For the math type brainiac hambers out there. I am in the process of putting together a blown 8ba flathead with a 471 on top. My manifold is on the way out to get converted to work and my question is, I have a 239cid engine, a 471 blower and pulleys that are the same size crank and blower hub, so I believe it is 1:1 ratio. How many pounds of boost will i have with the factory compression ratio head (7.5:1) I believe. Thanks in advance
http://www.blowerdriveservice.com/technical-charts.html Per the charts at 239 inches you'll have a little over 11 psi at 10% underdriven. Compression ratio isn't a factor of the boost. With 1:1 pulleys you'll be over 14 psi. That's a lot Then look at the boost/compression ratio chart - you'll be right on the verge of needing good gas. You might want to consider more cubes to drop the PSI you'll be making or utilizing a little less compression ratio to move you into the normal gas area of the chart.
Step one is to obtain a boost gauge; anything above 4-6 lbs without "beefy or altered main caps" and your asking a lot for a 3 main cap engine to not self destruct.I ran several SCOT blowers on the street in the 70's and had trouble "pushing water past the HEADGASKETS" at psi's exceeding 3-4 lbs.Upon tear down the thin trail of water would enter the cylinder in the same place time after time even though the block and heads were both perfectly flat.Another recommendation would be to obtain a "blower cam" for your engine.The performance was great,but I chose to sell(mistake) the blowers after replacing the HEADGASKETS 3-4 times with the 276 cubes in my then 32 Ford 5 window.
That's good information from Flatheadjohn there - He's saying he was having trouble past 3-4 lbs. You're set up is going to provide way more than that based on those charts. Getting a boost gauge is actually one of the last steps because at that point you'll only be verifying you've got way too much boost. You'll need to underdrive it 30% or more. Wonder why they say less than 10% underdriven is not applicable to the chart.
I've never ran a 471 but on my blown flathead I ran about 4 lbs and on the kits I used to sell all of of them were set up the same. Seemed to work good with no motor problems. These kits used a Road Runner manifold with Wieand blower.
Those figures from BDS are a bit misleading for this case since they are, as stated, averages of "max boost, wide open throttle at 6000 RPM". Not many flat heads wind out to 6K. Much lowers numbers can be expected at 3500-4000RPM.
I'll roll with that, but you also have to remember boost is not linear with RPM or RPM dependent. It's all about the air you're putting in. Open the carbs all the way it up at 1500 RPM or 2500 RPM and you'll see the gauge going to boost - maybe 75% of the max boost. A 4-71 blower is a good size blower for 239 inch motor. I had a 471 on a 283 and had no problem making 9 psi throughout the RPM range. I've found those charts pretty accurate.
Get a boost gauge. Anything < 5psi should work great for your project. Lots of variables here way too many for any of the pundits to accurately comment on. Again get a boost gauge install a check valve in line. Have at least one pulley larger or smaller won't matter. Start out under driven (small pulley on bottom) and drive the puppy. Call back...
Holy smokes thanks guys. That's WAY to much boost for what I'm doing. I'd like three pounds of boost. Soooooo like stated here, a boost Gauge and small pulley on the bottom to start. Check! Thanks a lot guys
Without a bottom end girdle, I wouldn't run over 5 lbs, with only 3 main bearings, you want to be careful. A friend of mine mounted a 6-71 on his flathead back in '47, I wrote a story about the experience, here is the link. http://www.hotrod.com/hotroddeluxe/1401_the_first_gmc_blown_roadster_jack_gillis/
Beaner. How is the amount of boost you'd see from a particular blower not related to cubic inches of the motor it is on?
