Ok so I finally have the parts of my fuel system I'm designing and have some questions.. Using a 471 blower on a 8:1 motor ( have to figure the proper cam still ) This is an old (50's/60 ) style drag motor. I see like a million 97's on some of these cars all lined up side by side and stuff ( 6 and or 8 = a million ) how much fuel is to much ?? or do the rules change with a blower ? What would and old Hilborn uint put out fuel wise ? Is Fuel Injection rated at CFM same as a carb ? Thanks
I'm confused. What's the 8:1 motor? Is it a 260 ford or 394 olds? I'm quite curious how you're already acquiring fuel system parts if you don't know if it'll be carb'd or injected. But if you want hack advice, know this.......the blower fixes all evils. Huge cams, oversize carbs, no problem. That huffer will tame it all down and provide a level of driveability that you've never imagined if this is your first roots-blown car. Good luck! But be careful, the true cost of your first huffer has nothing to do with this car, because when you go to build other cars, naturally aspirated suddenly seems so......lame
rule of thumb...run about 50% to 100% more carb with a blower than without. So if 2-3 strombergs would be right without the blower, then 4-6 would be right with it. Or if you'd run one medium sized 4bbl, then run two small ones with a blower, or move from a small one to a big one. Keep in mind that carbs let air into the engine, and mix the appropriate amount of fuel with the air. Having a lot of carb means it will be able to let in a lot of air if the engine needs it, it does not mean that it will flood the engine with fuel (if everthing is working right)
Using pressurized carbs,,but just wondering more about the 'math' ?? of the whole thing.. Motor is a 287 Pontiac.. So not knowing a whole lot about motors,,don't wanna 'over do'
how are the carbs pressurized? The math works pretty simply, the blower adds more air to the engine, so you need more carb capacity to handle the added air. 7 psi boost means about 50% more air, 14 psi boost means about twice as much air. Fuel needs to be added to the air in the normal ratio (although it's reasonable to run a blown engine a bit rich under boost conditions, just to be safe, lean is bad)
There were some books printed on what you want to do "Street Supercharging" comes to mind. I have never read it so I can't really recomend one or the other. But it sounds as though you might want to read up a little and be sure you understand the basics before you invest much more money in this.
Carbs won't be pressurized, they're sitting on top of the 4-71, they are at atomospheric pressure and assuming you use period carbs (97s'; 94's etc) they won't have any boost referance at all. The carbs are pretty dumb in this type of setup, they don't know they're sitting on a huffer but will supply on demand. Simple and effective. The only accomodation to the blower is in the idle circuit, and that'll be easy since you are using multiple carbs each with an idle ckt.
So your front mounting a 4-71 on an 8.0:1 287" Pontiac? At 1:1 that should put you around 8-9#'s boost. Now, what might you be considering for carburetion and are you sure you wouldn't rather have them mounted on the supercharger? Hilborns and blow throughs can be tricky to set up. If it was me and I was going for a period look, I'd probably build a plenum that mounts 3 Rochesters or better yet 2 AFBs on the front.
Also, after reading your post again, I think I get your main concern. The main reasons so many carbs had to be used (Strombergs in particular) was part because they were small CFM wise and also because they had small bowls and operated at low fuel pessures, making it easy to run out of fuel. Going lean under boost is no bueno. If you look at pictures of Don Garlits last carbureted and blown rail, you can see an extra set of fuel lines going to each carburetor. I'd assume he somehow dumped a metered amount of extra fuel at some point during each pass. I found an interesting read on it here with a picture http://www.nitrogeezers.com/Contents-Legends.htm Go to The Innovators column and click on Leonard Van Luven.
Don ran Nitro. If your running a high percentage of pop and Strombergs the deal was to throw away the jets and drill extra metered holes for fuel transfer. It was very hard to get enough nitro into the old blown fuel cars. But that is for guys running the can and label. This is going to be a gasoline burning car, witch I know by way of PM will be running suck through aircraft pressure carbs, not blow through 97s
OK fellas, riddle me this... I'm doing a 4-71 on a 255CID flatty. Alot of the flathead blower rigs (including Roadrunner Engineering's) use a pair of 94's or 97's. Wouldn't a pair of those be in the ballpark of only 300cfm total? (give or take). What's going on there? I ask seriously, I'm looking at either a pair of 94's or a pair 2GC's (since they flow more and should be able to be had on the cheap) on top of mine and honestly have been wondering this exact same question for a while now (especially since RRE makes a crank pulley that I can use for my 4-71 giddyup for $80!).
Gotcha. I've seen a few healthy blown cars on gas with 4 barrels that were hard to keep fuel in and regardless of fuel type, I'd think Strombergs would be even more of a challenge. The aircraft pressure carb idea sounds interesting, but I'm pretty sure they are very expensive to rebuild and even more difficult to set up.
I've never played with a blown flathead, but with the small inches, low RPM and boost levels they run at, it would seem less of an issue. I'd talk to somebody like H&H Flatheads about that.
yes they are a challange but I am making great progress ( even as of yesterday thanks to those I talked to and a book I found ),,The carbs in question are designed to have no lag in fuel being supplied at any time so figured they would be great fro dragster 'idle - mash pedal' application,,,still trying to figure my cfm's as I can't find that spec,,though I know on a 470 in engine it used just one of these carbs,,I have 2 but have to think now that maybe 1 is enough or 2 will make me invincible These carbs are insane $$ to rebuild but one is from an old aircraft shop in Dallas and is good to go,,the other I have to check and yes my concern was feeding to much fuel,,,I used other forms of delivery as examples cause I have a basic understanding of them and would then translate that to what I'm working on.. will be neat when its done and it will work,,just one of those cases where ,,finding specs and info on such old stuff,,gets really hard,,and when working on the backyard level ,,you get the idea
I'll be interested to hear how it works out. Also, it may have been a big engine, but it operated at low RPM and it was most likely naturally aspirated so I'd go with 2, but then I know more about women than I know about Bendix pressure carburetors and that aint shit.
Oh ,,gezzzz your bring up the RPM''s is that something I need to toss into my math with this as this motor will turn about twice the RPM as the aircraft motor ?
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