Don Terrill’s Speed-Talk

Will a 1 5/8” header support 500+ HP without completely choking it to death?

Application will be a 408 sbf. Unfortunately for us truck guys there are no reasonably priced options for larger tubes (that I know of). SBF 1 3/4” tubes or larger typically go to a different flange bolt pattern, of which is available only for more typical performance platforms like mustangs.

This is kind of a bucket list type build for me...and I want to get it done before the arthritis in my hands gets to the point that I can’t do this anymore.

I believe that Calvin Elston stated that a 1 5/8 tube is good for 73.7 HP per cylinder, which is good for 589.6 HP total. If I understand correctly.

it wont choke it to death, but IMO it will be happier with a 1-3/4".

I have made close to 770hp with a 1.75 primary. I think shape, bends and fitment to exhaust port are equally as important. Most headers to fit common applications can't be the way they should be....therefore they need to be bigger.

When they have bends like this, 1.625 tubes could make a bunch of power, provided the exhaust port wasn't too big. But they would be pretty tough to fit into a mustang

When i talked to Calvin to get some tips on building my headers, he said always use the largest radius that will fit & you can usually go with the smaller tube.

I feel like with typical off the shelf car headers for that build, the 1-5/8” headers will end up costing you some power.

If that’s the only thing available for the vehicle, then I’d think about doing something with the cam where you’re not relying so much on the exhaust system for “help” in making power.

1-5/8" will "support" far more than 500 hp, it will simply suffer worse pumping loss. When the exhaust valve cracks, the flow rate is off the chart until the pressure drops. It will be higher at (nearly) any crank position. Opening the valve 5 degrees earlier may erase it.
The upside: less reversion, stronger returning pulse (greater vacuum) during overlap.

travis wrote: ↑Fri Aug 21, 2020 10:43 am Will a 1 5/8” header support 500+ HP without completely choking it to death?

Application will be a 408 sbf. Unfortunately for us truck guys there are no reasonably priced options for larger tubes (that I know of). SBF 1 3/4” tubes or larger typically go to a different flange bolt pattern, of which is available only for more typical performance platforms like mustangs.

This is kind of a bucket list type build for me...and I want to get it done before the arthritis in my hands gets to the point that I can’t do this anymore.

Chris at C & S Performance "radically detuned" our best / fastest SBC442cid that made 816 HP @ 8200-8300 RPM
to around 670's to 680's HP with 1 5/8ths Headers + less Cam , i think less CompRatio , Carb CFM ,
i came across that Dyno sheet a couple of Days ago by accident , i'll post more accurate info ,
and call Chris why he had to do this for the new Owner of that engine ????

Danny Durand sold his SBC442cid 815.8 HP @ 8200-8300 RPM to this Guy
to upgrade to 470cid SBC ... which Danny then made 951.5 Peak HP @ 7700 RPM 702.5 TQ @ 6600 RPM

Customer wanted a strong "Pump Gas" Street Engine ...
so that was main reason of "radical detune"
Customer put the SBC442cid into a 1934 Willys Coupe to drive on the Street
Chris actually drove the Willys around the street a few times , sorting it out ,
for his Customer .

all Chris did was put Arias flat-top Pistons , new Cam with less Lift & duration
and went to Chevron 93 prem Pump Gas + reduced 1650CFM Carb to my 0-9375 1050 CFM Carb
175 cranking psi at 188 RPM
38.0 BTDC

went from 815.8 HP @ 8200-8300 RPM to
700.0 PeakHP @ 6700 RPM and 577.9 Peak TQ @ 5200 RPM
with 2-Step Headers 2.000 x 10.5 , 2.125 x 20.5 = 31.5 inches, 4.000 x 10.5 Collectors

then with 1.625 ( 1 5/8ths ) Headers with 4.000 Collector
642.0 HP @ 6800 RPM -to- 641.9 Peak HP @ 7000 RPM and Peak TQ= 578.2 @ 5000 RPM


then add on Mufflers + exhaust system
636.7 Peak HP @ 6900 RPM with 565.0 Peak TQ @ 5000 RPM


so 815.8 - 700.00 = lost 115.8 to Cam, Carb, Compression Ratio, Pump Gas changes
maximum HP lost 815.8 - 636.7 = 179.1 Peak HP

Note : i'm very sure this Engine would have made even lesser HP if we would have put a 3.000" or 3.500" Collector
instead of the 4.000" Collector on those 1 5/8ths Headers

I ran 10.90's with a 1 5/8" header this year. Not sure if it makes 500hp but I bet it's close. 120mph also.

a friend dynoed his 400 sbc with 1 5/8 and 1 3/4 inch headers, 1 5/8 made 502 hp and 1 3/4 made 510 hp. not worth buying bigger tubes over about only 8 hp

Some of stock eliminator racer would pay 2500-3000 for that 8 hp header.

A couple months ago a friend wanted to dyno one of his motors that he hadn’t previously run.
368”, 450hp with the headers he runs in the car which are 1-3/4 to 1-7/8 step with a 3” collector.
Tried some 1-5/8 x 3” headers on it just to see what would happen(maybe more tq down low, etc)...... they lost everywhere........ the big number being 15hp lower.

On a stroker SBF, in the 500hp range....... I’m pretty sure 1-5/8” headers are going to be down “some amount” compared to some slightly bigger tubes.
But, without testing a particular combo, who knows if that number is 5hp or 25hp?

1.625" od will support well over 500bhp, more like over 600bhp, but here are the key words, WHEN BUILT CORRECTLY.

I would not be afraid at all of using 1.625 at all off an even larger port area. I do it all the time for Cobra Automotive and their nostalgia mustangs and cobras. 1.63@8-1.75@10, 2" secondary for 12-14", (not a typo) and a 2.25" choke at the final collector. 620hp no problem and all sorts of bottom end. Especially for such a small motor and road racing

All feeding into a single 3" tailpipe.

https://youtu.be/SAZXMXuBbrM





What Bill said who builds those engines:

BILL-C wrote: ↑Fri Mar 16, 2018 10:05 pm I have multiple sets of sb ford dyno headers that Calvin made for us. They are configured in a way that allows us to experiment with secondary pipe lengths and swap firing order easily. We have made 650+ hp with the dyno configuration of the headers in the picture posted by hoffman 900. They match or beat the lower end power of any 1.625 dia straight tube header and beat the top end power of any 1.75 header we have ever tested. They are far superior to any "catalog" header .

The big qualifier is you will not buy what Calvin builds from a catalog (well you can order them from Cobra Automotive), and they will not be cheap. See here: http://store.cobraautomotive.com/compet ... t-headers/ .

You also cannot apply Calvin's sizes to catalog headers. You're only going to get that level of a header from ProFab, GoodFab, and other shops who are doing NASCAR, Indy, FIA level work, or a great fabricator who is capable of that caliber of work.

This is also the kind of work that separates Joe-Blow bracket racer / builder from people who race top level semi-pro / pro level racing. They don't blink an eye at this level of detail (and the costs associated with it) as it every last bit matters.