You’re correct, Bill.  One thing to keep in mind is that because a car can shift gears and put a much lighter load on the engine, the engine can rev to a higher rpm with the same throttle setting – unless you have a constant speed prop you can’t do this with our applications.  I use a 65mm TB off an old mustang – very light - weights a few oz while the old 3 hole stock Rx-7 TB I originally used (which worked fine) weight nearly 5 lbs. 

 

You can’t just isolate the TB, it’s of course the entire induction system that matters.  But, since the TB is the hole to the atmosphere it naturally plays a crucial role in the system.  There are also other important factors such as maintain an adequate air velocity in the intake runners.  Numerous studies have shown that you need to keep the air flow within certain limits for best results.

 

 Too small a TB can restrict air flow in the system when then prevents the manifold pressure/air density from reach ambient and therefore you are giving up power.   Down side of too large, it may cause your intake  mixture velocity to be lower than optimum for best chamber stuffing.  This entire induction process for a internal combustion engine is a very complex and fascinating topic.  There are all sorts of forces and variables interacting, conflicting and aiding – depending.  You have air density, the inertia of the air mass flow, the Finite Amplitude Waves (causes the DIE effect), resistance of intake to air flow, air filter, etc, etc, etc.  This is one reason why something may work fine for Johnny, but Billy’s adaptation with just has a few “minor improvements” ends up not working well. IF your system is not an exact copy, working in the same environment – then it’s a different system, so don’t be surprised if it produces different results.

 

My first TB was a dual throated Webber with each throat 2” (50mm) in diameter going into a racing beat Weber manifold which divided the two throats into the four intake runners.  This was back in 1992 when the only folks to ask about the rotary were the Rx-7 racing crowd.  They swore this set up was the cat’s meow – well, it turns out they were right - IF (big IF) you were capable of turning 9000 + rpm on the engine.  The engine ran fine, but just never developed more than an estimated 130 HP.  Basically, at an engine rpm of 5000-6000 it sucked – just but not very well {:>).  It was simply a case of the wrong system for my operating conditions.  Once I ditched it and went to much smaller runners and TB, engine performance improved dramatically.  Does that mean the Rx-7 racing crowd was wrong – not at all, I just attempted to use a system design for  different rpm regime – a failure on my part in understanding what I was doing. 

 

Later, I tried a 75MM at one point and while it did nothing to improve engine power and performance, in my case it had an undesirable down side – now to be fair it could have been partially the results of having all 4 injectors back near the throttle body – but in any case, if you suddenly opened the throttle like in a panic go-a-round – the engine would bog and hesitate for a fraction of a second (seemed like minutes {:>)), the engine never stopped, but I just didn’t like it.  So I went back to the 65mm.

 

So the important thing is to match your induction system to your real operating regime – NOT what you would in your wildest dream like for it to  be {:>).

 

Ed

 

 

 

From: Rotary motors in aircraft [mailto:flyrotary@lancaironline.net] On Behalf Of Ed Anderson
Sent: Friday, February 12, 2010 4:38 PM
To: Rotary motors in aircraft
Subject: [FlyRotary] Throttle limits was Re: N.A. Renesis to turbo

Bill, if Dennis had a TB just big enough  that it exactly provided 100% power at sea level, then as he increases in altitude, he will find more and more throttle travel that does nothing to increase power.  This is because it takes less and less throttle plate opening to provide sufficient airflow for the pressure/air density in the manifold to reach the ambient pressure/air density (ambient density is decreasing with altitude).  Since that air density is less than at sea level, it requires less throttle opening to reach than at sea level.

 

The position of the throttle plate affects only the air flow volume (not the density of that air airflow), but the amount of air flow volume does determine the air density in the manifold.  The external atmospheric pressure is of course what is forcing the air through the throttle body – The less air pressure at altitude means less air is flowing through the TB as you gain altitude.  Remember even though to simply the verbiage - we often talk of the engine “sucking” but of course it does not.  The engine simply opens its ports and whatever pressure is in the manifold  forces that air into the essentially empty combustion chamber.

 

Think of it this way, there are two ways you can have ambient air density in your intake manifold. Those two ways are with the throttle open or the throttle closed – a contradiction?  Not, so.  You may have ambient air pressure/density in the manifold when the engine is running and TB open sufficiently to provide ambient pressure in the manifold OR when the engine is not running.

 

When the engine is stopped - you also have ambient pressure in your intake even with your throttle 99.99 % closed because the engine is not “sucking” any air out of the manifold when stopped.  So a small leak in the throttle bottle is sufficient for the manifold pressure to reach and maintain ambient when the engine is not running.

 

What Dennis has done with his DIE intake does not change this factor, but what the pulse in the intake does when it reaches the other end of the intake manifold (note how his intakes are tied together so the pulse can travel from one intake port of one rotor to the intake port of the second rotor) is briefly create a localized pressure increase in the intake manifold right at the intake port as it is open.  This in effect shoves more mixture into the combustion chamber – in effect it is a very brief supercharger effect.  It only lasts for milliseconds and therefore you don’t get the same power increase that you would if you had a turbocharger pushing in denser air the entire time the intake is open.   On the other hand you don’t have 30-50 lbs more weight and for basically no more than fabricating your intake different – it’s basically a free lunch.

 

 But, as Dennis will tell you - doing the analysis to get the equations so you get dimension correct for the effect – can give you a headache {:>) – right Dennis??.

 

So, anyhow, back the throttle travel, if your TB is larger than needed to reach 100% power at sea level, then you would also find “excess” throttle travel sitting on the ground with the engine running at WOT.  Once the throttle is opened sufficient for manifold pressure to reach ambient, then opening it further will provide no increase in power.  You have in the vernacular- “Maxed Out!” {:>)

 

Once the throttle plate is open sufficient to permit the manifold pressure/air density to reach ambient conditions – no further opening will produce any more power even though you may have 5” of throttle travel and 50% more throttle plate opening to go.

 

 

 

Ed

 

 

 

 

 

From: Rotary motors in aircraft [mailto:flyrotary@lancaironline.net] On Behalf Of Dennis Haverlah
Sent: Friday, February 12, 2010 2:20 PM
To: Rotary motors in aircraft
Subject: [FlyRotary] Re: rpm vs Power was : Throttle limits was Re: N.A. Renesis to turbo

I used the same 65 mm Mustang throttle body on both my original and new intakes.  If  I remember correctly, I believe I had a longer  unused throttle travel with the original intake but I never measured it.  I don't know the static rpm increase with the new intake - probably have that in my notes some where but my max. rpm at high altitude (8,000 - 10,000) increased 400-500 rpm.  I estimated I went from 165 -170 HP with the old intake to about 185 HP with the new intake.  This in in line with Ed Anderson's recent note that Mazda got about a 16% power increase at 6000 rpm with the DIE effect.  I felt much improved acceleration the first time I took off with the new intake.

the new intake is based on dynamic intake effect  (DIE) where the closing of an intake valve caused the moving intake air to bounce off the valve creating a pressure wave.  The wave travels at the speed of sound to the other intake valve and arrives there just before that valve closes.  This increases the amount of air and fuel that enters the combustion chamber.  (As we know the rotary has no valves but uses the sides of the rotor for opening and closing the intakes.)  I designed my intake to give max. performance at 6300 rpm.

If I had cut down my prop to 74" it would give me more clearance for the prop on my RV-7A and increase my top end rpm.   Max rpm is about 6400 rpm with the new intake and the 76" prop.  I'm really not needing higher RPM now.

Dennis Haverlah

Bill Bradburry wrote:

 

 

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