Don Terrill’s Speed-Talk

I'm new to this forum, and boy am i glad to have found this place. I've been around cars all my life, mostly indy repair shop and DIY stuff with my Dad, until Formula SAE in college (BSME 1991), and I've been a brake engineer in Detroit since 1993. I'm building a car for the Grassroots Motorsports Magazine $2000 Challenge, and I have the opportunity to build a proper set of headers for my car. it's a 1965 Corvair with a 1994 LT1 where the back seat used to be. the LT1 is fitted with an LT4 Hot Cam Kit including the 1.6 roller rockers, mated to an Audi 01X 6-speed manual transaxle. This is my fifth GRM Challenge build over the years, and the second mid-engine Corvair. The Challenge consists of 1/4-mile drags (best time slip, not elimination), autocross, and "concours" where the story behind the build is almost as important as the build itself.

i have plenty of room to package tubes and collectors, and i will use this car as a sunny-day commuter as well so it will have a full, albeit short, exhaust system with C7 Z06 mufflers exiting at the rear bumper. Mufflers and pipes were selected because they were super-cheap on craigslist. I've never built headers before, but i'm looking forward to giving it a try. Per the Challenge budget rules, my labor is free and I only have to hit my budget for the materials that are actually on the car at the event. i have a set of "street stock" 1.5" and a set of "twin turbo" 1.625" headers to chop up for parts, and I can add bends and straights as needed.

Advertised Lift 0.525" both I and E
Advertised Duration 279 I / 288 E
EVO 79 BBDC
LSA 112
Standard SBC firing order 1-8-4-3-6-5-7-2

after a lot of reading here and on exhausting101.com, I believe a 4-2-1 configuration with 1.5" off the head is the way to go, and i read that the cylinders should be paired (1-5), (3-7), (2-4), and (6-8). I have some questions and i welcome all feedback:

1. With a realistic redline of 6,000 rpm, would a single step up to 1.625" in the primaries be beneficial?
2. Primary length?
3. What diameter secondaries?
4. What secondary length?
5. What merge length,
6. What choke diameter where the secondaries meet? The C7Z pipes are 2.75", in case that plays into the math.

I'll keep my eyes open for any braking questions, as that's probably the only area I'll be able to offer valuable contribution.

Thanks all!

I've trusted my header choices to the PipeMax software. I think any and all of your questions could be answered using that program.
Here's a discussion regarding Tri-Y/4-2-1 vs 4 into 1 headers by the PipeMax author. Maybe something to be gleaned from that (although it's several years old).

viewtopic.php?t=374

Screenshot of what you'll get as an example.


That's for a shorty style header but the drop down box on the upper right has a Tri-Y input (IIRC).
Hope this helps.

Skinny Z, thanks for the reply and the link to the other thread. I read that and most of the “related threads” that showed up at the bottom of the page, and several of the threads that showed up at the bottoms of those pages. So much good info!

One thing I didn’t see in any of those threads is what point along the length of the primary collector is considered the end of the primary and the beginning of the secondary?

Another question: does PipeMax do merge collectors and chokes, or is it straight only?

Thanks again!

Calling maxracesoftware...
I sent him a PM. He may jump in and address your questions.

memberlist.php?mode=viewprofile&u=54

angrycorvair wrote: ↑Fri Jan 01, 2021 9:30 pm Skinny Z, thanks for the reply and the link to the other thread. I read that and most of the “related threads” that showed up at the bottom of the page, and several of the threads that showed up at the bottoms of those pages. So much good info!

One thing I didn’t see in any of those threads is what point along the length of the primary collector is considered the end of the primary and the beginning of the secondary?

Another question: does PipeMax do merge collectors and chokes, or is it straight only?

Thanks again!

PipeMax 4.50 calculates Merge Collectors and Choke sections
and this Pic shows parts of TRI-Y Headers :

Larry, thanks for the reply, I appreciate the info.

angrycorvair wrote: ↑Sun Jan 03, 2021 11:41 pm Larry, thanks for the reply, I appreciate the info.

your Cam Specs look close + good enough
PipeMax predicts same Cam specs except a 110 deg LSA might make more HP , your 112 LSA will be good too .


PipeMax v4.50 calculated TRI-Y Header Specs :

--- Primary Tube Specs : Tri-Y Street / Strip Header • Multiple Steps ---
--- 2-Step Tri-Y Headers ---
Primary Y Segment 1st Step Diameter = 1.451 Length= 16.134 to 17.352 inches
Secondary Y Segment 2nd Step Diameter = 1.576 Length= 16.134 to 17.352 inches
Primary + Secondary Segment's Total Best Length = 32.268 -to- 34.704 inches

Closest commercially available Tube diameters would be :
--- 2-Step Tri-Y Headers ---
Primary Y Segment 1st Step Diameter = 1.500" OD Length= 16.134 to 17.352 inches
Secondary Y Segment 2nd Step Diameter = 1.625" OD Length= 16.134 to 17.352 inches
Primary + Secondary Segment's Total Best Length = 32.268 -to- 34.704 inches

