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1.9 SOHC Exhaust Chord Length Calculations?

Posted: August 31st, 2018, 8:35 am
by lowtechprime
Greetings All!
This is my first Post here and I'll jump right into it.
I'm building a 1.9 SOHC for my 128 Rally. It will be a Fast Road/Canyon car. I'm using a 1.6 punto, sporting block bored to 87.02mm with a ZRP 78.4mm crankshaft and 128.5 mm ZRP rods. Head has 32.5ccm chambers with 40/35 Int/Ex 6mm stem valves, 10,000 R.P.M. springs with Titanium retainers, Cam specs are; Duration: 242 degrees duration (intake / exhaust) @ .050 lift (seat-to-seat) Lift: (.430") maximum lift, Lobe center: 108 degree. The Pistons are 28mm compression height Becker Telle's. I'm on the USA West coast so 91 octane is what we get, unless race fuel is used, which I'd like to avoid. My Compression target is somewhere between 10.6 -11.0 :1. Those Pistons were supposed to be Forged, but they look cast to me. Also, they have a sizable dish in them so I'm not sure they're going to get me to my compression target. Fueling is coming from a Bank of 4 Yoshimura TMR-MJN32mm Carbs on a LaRussa custom Manifold.
Now, with that said, I am trying to design my exhaust Header and have been using information from this forum to calculate the necessary specs and what considerations need to be taken into account. I've found a lot of information on Layout, tube diameters, Radius' and the need to tune for the resulting pressure waves from the transitions between Primaries and Secondaries. For my 'Optimum' 4-2-1, what I am trying to figure out is how do you calculate the necessary tube lengths for these pressure waves? Can this be calculated, or approximated without back and forth trips to the Dyno? (Obviously that would be ideal, but not practical or attainable for everyone.) Also, what is my final Pipe I.D. at the collector and through to the Muffler at the rear of the car? The EX Valve is 35mm, but the port as it exits the head is only 32mm. I would be grateful for the passing on of any experience from the Collective!
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Re: 1.9 SOHC Exhaust Chord Length Calculations?

Posted: September 2nd, 2018, 5:58 pm
by Guy Croft
Have a careful read of this to begin with and then come back with any questions,


best wishes,


Re: 1.9 SOHC Exhaust Chord Length Calculations?

Posted: September 3rd, 2018, 3:06 am
by lowtechprime
Thanks Guy!
I've actually read that thread a few times, and just about every other post relating to Exhaust headers that I could find in the various Sub Forums (Gen Discussions, Q&A, etc...) As far as diameters, Chord Radius, Layout, collector degree angle, and muffler, those questions have all been answered. What I am trying to determine are my Primary and secondary chord lengths. The example you present as a 'close' to optimum, is for a 1600cc X-19:
1.5"OD Primaries 28" long,
1.75"OD Secondaries 12"-14" long,
2.25"OD Exit...

Given my 33mm EX valve and 32mm EX Port exit, those diameters will work for me as well.. What I want to know is, how were the 28" and 12"-14" lengths arrived at? You go into some detail about the 'Critical' wave that is generated when the EX Pulse meets the enlargement at the collectors, and that the collectors themselves are not counted in the chord lengths, but not how those lengths were determined. Is it just that given your considerable experience, those are lengths that you've found to work well, or are there calculations of some kind?
I don't have any data on the example 1600 motor, but I assume that the head was ported, and flows as well or better than the one that I have. The 'breathing' capacity of the motor will most likely factor heavily into it's exhaust needs, and that is mainly in the Head, but I also have an extra 300cc's. Also, since this is going in a 128 and not an X19, I have more room to the rear of the motor to be 'generous' with my curve radii. So to sum up, (sorry, I may have rambled a bit..) I'd like to know how the Primary and Secondary chord lengths are determined.
Thanks so much!

Re: 1.9 SOHC Exhaust Chord Length Calculations?

Posted: September 13th, 2018, 5:04 pm
by Guy Croft
Sorry for taking ages to reply..

The layout you've referred to comes from advanced software simulation using Virtual 4T and very accurate rolling-road testing in the hands of a world-class expert I used to work with. Between us over a 7 year period we collaborated on some of the most powerful engines ever seen in Irish Tarmac Rallying - the world's toughest proving-ground by far. Fiat, Vauxhall, Ford, Honda, Suzuki, Rover, Peugeot - 8v and 16v, on carbs and FI. We also collaborated on many GC engines for other applications...

Without software like that you can't 'model' your system. I'm not referring to a simple buy-on-line package for simulation (which pumps out dimensions in seconds pulled from a library of preset values - but software where the annual licence is a cost of many thousands of pounds and where the computations could take a week with all the computing power of an entire university..

FWIW insofar as 1000cc - 2.5 liter is concerned - exhaust pipe lengths are LITTLE affected by bore-stroke and cubic capacity - but the internal diameters are certainly governed by the smallest ex valve throat or port 'effective diameter' and the primary pipes (with 4-2-1 or 4-1 or any other layout) should always be measurably larger than that in order to create an anti-reversionary' step. The latter was also proved-out by me with my friend many years ago. Tuning expert David Vizard was the first to talk about this and he was right - though of course when he first wrote about it in his book about tuning the For Pinto there was no such thing as computer software available to model it.

Computer simulation takes into account a vast array of engine features - too many to write about in this short section.

That data will work well for you.


Re: 1.9 SOHC Exhaust Chord Length Calculations?

Posted: September 14th, 2018, 7:27 pm
by lowtechprime
Alrighty, got it! Thanks for the useful information and explanation!