rotarygod

In the past I would have said to buy an exhaust first. You can't take any mor air in if you can't get rid of it any faster. This also applies to engine with overlap and scavenging ability. If you have an exhaust that flows better, and you have some degree of overlap, during this overlap period you will get more air pulled in through the intake from the pressure change. A better flowing intake will aid this. The exhaust and the intake effect each other. However, the Renesis does not have any overlap. This is why exhausts and intakes aren't getting much in terms of a power increase. If we had overlap, we'd see larger gains. My vote still goes to the exhaust but in truth it really doesn't matter on this car.

rotarygod

In regards to the first part, I was saying the exact same thing you. Just reworded. Too little flow through a large pipe is the same as too much flow (area) for the required application and therefor too little velocity.

180 ft/sec actually isn't a Corky Bell number although he probably mentions it. It is a number in Helmholtz theory equations that dictate that you don't want air going through the main intake to be any faster than this. There is nothing wrong with flowing faster than this though. The intake runners themselves could actually see up to .6 mach or about 450 mph. Again that number comes from mathematical equations but they aren't Helmholtz based. I've got alot of big technical books with graphs of the effects of airspeed to efficiency lying around. It's pretty neat how these numbers all work out the way they do.

alnielsen

iTrader: (4)

You will get a ram air effect if you get the air from a high pressure point on the car. In front of the windshield is one place. Some of the street pony cars and drag racers use this effect to boost the amount of air moving through the carburator. You can tell these cars by the air scoop opening to the rear of the hood.
This should work better on a EFI car. The computer would sence the amount of air moving through the system and increase the amount of fuel to compensate for the lean condition.

wakeech

...but when you have a cowl in front of the windsheild you now no longer have a point of high pressure, not to mention it's all about the velocity that the air is being drawn in (where you have air being pulled into a hole faster than it can be pushed in you actually have a net negative pressure), etc etc etc.

this's all been covered. ram air is garbage.

thanks for the clarifications RG.

brillo

his thoughts do explain the designs of most IRL and CART cars with the ram air duct right above the drivers head....they however, are going 180mph + so I bet they get some benefit.

globi

Interesting read regarding RAM air:
http://www.sportrider.com/tech/146_9508_ram/

globi

I'd suggest that you should still do the calculations with a 2.6 l engine. The flow in the engine might be higher, but the Renesis engine probably also recirculates much more air or better exhaust gases than an ordinary piston engine (as you actually point out in that quote anyway).
Otherwise if it really were comparable to a 4.0 l engine at 9000 rpm it should be in the power range of a Corvette Z06 or run extremely lean or be extremely inefficient (and neither is really the case - although I'm sometimes not quite sure about the latter ).
After all any engine's power, be it a little 2 stroke, a gas turbine or a coal power plant equipped with a steam turbine, is proportional to the amount of oxygen it pumps and ultimately burns.

canaryrx8

iTrader: (3)

thanks for the input guys, I'll probably do exhaust first if my wallet ever decides to cooperate, good to know that either one would help out a little though

rotarygod

Actually you hit it. The engine is very inefficent. It wastes alot of air. Ask Richard Paul what he's learned about this.

NomisR

So what's the best way to make it more efficient? :D

RX8-TX

Maybe I am paraphrasing this incorrectly....but I think I read somewhere that any means of forced induction increase efficiency.

