Carb Selection

[Ron Earp]

I've been looking around for a carb to go on my Lola motor. Motor specs currently are:

342 inch SB Ford
AFR 185 heads, custom CNC porting job
Solid Roller Cam
0.676" lift Intake, 0.673" lift Exhaust
Duration at 0.020 287 Intake, 295 Exhaust
Timing @ 0.050" IN Open 18 BTDC Close BTDC 58
Timing @ 0.050" EX Open 61 BTDC Close BTDC 21
110 Lobe separation
Comp Cam Valve springs and titanium retainers, 600lb open, 260lb seat
Funnelweb single plane intake - if we use 4bbl

I'll probably leave the motor as is to cut costs on the build a bit - I'm poor, don't know why.

However, in speaking with Barry Grant, company that does Demon carbs, they recommend a 675 Race version with removable venturis. That carb can flow up to 737 cfm of air which should do the motor to about 7800 RPM or so. Plus, they have the removable sleeves that are nice for adjustment. Problem is, this carb goes for about $1000 - quite a bit more than I reckoned. That, plus the manifold, lines, etc. it'll be about $1400 all told.

That isn't too far off, relatively speaking, to a set of webers. Of course, back to the same old argument on the webers - I don't think a 48mm weber flows enough for a 34X inch motor at 7500 RPM as we debated last year.

Any ideas, suggestions, or connections for a Demon carb on the cheap? I will look at some alternative carbs too.

R

 

[Mike Dozier]

Ron

You're right..those "race ready" carbs are not cheap.
Any idea how much power you'd be giving up vs a "stock" double pumper?
If it's only 20 or 30 hp...I think I could live with the $ 400 carb.

MikeD

 

[Ron Earp]

Me too, exactly what Fran suggested as well. That sort of money for a 4bbl is getting high!

 

[Mark Worthington]

Yeah, at that price I think I'd either reach a little higher for the Webers or go low-budget and order someting from Jegs or Summit.

 

[Howard Jones]

I would put on a single plane manifold, your choice, and a out of the box new 750 double pumper. Spend a little time on a dyno to tune up the carb/timing curve and enjoy the money left in your pocket.

The difference in performance of a 2400 pound car with 450hp verses 480hp is beyond what I would be able to differentiate.

Tires are where the real performance gains will come from anyway. At least as it relates to lap times. Proper selection of brake pad material and dialing in brake bias along with chassis set up will decrease lap time much faster, and safer, that adding 50 or so more hp once you reach about 350ish.

 

[Russ Noble]

Hi Ron

Or do what us impoverished Kiwis do.... Get a secondhand one off ebay!

I got the 825 version for about half price but it took a couple of months to find the right one. Very nice piece too with the removable venturis.

The good thing about buying secondhand from the States is that most of the items are fairly new and haven't been mucked around with. Out here in NZ anything secondhand is probably twenty years old and every man and his dog has been at it!!!

Yesterday I obtained a dry sump tank and in the last month or so I also got a six stage dry sump pump and VJ inlet manifold. Still looking for the right dry sump. I wouldn't buy heads or reciprocating assemblies though.

Just a thought.

Regards,

Russ

 

[Ron Earp]

Hey Russ,

I am a huge Ebay fan - the Funnelweb came from Ebay on the cheap, and I've run over 450 auctions on Ebay. I've been scouting Demons on Ebay, no doubt, and there is one seller who offers re-man Demons for pretty good money.

I think a 750 is just going to be put on it, and be done with it. I imagine that as you say tuning it will result in good power and drivability.

Ron

 

[Jac Mac]

Hi Ron, Go for the Mighty Demon @ half the price and just play with the jets. You have had enough hassles lately. A couple of things to watch with all Demons, the floats seem to be very sensitive to hi G cornering forces and in situations where cresting a rise under power, there have been a couple of high RPM leanout problems over here due to this. I have also had to richen these carbs by up to 8/10 numbers , but this may be due to the fuel we use(100/130 Av Gas) or perhaps we Kiwis dont have as much pollution to digest in the combustion process.

Cheers Jack.

 

[Dan Haynes]

Dano says.........

I know the Demons get some good press. But most of the folks I know have had troubles with them. Just my experience.

I long ago decided I could not be an expert at everything. Carbs and Distributors I leave to the pros. My carb guy is Jesse Bigs. I can't sing his praises high enough.

Tell him what you are going to do, give him all the info he asks for and write the check. You will be glad you did. If not, he'll buy the carb back. Not the cheapest, but the results are worth it.

I have not touched my carb since I got it back from him. It runs perfect. All day. Never a worry.

I just checked the website, to grab the link. Currently offering $150 off.

http://www.bigsperformance.com/

Contact him before you do anything. Trust me.
Congrats on your first solo....I remember mine well......

Dan

 

[Wayne M.]

Just some thoughts on carb selection.
There is always lots of decision when selecting the size of a carb. Here are some calculations that may or may not surprise you. Lets assume you have a 302cid (5.0 ltr) engine.
Lets take the 302 cubic inches and convert it to cubic feet to make things easier when looking at carb capacity. 1 cubic foot is 12X12X12 = 1728 cubic inches.
So 302ci/1728 is .1748 cubic feet of displacement. Sounds small doesnt it??
Then lets assume a max RPM of 6000. Remember that 4 stroke car engines fire every other stroke so there is one intake stroke for every 2 revolutions. That means there are 3000 intake strokes at 6000RPM. So now we can calculate the max air flow at 6000RPM by multiplying the engine displacement in feet times 3000.
.1748X3000= 524.3 cubic feet per minute.
This 524 CFM is for normally aspirated engines and assumes that the cylinder is being filled 100% on each stroke. We know that isnt the case but would be nice. If we further assume we get 90% cylinder filling for each intake stroke the 524 is then reduced again.
524.3/80% giving a max carb size of 471 CFM.
This sounds very small doesnt it, but the numbers dont lie. Has anyone ever put that 780CFM on their 302 only to have it run worse than the 600CFM they took off. Maybe we should give more attention to jetting and mixture rather than a bigger carb is better. Remember that NASCAR uses 390CFM carbs. Now that I have really confused things, happy carb hunting.....

