Port taper and velocity 1

[IROCRT]

Hello, this is my 1st post and I am grateful to have found this forum. I hope my asking of questions doesn't become to overwhelming for some, as I am here to learn from the many advanced discussions only found here.

My first question petains to the relationship of port taper and it's velocities. It is my understanding that velocity will increase with the taper in a given runner, and that increased velocity will help out greatly in the cylinder filling... especially at low rpms. But, the part I can't fathom is this taper in my eyes is like a restiction. It causes a pressure loss. In order for the air to speed up, there must be some form of energy that is being lost to speed up the air. I can guess that it is overshadowed by the increased scavenging and cylinder filling, but I'd like to understand the "why" aspect.

I have only an Associate's degree (Machinist) and my math skills have faded, so please take it easy on me ;-)

If I am out of line with my questioning and credentials, please forgive me. The analytical approach to the internal combustion engine continues to amaze me.

BTW... I own JB Heywood's in-dpeth book on the internal combustion engine fundamentals, but have ahard time relating to most of it.

Thank you for your time.
Steve

 

[thundair]

One other advantage of converging port walls is it prevent delamination, and works best in straight runs.
There are engine head designers who do not believe in converging walls. I know Jim Feuling swore by straight bores and said he had dyno proof of his experience.

If you are looking for hard math to resolve conditons I doubt that it exist. Most will be empirical and need to include RPM, reversion waves and port length.

Lets see if we shook the tree hard enough to get some real pros on line..

Regaqrds

 

[EngJW]

Steve,

Don't let your associates degree stop you. When I was in your position I told one of the engineers that I wanted to learn more about engines but was afraid that engineering books were beyond my level. He replied that while there is math, you can usually skim over it and get the general idea of what the topic is about. Also if you read enough about the subject you will eventually understand what the math is about even though you would not be able to solve a problem in it.

This advice really worked for me, so I want to pass it along.

 

[IROCRT]

Thanks for the comments so far from the both of you. EngJW... Yes, I have done plenty of skimming mainly through JB Heywood's book and even though I have picked up 1/2% of the total information his book contains, I have learned 100 times more from it than all the "performance" car books and magazines I have collected over the years. I really would love to learn how to apply alot of the subjects discussed in his book. Not sure where to start, so I posed the question about port taper and velocity.

 

[EngJW]

Get the 2-volume Tayor book. It is a good companion to Heywood. While Heywood is heavy on theory, Taylor is more intuitive and is based on a lot of lab testing, much of which was done before the days of computers. Both are excellent books and they represent different approaches for different times. Taylor also includes a lot of design information.

 

[Marquis]

This is a question I've been asking myself for years.
I've seen huge benefits in using tapered intake runners on the dyno by several percent. It seems to benefit top end performance more than bottom end- however the overall area under the Volumetric efficiency curve is increased- so some of the bottom end can be reclaimed by re tuning primary lengths- in this case the overall area under the VE curve will still be more than constant sectional area runners. Packaging allowing all engines I design now have some for of taper- but it must be optimised to specific application-we're not talking much- maybe 1-2 degrees.
I've seen non satisfactory explanations- one of them was from Blair that said that the tapering of the duct sets up little minute discrete reflection pulses back, but I don't buy that.
For a given cylinder and engine speed- if we compare a taper duct (which converges down to the diameter of the inlet port) versus one which is of constant cross sectional area ( the same area as the port- for sake of argument).For both these cases let us assume that the ports , valves and port flows in the head are identical.
Assuming we haven't gone above the 7 degree taper so that seperation isn't encouraged also.
For the taper runners the entry conditions will be less "lossy".
This lowering of entry CD will benefit top end more than bottom end- that adds up. The effective inlet tuned length is now altered however and that is why often you must lengthen slightly to retune. However what doesn't add up is that you can achieve the same lowering of entry losses by simply putting a fully eliptical bell mouth on a constant cross sectional runner. This will NOT achieve the broadening of the VE curve apparent in tapering runners.

All I can think of is that the velocity is increasing in the runner continually- and VE and tuning is intimately related to the momentum of charge- momentum is MV^2. As the square of the velocity is increased so goes up your momentum. If you tried to achieve the same thing by simply increasing your diameter throughout (again constant cross sectional)-including the port- you would increase your mass of tuned pulse but lower your velocity. However why does the above example of the original constant cross sectional area will fully eliptical low loss bell mouth NOT show the same benefit (it doesn't take my word for it!) of the tapering runner?
The acceleration of charge must be "cramming" in more and more charge ahead of itself continually as well as the benefits to momentum from the increasing velocity. This DOES cost energy, but perhaps it is negated by the lower entry losses.

 

[ricdat]

Thanks for the valuable post (and reassuring, considering how I am locked into tapers for my engine currently being built).
One small thing: momentum is actually MV (mass x velocity) while KE (kinetic energy) is 1/2MV^2.

Regards,
Rick Dathan

 

[MaxRaceSoftware]

For Carbs only=>
for an all-out max-effort DragRace engine
Max_Plenum_Entry_Area = (Int_Valve_OD ^2) *.7854 * 1.4
figure your Taper angle from that point to Valve

if your Total Induction Length is on verge of being too
short, then too much Taper will hurt overall ET

playing around with Taper to see how far i could
could go on a Chevy 355cid 720 HP at 8500
the 1.40 ratio with Carbs was the most it wanted
without hurting Torque

on the DragStrip , it ran 7.50's ET at 1420 Lbs
with PowerGlide , overall that amount of Taper
made very little difference in 1/4 mile ET,
but the MPH was almost 2+ MPH better
from 179 before to as much as 181.459 MPH after .

it looks like the Taper showed more at
hi mph speeds with a HoodScoop increasing pressure
i think a 3-speed Trans might have netted a little better
ET also

with Carbs , light Car, 5-Speed Manual Trans
like a ProStocker ...a 1.40 Ratio should work OK
especially at their MPH (205+)

with EFI , usually much more than 1.40 Taper Ratio
just look at LS-1 or LS-6 Induction
and thats off showroom floor and designed to work
pretty well around 5500 rpm OEM













Larry Meaux (maxracesoftware@yahoo.com)
Meaux Racing Heads - MaxRace Software
ET_Analyst for DragRacers
Support Israel - Genesis 12:3

http://www.maxracesoftware.com/

 

[facty]

The phenomena described in detail by Marquis are explained thoroughly by 1D Unsteady Gas Dynamics. Unfortunately, the explanation that he recites as “little minute discrete reflection pulses back”, to explain why tapered intake runners might help engine performance, but which he does NOT “buy” as a feasible explanation, is indeed an acceptable explanation for this phenomenon.

The theory of Unsteady Gas Dynamics to explain the whole phenomenon of engine tuning is now almost universally accepted. The aspects described by Marquis, such as the difference in performance with and without a Bellmouth of a straight runner compared to tapered runners, can also be thoroughly explained in 1D terms, as the tapered runner is having a direct effect on the Wave Dynamics in relation to the straight runner.

Using the 2-stroke racing exhaust system as a most extreme example of UGD should go part of the way to help understand that any discontinuity in a ducting system, whether in the intake or the exhaust, will affect the unsteady gas dynamics of that whole system and hence the overall performance of the engine.