Inliners International Forums General Discussion Hi Performance What Carb set up?????

hello all. im currently building a turbo 250 and have a question for what type of a carb set up. i originally considered using a dual two barrel set up utilizing a pair of holley 350 cfm two barrel. now i am considering a single 4 barrel setup with a demon carb. i know its generally better to have at least 2 carbs because of the better air/fuel distribution to the cylinders but it would be so much easier and cheaper to do a single carb. what would be the power benfits/ fuel mileage benefits between the two.
also this will be in a daily driver so im going to limit boost to 15lbs with intercooler, forged pistons, etc.

when your car is done i want to drive it...drool, 15 lbs of boost would be sweet, most ive hear of in another forum was 10 lbs, not sure if hes got forged pistons though or not. but u can use a single carb easy enough, but u will possibly have to run richer with the one than if u had 2 but i think 2 carbs would still be worse for mileage, but a setup like that i am sure mileage shoulnt be too much of an issue....but a 500cfm holley carb would be fine I'm sure with a few mods...thats what the guy with 10 lbs has, it should be fine for the 15lbs too. although i think power benifits would be more, u need to be sure that the $$ spent is worth it, i think others should comment first on that though, 1 is always cheaper and works fine, but 2 is always better. do u have a ford or chevy, log head or not?

i have a chevy 250. i cant believe that a 500 holley could feed enough fuel to the engine even on 10lbs of boost, unless the engine is really inefficient. the only reason i say that is becuase i ran a 600 holley 4 barrel on my old 250 that had some mods to it and it never ran rich. also, i did some calculations and the fuel requirement for 15lbs of boost would be upwards of 600cfm so i was thinking it would be safer to go a little bigger that way i wouldnt have to worry about starving my engine and detonation occuring.
please, all you knowledgeable people out there speak up on this.

I'm interested in what kinda calculations you did to figure out what carb size you need. Cause I suppose there are few different ways to do it. You could take your horsepower and times it by 10% for each lb of boost to get a new HP number and then see what carb supports the necessary power.

I tried a carburetor calculator on the internet. It needed engine size as one of the parameters. I know your engine is a 250, but under 15 lbs of boost (twice atmosphere) it'll take in twice as much air and therefore be equivalent (if I'm right) in airflow needs to a 500 CI motor. And when I put in those parameters (assuming 5500 RPM redline) I get 800 CFM as the carb requirements. I have no idea how good a calc it was.

And yeah as far as turbo motors go it's always best to go too rich and with too much fuel rather than too little. You break a lot more stuff leaning out a turbo motor than a NA one.

I'd love to drive such a car too. She'll be amazing. Just like I said above, it'll make power under 15lbs like a 500 CI motor would NA. Jeez that'll be nice. And as soon as you let off the gas *poof* fuel mileage of a 250. Not bad at all!!

the 500 cfm carb supplies enough fuel because it has been jetted to do so...the primary jets were made smaller, so gas mileage wasnt so bad, and the secondaries were made honkin huge to supply more than enough fuel when opened, this way u dont have to worry about leaning out the engine under boost and can still have reasonable mileage when just cruising.

That's a great idea as far as I can see. I have a feeling it's easier to make a small carb bigger (big jets) than to make a big carb seem smaller (might just lean out after a point, don't know).

I have a book on turbocharging that had a bunch of carb setups. And one it showed specifically had a smaller than necessary carb to restrict the turbo. It sounds backwards but the turbo was making slightly less flow but at more pressure. That increased pressure put it more into it's operating range and made it more efficent. In the end the car made more power with what you'd think was a restrictive carb cause the turbo was more efficent and the air that was getting through was colder and denser (more power). Interesting idea.

theone61636 how are you constructing your turbo manifold and such? or more like turbo headers? I'd love to hear as well what pistons and rods and whatnot you plan on running cause I'm slowly working towards a turbo conversion.

