Exhaust Diameter Sizing Chart (Street Use)

[bugguy1967]

Posting this for whoever uses the search function to figure what header size they need. The rpm depends on your camshaft's powerband. Engle 100 would only be around 5000, 110 around 5500, 120 around 6000, 125 around 6500 just to give a few examples. I only use Engles so I don't know anything else really.

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"A petrol engine can start readily, run smoothly and give every appearance of being in good order, without necessarily being in good tune." - Colin Campbell, "The Sportscar Engine"

[slalombuggy]

Damn, my 1 3/4 isn't big enough

brad

[jfats808]

For general sizing, I suppose that graph is as good a start as any. Just FYI, there are much more factors that decide the pipe sizing besides cam profile then just CC size. For instance my 2276 pulls to 8300rpms ( plateaus about 7800) with a 1 5/8"s. Head choice is obviously a big factor.
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[mark tucker]

head choice is also a big factor as to how well or how far the cam will "pull""powerband" "rpm range" as you say it.there are somany factors to consider. a fk89 on a 2387 with stock heads wont quite do as expected,by just going off the cam expectations. it all needs to work togeather.

[bugguy1967]

Ahh yes, you're right. Trying to get all my thoughts in there but I left out some. I knew people would add stuff as long as I threw it up here.

Edit: I should also add that HVWs used a computer program to get the information. Of course any 1600cc @ 9999 rpms is not going to be street rpms.
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"A petrol engine can start readily, run smoothly and give every appearance of being in good order, without necessarily being in good tune." - Colin Campbell, "The Sportscar Engine"

[flemcadiddlehopper]

Is that graph from "How to Hot Rod a VW Engine" ?

I think I've seen that before, and studied it, and dreamed of bigger exhaust....and I still run 1 3/8"

Gordo.
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[mark tucker]

other than copying or transfering the chart I doubt that hvw's used a computer for that chart

[flemcadiddlehopper]

[andk5591]

I loved seeing that in Hot VWS. Gives some validation to what a lot of folks have been saying for years that a LOT of folks run too big of a primary particularly on street cars.
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[mark tucker]

restricting the exhaust is a good way to over heat the chambers&crack some heads,along with other issues.
but dont get confuzed by the start rpm listed, a big motor already has better torque, so dont undersize the header thinking it's going to give you more power down a thousand or so rpm lower, it may not change it enough to feal but it may hurt it on the upper rpm side(both power&heat related)

[bugguy1967]

You're probably right. People shouldn't use this as a bible, but many do use too large primaries for their combos. I knew a guy in my old club who installed a shiny new 1 5/8 header on his 1300 sp. I was in Bill and Steve's a few months ago and there was a guy in front of me purchasing a 1 5/8 for a 1600, Weber 40, 110 cam that a pro was building in Mexico for him. I tried to talk him down to a 1 1/2 but he was saying the engine was going to make 150 HP and the 1 1/2 would hold it back. I guess you can't cure stubbornness.
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"A petrol engine can start readily, run smoothly and give every appearance of being in good order, without necessarily being in good tune." - Colin Campbell, "The Sportscar Engine"

[modok]

Sometimes stuff works right for the wrong reasons.

Say you go up to a 1 5/8 bugpack header from a 1 1/2, and get an improvement.
Is it because the engine likes the big tubes? or because the tubes are now only one size smaller than the choke point in the collector? what if we try the 1 1/2 with a smaller choke point or different muffler setup? maybe it works even better! or not

[bugguy1967]

So ideally every different size header should have a different size collector to be truly efficient?
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"A petrol engine can start readily, run smoothly and give every appearance of being in good order, without necessarily being in good tune." - Colin Campbell, "The Sportscar Engine"

[modok]

Unfortunately all rules of thumb are not true in all cases, but as far as that.
If using a choke point in the collector with the rest of the system going big right after or with muffler very close, the optimum size is often one or two tube sizes larger than the primaries. I see alot of the off the shelf systems made in this style.

The idea is only true sometimes, with a long secondary length, like a hideaway, then maybe no choke point is needed at all!

Whatever system, often some improvement can be had by changing this, but then deciding what type of system is best is......... more difficult!

[vwracerdave]

Lots and lots of people and vendors used the 1 5/8" exhaust because that is what is commonly used on most street v8 engines. Unfortunatly it is too big for 1600-1915 mild street VW engines.
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[bugguy1967]

Did a vendor tell you that? That's an odd basis to select exhaust size.
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"A petrol engine can start readily, run smoothly and give every appearance of being in good order, without necessarily being in good tune." - Colin Campbell, "The Sportscar Engine"

[vwracerdave]

No it's a huge problem in the VW hobby. Guys read a Chevy book and try to build a VW engine. I guarentee lots of parts vendors whore out the 1 5/8" exhaust because that is what is needed on a 350 SBC. Lots of young guys wih a 1600 or 1776 buy a 1 5/8" exhaust because that is what their father ran on his '69 Camaro.
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2017 Street Comp Champion - Thunder Valley Raceway Park - Noble, OK
2010 Sportsman ET Champion - Mid-America Dragway - Arkansas City, KS
1997 Sportsman ET Champion - Thunder Valley Raceway Park - Noble ,OK

[stault]

I run a total ceramic system, from heater boxes to the muffler, 1 3/8" on my 1835, I bought this system years back while I was collecting parts for my car, the add said the ceramic had better flow and I admit I hadn't done a lot of research on what size may have been the best, although I'm very happy with the car and how it pulls hills , expressway driving and if I do a WOT no hesitation , would a larger system be better ? some would probably say yes, years back I had a dual muffler on another car I had and it was always hitting speed bumps so I bought a single and put it on and I noticed the difference as soon as I drove the car, had more bottom end torque and ran smoother through the gears and like some said, you just don't know until you try it and my pocket book can't afford another $6 hundred pluss dollers to try it

[flemcadiddlehopper]

This article takes things way beyond a simple graph and show the actual math in the headers and how changes in primary tube cross section and length change where the peak torque occurs.

http://www.carcraft.com/techarticles/header_basics/viewall.html

Good article, but break out the calculator and sharpen your pencils if you want to apply this to your engine. Much easier to go with the tried and true method of what others are using.

Interesting note that primary tube length and size are a factor in peak output of an engine and collector size and design only help to move the peak torque lower in the rpm.

Gordo.
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Retrograde Garage. Vintage Aircooled, and others.

[ALB]

Stault- What's in your 1835- cam? valve sizes and porting? carburetion?
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