What is the aim to design and how to assess the performance of restrictor ?

[Mbirt]

Originally posted by Z:
<BLOCKQUOTE class="ip-ubbcode-quote"><div class="ip-ubbcode-quote-title">quote:</div><div class="ip-ubbcode-quote-content">Originally posted by Mbirt:
Running a restrictor and diffuser at the end of an intake tract has been a consistent recipe for 25 hp on a 450 cc single in my testing experience.

Mbirt,

Do you mean that a 450cc single with an intake pipe that consists only of throttle-restrictor-diffuser-head, and nothing more (no plenum), will only put out 25hp? I can see why this might be the case, but just checking if this is what you meant.

Furthermore, what size plenum would you recommend for a 450 single? (I guess I could use "Find", but while I'm here... .

Z </div></BLOCKQUOTE>Z,

Yes, except the setup I tested was throttle-restrictor-diffuser-about 5" of intake runner-head. The setup you describe would make even less power because the intake runner, without a plenum volume into which it would normally terminate, acts as plenum volume itself.

As for plenum size, the general consensus in this thread is too small. In testing (on a 470 cc CRF450X) a 1 liter plenum, I found that intake runner length was still behaving more like plenum volume and less as a tuning tool. As the runner was shortened from a length that was good for the 2nd ramming wave at somewhere below 5000 rpm, power was lost everywhere and the shape of the curve did not change. An increase to 2 liters changed this and finally allowed intake tract tuning to behave as expected. Above 2 l, the volume of diminishing returns will depend on the engine system design and its VE. I would thus start in the 2-2.5 l range and go up from there.

Our WR450f stock bore/stock stroke likes 3.5 l, making 50 hp from 7800-11500 rpm with a nice peak at 60 hp at 9800.

[Z]

Mbirt,

Thanks. That makes sense.

The 25hp suggests the restrictor is sonic ~20% of the time (given that 100% is ~125hp). So sonic for a bit less than the full intake stroke. And 3.5 litres is ~8x swept volume, with more being better (space permitting). Although 50+hp sounds good.

Z
(PS. Still wondering about that rocket nozzle???)

[Wesley]

Originally posted by mk e:
<BLOCKQUOTE class="ip-ubbcode-quote"><div class="ip-ubbcode-quote-title">quote:</div><div class="ip-ubbcode-quote-content">Originally posted by Warpspeed:

But I believe there is far more to be gained by trying to build a mountain of a torque curve at mid rpm than trying to squeeze an extra top end Erg (or two) by sacrificing everything else.

You know....it really doesn't much matter. if you build an engine that makes say 50 ft-lbs torque at 5000 or an engine that make 25 ft-lbs at 10000 rpm becasue you still have the transmission and final drive sockets with the goal of turing the wheels at say 2500 rpm. The first engine gets a final drive ration of 2:1 and puts 100 ft-lbs to the wheels and the other gets a final drive of 4:1 and puts.....100 ft-lbs to the wheels. The 2 engines woulds have the same hp so you get the same final result.
</div></BLOCKQUOTE>

Problem is you compress your usable torque curve significantly, which means your accel score lowers from increased number of shifts, your lap times suffer because newbie drivers can't keep the peaky engine in the power band, and you operate the engine at a less efficient flow range through the restrictor, increasing your pumping losses and decreasing efficiency.

Originally posted by M. Nader
On the other hand i think that a properly designed restrictor with an inefficient plenum would be just wasted, if for example cylinders 1 and 4 get just 50% of the air needed the power will drop by 25% even with the best restrictor this is.

If you can make more overall power with a biased plenum design, do it. You want more power always. You can tune out individual cylinder trims if it's a big problem, and you'll be losing some ultimate power, but at least youll be making more than if you used a rocket nozzle for a restrictor.

More power is more power, regardless. Yes, you should focus on a plenum with good cylinder distribution AS WELL AS an efficient restrictor, but there are reasons we have the ability to trim cylinders.

Plenum volume considerations are manifold. The larger the plenum the closer to steady state the restrictor acts, which means you aren't bringing it to choked flow every time a valve opens and wasting the rest of the time. It also increases drawing volume for the cylinder - you have a larger volume of air you can induct with less pressure drop within the plenum.

At the same time, that buffering effect that helps total top end flow through the restrictor by dampening pressure drops from individual valve opening events, it takes that much longer for pressure to start building in the plenum in high vacuum situations and you see a resultant drop in throttle response.

