Don Terrill’s Speed-Talk

exhausted wrote: ↑Wed Jan 08, 2020 7:33 am

cjperformance wrote: ↑Tue Jan 07, 2020 7:33 pm

exhausted wrote: ↑Tue Jan 07, 2020 7:07 pm

I would pair 1&7 and 3&5, 2&8 and 4&6.

YES Thankyou Calvin! 1&7 , 3&5
(NOT 1&3 , 5&7as i had stupidy said !!)
Then ,, 2&8 ,4&6 ,, as I had said and as you can see im not a flat planer!
(Not a flat earther either )

Hey, I thought you were just showing how to do the close timing as opposed to the most separated timing. I was not trying to say that is wrong. It gets different results and is not the most effective 421 layout for minimizing reversion and I would never build like that. IMO

Definetly the most seperated timing for HP/rpm and being able to use the tightest pipe diameters(highest gas velocities)
These's some interesting 4.2.1 pipe layouts in the motorcycle world, with different firing patterns and balance tubes between some pipes etc. I have "heard"(but dont know) among other theories that some of the pipe layouts are specifically to taylor the Tq curve to enhance ridability moreso than HP when talking high powered street bikes.

exhausted wrote: ↑Wed Jan 08, 2020 8:17 amI have a bit more time so I want to throw this out. If we look at the combined lengths of 421 paired cylinder exhaust tube lengths and are shooting for say 16" each or 32" total, I am willing to shorten one and lengthen the other while still maintaining that 32" number for both of them.

How do you productively model the "cross-tuning" in a four cylinder 421 header, given a fixed primary+secondary length? There's at least some of the positive pressure blowdown pulse being reflected back from the closed exhaust valve of the other cylinder connected to the first Y, how does that need to be timed?

ptuomov wrote: ↑Wed Jan 08, 2020 4:25 pm

exhausted wrote: ↑Wed Jan 08, 2020 8:17 amI have a bit more time so I want to throw this out. If we look at the combined lengths of 421 paired cylinder exhaust tube lengths and are shooting for say 16" each or 32" total, I am willing to shorten one and lengthen the other while still maintaining that 32" number for both of them.

How do you productively model the "cross-tuning" in a four cylinder 421 header, given a fixed primary+secondary length? There's at least some of the positive pressure blowdown pulse being reflected back from the closed exhaust valve of the other cylinder connected to the first Y, how does that need to be timed?

Good question...

Just one further thought. You mention having an AR chamber at or after the final merge, would it be beneficial to add AR chambers elsewhere in system? I see it is quite common to have them in the primaries in 4-1 systems

oneaves2005 wrote: ↑Thu Jan 09, 2020 7:32 am Just one further thought. You mention having an AR chamber at or after the final merge, would it be beneficial to add AR chambers elsewhere in system? I see it is quite common to have them in the primaries in 4-1 systems

Do factory race cars use anti-reversion chambers?

To me, it seems like any application that actually uses the pulse energy from one cylinder to scavenge other cylinders would work less efficiently with the additional volume of the "anti-reversion" chamber.

To me, anti-reversion chambers are like those pressure-wave termination boxes: Popular on the internet but never used by engineers who can design the whole exhaust from scratch.

ptuomov wrote: ↑Thu Jan 09, 2020 7:39 am

oneaves2005 wrote: ↑Thu Jan 09, 2020 7:32 am Just one further thought. You mention having an AR chamber at or after the final merge, would it be beneficial to add AR chambers elsewhere in system? I see it is quite common to have them in the primaries in 4-1 systems

Do factory race cars use anti-reversion chambers?

To me, it seems like any application that actually uses the pulse energy from one cylinder to scavenge other cylinders would work less efficiently with the additional volume of the "anti-reversion" chamber.

To me, anti-reversion chambers are like those pressure-wave termination boxes: Popular on the internet but never used by engineers who can design the whole exhaust from scratch.

If you look at the exhaust on the Corvette C8R, I suspect they’re using an AR chamber... I cannot confirm it though.

This one?

ptuomov wrote: ↑Thu Jan 09, 2020 11:21 am This one?

B3DC249F-83A0-4396-9424-B191D92B0CD5.jpeg

C8R is flat plane DOHC... pretty sure that is the "old" LT2 variant pictured.

ptuomov wrote: ↑Thu Jan 09, 2020 7:39 am

To me, it seems like any application that actually uses the pulse energy from one cylinder to scavenge other cylinders would work less efficiently with the additional volume of the "anti-reversion" chamber.

