I did a search but couldn't find what I was looking for. What is the trade off with runner length (I think I read HP vs. Torque somewhere? )? I'm starting to fab a tube type EFI intake for a stock rebuild 225 and wondering how long the runners should be. Was considering an interchangeable section to make it variable but not sure yet if I need to.

This is the proposed evoloution of my project. The stock rebuild is phase one, the (buick) EFI is phase two, efi turbo is phase three, and the final phase is strapping that all on to a slightly overbored block, forged pistons, lots of headwork, maybe Doc's stroker crank as described in the articles, megasquirt, etc. Intention is to push the limits of the stock rebuild and have all the bugs worked out of the EFI and turbo by the time I start getting lots of machine work done.

Thanks

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Grip it and rip it

In simplest terms: long runners for torque and short runners for top-end horsepower.

long runners reach resonance at low rpm and will therefore boost power at low rpms where torque is dominant. Short runners won't reach resonance until higher rpm; so the boost occurs as top-end horsepower is building. There are cases where a long runner will work at high-rpm because you're hitting 2nd and 3rd harmonics.

It sounds like tuning a musical instrument because it is like tuning a musical instrument. Longer tubes produce lower pitched notes.

There are some formulas for runner length vs. engine speed and volume, etc. If you're interested I can post them tonight when I get home (supposed to be writting code right now...)

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My truck is a Frankenstein creation
Built from the dead carcasses of others
And brought BACK TO LIFE!

Yeah, aren't we all supposed to be doing something else . Too bad I can't get paid for this .

Anyway, some formulas to fiddle with would be great. The goal being something that will function on both sides of the spectrum, Stock vs. mod. One thing I did fail to mention was that this is supposed to end up as a summer cruiser so street manners are important too.

Sounds like an interchangeable section might not be a bad idea in order to keep it 'tweakable' (if that's a word) until I find the sweet spot that I'm happy with.

W

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Grip it and rip it

sorry it took me so long to reply. Had a bad weekend ending with me rebuilding the carb in my truck. Also discovered the lean-burn computer in the truck was the wrong one -- explains a lot...

Anyway, there are several ways to approach this:

My book from Motion Software Inc. gives some equations which they claim are "conservative" and I'd assume are somewhat generic (they've made a bunch of assumptions...)

They claim the minimum port cross-sectional area required to maintain a reasonable air velocity (in inches) = (RPM X Stroke X Bore X Bore) / 190,000

Using the above sized port (runner) area, Ball-park runner lengths (in inches) to use the resonance pulses are:
2nd order = 108,000 / rpm
3rd order = 97,000 / rpm
4th order = 74,000 / rpm
5th order = 54,000 / rpm

Each later pulse is weaker (i.e. 2nd is strongest and 5th is weakest). Anything below 5th order is negligeable. Chrysler's long runner intakes are generally tuned to the 2nd or 3rd order resonance.

Another site I've found ( http://www.grapeaperacing.com ) says that runner length can be found as:
L = ((ECD x 0.25 x V x 2) / (rpm x RV)) - 1/2D
where:
ECD = effective intake duration (crank angle) = 720 - Adv. duration - 30
V = wave speed (speed of sound in fuel air mix) (1200 -1300 ft/s)
rpm = rpm you're tuning for
RV = reflected wave number (1-5)
D = runner diameter

and the required runner diameter can be found as:
D = square root (CID x VE x rpm) / (V x 1130)
where:
CID = cubic inch displacement
VE = volumetric efficiency of engine
rpm = max rpm
V = air/ fuel velocity in ft/sec (they recommend no more than 180ft /sec, dynomation recommends 0.45 - 0.55 speed of sound...)


If you're really good at math, They're all based on Helmholtz resonators...

If you want to do some modelling on computer to see how changes in the plenum or runners affects the performance, there's software available called "Dynomation" http://www.dynomation.com It's around $600 which is kind of pricey unless you're racing or have money to spare...

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My truck is a Frankenstein creation
Built from the dead carcasses of others
And brought BACK TO LIFE!

Well this throws my "grip it and rip it" theory out the window

Thanks though, I think this is what I'm looking for. It's gonna take me a month of Tuesdays to get through it all. I've been using the SDS tech page as a starting point for the build but they don't go into specifics about determining runner length just that street engines should be minimum 9 inches.

Thanks again.

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Grip it and rip it

Hey mnecaise,

That's some cool info I had seen some theory on the long rams before but I was curious what effect the size of the plenum would have on the tuning I assume the length between the valve and the plenum is the critical dimiension but if you had a large plenum would it affect the resonance on the reflection?

Hope you understand the question I can't think of a better way to state it.

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It is easier to REACT,
than it is to THINK!

Middy.

That is correct. You also have to take into account the cross-sectional area of the runner (because it affects airflow velocities and capacity).
It's not supposed to. The signal comes from the "sudden" change in cross-sectional area.

The equations should get you in the ballpark. They don't take taper angles, steps, and other restrictions (like the reduced flow area around the valve pocket) into account -- they assume the runner is the same diameter from the inlet to the valve...

Now, I gotta throw in a warning: I've never built a custom manifold. I've got a working knowledge of the concepts and theory (damn that physics degree); so, I believe the information I've posted to be fundamentally correct. My point is: Although I've never personally done it, this is where I would start...

