tweaked

iTrader: (4)

I was wondering what was the limit on a 750 cfm carb on horse power.
such as: a 2 1/2" exhaust is capable of 300 horse power. any more and you would have to go bigger.
can a 750 carb flow 400 horses?

RX-7Impreza

what are you putting a 750 carb on?

tweaked

iTrader: (4)

you guys are going to think I am wack!

I am thinking of putting one on my seven!
I have figured out how to make an intake out of carbon fiber.
and I am going to have it set up like the air hones or what ever they are called. the gradually become narrower so that the air is rushed in and becomes moree dense and compressed. the end is going to be perfectly mated with my intake ports and the carb will be right about where the secondary injectors are sitting now.
I know this can be done because some guys made and intake for their gen 1 but they didn't think along these lines. I would have to go and search a bit before I could find it ma know why. I think they just wanted to know if they could.
so any way I have a 600 4 bbl carb in my shop sitting there. so I am going to start with that and see how hard it is going to be. But I think I am going to run a holley 750 dominator.
the cool thing is, if I don't like it I have only lost a few hours of time and carbon fiber. I can always go back really easy.I had to take my intake off a few weeks ago and I know haw easy it is. I just run a rubber fuel line from the filter to the carb. and I get a throttle long cabel from RB for the side draft carbs. everything will go back right if I dont' like it.

anyway it is something to play with for now. I get bored easy and my car pays the price most times.

MikeL

CI = Cubic inch displacement
RPM= Max expected rpm
VE= Volumetric effiency (below 1 for n/a. Above 1 for turbo/super

The formula:

CI * RPM * VE / 3456

So, for an 80 ci, not really radically ported or cammed n/a piston engine, all you need is about a 157 cfm carb:

80 * 8000 *.85 / 3456 = 157 cfm

I guess we could increase the VE for the n/a rotary and then double the cfm result:

80 * 8000 * .90 / 3456 = 167 * 2 = 334 cfm

What's the VE for a turbo engine?:

80 * 8000 * 1.15 / 3456 = 213 * 2 = 462 cfm

My old 351C mustang, about 350 bhp:

357 * 6500 * .85 / 3456 = 571 cfm

But..., if your intake is a low-rise, dual-plane type, it's generally better to go above what the formula says. I had a 750 on it.

How much bhp will a 427 ci piston engine that revs to 7000 and has some pretty radical porting and cam do?:

427 * 7000 * .90 / 3456 = 773 cfm

All it needs is a 773 cfm carb. I guess that one would get at least an 850.

MikeL

Aren't thos NASCAR engines often restricted to a 350 cfm carb? Don't they make over 600 bhp?

relvinnian

Here's how I do it... same thing. General forumula: CID * RPM * .5 * VE / 1728 = Engine CFM. Now .5 represents the fact that a piston engine only realizes half it's displacement per revolution. I can see you multiplied the 1728 X 2 which is the same thing. But if you do this you have to also multiply the CID x 2, since a rotory realizes it's full displacement per revolution.

When doing rotory calculations I just use the main formula I stated, and take out the .5.

Just want to let people know what the numbers mean

Also, a stock port N/A will not hit 90% VE. A decent street port setup, with free flowing exhaust and intake will. Strong street port will get around 95% if the port design is done correctly. As far as turbos engine flow: Airflow = Basic CFM * Pressure ratio. Pressure ratio = (ATM + Boost (PSI))/ ATM. Where ATM is atmospheric pressure (14.7 @ sea level)... Using a static number for VE of a turbo is much less accurate.

Edit: P.S. - If you see something you think is wrong let me know... I'm just going by my experience, and what makes sense to me

MikeL

You'll get the same answer using the *.5 and the /1728

I'd rather leave out the *.5 and go back to the /3456 and double the cfm for rotary...

...but, my mind works from the piston engine way of thinking and I adjust for the rotary

Felix Wankel

I had a dual line Holey 750 on my worked 400 Pontiac (~450 hp) and the carb was a wee bit big for it

FPrep2ndGenRX7

Not to be a stick in the mud but wouldn't the time and money spent to make a Holley work right on a rotary be better spent on a known good intake making it work better? Never having the time and money required to test different intakes I could be wrong stating this but in my mind and after many years of reading everything I could it just doesn't seem worth while to me. Correct me if I'm wrong.

Later

RX-7Impreza

with all of the complications and pulses made by a 13b fuel system i think you will need to do more that just match a carb up to the intake...

