Technical What's the story on Pruned Blowers?

Someone school me, please. I've heard about "pruned blowers" in the past and I just noticed this one in the classifieds. What is the purpose of the shrunken cases? Is it a matter of less volume of air but greater pressure? Thanks for any input. I'm not looking to buy or use one, just curious about why they exist......Don.

 

It's mostly just for esthetics...it makes the sides of the blower rounder instead of having a flat, square mounting flange.

Edit...maybe there other reasons to prune blowers...bolt to a smaller pattern intake or ??

 

Technical - How is a pruned blower case mounted? | The H.A.M.B. (jalopyjournal.com)

Looks like a whole lot of extra work all the way around after you get rid of the flange. I'd imagine That more important than looks would be clearance on some engines.

 

WEIGHT! Them things are Heavy!

 

Wow, very interesting. The one in the classifieds looked like the cases were "pruned". So just the mounting flange is shrunken.

 

@lothiandon1940 I was wondering the same thing recently!

 

https://www.jalopyjournal.com/forum/threads/how-is-a-pruned-blower-case-mounted.999144/

https://www.jalopyjournal.com/forum/threads/holy-grail-find-tom-beatty-blown-olds.1155299/

 

Can you prune the v6 version (usually less desirable than the inline version) to use it on a gas v8?
V6 version:

 

Pete Robinson ran pruned blowers, as he was very much concerned about any extra weight. Many years ago, I had one of Pete's pruned 6-71 blowers as well as a Pete's drive for the blower.

 

Blowers are often pruned so they fit the application. For example 71 series GMC blowers will not fit a Ford flathead V8 as nicely without pruning. Ron

 

Hello,

The big question is…WHY? The shaving the edges and bolt hole savings in weight is minimal compared to saving weight elsewhere. There were plenty of nice blower to engine manifolds to adapt the regular 671 to just about any motor. It did not matter if the drive was an old style chain drive or a 1960 newly created Isky Gilmer Belt Drive that just came on the market, then. The complete blower was something that was put together, then able to be moved from the workbench to the installation on the engine manifold. Then the drives were added.

Having access to the installation nuts and bolts made it easier to install and not worry about having to do a complete rotor, end plates, pulley adjustment when installing. When necessary, the removal of the complete 671 was done in a normal way and that in itself was easy. Now, the block could be worked on or taken out without having to rebuild the whole super charger from the inside.

While the shaved housing might save some weight, the extra time it takes to take everything apart outweighs any savings, minimal as it is. But, having to install from the inside just does not make sense. It might look thinner, as it does, but always having to disassemble it and reassemble it just does not sound very efficient. IOHO

Jnaki

When we got all of the new Isky Gilmer Blower parts together, it was an additional puzzle. But, we knew where everything would go during the assembly. Our initial 283 SBC motor had 6 Strombergs on top and we had to remove all to get access to pulling the engine out of our 1940 Willys. We told ourselves the assembly of the completely new, 671 SBC 292 ci motor would be done in stages.

The last thing would be dropping in the completely assembled 671 supercharger down on the Edlebrock blower manifold that was specially created for this SBC application. We did not want to mess around with taking apart a custom machined and measured blower parts/rotors/plates. The 671 just simply dropped down onto the new manifold with the supplied bolts.

We were happy with our new 671 supercharged 292 SBC motor. It ran well and came close enough to the 1960 National Record in C/Gas to make us want to increase our involvement with the next level of money and speed parts. YRMV

 

Pruned for my pleasure.

 

My old buddy Jack Gillis pruned this one in '47, to fit a 6-71 on a Flathead, perhaps one of the first to do so.

 

As mentioned before some are fanatical about weight savings and every ounce removed helps. There was a local funny car team, Fontanini and Nanini that built cars with that in mind and bolts on their car were rifle drilled in their quest for the lightest car possible.
Pete Robinson had pruned magnesium blower cases cast BITD and they worked well. Boost was not at the levels seen today and Teflon and nylatron stripping of rotors hadn’t been thought about yet so blowers ran all through a race meet, even a multiple day National event, and unless there was a major tuning mistake made the blower stayed on the car.
@jnaki the Willys you guys ran was in C/GS and with 292 cubic inches it could have weighed a minimum of 3680 lbs. If it had weighed in at 3750 for example weren’t you guys concerned with reducing the weight?

 

Hello,

I am not sure where you got the 3680 lbs for our 671 292 ci SBC 1940 Willys Coupe, but it certainly did not weigh that much. Your estimate of 3680-3750 is more inline with our black, 1958 Impala, not a lightweight 1940 Willys Coupe. Stock, it weighed about 2200 lbs. add 600lbs for motor and trans. 200lbs for rear axle housing, etc. That still comes out to about 2800 pounds. It was light weight for the cars running in those days.

