I went to a swapmeet sunday and noticed a guy had a 6-71 blower and a B&M mini blower for sale , both were polished but dirty . I always dreamed about having a blower on a hotrod but probably wont ever be able to afford one . It got me wondering Why would someone be able to have not one but two different types of blowers but not want to run one ? What are the bad sides of a blower motor ? I know the 6-71 would be hard on gas but are the mini blowers with a single 4bbl carb hard on mpg's ?
I've never found blowers to be what kills the gas mileage, it's how I drive them, if You really want one go for it, the last one I bought was all stock and I'm converting it, there are two very good tech posts about doing that right here on the hamb, and about mpg My Hemi with a GMC blower got 20 just cruz'in around and not tromp'in down on the qauds, so just go for it. Von Doc
The mileage in my Hemi is surprisingly good...till I nail it. It's like any other engine...till you nail it...
The gas milage wasn't my concern as much as I'm just wondering why someone would remove a blower and sell it if there wasn't a big problem with it . What are some reasons someone wouldn't want one after it was obviously bought and installed (but never cleaned !) One possible reason I might understand is If it was installed something like a camero or vette it would block some of your frontal view - NOT a biggie in my book . On my truck I bet it would fit under the hood - IF I left the hood on !!! I'm just wondering WHY ...
Some swap meet vendors buy parts that they believe are under-priced and then mark up the price to make some $$$. That might be the case here.
He probably bought or traded someone for part that they were no longer using. The mini blowers don't work as well as the bigger blowers and either one is going to get comparable mileage to a naturaly aspirated engine built along the same lines. Some fellas get a blower and then discover that it isn't just bolt it on and go. Or they blow their engine or wreck their car and are just trying to recoupe some of their investment. Then there is always the possibility that the blower is worn out. It is not cheap to rebuild a blower especially if you are not a machinist and have access to all the equipment to rebuild one. This is really an open ended question but I may have hit some of the high lights for ya.
Yep, GMC "Screamin Jimmy's" The first number, 6, denotes the number of cylinders in the [diesel] engine. The second denotes the CID of each individual cylinder. So a 4-71 comes off a 4 cylinder that's 284 cid. A 6-71 is a 6 cylinder diesel of 426 cid, etc, etc.
You can get a refurbed blower converted for gas use including core 4-71 or 6-71 for $700 bucks from Rick Dean. You can get a snout for 150-300, pulleys cost around 75-100 each, , tenioner, 150, belt 100, carb adapter 100-175, carb linkage figure 100, fuel lines, 50-100, manifold 200-500 Set up your distributor for the correct advance. 9:1 or less compression, adjust boost accordingly with pulley selection enjoy. It's not bad if you save and space out buying the parts.
Funny little known fact about the " blowers " that originally came an old GMC 2 stroke diesels. They are actually cl***ified as " exhaust scavengers " . Gear driven Roots type blowers mounted to the outside of the engine block. Have fun with that fact. Turbochargers were actually added on in later years to increase torque output. The " scavenging devices ' were pretty much needed for the engines to start the combustion process and run.
For the younger guys who were not around before these motors were made obsolete by emissions laws. The Detroit diesles are a two stroke design. Its called a blower because it presurizes a chamber along the center of the block, when the piston uncovers holes in the cylinder, the presurized air forces the exhaust out and fills the cylinder again. Some models have two valves per cylinder some four, all are exhasut valves The Detroits came in 53, 71, 92, and 110 serries. For years, the most common heavy truck engines were 4-71 or 6-71. From the early fifties untill the late 70's, these were everywhere.
Rick Dean also has a pretty cool CD ROM for sale showing how to convert an old diesel blower for automotive use, with part numbers.
A stock GMC 71 series will have the lip casting edge that enabled hanging it on the engine to locate during install. usually milled off when adapting over to other uses.
To the O/P's original inquire, you find Roots type blowers at swap meets for several reasons. Here are 2 popular ones, 1- they are worn out, plain and simple and need a complete and quite costly rebuild/clearance when done right. Blowers take or consume power and lots of it to even turn them. If they are not capable of providing positive boost pressure, this is no diffrent than dragging a trailer loaded with dumbells behind your hot-rod. 2- they are blamed for the engine they were on top of- blowing up. Forced induction of any type requires a knowledgeable person to build an engine and the correct set-up to keep things in one piece. Hapharzardly installing forced induction on a engine will sooner or later result in a blown engine, the blower gets blamed, and off it comes. The next swap meet, there it is for sale. You never hear that blow-up story though, you only hear how incredible it was.