to trueslayer:
i didnt know holley made a 500 cfm 4 barrel and the reason i say this is you mention secondaries but unless im mistaken 2 barrels dont have secondaries.
To greg:
for my exhaust header im going very smple since this will be my first attempt at welding any kind of manifold. im going to take a piece of 14 gauge stainles steel 3" pipe and make a log manifold with 1.75" primaries feeding into it. the turbo will be placed at the rear section of the manifold and the wastegate will go wherever i can fit it. LOL
for rods im going to go with forged steel small block v8 i-beam rods. i dont care if the ends are skinnier, i think theyll be able to handle anything i can throw at them. and for pistons, i think ill get some forged ones from that flatlanders website that was recommended to me in another thread.
as for cfm: i just used two calculations
the easy one
(RPMxCI/3456)xVE=CFM of the engine at 100% Volumetric Efficiency and for theoretical purposes only i assumed with a turbo i would achieve 150% VE, which is high but like i said id rather be overoptimistic. oh yeah i assumed a redline of only 5000 just to be safe.
i also used the best turbo calculator site ive seen at http://www.turbofast.com.au/tfcalc.html. i love this site as it has all sorts of calculators including a turbo matching calculator which is what i based my turbo size off of. its also fun to play around with the power calulator just to see the possibilities. what i did was enter all the parameters i knew with a stock 250 and then guessed the VE to get the appropriate horsepower according to the Chiltons manual i have which is pretty low around 46% VE. Then i added boost and added 5%VE for each mod i made like cam, head mods etc. but for cfm i used 100%VE engine and added 15lbs boost to see what the most id ever need would be.
hope this helps. any more questions please ask.

whoops...i just checked out a bunch of the guys threads, and I misread a few things and mixed up the one guys turbo setup with another guys supercharger setup.

this is what he has...quoted. and my mistake is that its not staged, but thats how some people were discussing how to do it, so if u do go with a 4 bbl then stage it like i mentioned..sorry bout that. this is what he has running on 10 lbs.
but hes working on and most likely has it installed already, a turbo manifold as well...heres a pic.


and heres a guy with a supercharger..i know i should do a different thread to show it but whatever, hes also running 10-12 lbs with a holley 2300, he states that the 500 CFM is actually 374 cause theyve done some calculations or something to figgure the actual airflow of it...supposedly a carb is hardly ever flowing the amount of air it says it is. this is what he has and says about it:

lol wow. first off tha turbo manifold is a great reference. thanks. that supercharger is a trip though. it looks bigger than the freakin engine!!!! cool stuff though. have you thought about getting a bigger carb and letting that engine breath properly?

i was thinkin...with a turbo or supercharger, do u really need such a high CFM carb that the specs call for, i mean...its not naturally aspirated, so when u force air into it, you force the CFM into it, therefore your turbo would be most of the air supply and the carb would only need to be big enough to allow for good unrestricted flow and good fuel distribution...so a 500 cfm 2bbl should be plenty, right? seeing that your putting an extra 200-300+ CFM through that same carb...u shouldnt need an 800 cfm carb if the specs call for that, only if it were naturally aspirated, with the 500 CFM carb and the extra couple hundred cfm coming from the turbo u will have that 800 CFM, and wouldnt that result in the best efficiency being the perfect amount of air for the setup (higher volumetric efficiency?)

seems like a logical assumption, that why i was going to go with 2 350 cfm 2 barrels,...but i guess eventually the air being blown through the carb, if too small of a carb is used, will be too fast and the size of the venturis will actually restrict the flow, thats why a larger size carb may be better.

but then u just look at what Greg said and it makes u wonder again?

That's a really good question. I guess draw through turbo setups wouldn't be any different that normal cause they're at atmospheric. But pressurized carbs I don't know. I'm not sure how the airflow would work. But I know this: you would need to be able to flow a lot of fuel. If the carb physically doesn't have big enough jets or gas passages to get enough fuel through you're in trouble. Which makes me think that a 300CFM carb might not be up to the job of supplying 600 CFM worth of fuel (at twice atmospheric pressure). I don't know.

any carb can be made to supply a heck of a lot of fuel...u can easily double the jet size which will supply way too much fuel...and if the jet aint big enough then drill it...so fuel isnt really a problem as much as airflow...but your right about the draw through and pressurize...the draw through needs to be bigger for sure as the air is atmospheric like u said and its being sucked...a smaller flowing carb would work fine as most of us use carbs that are too small anyways (stock setup) also i dont think u need as much fuel for less flowing carbs as with higher flowing carbs...at least thats what i find with naturally aspirated engines...although with forced induction extra fuel is needed anyways...hmmm

okay, wll here's the next logical question...how much air can a carb flow when uder boost???are there formulas to determine this? or would testing be involved?

