FI Plenum Design, MS'ing my 2270 Options

[Tom Perso]

Bringing this post back to life....

Any thoughts on plenum design? I see we have the size area covered (somewhat) but what about shape? Square box with 4 tubes sticking out for runners? 5" steel tube with runners? Any better places to stick the TB? Top, front, side, backwards, up my... ?

I found a Ford TB that is nice and simple, just has a TPS on it... But, 62mm in diameter. Sounds a little big?

Also, what injectors will fit into the stock runner injector bungs? Looking for something a little smaller than the 2.0L and bigger than 1.7/1.8.

Thanks,
Tom

[Bleyseng]

For starters, lay a 1 gallon paint thinner can on top of the motor, flat with the long sides facing the runners. Thats a start, hell, fab something up that basic shape or alittle bigger.

Geoff

[TimT]

Try to make a plenum along the line of this one.

This pleneum is from a GT3RS. I have a feeling Porsche Motorsports knows a thing or two about sizing and designing plenums.. If I get a chance Ill measure the volume f this particular unit.

Anywayz... it a box with runners molded to it. It appears to be one ply of CF cloth

[TimT]

nuther

[Brett W]

Tim, if you would be so kind as to take a tape measure and the external dimensions of the Porsche plenum. I roughed an estimate at 18x12x3, that should be about 10 litres. Close to my 2.5 rule.

QUOTE

However, it seems to me that just because engine parameters change slightly doesn't mean you have to start from scratch. I just don't see how you need a plenum that is 3-5 times bigger, and a TB that is 1.3 times bigger with an engine that is only about 13% bigger, even with the other changes you are talking about.

You are assuming the factory dimensions are correctly sized for a stock motor? My stock 1.6 litre Honda motor made 108HP from the factory at something like 6500rpms. It uses a 56mm TB. Although I believe the plenum volumn to be less that 3 litres though.

A smaller plenum will yeild good throttle response and drivability. The plenum should not have a flat bottom with defined corners. Airflow doesn't like corners. I would not bring the runners in at a 90 deg. angle like the manifold pictured in a previous post. Airflow does not like radical changes in direction. Notice how Porsche brought their runners out the bottom. You could do the same or you could bring them out of a rounded plenum at the angle where the corners should be.

[Bleyseng]

Brett,
What are the valve sizes of that motor?

[Brett W]

QUOTE

What the goal is, is to use the small venturi ( velocity increases magnanomously, with no loss of flow) which rams the air into the large plenum, changing the velocity to pressure---hence, " Ram Effect "

Sort of. The ram effect does not necessarily come from that. The reason plenums taper into the TB flange is that you can just bolt a flange to the side of an open box, you will kill the airflow. Air does not like radical changes in direction, it will seperate from the boundary layer and create turbulence, killing your flow. Airflow likes to "see" less than a 15 degree change in boundary conditions, anymore and it tends to seperate.

QUOTE

This is the only way turbines will survive at high speeds--just extropolate this practice into automotive--the practice is the same--but different--with turbines, the air has to be slowed down, or the engine will stall and flame out--piston engines are spark dependant, and never get to this speed, and hence can use this effect to an advantage--much testing has shown that when you increase the velocity of the air into the plenum beyond supersonic--and then have a large plenum to extract the velocity and turn into pressure ....

Turbine engines have to slow air down because it chokes at levels supersonic and beyond. When airflow through an orifice chokes it will stall. Supersonic airflow is a very different animal than subsonic flow. I have seen what happens when airflow goes supersonic inside an induction system. An engine I was working with last year would flatline at 11200, exactly the point at where the restrictor caused the air in the intake to go supersonic. It is a phenomenon know as choke flow. This engine should have continued to make power into the 15500 range.

Velocity is good to a point in an intake tract. Too much is just as bad as too little. The airspeed in the intake ports should not see any more than about 300-400 fps, any more and you run into problems. Remember High velocity does not create high pressure. High Velocity creates low pressure. In the intake you high pressure not high velocity.

The "ram effect" that you mentioned is caused by "wave" tuning. Air is a really cool medium. It is a fluid but it also behaves as a gas. A fluid has mass and direction, but you can't compress it. A gas on the other hand can be compressed. Airflow can not be stopped and restarted instantaneously.

When the valves in a cylinder are opened pressure drop moves air into the engine, it will keep flowing even when the piston is starting to move back up the cylinder. This inertia will draw more air into the engine. When the valve closes the column of air moving through the intake tract can't just stop and wait for the valve to open again. It bounces off the back of the valve creating a "negative direction wave". As this wave reverts back through the intake port it has a certian velocity and mass. It will hit the plenum and either proceed to disipate somewhat or it will flow into another cylinder. The size of the plenum will determine how much a dampening effect is applied to the "wave" . When the wave bounces off the plenum wall it is headed back for the intake port. If the cam timing is matched correctly to the intake characteristics then this wave will have some extra velocity and can help push more mixture into the cylinder. The problem with "ram tuning" is that it only works in certian rpm ranges.

