Mach2

I was reading the August issue of Turbo and there is an article on RC engineering and they have a couple of tables with formulas and such. Well I was sitting around and decided to plug in some numbers and see what I got. These are the results:

At 80% duty cycle:
550cc primaries = 167.6hp
850cc secondaries = 259hp
total = 426bhp or 362rwhp (considering 15% loss)

At 100% duty cycle(only for secondaries):
550cc primaries = 167.6hp
850cc secondaries = 323.8hp
total = 491bhp or 417rwhp (considering 15% loss)

Then I tried to calculate what size secondaries were necessary to make a certain amount of Hp and found this:

400 rwhp = 550cc pri + 960cc sec @80% duty cycle(total 3018cc)
450 rwhp = 550cc pri + 1150cc sec @80% duty cycle(total 3400cc)

Then I calculated the max horsepower I could make with my 1300cc secondaries and found this:

550cc pri @80%duty cycle = 167.6hp
1300cc sec @80%duty cycle = 396hp
total = 563.6hp or 480rwhp

The question I have is are these formulas correct? I think they should be since RC engineering is a very respectable company that has numerous programs to calculate fuel requirements for different power levels. If so then why do people go ahead and install 1600cc secondaries? Is there something I'm missing?

spigot

iTrader: (4)

The numbers for RWHP seem a little high. Did you use .5 or .65 for the BSFC?

Besides that I think the discrepency between real world and the calculations would be the Fuel pump. If the Fuel pump is upgraded and you can actually supply the required preassure/flow then thes #'s seem possible, although ideal.

From what I've seen the stock system seems to max out at 350 RWHP at most. (usually 100% duty cycle, and the pump trying to keep up.)

my guesses would be ( without doing any calculations)
stock Fuel pump
550 850 300RWHP at 80%
550 850 350RWHP at 100%
with upgraded pump
550 850 330RWHP at 80%
550 850 380RWHP at 100%

Matt

Mach2

Matt,
I used .5 as the BSFC. I agree that these calculations are on the optimistic side. These numbers would be valid only in a hypothetical situation where all factors are optimized(fuel pump, fuel pressure, fuel flow, etc). I don't understand though why you think that the stock system can only handle 300rwhp at 80%dc and my calculations(or RC's calculations) state that the system is good for 362rwhp. That's a huge difference(71bhp). Is it possible that these calculations are "that much" off?

spigot

iTrader: (4)

550cc = 52lbs/hour so 2 x (52x0.80/.65) = 128 horsepower
850cc = 81lbs/hour so 2 x (81x0.80/.65) = 200 horsepower

Using the .65 (typical BSFC for the rotary engine), at 80% at 43PSI
so 328 BHP ~ 279 RWHP (at 15% loss)


@100%, 43PSI, .65 BSFC
2x(52 x 1/.65)=160
2x(81 x 1/.65)=249
so ~410 or 348.5

this is close to what is typical for the max HP, Like I said before those were just guesses.

Matt

maxcooper

YEAH! Nice job on the calculations -- most people just buy stuff before they take the time to figure out what they really need. I am glad to see someone else that cares to apply the formulas -- you can calculate some useful stuff with these.

I concur that the BSFC should be more like 0.62-0.65. 0.5 is way off for a turbo rotary. The rotary is higher than the 0.60 RC recommends because rotaries lose a lot of heat the cooling system and thus require more fuel to make the same power as a piston engine. The 0.5 and 0.6 numbers that RC suggests are for piston engines.

Also, don't forget to adjust for fuel pressure. The effective fuel pressure (gauge pressure minus boost pressure) on an FD is more like 39 psi. Injectors are rated at 43.5 psi, so you have to adjust the calculations. A 550 injector at 39 psi gives (SQRT(39/43.5)*550=) 521 cc/min. An 850 at 39 psi gives (SQRT(39/43.5)*850=) 805 cc/min.

If the fuel pump cannot maintain the desired pressure through the rev range under boost, the fuel system is out of control and the fuel pump must be upgraded before you can expect to get the car running in a safe and consistent manner. If you can maintain pressure under all running conditions and buy a bigger pump anyway, there will be zero benefit. All you need is "enough" -- more than that does not help.

-Max

spigot

iTrader: (4)

Max, I usually let you do all the research and then just buy what you buy :P
(but I did go with the Bosch fuel pump and the M2 RRFPR, since I've used bosch stuff before without any problems. And I can't justify[yet] a set of 18" wheels just for racing)

Matt

I think I'm going to have to start the Starving Students Racing Team.

Mach2

Max,
I calculated the equivalent injector size for a system running at 39psi instead of 43psi and I got:

550*(39/43.5) = 493cc
850*(39/43.5) = 762cc

Why are my numbers different from yours??

If my calculations are correct then the numbers would look something like this:

At 80%DC@39psi (BSFC .65)
(550cc)493cc = 115.6hp
(850cc)762cc = 178.6hp
total = 294.2bhp or 250rwhp (at 15% loss)

At 100%DC@39psi (BSFC .65)
(550cc)493cc = 144.5hp
(850cc)762cc = 223.3hp
total = 367.8bhp or 312.6rwhp (at 15% loss)

These results mean that the stock fuel system is only good till 312rwhp, but why is it that people are able to make 340-350rwhp and still have enough fuel? If the numbers are right then it should be impossible. Correct?

spigot

iTrader: (4)

you left out the sqrt... it should be

sqrt(39/43.5) x 550

Matt

Mach2

Matt,
I calculated that here:

spigot

iTrader: (4)

yes but you left out the Square Root (sqrt)

850 * SQRT (39/43.5)=804.834
550 * SQRT (39/43.5)=520.775

it's not just the ratio of the preassures. Its the square root of the ratios. (39/43.5) = .896 The square root of any # less than 1 is always bigger than the original number. So sqrt(.896)=.94

Matt

Mach2

Now I get it. Didn't see that. That still makes the hp very low though.

at 80%dc
(550)520.7cc = 122hp
(850)804.8cc = 188.67hp
total = 310.67bhp (265rwhp)

at 100%dc
(550)520.7cc = 152.6hp
(850)804.8cc = 235.8hp
total = 388.4bhp (330rwhp)

Isn't the fuel system capable of handling "a little more" than that??

Mikey

AHHHHH FORMULAS...NO MORE....CANT GO ON...MUST FIGHT BACK...
electrical engineering

school starts back in two weeks but only a little more left
haha...hey can i join?