Do appropriately sized and designed Tri-Y headers show any advantages vs. appropriately sized and designed 4:1 headers at part throttle?
My thought is that, especially with a slightly large or higher overlap cam, the improved low RPM torque and exit velocities you see with a Tri-Y design will still show benefits at part throttle cruising. (Otherwise we wouldn't see fuel economy improvements in highway cruising with Tri-Ys vs. 4:1s, right?)
The trouble is how do you define "benefit"?
It's going to be a big chain of THIS, allows THAT, which makes THIS OTHER THING possible.
A tri-y could allow you to have a clean enough vac signal to make the carb work right at a light throttle setting where it would not before, OR reduce reversion enough to get a better burn then that's good. You'll like that if it solves a problem you were having. Or let you USE a more radical cam or something where you could not before, or raise the CR somehow then great.
If there wasn't any problem to solve and you make it start scavenging sooner maybe more of your fuel will be going out the tailpipe and that's possibly bad?
If it moves the happy and unhappy rpms around then the response might be might be better or worse coming out of cruise depending on what RPMS you are cruising at.
Nothing ventured nothing gained, so, try some different exhausts and see what happens. Does not have to be a tri-y but something different.
You will see what changes and what does not and that may tell you what you have to gain or lose. Want to have your cake and eat it too but there is a lot of ingredients in a cake.
Well, a 421 header is able to reduce reversion at a greater level than a 4into1 header. The level of reversion a given engine experiences is relative to rpm OR intake restriction. The higher the intake depression the more severe the reversion and the need for a 421 system. Your engine will be more efficient.
When I made all the 421 headers back in 95 they were done for the plate motors in Cup. Talk about intake depression. They even made more power at maximum rpm as the reversion was still strong. They were able to increase the pressure differential across the engine even at 7500 rpm.
There are several things going on in a "merged" collector.
One aspect is the area of the highest restriction or the "choke" as I like to refer to it. Most people see it as a flow restriction and it most certainly can be from the "flow" side of things, (or what it looks like from the engine side), but it also is a restriction from the atmosphere side. The smaller the orifice between the inside of an engine and the atmosphere the more time it takes for atmospheric pressure to equalize with any depression on the engine side. As rpm's are reduced, there is more time for atmosphere to equalize the pressure inside the engine with atmosphere pressure and the result is a reduction in the engines ability to maintain a depression in the cylinders without which the motor will not work, (can you say reversion?). The lower the rpm and flow level and uneven pulsing, the smaller you can and need a smaller "choke" if you need a broader rpm band. Overall it is why you do not put a 3.5" collector on a 2 liter 4 cylinder.
All else being equal, doing that increases reversion and you lose the ability to fill the cylinder on the intake side as rpms decline. It functions in its effect on the delta P of an engine. A simple observation would be if you change the choke size and see no or little effect, starting with your exhaust system, something is not right at all.
exhausted wrote: ↑Sun Jan 28, 2018 9:54 pm
The higher the intake depression the more severe the reversion and the need for a 421 system.
I'm intrigued by this statement.
Are you saying that for a car rolling down the highway at 3000 rpm with 20" hg and nearly closed throttle will see some efficiency (mpg) gain by running a 421 header instead of a 4-into-1? Is that because the 421 tends to kill some of the wave tuning.
exhausted wrote: ↑Sun Jan 28, 2018 9:54 pm
The higher the intake depression the more severe the reversion and the need for a 421 system.
I'm intrigued by this statement.
Are you saying that for a car rolling down the highway at 3000 rpm with 20" hg and nearly closed throttle will see some efficiency (mpg) gain by running a 421 header instead of a 4-into-1? Is that because the 421 tends to kill some of the wave tuning.
