ttg35fort

iTrader: (2)

+1

EDIT: One note here, in the Cosworth Plenum the runners and velocity stacks are integrated together. I therefore used the terms interchangably, which is not completely correct. I should have merely referenced them as "runners". The article discusses a "bellmouth", which is the opening to the runners. I believe that it is the bellmouth that is typically referred to as the velocity stack. Based on the article provided by Copec, perhaps there is additional performance to be gained by optimizing the shape of the runner openings in the Cosworth plenum. It's hard to say without doing a fluid flow analysis on the design.

+1

Remember, though, that the greater the diameter of the runners, the longer they need to be to achieve the same resonant frequency.

Thank you for sharing this article. It is very good.

Please note, however, that the article is only addressing the design of bellmouths at the end of the intake tract (i.e., these would be the upper ends of the tubes inside the Cosworth plenum ). The article does not address the total length/diameter of the intake runner/velocity stack system.

copec

You have an astounding build there ttg35fort! Looks like you have had a lot of fun.

I'm pretty sure the intake runners are too narrow for RPM's outside of the stock engine's range. I looks like every other factor has been changed for high-RPM use except the stock runner cross-area.

On Cosworth's own build for the castrol challenge, from what I read, they CNC'd the lower intake collector (and I assume matched their manifold). So they changed the diameter and shape of the intake manifold runners for that application.

The higher the RPM, the smaller torque differences it takes to make HP, and the more important it is for all the factors to be precisely in tune, the geometry of the intake manifold, the head flow, the cams, the exhaust. One of those factors not being right can ultimately throw off the whole 'equation'.

ttg35fort

iTrader: (2)

Thank you!

It is a lot of fun and I have learned alot. The technology has greatly advanced since my first Honda build 25 years ago.

I had my heads match ported to the intake runners and opened up a bit, and that does help. The Cosworth intake runners do have a bigger cross-section than the stock plenum runners, which also helps.

I didn't open the intake ports up too much for fear of decreasing the velocity of the intake charge too much in the mid-rpm range. I had the head ported when I had the 3.5L motor. In hindsight, had I decided to go with the 4.0L motor before ceramic coating the intake runners on the head (I also coated the exhaust runners, combustion chambers and the valves), I would have ported the heads even more. I'll still be able to achieve my HP goals, but it will take a little bit more boost.

On a positive note, my current setup does provide a very broad torque curve, which I like. Also, given that I have increased the stroke to 90mm and that I am using JWT high lift cams, I really shouldn't push the motor past 7500 rpm anyway. The Cosworth plenum works well there.

XKR

iTrader: (1)

What stage in the build are you at Terry??.....

ttg35fort

iTrader: (2)

Waiting for the short block. The most recent delay is my fault. I asked Sam to weld in supports for the upper portion of the cylinders, kind of like Jasonvs2146:

https://my350z.com/forum/forced-indu...5hr-build.html

I have always been a little concerned about the open deck design in the VQ35DE, but handn't really thought of addressing it until I saw that thread.

With respect to the underbody, I'll be working on the 3D modelling this weekend. Mike, call me to discuss this.

copec

It always fun annihilating people on the freeway without even having to shift out of 6th gear.

Have you been watching <a href="https://my350z.com/forum/na-builds/284761-na-build-inside-soup-to-nuts-45.html">Adam of Z1 Performance thread?</a> I'm excited to see his results!

ttg35fort

iTrader: (2)

I haven't, but I'll check out the thread.

QuadCam

iTrader: (8)

as some of you sadi, the length does not totally determine the "rpm band" for an intake manifold. you have to look at runner length, shape, area, and total volume....all these also affect the air velocity. Hogging out a runner (or port) is usually not the answer.

You need to design an intake manifold to work within the rpm band that you have designed your motor to ideally run in. at that point, coming up with the correct volume, length, and shape of the runner to get the right Helmholtz Resonance of each runner will allow you to maximize the intake manifold to your engine combination.

It is similar to designing the perfect port for a world-class SPL sound system. You can be close to what you need and have things work well, but if you are trying to squeeze out every last bit from your combination, a properly designed port will allow the combination of the sweet spot of the subwoofer to work with the optimum enclosure volume, creating the perfect resonance in the perfect port to allow the total combination to accentuate the perfect resonant frequency of the vehicle.
you can look at an engine with its intake port-head port in the same manner.

the design all comes down to the desired power over a specified rpm-band.

ttg35fort

iTrader: (2)

^+1

I have come accross a number of calculators on the Internet for computing intake runner length, as well as various forum posts that describe the mathmatics. Unfortunately, the ones I have found so far ignore the interaction of the runners with the plenum. Nonetheless, the information does provide some good insight for those that are interested. Here is one post that provides pretty good information:

http://www.team-integra.net/sections...?ArticleID=471

The Hemholtz resonator calculations would be applicable to us, but the volume (V) variable would need to be changed to further consider the effect of the plenum on the system. The plenum should act to reduce the optimum length of the intake runners by increasing the effective value of V, which is in the denominator of the given equation. Since the plenum is not a sealed box (i.e. it is fed by the intake tube), getting to the right value of V will be a little tricky.