Remember all engines are a compromise. Factory stock engines will have compromises made in order to:
make the engine fit the engine bay
Meet emmisions and noise regulations
Make the car easy for grandma to drive to the grocery store
ETC.

Assuming that Porsche built it right from the factory is not the way to settle an argument. Porsche made the 3.0 litre S2 with 208 hp. Not really impressive if you ask me. Yes it is good but hell the 1.8 litre Intergra engine made 180hp from 1.8 litres. The first production car to make 100Hp per litre.

[Brett W]

My SOHC 1.6 has 29mm intakes and 25mm exhaust. Not a great performance motor by any stretch but impressive for what is is.

[Reiche]

QUOTE (Brett W @ Feb 3 2005, 09:41 PM)

Remember all engines are a compromise.  Factory stock engines will have compromises made in order to:   make the engine fit the engine bay   Meet emmisions and noise regulations   Make the car easy for grandma to drive to the grocery store   ETC. Assuming that Porsche built it right from the factory is not the way to settle an argument.  Porsche made the 3.0 litre S2 with 208 hp.  Not really impressive if you ask me.  Yes it is good but hell the 1.8 litre Intergra engine made 180hp from 1.8 litres.  The first production car to make 100Hp per litre.

Hey now, take it easy Brett W. Assuming Honda built it "right" isn't going to settle any arguments either, especially in a Porsche forum. And I doubt Porsche made many compromises for Grandma. Ever. (IMG:http://www.914world.com/bbs2/html/emoticons/smile.gif)

Back to the original question.

QUOTE (Tom Perso @ Feb 1 2005, 05:41 AM)

I am designing a MS system for my 2270 (163.86b cam, 48x38 valves, future Triad header). I am looking at running stock intake runners and possibly making a new plenum to run my Ford TB (probably from a 4.0L Exploder/Ranger). Are there any "design guidelines" that should be followed when making a new plenum, such as a certian volume required for engine displacement?

I never claimed to be settling any argument by what I wrote. I was asking how you figured a 300%+ increase in plenum volume for a 13% increase in displacement, given the stock intake runners. I'm still not sure. But I can respect what you know. Just don't read more into what I wrote than I intended. Like most here, I am trying to learn.

My suggestion was simply to look at something that works and start from there. You can look at Hondas, I am looking at Porsches (including type 4s.) Maybe Porsches don't work the same way your Hondas do, but I think I can assume Porsche's engineers know a thing or two about engine design. Did they build it "right?" I think you answered that yourself: "all engines are a compromise..." Not just factory stock engines. Yes, even Honda engines. So why dis the Porsche engineers for the compromises they chose? Don't agree with them? Fine. That hardly makes them wrong.

Speaking of compromises, just for fun let's look a little closer at the two engines you are comparing.
944S2 3.0: 208 HP @ 5800 RPM, 206 lb.-ft. @ 4100 RPM
VTEC 1.8: 178 HP @ 7600 RPM, 128 lb.-ft @ 6200 RPM
Obviously they are tuned for significantly different results. While the specific output of the Acura is impressive, that is only one of the many compromises that make the whole package. And those compromises may or may not have anything to do with the original question that started this thread.

I'm serious about that dollar. (IMG:http://www.914world.com/bbs2/html/emoticons/wink.gif) All good bro.

[Tom Perso]

QUOTE (Bleyseng @ Feb 3 2005, 06:07 PM)

For starters, lay a 1 gallon paint thinner can on top of the motor, flat with the long sides facing the runners. Thats a start, hell, fab something up that basic shape or alittle bigger. Geoff

I have already fabbed something like that up. It was a cardboard box that was 10x4.5x5.5 and was 3.4L in volume (if I remember my math correctly). Looked like hell though (IMG:http://www.914world.com/bbs2/html/emoticons/dry.gif)

Half of me thinks to mount a Chebby 2.8L v6 TB on a stock manifold to get it running, then dink around with it. Hell, Tom Notch has the same setup on 2.7L T4 in his Notchback running Autronic and it works great. WTF.

And to put this in perspective, I'm not planning on getting this right the first time... (IMG:http://www.914world.com/bbs2/html/emoticons/smile.gif)

Later,
Tom

[Brett W]

QUOTE

I never claimed to be settling any argument by what I wrote. I was asking how you figured a 300%+ increase in plenum volume for a 13% increase in displacement, given the stock intake runners. I'm still not sure. But I can respect what you know. Just don't read more into what I wrote than I intended. Like most here, I am trying to learn.