[Disclaimer: I'll delete the post if wrong...]

shawnio

this is a quote of Rotarygod's post in why 1.3l

"In case anyone is curious I did some math to determine what the 13B rotary would be sized at if it were a piston engine. The results are pretty neat. First of all the rotary would be a 3.9 liter, 6 cylinder engine. It would be a 6 stroke. Each cylinder would be 6.54" across (damn big piston!) but the stroke length would only be 1.18" in length peak to peak. Not much there. Interesting isn't it. Now just imagine a way to make all this work with only 2 intake runners!"

just as an example a ferrari 360 modena has a bore and stroke of 3.34" and 3.11", these numbers make a big difference on an engine. Truck and econobox engines typically have a longer stroke, it's more efficient -- More time for the heat to move the piston. As opposed to an engine with a bigger bore and equal displacement. If you really care, look at how different engines of the same configuration and displacement with different bore vs stroke compare. Anyway, shorter stroke means higher revs are possible, bigger bore doesn't effect the revving ability as much. It's about piston speed more than the added weight of the bigger pistons, those things stop and go twice every revolution. Though there is more to it, rod ratios and such ... but my understanding of that is rather elementary

i think someone told me that shorter stroke engines are more efficient at higher revs ... though i'm not sure if it was in power to weight to fuel burned or whatever ... i think, she was just saying that they are more ideal than a longer stroke engine in that case.

globi

Ok, so if it pumps a lot of air without burning it wouldn't this basically ask for a turbocharger to make it more efficient? So that the energy in that air flow would at least partly be regenerated?
Or a pressure wave supercharger (comprex) would do the same. After all Mazda has been using this concept on its Diesel engines.

Links regarding pressure wave supercharger:
http://archive.greenpeace.org/climat...7pressure.html
picture 1221: http://www.technolab.org/Hako/Katalog-e/Section9.htm
http://www.swissauto.com/uploadfiles/EN_200056.pdf

This would also mean that this is not the engine for a mechanical supercharger, since it would have to pump a lot of air for nothing.

Regarding a 'bigger bore short stroke engine' being more efficient at high revs (apart from the slower piston speed): I believe (as probably most of you too) it's partly because the valves, the intake and exhaust manifold have larger diameters and therefore there's less friction at high air speeds or high revs (which should also lead to a higher volumetric efficiency at high revs).

rotarygod

The rotary lends itself very well to turbocharging. You can use a very large exhaust wheel for better flow but still have great spool up times. The problem with thinking that the rotary would be better suited to a turbo over a supercharger based on efficiency is that you are assuming the engine uses disproportionately more air than it does without forced induction.

You're assuming that if we have more air that we aren't burning and therefore less efficiency, if we add more air, we burn more air. This is true but we are also wasting a proportionate amount of air with forced induction. Remember just because a turbo rams more air into the engine making it over 100% efficient, this does not mean that we are burning all of the air in the combustion chamber. We are adding more than 100% volume to the air but we still aren't burning it all. I'm not sure if I'm explaining this clearly or not.

Think of it this way, compressed air is just that, compressed. We have more air in the same space. If we aren't buring a certain amount of air normally, increasing the pressure and therefore amount of unburned air isn't going to help us any. This extra airflow is only good in the standpoint that there is alot of flow to spin a turbo. However turbos cause backpressure and backpressure hurts power. You NEVER want backpressure and it never helps you make more power anywhere. This includes the low end. Having too much exhaust flow area and low velocity are what causes poor low end, not lack of backpressure.

This isn't set in stone but it is a good rule of thumb. For every 1 psi of exhaust backpressure you have, it takes 2 psi more of positive pressure to offset it. If we get 3 psi of backpressure through our turbo, it will take about 6 extra psi to break even. Boost does not equal power. Volume does. If we have extra backpressure in the exhaust, some of the gasses stay in the engine and get carried over to the next cycle. This how dilutes the incoming mixture which heats it, and leaves less room for oxygen. It then takes more boost to offset these effects.

A supercharger robs power off of the crank. Everyone acts like this is terrible. What's worse is people losing power to the exhaust when they think they aren't. Meanwhile the supercharged engine can have a nice free flowing exhaust which doesn't need extra boost to overcome. Suddenly we are faced with which system uses the most power to make the most. This is debatable depending on what units are being compared but the point is made.

I got more technical than I intended to.