To me, anti-reversion chambers are like those pressure-wave termination boxes: Popular on the internet but never used by engineers who can design the whole exhaust from scratch.

By "pulse energy" do you mean pressure wave energy or particle velocity and momentum? IMO the former is only really effective by reflecting at the end of Particle blowdown length or so, and that is where I have installed them. beyond that they are a detriment, The latter, I would agree which is why I would never use and AR device only at the first tube step on a 4into1 header, would not bother with a 421 and after the header in a system that uses a tail pipe and.or mufflers.

second point I would agree, I only use them in primaries on 4into1 headers in those situations where a 421 isn't wanted but I can talk customer nto that...It can be difficult to fit them in most cars which would be another reason why I would always do a 421, easier to package actually.

It seems most people do not understand my view of exhaust systems. Pressure waves hurt things more than help in uneven pulsing systems. The only reason I do not use a 421 header on high rpm engines is it is usually not possible to get the primaries short enough on most engines and keep them equal length. I am not aware of any time I was not able to maintain the power level of the best 4into1 header with a 421 and the 421 always broadened the power curve. It is true though that the higher the rpm, the less reversion is a problem, and most high rpm engines in racing use multiple gears in order to keep a tight power band so they can stay away from reversion ...Ha. You still have to wonder why 4 cylinder 4valve motorcycle engines still use a 421 design. It is because you still have to drive them around and they do not hurt the power, it really is a no brainer. (I would say thought that they are missing power as they usually use long primaries and short secondaries, but they still benefit).

I recently posted some comments on 4cylinder stuff. It may have been Yellow Bullet, Can not remember. Search for "KIA". We are talking about 4 cyl, so even pulsing with 13" or so primaries and VERY long secondaries. I tried even shorter primaries with no gain.Impressed that the factory designed them I believe.

I don't really know what I'm talking about, I'm just thinking out loud.

I think in terms of the pressure wave model, where the pressures cause gas flows and wave travel speed is influenced by the gas flow speed and direction. I don't know if it's right or accurate enough.

I think I understand why one wants a big step some distance from the head, where the distance is not that dependent on the rpm but is quite dependent on the exhaust port flow relative to the cylinder displacement. I read somewhere that the step is there to reflect back a rarefaction wave at the point when the exhaust valve seat no longer has a sonic pressure ratio. Sounds plausible to me. If the "anti-reversion chamber" is placed at the same distance where the factory cars place their big stepped header first step, it makes sense to me. The AR chamber works like a step. That doesn't mean I understand it all, just that this is what I think.

Playing with Vannik's simulator and thinking about it, I also now think that bore spacing is a first-order issue for high-rpm header design. With ports far from each other, one will have to start compromising the lengths to get sensible merge angles.

exhausted wrote: ↑Thu Jan 09, 2020 6:25 pm I recently posted some comments on 4cylinder stuff. It may have been Yellow Bullet, Can not remember. Search for "KIA". We are talking about 4 cyl, so even pulsing with 13" or so primaries and VERY long secondaries. I tried even shorter primaries with no gain.Impressed that the factory designed them I believe.

Well Sorry, search for "KEA" if it works, and turns out I posted the pics in this topic...PM me if you have more questions

Had an interesting chat about specifications with the exhaust company the other day, the guy is fairly insistent that in his experience a 2.25" pipe off the bank collectors will limit power to 185-190bhp (per side so 370-380bhp total) and has advised that I should consider a 2.375" pipe instead. Now, these are one of the highest regarded exhaust fabricators in the UK (www.simpsonraceexhausts.com) and they build systems to the engine builders/customers specs so are not design experts, but obviously see a lot of systems used by top race engine builders/teams.

This left me really confused, until recently. We had a customers car in that we built ages ago, its an 1800cc 4 cylinder aluminium engine, with 4 valve heads, throttle bodies, 12:1 compression and fairly mild fast road camshafts. The car makes a solid 190-195bhp @7,800rpm on conservative rolling road dynos with around 150lbs/ft torque @5700rpm. I measured the system today:

Primary - 1.625" x 27" long
Secondary - 1.750" x 12" long
Collector - 2.0" all the way to the silencer and out the back

On this basis, i'm sticking with the advice from Exhausted

I would question that long a header for that high a rpm band…

Power Graph attached, two lines are two different camshafts.....