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My truck is a Frankenstein creation
Built from the dead carcasses of others
And brought BACK TO LIFE!

The magic of MS excel. Hopefully I interpreted this correctly. By this formula a runner 30"-36" should function well in the 1500 to 3500 RPM range and a runner 24" - 30" should function well in the 1750 to 4250 RPM range? Runners outside of this range would function but not take advantage of the ram effect, correct?

Just as an aside, how long are the Hpak runners?

mnecaise, I hear your warning... I've never built an intake either but I appreciate the fundamentals.

RPM 2nd order 3rd order 4th order 5th order
108000 97000 74000 54000
1000 108.0 97.0 74.0 54.0
1250 86.4 77.6 59.2 43.2
1500 72.0 64.7 49.3 36.0
1750 61.7 55.4 42.3 30.9
2000 54.0 48.5 37.0 27.0
2250 48.0 43.1 32.9 24.0
2500 43.2 38.8 29.6 21.6
2750 39.3 35.3 26.9 19.6
3000 36.0 32.3 24.7 18.0
3250 33.2 29.8 22.8 16.6
3500 30.9 27.7 21.1 15.4
3750 28.8 25.9 19.7 14.4
4000 27.0 24.3 18.5 13.5
4250 25.4 22.8 17.4 12.7
4500 24.0 21.6 16.4 12.0
4750 22.7 20.4 15.6 11.4
5000 21.6 19.4 14.8 10.8
5250 20.6 18.5 14.1 10.3
5500 19.6 17.6 13.5 9.8
5750 18.8 16.9 12.9 9.4
6000 18.0 16.2 12.3 9.0

Added up to 6000 rpm (don't know why I didn't do it in the first place )
Can't get it to space out any better Sorry for the clutter
Thanks again

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Grip it and rip it

Oh I understand it's all theoretical.

I was tying to figure out a plan for an adjustable runner length manifold controled with a stepper motor that would use rpm inputs at intervals of 250 rpm. When I first found the info on the "long ram" page. I won't be able to work on my slant for many years yet, It's in storage until I get a house with a garage. It's all just fantasy but who knows someday it might be possible.

I appreciate your knowledge and willingness to share.

_________________
It is easier to REACT,
than it is to THINK!

Middy.

Here is a link to that thread on adjustable runner length.
http://www.slantsix.org/forum/viewtopic ... ram+tuning

The factory H-Pak has 17-18 inches of runner to the center of the plenum.
add 4 inches for cylinder head runner and another 3 inches to get up through the carb. venturis and that gives you a total of appx. 25 inches.

These manifolds tend to "tune" in the 4500 to 5500 RPM range.
DD

I think it's possible to build a 4 barrel dual plane manifold with longer runners for the primarys and shorter runners for the secondarys. I'm presently looking for a dual two barrel manifold at a resonable price that I can slice and section, It most certainly will only fit under the stock hood of a truck. I too am going to take a month of sundays to get through those equations but I'm really gratefull they were posted. This is a great place to share not only experiances but also theorys. Allot of what we do never gets scientificaly tested, but allot of educated tweeks here and there and we will get lucky, the manifold I'm using now is an example its an "Offy" 4 barrel with a divided plenium and a 4" long divided riser that slopes 45 degrees followed by a 1" phonelic spacer that matches an edelbrock 600, care was taken to match venturi areas and to eliminate bumps and jogs, but thanks again Mnecaise for the equations I plan to put them to good use! Bruce J.

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78b4ramSnortbed/6

Bruce,

I've been working on a design to cast in aluminum, and now I think you've given me something else to throw in there. The longer runners for the primaries and shorter for the secondaries. Now if I could only get the time to work on it!

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Slant Six... out of the ordinary - just like me!

Hey Andrew or anyone else, How do you overcome the round peg square hole issue with the intake, round tube runners and square cylinder ports? I'm thinking of welding 2 1/4" flanges together, one will have square ports (head side) and 1 will have round ports (manifold side), then grind the welds and blend them nicely from round to square. Anyone got any other ideas?

Doc, thanks for mentioning the port to valve distance... didn't think of that.

*** The following is a mental meandering I had while reading this and other runner length posts, there is no basis to it just a crazy vision***

If you were running injection you could build a tube manifold with a vertical section running the length of it. Short runners straight to the head and long runners that start opposite the head and bend around back to the head? Orient the primaries over the long runners and the secondaries over the short ones. Some kind of baffle to direct the air from the correct tube at the correct rpm, throttle position?

I don't know if it would work, just something that popped into my head as I was reading

Later

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Grip it and rip it

I was looking at making one 1/2" thick flange and adapting within that. I was thinking that I would use it like an adapter plate between whatever my design would be and the head. Perhaps being 1/2" thick it could be treated as if it were part of the head. Instead of making a bunch of smaller adapters?

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Slant Six... out of the ordinary - just like me!

I'm getting my flanges laser cut (once I finalize my idea) and 1/2" is a little beyond the capabilities of the local guys here. It can be done but the edge is really not that clean. 'Laminating' 2 x 1/4" will give a smoother cut.

One of the engineers here is aiding me with the design ideas and CAD work he's also a wrenchhead (has an RB26 in a Z car) and has experience with EFI and turbos.

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Grip it and rip it