Justin

tweaked

iTrader: (4)

yeah of course I am gong to do more I was just over simplifying.
I am going to have to put on a fuel pressure regulator, which is gong to mount to the side of the manifold. I am going to have to adjust runner lengths. I have a six port so this is my setup. the center ports (idle) are gong to be the shortest, slightly larger at the top for the velocity effect.
the "main" ports are going to be longer since they have a higher rpm operating range than the middle ports. the 5/6 ports are going to be a little longer still. and the 5/6 ports I am not sure yet if I am going to make these work only from the vac secondaries or or if the 4 thru 6 ports are all going to work from the same chamber which all ports wil "pull" from.
I have the carter 650 and if it is too much I will just change the venturies, jetting and things. I know I can set it up to work perfectly. it is just a matter of proper tuning.
does anyone know what thos weiber downdraft carbs flow in cfm? the ones they sell for the n/a's?

Evil Aviator

Hehehe, I think you would get about 100-120 hp with that carb on a stock port 13B. If you can make your own manifold, why don't you get an aftermarket throttlebody, aftermarket EMS, and a handful of good used fuel injectors and make some real power?
http://www.injectionperfection.com/

Most carbs are rated at 3inHg for 1-2bbl, and 1.5inHg for 3-4bbl. You will need to do some math to convert them to the lower vacuum signal of the rotary engine. It would probably be best to get a Racing Beat setup which has been properly modified and proven to work. However, my personal opinion is that EFI is the way to go.

Samps

I took this directly from the Mazdatrix web site and was wondering if it is acurate.


The WEBER 45mm side-draft intake system is an excellent performance upgrade for your RX7. Kits are supplied with carb, filter, and linkage brackets. Note: fuel pump and/or fuel pressure regulator are not supplied. The 12A kits use a one-piece manifold, 13B 4-port kits include the adapter needed to bolt the manifold to the engine. The 6-PORT kits are supplied with the upper manifold, which bolts to the original equipment lower manifold. A low pressure fuel pump is required (not supplied) if the WEBER system is being installed in a formerly fuel injected car.


So, is a carb set-up "an excellent performance upgrade"?

Felix Wankel

A 45DCOE woke up the 12A in my FB.

Samps

I understand that a carb set-up to a better carb set-up is going to be an upgrade. I'm curious to know if dumping a FI system for one of these carb set-ups is going to be worth the effort.

Felix Wankel

I wouldn't do it. Unless I was putting a 6 port engine in a first gen.

MikeL

FPrep2ndGenRX7

I'm with evil on this one. I've talked to the people at MazdaTrix and on the phone they suggested a good working fuel injection system is hard to beat. I know there web site says something different but thats what they told me. Personally I'm still going to end up with a stand alone computer controlling everything through a 86-88 intake till I can afford the TMW intake. I can still use the same injectors I'll just swap out the intakes.

MikeL

FPrep2ndGenRX7, what's TMW intake?

tweaked

iTrader: (4)

frirst off where did you get that pic of the carb setup? What web site is it on. you mean to tell me someone has thought of this first?
seriously where is the site?

I am just doing this to play around. I have everything I need mostly. I am not going to buy a carb setup and intake to use it. I just need something to do while there is snow on the ground. I don't have doors on the garage. I can to all the layup work in my heated shop on a bench and put it in when it is warmer.
There is alot to be said for both fuel delivery systems. injection can't touch the responsiveness of a well setup carb. carbs can't touch the near perfect ratio. on carbs, if your fuel needs change it costs about $3 and 15 minutes to get it right again, ( depending)
injection is more precise because of the computer management but it takes more to get more power from it.
injection also has better economy, but you can consistantly get more reliable power from a carb than from injection.
i am not knocking injection. I am going to probably make it so I can just change the upper manifold so I can put them back and forth fairly easy.
this is just to tinker and play.
So all I am asking for is a little help and some tips from my rotor family so I can keep occupied and not cunvulse and implode from stagnation.

RETed

I've seen a 700cfm Holley power a bridgeport 13B to about 200-220hp to the wheels.



-Ted

tweaked

iTrader: (4)

where?
I would like to see other setups so I can get tips and ideas

HWO

i read somewhere a few years back that a fully tweaked 550cfm holley can feed a 13b bridge port to 7000rpm, and thats the carb maxed out

FPrep2ndGenRX7

I meant to say TWM. They are fuel injection intake specialist.

www.twminduction.com

Expensive stuff but nice.