 

[Jac Mac]

I agree with 99% of this but you have not allowed for the amount that is lost during the overlap/ scavenge period which of course varies due to the conditions the motor is operating under.

Jac Mac

 

[Ron Earp]

Also must bear in mind a 650 cfm carb may not flow 650 cfm just because the advertisers says that it will. And, in this case we have 342 inch motor with a high RPM target of 7500 RPM instead of 6000 RPM. I think the right carb will be somewhere with a carb advertised to flow in the 650 to 750 cfm range. Erring on the small end, in my limited experience, is always better. But, with a steep rear gear, aka 016 box, and a light car, you can get away with more.

I'm going to see what some of the good builders, like mentioned above, suggest and probably go from there. Naturally it'll need some tweaking, but if I'm at a good starting place that really helps.

Ron

 

[colin artus]

A well tuned race engine can easily achieve a peak v/e (volumetric efficiency) of over 100% - good exhaust scavenging being particularly beneficial in this respect.

 

[JCoop]

Ron, I thought I saw 51mm chokes available for the 48IDAs? I took your comments about flow to heart when I went for the Webers on my 302 and was surprised to have to jet down. Now I realize what you mean is flow up top but I tell you, the Webers' response and performance has been incredible for me so far, up to around 6000 RPM anyway.

 

[chuck smith]

Ron, I agree with the EBAY route,that is where I got my Demon 750 for the Kirkham 427. I have found the Demon Tech line to be helpful in carb decisions and sizes,and they are usually smaller than I would have thought. Good luck on the build. Have you ever seen Johan's Lola? It is a beast and he knows more about them than anyone around the south. chuck smith

 

[Phil]

Ron
I can tell you that I have seen most of the setups mentioned above, and then some. I have the opinion that the Holley carbs have a driveability problem..just seems like they always need tweaking, and always in the mid range. I have not had experience with some of these other units mentioned above specifically, but we have had really good results with the Edelbrock performer, essentially a copy of the Carter AFB, and this unit seems to give the least trouble with excellent driveability. I think about $400. new. Just my two cents.
Phil

 

[Ron Earp]

Ron, I thought I saw 51mm chokes available for the 48IDAs? I took your comments about flow to heart when I went for the Webers on my 302 and was surprised to have to jet down. Now I realize what you mean is flow up top but I tell you, the Webers' response and performance has been incredible for me so far, up to around 6000 RPM anyway.

Yep, they are available, at a high cost for re-working the entire carb. Jetting down with respect to fuel flow I'd expect. And, with 302 inches, you'll have to check the chart again, you're probably good to 6800 RPM with airflow. 6800 RPM might be beyond your cam and lifter capabilities depending on what you have. But for a solid motor built for high RPMs the 48s are not enough it appears.

Those Eldebrock carbs are good with those removable needles. I have fiddled with them before and they seem like good carbs, but, I will use a worked Holley/Demon carb. I've a fellow that contacted me through the forum that has offered services and comes well-recommended via other folks too.

Ron

 

[John Fitzpatrick]

Tires are where the real performance gains will come from anyway. At least as it relates to lap times. Proper selection of brake pad material and dialing in brake bias along with chassis set up will decrease lap time much faster, and safer, that adding 50 or so more hp once you reach about 350ish.

I just came across this post from a month ago, but it is so cogent it deserves a bump. I seem to remember reading in one of the car mags that the BMW (!) lap times at Daytona are significantly under what the 7 liter GT-40s posted back in the day, due largely to tire and brake improvement.

We're suckers for massive engines, as though quarter mile times were what a GT-40 is about. In our heart of hearts we know a 5% increase in horsepower is barely noticeable, but "bolting on" 50 HP is ever so much less tedious than fine tuning the suspension. I know I've spent MUCH more time tweaking my Webers than I have fretting about my front end geometry.

Perhaps that’s because back when I started working on cars back in the late fifties, drum brakes and leaf springs weren’t held in the same esteem as four-barrels, and Fangio and Moss just didn’t have the same stature as Garlits. Perhaps it's because we raced between mile-markers on the Turner Turnpike, not on a proper track like the midget racers down at the Fairgrounds.

Whatever the reason, I know I ought to learn more about tire compounds. Still, I can't help wondering how much horsepower I could gain by underdriving my water pump…

 

[Jac Mac]

Well said John, Virtually every race car that I have been associated with has benefitted more in improved lap times by small changes in suspension geometry or finding an area of the chassis that is structurally weak. Unless the motor has an obvious power deficit, making the chassis more comfortable and predictable to the driver will reap large gains by giving the driver more confidence and allow him/her to relax & drive the car rather than hang on for dear life!

Jac Mac

 

[Ron Earp]

Well said John, Virtually every race car that I have been associated with has benefitted more in improved lap times by small changes in suspension geometry or finding an area of the chassis that is structurally weak.
Jac Mac

As a driver and mechanic I find this completely true too.

But, bear in mind this thread was just about getting a proper carb on a high revving V8, not getting every hp from the engine. No sense in bolting on a carb that causes the last 1500 RPM of rev range to be flatter than a pancake.

I already feel I won't be able to use the power generated, and I think a lot of people will find that true if they can admit it. I think the little Lola will be well served with a 250-300hp motor and I've a backup plan for that.......and still be faster than a lot of things on the track.