Well there are formulas for this kind of stuff. But I could get them to work the other night when I was trying (this happen a lot, just takes time). Basically if you double the pressure (15 lbs of boost) you double the flow. What I'm not sure about is how well a carb would be able to handle the huge range there. You go from very little at idle to very large amounts of fuel under boost. If you've got huge jets in won't you have some issues at part throttle or something? I'm not good with carbs at all. But I have a feeling that doubling the fuel delivery of this carb might hurt something. I guess it depends how much it hurts whether it's worth it.

from my experiences with modifying my 1bbl carb...i bored it out...but i played with different jet sizes and accelerator pump adjustments and stuff like that...and my very first attempt was actually the best...i had a massive jet and the accelerater pump was set to shoot way earlier than normal...the result was good power...just a nasty rich smell...I have tried smaller jets...i can get it running without the rich smell but the performance isnt there...I have a few more things to try out now...but i have a larger jet thrown in again..so i think that it should be fine having all that extra fuel...just not so much for gas mileage

here we go...a quote from another forum...there is a progressive 2bbl carb...
it is possible to rejet that one for some sort of turbo setup...that would just eliminate the economy part...and i dont know how many CFM it is either...so if someone knows that figure that would be great.

Well, Ive decided to go a different route than the dual 2 barrel setup. Im getting a 4165 series holley 4 barrel. This is the spreadbore quadrajet replacement series. Its 650 cfm, mechanical secondaries and the best thing is that the primaries are small and the secondaries are huge so it should be perfect for low speeds before the turbo kicks in for mileage and still provide me with the power needed for after the turbo spools up. i just need to do basic mods like solid floats, jet extensions, mill choke horn, etc.

awsome man...let us know how it works out for ya

Pressure Ratio is the total absolute pressure produced by the turbo divided by atmospheric pressure.

Pressure Ratio= (14.7+boost)/14.7

(14.7+15)/14.7 = 2.02 This means that with 15 lbs of boost, 102% more air will be going through the system.

Airflow rate (cfm)= (cid*rpm*0.5*Ev)/ 1728

Ev= volumetric efficiency

(250*5000rpm*0.5*80%)/1728 = 289.35 CFM

I'm guessing on the volumetric efficiency of the 250. I think 80% is kinda generous though.

The pressure ratio times the basic engine flow rate then becomes the approximate flow rate under boost.

Airflow rate= pressure ratio*basic engine cfm

2.02 * 289.35 = 584.487cfm

This is taken directly from Corky Bell's book "Maximum Boost." It's a VERY useful book. Hope this helps. John

Yep, I've seen those calculations and formulas run through before. But what we're questioning is the performance of a carb under the influence of boost. Carbs are by definition rated in standard cubic feet per minute (SCFM). Since a car will operate in roughly speaking the same conditions (as far as pressure goes) the SCFM works well. But what happens under twice the pressure? I wonder if that carb would flow twice the air with the same pressure drop? So wouldn't one wonder if a turbo engine doesn't need anymore carb that it did naturally asperated (given it would need to be retuned)? It could be that I've got a gap in my understanding but I'm wondering.

Greg

Quoting:

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Caburetor Capacity

In general, a high output turbocharged engine requires the same or even less capacity carburetor than a highoutput naturally aspirated engine. For example, a normally aspirated 1971 Oldsmobile 350-CID engine used a 725 CFM quadrajet. When Rajay turbocharged the car, they got best results with a single Holley 600-CFM four-barrel.

Crown Manufacturing's Datsun 240Z installation used only one of the two SU/Hitachi carburetors provided on the engine, yet achieved near 100-MPH performance in the 1/4 mile.

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Taken from Turbochargers by Hugh MacInnes

I can't believe it took me this long to find that. I've had this book for years.

I was about to look in the same book. I just loaned my "Maximum Boost" to an instructor of mine. John