When we were ready to race the first version of our 1940 Willys Coupe, 283 with 6 Strombergs +weight put us in the 9.00 to 10.99 lbs per square inch. B/Gas Class. It was fast, but not as fast as the veterans already racing in the competitive class. So, we tried for several months from January to March 1960, the results were getting better, but still not enough for winning in the class eliminations.

By May 1960, we had taken out the 283 SBC and rebuilt the whole motor with new speed parts specifically for a supercharged motor. A new Isky-Gilmer Belt Drive for the 671 and SBC was just on the market and we happened to be able to get one of the first ones. The 671 and drives added a few more pounds, but nothing that would make it go from 2800 lbs to 3700 lbs.

When we first went back to Lion’s Dragstrip, we were measured, weighed and given a B/Gas classification. It was as light as we were going to be able to do to qualify for the B/Gas class. That was our original goal to race against the likes of K.S. Pittman, Jr. Thompson’s Studebaker sedan and Doug Cook in his 37 Chevy. You only get better if the competition is better and that was a lofty goal.

We ran that version for a while with the speed results getting much better and E.T. also, quicker. Then we decided to add some steel plates welded into the trunk area for structural support and welded over the rear axle area. That extra weight gave us about a 10.60+ pounds per cubic inch rating and now, we were in the C/Gas class until August 1960. We were so close to the national record and had hopes of doing well at the U.S. Nationals in September.

Jnaki

So, if the confusion of 3800 lbs was for the 348/280 hp 1958 Impala, that was our tow vehicle and not the racer in the C/Gas coupe class. The powerful 671 292 blower spec SBC motor was powerful enough to overcome the additional weight welded into place. Even with the additional weight from the welded-in steel plates, the times were better and the speed increased, too. Plus, it handled better and gave us better traction over the rear slicks. The final thing on performance was that my brother was now getting into the swing of things and his reaction at the starting line was pretty quick.

That fateful day in August, several people we talked to later on, told us that each race was better than the last one. Each race had the starting line power to get ahead before reaching the orange Lion’s timing tower. My brother’s learning curve was adapting to the many races he had since we built the 1940 Willys Coupe.

There was no need to “prune” or trim our 671 supercharger. It was easier to install and remove when we wanted and still had the racing specs from the rebuild. Plus, the new Isky-Gilmer Belt Drive and Edlebrock, blower to block, Manifold was just a perfect fit for us. Neat and clean.

Similar to our April 1960 version of the 292 c.i. SBC 671 supercharged, blower spec motor.

 

I was not confused but I did use the 1963 NHRA rules when the weight break had been 12.60 lbs/cu. in. for C/GS.
When I sent that post I couldn’t find the 1960 rules and I should have taken the time to track them down but I didn’t do my homework. Looking at the ‘60 weight breaks your choice to add steel plates and move to C/GS was a wise one that put you at the low end of that class weight wise.
My post never suggested that you guys should have had a pruned blower but rather that for some racers the quest for performance drove them to put together the lightest, most powerful package possible. Pete Robinson, Ohio George along with a host of other racers built cars to the lightest weight possible for a reason and that was to stay at the top of their game.
Keep in mind that in the “A”classes the weight breaks were 0.00 to x.xx pounds/cubic inch.

 

Pain in the ass to install, but sure looks cool!

 

Not to mention the giant pain in the ass to actually "prune"!

 

Thanks for all of the responses everyone. You continue to educate me...........Don.

 

When a blower case was / is Prune it No big deal , (71 style case ) Just adds about 1hr to Job , 30 mints to , pull front cover with Snout, then pull Front Rotors plate with Rotors, unbolt case & 30 back together,
For cutting flange off maybe 1 pound weight .May be it was first done for clean look, Clarence problems or adapting , Maybe for Thief deterrent,
If one has been Prune, No big deal to make a adapter out of a 1/4 - 1/2 aluminum to bolt to a 6-71 intake or Tunnel Ram , or the poor man blower intake "single Four intake "

 

Pete Robinson was a fanatic about saving weight. Did you notice how short his hair was ? He also supposedly trimmed his fingernails to save weight. His dragsters were probably the lowest weight in his class. He won a lot of races, too.

 

Had to be for a look, take a pound off the add a plate to mount it. Seems it would get a bit heavier?

 

^^^^
After One has been Prune, You can make a plate to adapt back to a 6-71 intake , Yes the adapter would be heavier, may be by 1/2 pd then original weight, 3-71 -10-71 manageable by yourself 12-1671 might need a second person , they get heavier, but I have done 14-71 by my self .

 

The photo is from @drumyn29 , he posted it some time back. You can clearly see the mounting flange has been trimmed off the manifold too, all in the interest of saving weight.
Seems to me there’s only one reason to use a mounting plate with a pruned blower- you had a pruned blower but your manifold wasn’t machined for it.

 

I just thought I would add that my Navarro intake and pruned 471 does not require any disassembly of the blower to remove or install. It is a pain in the ass but it bolts from the underside of the intake flange into holes tapped and threaded in the bottom of the blower. I have had it off a couple of times and I would rather not do it again.