Be careful when buying something like that. They are like a used "Condom" you never know what your getting into !!!!! LOL
I don't know if the statement mini blowers don't work as well as big blowers holds true on the street. I ran a 142 blower for 10 years never had a problem with overheating , or fuel mixture problems and it made great power. Many guys are under driving big blowers on the street leaving the front carb unhooked to gain gas mileage, and running hot due to increased friction. I will admit A 671 looks a lot better and has the potential to make more power, but in most case a mini blower works just as well, Carlg
No. 2 is real common. I know that there are some that think that a blower motor is just a motor with a blower hung on it but to be done right there is a lot involved in building one. Of course Trad and I are probably thinking along the lines of if you are boosting less than 12 pounds you are fooling yourself. Never the less even an 8 pound streeter needs to be properly ***embled with the blower in mind. I know that to most of you the 12 pound thing sounds crazy, but we didn't come up around street engines. We had to learn to build a street engine.
Only when they're making boost. 99.9% of the time, a street blower is just going along for the ride, it operates in a vacuum, so it does not take much power to turn it.
Never gave it a thought, but with a muffled car, I wonder where the sound comes from? Shrill whirring, from the toothed belt drive?
depending on your current compression ratio and cam you may not be able to run very high boost (5-8psi) which will still add power, but not nearly as much as if you did it right to begin with. If 5-8#'s is all you'll be running a nearly stock GMC blower will work. No teflon or other fancy stuff needed, just have it converted so it will accept a snout and self-contained oiling. (the old detroits used engine oil to lube the blower)
You can cure that by drilling holes in the flanges (If it has them) in the pulleys. A buddy of mine did that and quieted it down by a lot. The problem is trapped air between the belt and pulleys.
I agree with earlier posts, a cheap blower is cheap for a reason. It is probably completely worn out and very noisy, unless you have your own complete machine shop, paying someone else to rebuild it will be very expensive. I have sold old worn out stuffed blowers to guys, because I would rather just replace it with something better when I can afford to do so. Like buying old bald worn out tires. They may be very cheap, but are they really of any use ?
Squirrel and others always kick in some good info on blower threads. Thanks guys. I am finding this 10% thing to be real true. I did buy a ragged out 4-71 converted to gas w/double v-belt snout for a few hundred, and then a NOS 8v-71 for 150 more, but it's going to take a while, and a good bit of extra money to get a blower motor together. I am enjoying the journey and it would much harder w/o the HAMB. The threads on other forums are full of bad information from expert magazine readers.
You won't have the whole magazine reader problem here a lot of us can't even read. I have been trying to read Curious George Goes to the City, but some of the words are real hard.
I cant tell you to much on the difference from one blower to the other, you should talk to Rick Dean at deanblowers.com, he just joined the hamb a couple days ago but hes been doing blowers since the 90's. my company is currently working with him in designing a triple lobe roots 671 and he can tell you anything you need to know about blowers. I can comment on the efficiency on a blower and give you some science behind it. Roots blowers are less inefficient compared to the newer designed twin screw supercharger which are the ones you seen on the newer cars that are under the hood. the reason they are more efficient though is because of the way they compress the air into the intake. Roots spin outwards towards the casing wall and pretty much smash the air into the intake while newer twin screws spin the air in from the back and compress the air while its in the blower. When ever u move, smash, compress anything, whether it be a solid, gas or liquid it causes the molecules to heat up, which causes the volume of air to expand but u still have the same amount of air in your intake. the hotter the air gets the less you will have density wise going into each cylinder. If you where to install a boost gauge in you manifold and block off the intakes, then heat the manifold from 70°F to 150°F the gauge would actually register an increase in boost, although obviously its not running any boost and you still have the same amount of air in the engine. The faster you spin a supercharger the more it will heat up the air and the more the volumetric efficiency will drop but the boost will still increase. If you can build an engine to actually withstand it, you could theoretically spin a supercharger so fast and heat the air so much it would actually become parasitic to the engine. (I don't recommend that) To think of it on the reverse side take an engine that runs NOS, NOS cools the air down so you can pack more air into the same volume. You might be think though if u take a 1471 and throw it on a engine you can spin it slower than a 671 and still get the same amount of compression, and your right but you have to factor in the extra weight and resistance that the engine has to push. The newer superchargers spin the air in causing less turbulence which results in a lower air temperature, that also why they can slap a supercharger on a stock engine not designed for a supercharger, run low boost and still gain a considerable amount of hp.
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