I am not working on ratios from the stock parts. I feel they are to restrictive. Based on the calculations I have done on some Honda engines and Suzuki engines I feel that the plenum on the 914 is too small for good performance in the range where a good high performance engine will make its best power. Although a plenum system will always offer less throttle response than an IR type system, it can make more overall power. Most guys with a 914 will want throttle response over maximum power.

QUOTE

My suggestion was simply to look at something that works and start from there. You can look at Hondas, I am looking at Porsches (including type 4s.) Maybe Porsches don't work the same way your Hondas do, but I think I can assume Porsche's engineers know a thing or two about engine design. Did they build it "right?" I think you answered that yourself: "all engines are a compromise..." Not just factory stock engines. Yes, even Honda engines. So why dis the Porsche engineers for the compromises they chose? Don't agree with them? Fine. That hardly makes them wrong.

I look at Honda because I feel that, as far a production NA engines, they are some of the most highly developed. I am not "dissing" Porsche engineers, I am making a statement about what I feel is some drawbacks to how Porsche does things. Please stop thinking of the T4 as a Porsche motor. Porsche would hardly have anything to do with this engine if it weren't for the original success of the early 70s cars.


All this being said I am stating my opinions based on my experiences and research. When VW decided to make the T4 an oversquare engine they had to do something to create torque. That is the part reason behind the 54+ inch exhaust primaries and common plenum design with very long runners and a small throttle body. Increase torque. When you modify the factory engine beyond the stock characteristics you move the powerband up in the rpm range. Thus creating the need for more airflow.

[Mark Henry]

Not VW/Porsche but some tips on building a plenum here

[Bleyseng]

I just think that a inline 4 plenum design doesnt share that much in common with a flat 4/6. The VW 4 has too long of runners to make real hp above 5000 rpms plus a small plenum but who in stock form needs it? I think Porsche's design shows where to head with the design.

Geoff

[Reiche]

QUOTE (Brett W @ Feb 4 2005, 06:16 AM)

Please stop thinking  of the T4 as a Porsche motor.  Porsche would hardly have anything to do with this engine if it weren't for the original success of the early 70s cars.

Et tu, Brett?

It's bad enough when others NARP the 914, but one of our own?

[airsix]

QUOTE (Brett W @ Feb 4 2005, 06:16 AM)

Based on the calculations I have done on some Honda engines and Suzuki engines I feel that the plenum on the 914 is too small for good performance in the range where a good high performance engine will make its best power.

My input here won't be very scientific, but... When I was planning the turbo project I needed to move the TB to a location better suited for the turbo plumbing. This is on a 1.7 so the TB was horizontal on the driver's side. I cut my plenum neck off, rotated it 90 degrees, added about 1" to the length of the neck, hammered to shape, and rewelded it back together with the TB in the vertical possition. I estimated that I added about 5% to the plenum volume. The effect was a slight, yet noticable decrease in throttle response between about 2,700-3,100rpm. At other rpm's there was no noticable difference but it definitely lost some snap in that narrow rpm range. It was a little disheartening since that is typical cruise rpm and it's exactly the rpm I'm generally in when I decide to goose it. The good news is that for whatever reason, after increasing the plenum volume I was able to increase the timing advance by about 2 degrees (at that spot - not across the board) and got most of the snap back. I drove it like this for many months before the turbo was added.

-Ben M.

[Brett W]

Turbo plenums are different. If you are going to go turbo I would copy the Subaru manifolds. They have really long runners and small plenums. I have a complete article on the Rally engine complete with pictures, but no scanner,

[cnavarro]

Tom, I still have the original plenum that we fabricated. Based on models with Engine Analyzer Pro, we found that a configuration or 130 cu inches and runner lengths of 10.5 inches would have the proper tuning characteristics for the flat four. It comes out to a very odd shape for the overall width of the engine, that's why it had a sweeping profile matching the top of the cylinder tin and case. I can post pictures when I return from CA next week.

Charles Navarro
LN Engineering
http://www.LNengineering.com
Aircooled Precision Performance

[Mark Henry]

I'd like to see that too Charles....I'm still on the fence debating between a single TB/plenum or dual Jenveys.

Going to have to make up my mind soon.

[TimT]

QUOTE

Tim, if you would be so kind as to take a tape measure and the external dimensions of the Porsche plenum. I roughed an estimate at 18x12x3, that should be about 10 litres. Close to my 2.5 rule.

I measured the (GT3RS) plenum today, it roughs out to about 18L the engine it was on is a 3.6L

This post has been edited by TimT: Feb 5 2005, 02:14 PM

[Mark Henry]

Holy fuch! (IMG:http://www.914world.com/bbs2/html/emoticons/ohmy.gif) I'd need a plenum of 13L to follow the design of Tim's Porsche unit (IMG:http://www.914world.com/bbs2/html/emoticons/unsure.gif)

BTW my /4 is going to be almost 2.7L