Don Terrill’s Speed-Talk

If I were to run open headers or a pressure wave termination box would it make sense to design a variable length collector? I have something in mind where I could automatically change the collector length up to 6 inch depending on rpm. Obviously I would need a program such as Pipemax to see which collector length would be ideal for different rpm levels.
e.g. 18" at idle, 15" at 3000rpm, 12" at 6000rpm

I think it's simpler to find a length that works and stick with it. Plenty of tests over the years show how adding collector length to a too-short combination helps pretty much across the RPM range. Not sure I've seen anything for a typical build (i.e., no 10,000 RPM stuff) where a shorter collector added significantly except possible on the high side of the peak HP RPM. But this is just one person's limited experience & opinion.

Indeed it would be simpler to stick to one length and be done with it. I consider it sort of a science project How would the hp/trq curve change if I could lengthen/shorten the collector a few inchens when going through the rpms. Worth persuing or not - I don't know. Maybe having the ideal collector length at several rpm points is worth much area under the curve maybe only a few lousy points.

ChargerST wrote: ↑Fri Jan 08, 2021 2:10 pm Indeed it would be simpler to stick to one length and be done with it. I consider it sort of a science project How would the hp/trq curve change if I could lengthen/shorten the collector a few inchens when going through the rpms. Worth persuing or not - I don't know. Maybe having the ideal collector length at several rpm points is worth much area under the curve maybe only a few lousy points.

what would be your testing,
Dyno or track testing??
Street??

Testing would be done on the dyno first (with same exhaust that will be used on the car - so probably termination box) and track afterwards.
My idea is to have several dyno pulls with a different collector length and compare the charts. That should give me an idea which rpm likes which collector length. Second step would be to program that into the actuator for the variable collector and see if the new curve is better than the ones with fixed collector length.

The collector length is tuned for a certain rpm - especially on open headers or with a PWTB as the negative pressure wave that travels back up to the exhaust valve needs to arrive precisely when the valve opens - otherwise it has no effect as it can't create additional vacuum for a better cylinder filling. Now if I were able to broaden that rpm range (by varying the collector length) I should be able to broaden the power curve as well - at least that's what I think

Appreciate your response, your dyno and then track testing is really only way to get good data,
I spent 12 months track testing 450hp SBC
4 into 1headers and collectors and exhast/ no exhaust
Smaller 4-2-1 tri-y collector lengths etc
Including carb and jet chages,I have no real evidence of a specifc trend for my application.
With track testing there are so many variables it will make your head spin.

i dont think a few inches is worth the effort, if you changes from one of the pipemax harmonics to anopther then yes it would be

digger wrote: ↑Fri Jan 08, 2021 4:59 pm i dont think a few inches is worth the effort, if you changes from one of the pipemax harmonics to anopther then yes it would be

You can't change from one harmonics to the other as the required length is too far apart (maybe it's possible from 5th to 6th harmonics). Usually the 3rd and 4th harmonics are being used.
There are just a few ingredients to the formula that let's you calculate optimal header length for a specific rpm - maybe I give it a try or just get Pipemax
Basically I want to know how much does the toque peak move (in terms of rpm) for each inch difference in collector length.

ChargerST wrote: ↑Sat Jan 09, 2021 6:43 am

digger wrote: ↑Fri Jan 08, 2021 4:59 pm i dont think a few inches is worth the effort, if you changes from one of the pipemax harmonics to anopther then yes it would be

You can't change from one harmonics to the other as the required length is too far apart (maybe it's possible from 5th to 6th harmonics). Usually the 3rd and 4th harmonics are being used.
There are just a few ingredients to the formula that let's you calculate optimal header length for a specific rpm - maybe I give it a try or just get Pipemax
Basically I want to know how much does the toque peak move (in terms of rpm) for each inch difference in collector length.

they do it with flaps on the inlet, can be done on exhaust to IMO

You would probably fair better making an 8 stack efi intake that has adjustable runner length based on rpm and load. It has been done extensively in oem applications and to a lesser extent in the aftermarket.

Bill Chase wrote: ↑Sun Jan 10, 2021 3:24 pm You would probably fair better making an 8 stack efi intake that has adjustable runner length based on rpm and load. It has been done extensively in oem applications and to a lesser extent in the aftermarket.

That's something I also have in my mind - and the ultimate goal would be to combine both (variable length intake and collector). Something like this for the intake (but 8 trumpets and in a cross ram fashion): https://jalopnik.com/mazdas-all-conquer ... 1825651052