Most of our plenums were sized around 2-3x engine displacement, which didn't rob too much throttle response and allowed for steady WOT flow through the restrictor. We also used a larger divergent angle than a lot of teams thought was "correct." Because of packaging constraints the larger diverging angle allowed for a more gradual increase to the plenum interface instead of a dump into the huge volume of the plenum and the turbulence resulting from that, maintaining flow momentum within the plenum.

Like all things, it's a tradeoff. Either do some 1d simulation to determine what your pressures look like through the restrictor and investigate plenum pressure rise rate after sudden throttle position changes (throttle response) or build a few test plenums that you can test on a dyno.

[Warpspeed]

Never worry about the torque curve, it is the power curve that matters.
Gears are for multiplying torque, engines are for making power.

If you have the power (at ANY rpm) matching that to the road speed with suitable gearing will get you the tractive effort.

If you can keep your engine at peak power, and peak airflow, as limited by the restrictor, over as wide an rpm band as possible, you will be out in front.

[M. Nader]

Originally posted by Warpspeed:
Never worry about the torque curve, it is the power curve that matters.
Gears are for multiplying torque, engines are for making power.

If you have the power (at ANY rpm) matching that to the road speed with suitable gearing will get you the tractive effort.

If you can keep your engine at peak power, and peak airflow, as limited by the restrictor, over as wide an rpm band as possible, you will be out in front.

This kind of talk has always confused me, I understand that the torque values can be fiddled with various gear reductions as much as you need, but i have always thought of power as more of a tertiary property rather than the primary one, the engine produces torque and power is a function of torque not the other way around. for me i think a good torque curve is more important than a power curve is it is what actually does the driving

[Nick Renold]

While we are having this wonderful discussion of restrictors, I was wondering if any teams have investigated the effect of surface roughness on the restrictor performance. Is there a big difference between a powder rapid prototyped restrictor and a polished metal or smooth CFRP one? Are there any resources you can recommend on the effects of surface roughness on airflow? I know that RCVD has some brief sections on it, but those are more related to bodywork aerodynamics.

[Warpspeed]

Originally posted by M. Nader:

This kind of talk has always confused me, I have always thought of power as more of a tertiary property rather than the primary one,

If you could choose between fitting a V8 engine with 400 Ft/lb of torque, or a lawnmower engine that was heavily geared down with a gearbox to also generate 400 Ft/lb of torque out of the gearbox.
Which do you think would make a vehicle accelerate and travel faster.
Remember the torque is identical, but the POWER is hugely different.

Power is the primary factor.

[Dash]

I think Nick is more or less talking about looking at one motor. Do you look at the power curve, or the torque curve for tuning? Personally ( I'm not an engine guy), I'd probably look at the torque curve to see where to improve.

More power is more power though when looking at two choices, and gearing will help you out.

[M. Nader]

Originally posted by Warpspeed:

If you could choose between fitting a V8 engine with 400 Ft/lb of torque, or a lawnmower engine that was heavily geared down with a gearbox to also generate 400 Ft/lb of torque out of the gearbox.
Which do you think would make a vehicle accelerate and travel faster.
Remember the torque is identical, but the POWER is hugely different.

Power is the primary factor.

I don't think this example is the primary choice here as these engines are on two ends of the spectrum in terms of RPM range and weight which are critical choices for choosing an engine. for an FSAE team the RPM "useful" range in terms of power and torque is necessary to know as shift times vary depending on the system used and a lot of teams desire to do certain events (autocross) in only 2 gears for example.

I still claim that having a neat torque curve is more important than having peak power, the power curve will also be quite good as it depends on the torque curve!

my support is that power depends on torque and not the other way around, the second thing i would look at after torque would be RPM range

[mk e]

Originally posted by Wesley:

Problem is you compress your usable torque curve significantly, which means your accel score lowers from increased number of shifts, your lap times suffer because newbie drivers can't keep the peaky engine in the power band, and you operate the engine at a less efficient flow range through the restrictor, increasing your pumping losses and decreasing efficiency.

High rpm hp does not = peaky any more than more than low rpm power = board power delivery.

Your assertion that the usable rpm range is compressed by gear reduction is correct and needs to be considered, but the assertion that using gear reduction = slow acceleration and poor lap times is not.

Have you ever seen an F1 car accelerate?

I do agree with what I think were your under lying assertions though, balance is the key to success and in that balance focusing on easy of operation over absolute performance will probably produce a better competition result.