As to the remark about how a shorter stroke would be mroe efficient, remember that the flame front doesn't have to travel very far. My comparison was just that, a comparison. It can't actually work that way. The rotary has a long distance for the flame front to travel. The faster the engine spins, the less time the flame has to go this distance. This is why the rotary is inefficent. It can't effectively burn all of the air fast enough. It doesn't matter if the air is compressed or not. It can't usably get it all.

red_rx8_red_int

Thanks for the explanation! But regarding the quoted passage above, can't you just add more flame fronts to obtain a more complete burn. I'm sure it's very complicated, but instead of two plugs per combustion chamber (not sure what to call the ignition area), couldn't you add a third or even a fourth plug?

wakeech

yup, mazda's done 3 spark plugs in some of theri high rpm race motors like the R26B.

another thing that can work is while increasing lead plug advance, you widen the trailing delay so there is a broader gap between the two flame fronts and you can end yoruself up with a super-trailing type of combustion.

rotarygod

On early prototype side Renesis engines, Mazda had tried 4 plugs per chamber. Basically there were 2 leading and 2 trailing next to each other rather than a late trailing like the R26B had. They found no improvement with them. The R26B late trailing plug did not make hardly any more power. At least not an appreciable, feelable amount. It did improve fuel economy though. This is very important in a long endurance race where you are only alloted so much fuel.

Aoshi Shinomori

So would it be prudent for an aftermarket company to make these? Or is there room in the car to put extra spark plugs in? In theory, would I be able to go buy extra spark plugs, install them and expect better gas mileage? Seems a little farfetched as I don't think anyone has done. But it would be sweet if it were possilbe.

mlx8

FWLIMABW, from my investigations of airflow, "ram air" & popular concepts of what it does, if anything, for a normal car and driving, is wildly over hyped and generally misunderstood. Maybe a more descriptive term for what I think RotaryGod wants to discuss might be "inertial ramming" (aka "velocity head" or "velocity pressure.")

Don't get me wrong, a well designed intake system is a very good thing. But, IMO its the avoidance of (a) parasitic flow losses caused by sharp edges and sudden turns, and (b) siting a good "cold air" intake point, not "ramming" air that's of greater importance in designing a good intake system for the normal car.

From what I understand of engineer-speak, air, at any speeds below say 120mph, air is relatively in- (or is that "un)-compressilbe ~ i.e. you HAVE to be running at speeds approaching 180mph, or better (its interesting that RotaryGod happens to pick this number in his hypothetical speculating) with an appropriate (i.e. well) designed intake to hope for say, at sealevel, a power increase of maybe 3%. Since the effect of velocity on the compressability of air isn't linear, i.e. the benefits of "ram air charging" fall off very rapidly as you drop from say that 180mph velocity. At anything less than 100mph, I believe the effect is basically unmeasureable. No real world street driven vehicle, and really not all that many race cars other than F1 and the Nascar guys, can practically tap the "magic" 180mph mark. A good description of the issue, and some basic illustrations of intake dos & don'ts can be found in Forbes Alrd's "Automotive Math Handbook" MBI Publishing. I think David Vizard also has some stuff on this in one or more of his various publications on proting cylinder heas.

HTH

rotarygod

I agree with that entriely. "Ram air" as widely used in the aftermarket is just a marketing term. That is my whole point about people saying they have "ram air" and then expecting usable gains from it. Maybe up top if their car can get moving fast enough but not in real world driving conditions. Air pressure rises as a function of the pressure squared per double in speed. I probably worded that wrong. In order to get to a point where this pressure starts to help, you need to be going pretty damn fast. It gets into the same theory as to why ramjets only work at high supersonic velocities.

Celronx

iTrader: (1)

I don't agree that Ram Air Intakes are worthless.

Here are some good articles about ram air intakes and Bernoulli's Equation. I read them a while ago. Following these articles, I've created some custom ram air intakes, and I feel that I've had some success. I used to own an MX-3 GS, installed a true ram air intake (one that pulled unobstructed air from the front of the car into a sealed intake) and had good results at highway speed (60-80mph).