--- Primary Tube Harmonics --- ( One-End-Closed Tube = Odd Numbered Harmonics )
1st Harmonic = 102.850 to 105.286 inches long ... typically never used ( too long to fit any Vehicle )
3rd Harmonic = 32.268 to 34.704 inches long ... highly recommended , best Torque and HP Curve
5th Harmonic = 18.152 to 20.587 inches long ... shortest recommended ( Shorty or very Hi-RPM Header )
7th Harmonic = 12.102 to 14.537 inches long ... shortest recommended ( Shorty, Hugger, or Log Style )
9th Harmonic = 8.741 to 11.176 inches long ... shortest recommended ( Log Style or Cast-Manifold )
11th Harmonic = 6.602 to 9.038 inches long ... shortest recommended ( Log Style or Cast-Manifold )

===================================================================================

--- Collector Specs : Straight Tube Collector • or Straight with a slight Merge shape ---
Peak TQ Diameter Range = 2.621 -to- 2.746 Best Length= 21.557 -or- 43.114 inches
Best Mid-Range Diameter = 2.746 Best Length= 21.557 -or- 43.114 inches
Peak HP Diameter Range = 2.746 -to- 2.871 Best Length= 21.557 -or- 10.778 inches

you could run as small as 2.500" OD Collector if needed , or more like 2.750" OD Collector
if you can get a 2.750" OD Collector Tube

==================================================================================

H-Pipe Location= 21.557 or 10.778 inches X-Pipe Location= 86.228 or 43.114 inches
( both H-Pipe and X-Pipe locations are measured from Primary or Secondary Tube ends inside the Collector )

Dual-Exhaust System Diameter = 2.621 to 2.871 inches Dual-Exhaust's each Muffler CFM = 460
Single-Exhaust System Diameter = 4.075 to 4.200 inches Single-Exhaust's one Muffler CFM = 920

--- Collector's Harmonics --- ( Both-Ends-Open Tube = Odd and Even Numbered Harmonics )
1st Harmonic = 172.456 inches long ... longest recommended with Mufflers and TailPipes
2nd Harmonic = 86.228 inches long ... longest recommended with Mufflers and TailPipes
3rd Harmonic = 43.114 inches long ... greater Low RPM Torque -to- Peak Torque RPM
4th Harmonic = 21.557 inches long ... highly recommended , best Torque and HP Curve combination
5th Harmonic = 10.778 inches long ... reduced Peak Torque , higher RPM HP gains possible
6th Harmonic = 5.389 inches long ... reduced Low RPM Torque , even though Tuned Length

Best TQ + HP Tuned Collector Lengths= 5.389, 10.778, 21.557, 43.114, 86.228, 172.456 inches long
Worst TQ + HP Loss Collector Lengths= 8.084, 16.168, 32.335, 64.671, 129.342, 258.683 inches long

Collector definition: from the Primary or Secondary Tube's ending inside the Collector -to- Atmospheric exit point
Note : all Tube Outside Diameters are based-off your Header Tube Thickness choice's value

1. With a realistic redline of 6,000 rpm, would a single step up to 1.625" in the primaries be beneficial?

you could run a Single TRI-Y at 1.625" OD

PipeMax calculates a Single Tube TRI-Y ... closest commercially available Tube would be = 1.625" OD

--- Primary Tube Specs : Tri-Y Street / Strip Header • Single Tube diameter size ---
Primary Y Segment Single Tube Diameter = 1.656 Best Length = 16.134 -to- 17.352 inches
Secondary Y Segment Single Tube Diameter = 1.656 Best Length = 16.134 -to- 17.352 inches
Primary + Secondary Segment's Total Best Length = 32.268 -to- 34.704 inches

Larry, thanks so much for taking the time to run this for me. I really appreciate the info. I’m very early in my fabrication education, just a hack in a small garage trying to build stuff out of scraps, but I’ll post pix here as I progress, if that’s OK.

Thumbs up for Pipemax software as well. built around 20 sets of headers over the years and helped several others. If angry corvair is an engineer, he should buy the software just to sit down and run it on his engine combination. His math abilities would help him understand it really quick I would think.

I found my version very easy to use. I can't comment on the later stuff as V3.98 is undoubtedly limited in features compared to the latest edition.
I'll have to investigate the possibility of upgrading. There's a new engine project on the horizon and some analysis might be of real benefit.

I'm hoping the OP can find what he's looking for.
Great of Mr Meaux to drop in.

skinny z wrote: ↑Mon Jan 04, 2021 2:29 pm I'm hoping the OP can find what he's looking for.
Great of Mr Meaux to drop in.

Great indeed. I appreciate the analysis for sure. Due to the very tight budget of this build, I’m not going to run through multiple configs of intakes heads cams and exhausts. I have one combo and need to build the best set of headers that my $40 scrap pile and rudimentary fab skills will allow. Here’s where we stand today:

My first car was a 1967 Corvair convertible, 4-speed. I was 16 years old, it was in pretty good shape. I installed some "Thunderbird" brand headers from JC Whitney, and Turbo mufflers.

I still have the Corv-8 manual somewhere, even bought 350ci Chevy engine to rebuild but that's as far as I got, never ordered the Crown kit.

If you’re interested in reading a short blurb from Grassroots Motorsports Magazine on my last V8 Corvair (from the $2004 Challenge), here’s a link:
https://grassrootsmotorsports.com/foru ... 367/page1/

It’s cold up here in MI but I’m trying to stay motivated. My budget requires building with salvaged junk, so I chopped up a couple $20 sets of used headers and am doing my best to follow the advice here. Primary tubes for passenger header are mocked up for length (17”) and packaging. 1-1/2 off port, will step to 1-5/8” at 8-1/2” from port.

That's looking pretty good. Scrap bits or otherwise.