be able to weld up my own fuel tank (in either stainless or aluminum) and maybe even do all the finish welding of the rollcage in TIG instead of MIG. I'm not sure how TIG would be for sheetmetal work, but if it controls heat as well as I think it does.... it might save me from the endless "tack, tack, cool, move, tack, move, cool" that is required with MIG to keep everything from warping like crazy. Not being pelted with molten spatter, and burning holes in my socks and sweatshirts will be a huge plus as well!!!
-G
It's too bad that didn't work out. It sure sounds like a good idea. Maybe once you get the wheel fixed you could tape the pipe and then just score the tape?
"No, seriously honey, it's not a waste of money. Just think how much we'll save on tube socks and sweatshirts! I'd be stupid NOT to!"Not being pelted with molten spatter, and burning holes in my socks and sweatshirts will be a huge plus as well!!!
-G
Greg I use a hose clamp (the type with a screw) on my tube to mark it in a perfectly straight pattern. Tighten the clamp down then use a fine sharpie to mark it.
NICE!
That's a slick idea. I'm stealing that....
-G
Awesome! Ive been watching for 6 years waiting!! Finally found my opportunity to contribute!
*coughband
sawcough*
poifect cuts....
sawcough*
poifect cuts....
*coughbandsawcough*
poifect cuts....
Perhaps you haven't seen the size of my workshop?
Space is absolutely max'ed out, and I'm going to need to find space "somewhere" for that new rolling TIG machine.
A stainless hose clamp....now THAT'S something I can find room for!
....plus the new welder is going to bankrupt me anyway.
-G
