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optimal injector/throttle plate placment?

I am considering fabbing up a custom individual throttle body for my future EFI project. For reference the engine will be a 3.2SS with RSR pistons in SC heads (9.8:1CR), mod_S cams, likely twin plug, and the usual updates. Here is a link to the other thread

custom individual throttle bodies? fab your own

My next question is what is optimal placement of the injector nozzle relative to the head? For example on MFI the injector is down in the head angled similar to the valve stem. On a CIS its higher and more vertical. Even Jerry Woods has another idea with the Madonna type setup at the top of the runner/aircleaner. I am sure there is a mixing time associated with the air/fuel. There is probably some calculation about what the optimal position is for a given intake speed. Anyone know? I know John has done calculations for intake speed/port size vs. Hp. maybe something similar in this case. Is there any resonance involved with tuning the port length?

The obvious next question is the TB placement. MFI is down low, carbs a bit higher and CIS/Motronic are much farther away. I know these two are a comprimise for the single throttle body. What about the RSR high butterfly injection? At what point does fuel reversion play a role?

I know yield the floor for discussion...
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Old 08-24-2004, 07:06 AM
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I would think fuel reversion is more of a cam timing issue, not really a function of TB placement.

Could you elaborate on what you mean by resonance with tuning the port length?
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Old 08-24-2004, 08:59 AM
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"Could you elaborate on what you mean by resonance with tuning the port length?"

related thread

Head flow question
Old 08-24-2004, 09:10 AM
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Hey Brian,

I understand the reversion is an issue with cam timing and overlap. I am sure that at lower rpms this reversion will have a certain length that is rpm dependent. ie, the difference between the intake gas speed and the reversion speed. I just wondered if there was a height of the injector or tb that could take help control this process.

Exhaust systems are tuned to provide a scavange effect I didn't know if it could be done in reverse to help intake speed. I realize there is a large difference between mutliple gas streams recombining and individual gas streams for intake but wondered if there was an optimal intake length for a given gas speed. Does the intake need to be X cm long to allow for optimized flow? I realize its probably due more to diameter. Do you need laminar flow for optimal power, turbulent flow for gas mixing/ignition? I guess I am looking for more indepth analysis on this whole combustion engine flow process.

Come on Brian, You are getting a graduate degree in this discipline. Throw up some equations for us to model this. MathCad is waiting...
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Old 08-24-2004, 09:18 AM
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another related thread:

EFI injector mounting "outside the box"
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Old 08-24-2004, 09:20 AM
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Yes Andy, I followed that one too. AS far as I am concerned the injector will go below the trottle plate. But, how far below? How far away from the intake valve? I guess I could just put the TBs on the package shelf and have really long runners. Does this mean I need to dial in a time delay to compensate for the travel time of the gas at low rpm?

I really don't know what I am doing trying to optimize a fuel injection system. I want everyone to teach me what they know. I am a knowledge junkie... and everyone knows more than I do.

I love all these discussions.
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Old 08-24-2004, 09:26 AM
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Sorry Jamie, I mainly provided that link for the benefit of future searchers.

The main thing is that the length of the runner all the way up to the stack gives you a resonance at a particular RPM. the longer the runner, the lower the RPM this occurs at; and the shorter, the higher RPM. You can do math to figure this out (an engine simulator program definitely helps), but you could also model your stack length after an MFI or weber setup that you know works well, like perhaps an RS 3.0. I think it would also be reasonable to base the length off of an MFI stack for a smaller displacement engine, assuming that the diameter of the stack is correspondingly smaller.

I am quite sure others know more and can contribute.
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Old 08-24-2004, 09:40 AM
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Also, the higher the injector the more time the fuel has to atomize (this is good). I am still unclear about the disadvantages of a high mounted injector that is still under the throttle plate.

The convenient thing about the suzuki throttle bodies (there is a thread on this in the tech forum) is that the injectors are just under the throttle plates, as they are in the TWMs. TWM says in their FAQs that they wanted to mount the injector as high as possible but still under the throttle plate. of course the length of the manifolds and velocity stacks affect the overall (tuned for an RPM) runner length.
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Old 08-24-2004, 09:42 AM
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Great Idea. I will go search for the MFI specs in the archives. I wonder if the RSR specs are in there. NOw I need a copy of Frere's (sp?) book. The diameter and length (volume) can always be adjusted in the calcs.