Also, as described in various articles, the ideal runner lengths are far too long to be practical, and thus we choose runners that are a fraction of the length of the ideal runner length and rely on sub-harmonics of the runner's actual resonant frequency. For example, if the ideal runner length is 40", we could use a runner that is 20" long, which will have a resonance at double the desired frequency, but give a useful peak in airflow at that resonant frequency's 2nd sub-harmonic. We also can use a runner that is 10" long, which will give a useful peak in airflow at the 4th sub-harmonic of the resulting resonant frequency. Generally speaking, the quality factor (Q) of the 4th sub-harmonic will be lower than the Q of the 2nd sub-harmonic, and thus the air flow velocity will be lower.

I'll edit the posts I made herein to clearly state that for those who come accross this thread later while searching for this information.

copec

Correcting my own statements: This is totally correct, using just the runner length is just a course estimate at best. What I should have said was assuming certain runner cross-sectional area(s). Usually for a given engine in the aftermarket, I've noticed people don't usually very the runner area very often. So I've gotten in the habit of just using length. The higher the RPM the more precise you need to be though, because more minute flows and torque differences make a bigger HP difference. So just as Quadcam says a complete analysis does need to be taken into account.

One can simply use Helmholtz resonance of the volume of the runner, but even this is missing factors. The volume could suggest the runner would work well, however they have found that if the velocity approaches 0.6 mach in a runner it reaches choked flow, and that's basically determined by the runners cross-sectional area. So using a simple Helmholtz model would suggest the size and shape of a runner would be okay, but actually would reach choked flow. One would have to make runners with more cross-sectional area that were shorter (with the same volume as before) to perform according to the simplified Helmholtz model, or even the much simpler "length relative to the stock runners" model of mine.

This model can be as complex as modeling all known physics on the I.M., which is would be totally impractical, the engineering challenge is to use vastly simplified models but understand what they tell us and what the limit of the model is. Throwing bends into a manifold has many consequences for instance, which we haven't even talked about the model of.

I'm pretty certain the stock runners are indeed too narrow. I'm not purposing simply hogging them out, but precisely altering them to fix this problem, but I can't exactly say how. Its something I would like to find out though, because I'm interested in it. It seams the machinist for Adam's car of Z1 did this through experience, although I would like to see someone do a complete scientific analysis and put the results up here.

I think Cosworth probably just figured that they would fix the top end of the manifold, make that as optimal as possible, and people would take that piece and change whatever they needed to reach their design goals (such as changing the runner area for their target application). They made a decision to do what they could to give someone a much better starting piece to work with, but not necessarily the final model of how the stock I.M. should be.

Anywhozel, that's a lot of babbling for nothing, it seams there are many people on here that know more then me and know how to say it.

Alberto

iTrader: (14)

Id love to test a Cosworth or CJM intake manifold. I dont run a HUGE power but I run a Greddy kit to the max with BC3 cams. My setup would be closer to what most will be running, and I think would be nice to see. I run 13-20psi on my $hitbox. Who's got these "on sale" or up for testing?

Dynosty

iTrader: (27)

Alberto, Cosworth plenums are currently in stock

ttg35fort

iTrader: (2)

+1

Yes, I guess a good summary to all of this is that the cross-sectional area needs to be adequate to ensure that the impedance is sufficiently low to allow adequate air flow for the desired maximum HP. As you indicated, bends in the intake runners also will have an affect on the impedance (as well as the design of the intake ports on the head, e.g., https://my350z.com/forum/forced-indu...w-testing.html). The second piece of the puzzle is the intake runner length, which we have been discussing, and the article that you provided throws in a third puzzle piece, which is the bellmouth design.

It gets a little complicated, which is why computational fluid dynamics is such a valuable tool.

I'm not sure how well the Cosworth is suited for additional modification. From the looks of it, one might be able to play with the contour of the runner bellmouths, and maybe increase the size of the encaps to enlarge the volume of the plenum, but I'm not sure what else can be done.