Beaner - I'll answer my own question, the amount of boost a particular size blower produces goes down as the cubic inches of the motor it is sitting on increases. I would say the amount of boost produced is cubic inch related. You have a large cubic engine and you want more boost, you get a bigger blower. You don't see many big block Chevys with 4-71's on them. Wonder why? The volume of the blower needs to be matched to the volume of the engine. I've also put many many miles on a 4-71 on a 283 with 1:1 pulleys all over the street. Dyers actually suggested I run 1:1 when I bought the snout. Not sure how your comment on Not running 1:1 on the street is justified. What pulley set up (1:1, over or under driven) depends on the overall combination of the engine being built. It's all about the whole package. If you have low enough initial static compression then you can easily get away with the boost produced with 1:1 pulleys on certain combination or a little more if you need it.
Beaner - I'll answer my own question, the amount of boost a particular size blower produces goes down as the cubic inches of the motor it is sitting on increases. I would say the amount of boost produced is cubic inch related. You have a large cubic engine and you want more boost, you get a bigger blower. You don't see many big block Chevys with 4-71's on them. Wonder why? The volume of the blower needs to be matched to the volume of the engine. I've also put many many miles on a 4-71 on a 283 with 1:1 pulleys all over the street. Dyers actually suggested I run 1:1 when I bought the snout. Not sure how your comment on Not running 1:1 on the street is justified. What pulley set up (1:1, over or under driven) depends on the overall combination of the engine being built. It's all about the whole package. If you have low enough initial static compression then you can easily get away with the boost produced with 1:1 pulleys on certain combination or a little more if you need it.
Tear the bottom end down often, and replace the main bearings. Keep the oil changed and does it have an oil filter?
Well damn, Here all along I thought blower boost was the amount of pressure in the intake created by the blower over atmospheric pressure. I also was not saying the blower produces less boost because the engine sucks it all in. I was saying the amount of boost a blower can produce is a factor of the size or volume in cubic inches of the motor it is on. Why do you think all the blower charts that help you determine boost for a given driven factor are based on the blower size, the amount driven and the cubic inches of the engine it'll be on? Follow the chart. Same blower - same driven factor - less boost as the cubic inches of the engine increases. It's a fact - not a theory. Now start increasing the drive ratio and it'll make a little more. No one said you did or should have suggested the original OP should run 1:1 on his flatty. You said you wouldn't run "anything you planned on actually driving at 1:1 or over driven." Probably smart unless you have designed for it. Like if you have a 471 on a 454. You'll need to be spinning that sucker to make boost with such a little blower. Check the chart. We are probably saying the same thing -
No, "boost" is the amount of pressure created in the intake tract. A volume measurement produced by the blower is called "displacement". A blower only displaces so much air per revolution which is why the boost value (additionally other factors, cam timing, etc) is dependent on the engine volume . . . you have a larger volume to fill then the pressure is less given the same displacement of air. It's like taking a breath and blowing into a small balloon vs a large one.
I bolted a 471 on my 8BA but I bolted these on first to hold the crank in. Not cheap, I think I dropped a grand between the caps and the CNC machine work to size them. And Blue Moon is spot on, Joe's book will save you time, money and frustration. Everyone ought to have a copy if they build flatheads, that and a bible...and a number for a good attorney...and a copy of Deja Vue by Crosby, Stills , Nash and Young, not necessarily in that order.
I built a steel 32 roadster with a Roadrunner blower set up and I had a 8to1 compression ratio with a stock bottom and Joe set mine up to put out between 4&5 lbs of boost. I ran a 5 speed with 3.70 gears and 700/16 tires and the car was a blast to drive. Oh ya had two 94's on top. Car went to Portugal and I never heard anything negative from the buyer.
So many variables come into play to say with any certainty what kind of boost it will make. But I think at 1 to 1 you might be pushing it on a 239 C.I. motor
I did run a blown french flathead 255ci ( 4/71), isky max one cam. got 232Hp on the dyno, 6psi at 5000rpm, drove the car 15 000 miles like that and only one head gasket.... you really have to check your timing and you will be fine Dyno:
Brian Bass blew his main cap after a few runs down the beach at The Race of Gentlemen. He bolted on the reproduction SCOT blower that H&H sells to a basically stock flathead. It was an eye opener on how much pressure those pump out!
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