Now where would I get an engine simulation program?
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Old 08-24-2004, 09:44 AM
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Sorry for the jumbled thoughts, just thinking out loud..

In an MFI system, the fuel hits the back of the hot intake valve right before it opens, immediately flashes, and then is drawn into the combustion chamber. The mixing is mainly done by the flashing process, swirl and squish. A rough port surface wonít help the mixing and will reduce airflow. And airflow is the main restriction in an engine. In a carbíd system, the fuel is introduced further up the intake. The intakeís surface roughness helps create turbulence which helps mixing. But it also reduces airflow. So smooth vrs. Rough intake ports? Depends on where and when you introduce the fuel. So where to place the fuel injectors? It would depend on fuel pressure and delivery timing.

For intake gas speed (IGS), I am not totally convinced on the numbers Iíve seen. The numbers were generated using port diameters and displacement. The actual speed would be a function of the curtain area of the valve as well. The valve essentially acts as a diffuser (or nozzle), slowing down (or speeding up) the charge as it enters the combustion chamber. But you canít just use curtain area either, because the port diameter has a huge influence on the IGS. So now what? I have no idea! CFD?

Length and diameter of the runners? The pressure wave will travel at the speed of sound from the intake valve, to the atmosphere, back to the intake valve. You can time the wave to hit the intake valve right before it closes to increase volumetric efficiency, increasing torque, but only for a specific RPM. Port diameter doesnít really affect this wave. Taller stacks, better low end. So if you put taller stacks on a 2.4S it might keep up with a 2.4E around town, and still have more top end at the track. Sounds like an interesting project, eh?

This is just a tip of what actually happens, and is totally theoretical. I have no real world experience with any of this so I hope some of the more knowledgeable people chime in.
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Old 08-24-2004, 11:28 AM
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Interesting data Brain. I thought the MFI atomized due to the high fuel pressure. I did not realize it was dependent on flashing off a hot valve head. I thought delivery timing would come into play. I suppose that batch vs. sequential firing would also be a big deal, especially at low rpm.

I like the taller stack analogy. It would be nice to have a high rpm engine that was still drivable around town. That could be the best of both worlds. I could make them any length I wanted until I hit the package shelf or decklid.

Maybe you could ask some of your professors if they have any experience here. Are you still doing pit help for AJR? Maybe someone there has an idea.
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Old 08-24-2004, 12:17 PM
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The fuel does has better atomization because of the higher pressure. But it also flashes when it hits the hot valve. Like spraying a fine mist of water on a very hot pan.

I just got back from Road America. We were running 1st in class with the Flying Lizzards 2nd. Full course yellow and everybody made a dash for the last required pit stop. We beat them out of the pits by less than a foot! Held the lead, won, and passed them in the drivers point standings. Exciting race!
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Old 08-24-2004, 12:47 PM
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Sounds like a fun weekend. RA is a haul from FL.
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Old 08-24-2004, 01:04 PM
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I have been doing a little research on some of the harmonics theory regarding intake charges. Brian is correct that a longer runner is better for torque. Here is a good article and a runner length calculator.

http://www.hotrod.com/projectbuild/113_9907_efi/index.html
http://www.bgsoflex.com/intakeln.html

Why does all this work? The idea is to take advantage of the harmonics of a traveling column of air. When the intake valve closes the column of air is reflected back out of the runner. It then hits a maximum expansion when it leaves the end of the runner. This expansion sends another shock wave back down the runner. Each time the wave changes direction its based on a harmonic. It seems that the third harmonic is tuned at peak HP and the 4th is tuned for max torque.

In reading this infomation it seems that the butterfly placement could play a large role in the tuning of these harmonics. If they are placed at the two extremes (next to intake valve, top of stack) they would not bother the traveling wave. If they are placed in the middle they could restrict flow to a point. I wonder if they should be placed at a calculated "node" in the harmonic wave. Seems like this would be the best place.