This one is for a ram air intake for Corvettes. Though it is trying to sell an intake, I think it has some good information.

http://vararam.com/reality_of_ram_air01.html

This is a good explaination of the Bernoulli effect, and how lower pressure in a moving liquid is not necessarilly a bad thing.

http://vararam.com/reality_of_ram_air01.html

I give this link because of this quote

"Let's say we used a 6" diameter pipe that faced forwards into the airstream. We can't just taper this down into a 3" pipe since the added air will just reverse itself. We'd actually get less air into the engine this way since we'd have to speed the air up as it enters the pipe. when we speed the air up, we lower it's pressure. That is not the goal. Less pressure is less air. "

Remember that this is not always the case. If you take a 6" inlet and taper it to 3" you will be moving most if not all of the air (depending on the taper) into the intake. It will be at a lower pressure, but you will me moving 4x the air (6 in.= 28.26 sq. in./3 in. = 7.065 sq. in.).

This is some pretty complicated stuff and I have no formal schooling in this, but I'll try anyway.

CFM @ 65 MPH 3" inlet
65mph/60min = 1.0833mpm*5280 = 5,720fpm
1.5*1.5*3.14 = 7.065 sq. in./144 = .0490625 sq. ft.
5,720fpm*.0490625 sq. ft. = 280.6375CFM

CFM @ 65mph 6" inlet
3*3*3.14 = 28.26 sq. in./144 = .19625 sq. ft.
(From above) 5720fpm*.19625 = 1,122.55CFM

Now this is not taking into pressure, increased air speed through the taper, air density, and many other things I'm sure. Also, it would be before the filter and intake runners.

Giving the engine the ability to pull air as it needs without having to expend hp to pull it should increase the HP. It would be like putting in a wide open exhaust. The engine doesn't have to work as hard to move the air because there is a ready supply of it waiting.

Tell me if I'm wrong or if my math is faulty, but I still believe in true ram air and it's ability to free up hp in an engine.

Sorry for the long post.

Thanks,
Cel

Celronx

iTrader: (1)

Sorry, this should be the second link.

http://vararam.com/sub/tech_ref_article01.html

rotarygod

Ram air isn't totally worthless. It just isn't beneficial at typical everyday vehicle speeds. It is more advantageous the faster you go.

Tapering a 6" diameter pipe down to a 3" pipe can work as you've said but how you do it is very important. If you just taper it down from 6" to 3", you aren't going to get the gain in power that you think you will. A high pressure zone will develop right at the inlet. This will not cause high pressure air to enter into the engine but instead cause more air to bypass the inlet entirely and go around. What air does go into the engine will be at lower pressure as a result. If you were going fast enough this also would start to get affected but it would be much faster than the car can travel.

The 6" pipe into an airbox give the chance for air to slow down as the area increases. As it slows it rises in pressure. A high pressure in the box is a low pressure at the intake opening and more air gets drawn in. The engine feeds off of the high pressure air inside the box. Just the simple addition of a box allows the locations of the high and low pressure zones to switch. What was once a hinderance is now a benefit.

shelleys_man_06

You're on the right track, but you are thinking in terms of high-speed driving. What is the speed of air at 0 mph? Ram air is not going to have any effect whatsoever. Also, when you take pressure and temperature in account, you'll wind up with mass flow rate. Anyway, that's not important. What is important is ram-air is not ideal for all situations. Perhaps aftermarket ram-air systems will result in a lower average power gain compared to our stock airbox. I believe when choosing the right intake system, select the one that best resembles the stock airbox.

Here's a mathematical representation of Bernoulli's Law. I love conservation of stuff.

http://scienceworld.wolfram.com/phys...oullisLaw.html

Aoshi Shinomori

Shelleys, you love math way too much, it's sickening :p
Anyway, about my question on the 3rd page, can someone answer it? Or was it just a stupid question?