Injector placement is another issue. The injectors should be placed as far away as possible to promote adequate mixing and evaporation time. This is important at high rpm where air speed into the engine is at its peak. Teh problem is at low rpm. If the injectors are too far away the column of air can not support the fuel and it "puddles" in the intake. I don't know if this would be as much of a problem with the vertical nature of the stacks on a flat six. In modern cars, It seems that the injectors are placed close to the intake valve for purely emissions related effects. This is to prevent pooling of fuel for incomplete burn at the usual low engine speeds of modern drivers, (ie. sitting in traffic, stop and go). Makes me wonder about MFI injector placment.

I just wanted to throw some more data out there for this discussion.

Where are all our racers and engine builders opinions on this subject?
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Old 08-26-2004, 07:13 AM
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Jamie, I don't think the orientation of the stacks is too big of a deal. The problem is the fuel condensing on the intake manifold: falling out of suspension.

How about a WAG: the hot head temps of our air-cooled motors might actually help with this by keeping the stacks hot. OTOH, people put heat insulators under carbs to keep them cool for a denser charge in a maximum power situation.

I think if you are going to build a motor like this for street use, the main issue on injector placement is low load performance, and also emissions as you said. However, the low load performance should not be any worse than a weber setup if your injector is no higher than the main jet on a weber. It can probably be better due to timing the injector opening to match when the intake valve is open.

I have more to say about this but I guess I need to think more.
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Old 08-26-2004, 08:52 AM
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Andy,
I've also heard that the insulators are on the webers to prevent the fuel from boiling/evaporating in the float bowls, which isn't a problem with an injection system.
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Old 08-26-2004, 09:09 AM
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ah, thanks for that Tim.

I also wanted to mention that the 916 engine installed in the 911R has the MFI injectors mounted way up at the top of the stacks. I thought that I had seen a later engine with this same MFI injector mounting (maybe the late 3.0RSR) but I can't find that picture right now ... Anyone?

By contrast, the "high butterfly" injection used on 2.8RSRs and early 3.0RSRs uses normal low MFI injector mounting. On these manifolds, the throttles are higher than usual on MFI, and they are closer together like weber bodies, but they do not sit on a weber manifold, it is a unique manifold. I have no idea why they chose to do this because it seems a straight shot to the intake port would be better. some heads like the pinto straight 4 have angled ports on the outside cylinders, so you would want the manifolds to be going that direction, but I don't see why you would want that on our engines. Maybe you could clear taller stacks?
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Old 08-26-2004, 09:15 AM
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I know which injection systems you descibe. They have tall maifolds and then butterflys with tall wide stacks. Injectors angle in at the top of the stacks.

I don't know where any pictures are.
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Old 08-26-2004, 09:38 AM
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The manifolds I am talking about are:

911R injectors at the top of the stacks, otherwise the stacks are like standard MFI stacks.

picture: BA's book 2nd edition, pg. 89

"high butterfly" manifold brings the throttle bodies closer together, then there are the high throttles, then velocity stacks on top. MFI injectors are in the standard location way down at the head. I am still curious what the design rationale was on this setup, both why the throttles are high, and why they are closer together than the intake ports.

picture: BA's book 2nd edition, pg. 84
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Last edited by KobaltBlau; 08-26-2004 at 10:54 AM..
Old 08-26-2004, 10:51 AM
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Jamie, you may want to measure the runner length of a 911S MFI stack if possible, from the head to the top of the trumpet. That would be interesting to plug into the math to find out where the 3rd and 4th resonances are. I think this would be one of the most useful configurations to test because with a 6 throttle body larger displacement hot street motor you will probably end up closest to an 'S' cam as far as factory cams go.

As far as engine analyzer programs, Bill Sherwood (billzilla.org) says this one is good, so it probably is:

http://www.auto-ware.com/software/eap/eap.htm

it's $460 (engine analyzer pro), but there's another one from the same company down around $110 with fewer features (engine analyzer).
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Old 08-26-2004, 12:30 PM
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