Project Blown 305

[turbobill]

It's too bad that this thread is starting to degenerate.

Everyone has a point, however it boils down to what each individual wants out of what they are building. There is nothing wrong with a high reving small block NOR is there anything wrong with a boosted low speed engine, gasoline or diesel.

Over the years, I've done it about every way it can be done, but my favorite is boost. I've boosted sixes up to big block V8's and I've fooled with the diesels too. As my first turbocharged engine was in 1976, that's where my preference lies. Does that make me right or wrong? NO. It's what I prefer.

I love the idea of a supercharged 305. I also like high speed small blocks. One is no more right or wrong than the other. It's what the owner builder wants, not what someone else thinks is right in their eyes.

On the subject of GMC blowers, the 8V92 blower moves about 736 cubic inches of air every revolution. For every other GMC blower, multiply the first number by the second and it will give you the approximate airflow per revolution. Your 305 will breath 152.5 cubic inches per revolution (at 100% VE), so figuring out which blower at what drive ratio is easy depending on your boost requirements.

Personally, I think it is a neat project!

[Cuttyman9]

Yeah, to be honest until one dude popped up I felt this was one of the best mod useful forums.

Im from the same thought process. Anything can make power, it's more a matter of withstanding the forces applied.

I'm going to say at 50% under I should be making a decent amount of boost. I get different answers each time I search on specific blower info. Some say 8v92 was the cubic inches of the diesel it came on and the 8v71 was the same blower almost but mounted to a different engine id noting slight mounting changes(I hear its taller than the 71 due to the top flange being taller) but overall flowing the same amount and having roughly the same case dimensions.

I'm excited for this, I'm glad there's a few others who are as well. This group overall has only made me more excited as it's pushed me closer to my goal.

[Cuttyman9]

Whoops double post.

I'll snap pics when I get the blower here.

Definitely excited for that!


So since I ordered an adapter for the top to fit on the steel plate I have I'm thinking I can drill 6 holes to mount the dual quad plate directly on that to run NA first, then when the motor is sorted I can slap the blower on, move that plate up to on top of the blower and off I go.

https://www.ebay.com/itm/181486211512

[Cuttyman9]

If anyone's chasing these parts to do your own it's all on eBay through a few vendors.

The running total will prolly be under 1500 for the blower related stuff which I feel is really good.
I'm waiting for the next swap meet to find a test belt and gears.

[TJ's GMC]

Originally Posted by Cuttyman9

Yeah, to be honest until one dude popped up I felt this was one of the best mod useful forums.

There used to be two, one was finally banned. Yer gonna get those folks who think that what they say without proof actually matters. Both told me that the 4 barrel mod I did was a big waste of time and that I was dumb and ignorant for doing so because...Oh that math says blah blah blah....Who gives a darn? REALITY says it works and works well. lol

Anyhow, $1500 for boost ain't a bad start!

[Cuttyman9]

Yeah, some people just really only think an SBC is all that works...

It's kinda a shame we don't have a company that produces the flanges cuz we could prolly easily make a nice 4 barrel intake that takes a bit more advantage of flow with a more stand up intake.

It's a shame it would take a lot to convince edelbrock or someone to create the part.
A simple 4 barrel medium rise intake would bump performance and also offer better fuel mileage since a big 2 barrel doesn't have a small set of primaries to do the low speed stuff.

Yeah I don't think it's bad at all.


I did a bit more searching. 8v92 was the diesel it was under, the 92 was the cubic inches per cylinder equating to 736 ci. The blower is needed because it's a two stroke and it doesn't make enough compression or something
I'm going to need to find more info on how much air the blower pushes at a specific rpm so I can somewhat calculate the psi of boost.

[turbobill]

Originally Posted by Cuttyman9

I did a bit more searching. 8v92 was the diesel it was under, the 92 was the cubic inches per cylinder equating to 736 ci. The blower is needed because it's a two stroke and it doesn't make enough compression or something
I'm going to need to find more info on how much air the blower pushes at a specific rpm so I can somewhat calculate the psi of boost.

Detroit Diesel's nomenclature is the first number is the number of cylinders and the second number is the displacement in cubic inches per cylinder. Where a "V" is between the two numbers indicates it is a V type engine.

As these particular Detroits are two strokes, they aspirate every revolution. The reason for the blower in the first place is not for any supercharging, but to aspirate the engine as a two stroke does not have a dedicated intake stroke. Pressure to charge the cylinder must come from an outside pump.

The 2 stroke Detroits have been around since the late 1930's, so it didn't take very long for someone to figure out their blowers would make great air pumps to supercharge 4 cycle engines.

You really aren't going to need an 8V92 blower to pump up the 305. A 6-71, 4-71 or even a 6V53 would do the job. The smaller the blower, the faster you will have to turn it for any given amount of boost on your 305.

A few things to remember about the GMC blowers. The bigger they are, the more power they'll need to turn. Also, for any particular blower, the faster you turn it, the more power they absorb. (double blower speed, and the power requirement increases about 4 times depending on the internal leakage) Also, the GMC blowers become less efficient as you try to extract more boost from them.

I'm thinking a 4-71 (284 CI) or a 6-71 (426 CI) blower will give you the boost you want without absorbing to much power or becoming less efficient. Not to mention they are smaller and will probably package better on a narrow/shorter engine like the V6.

B&M used to make a couple of blowers, one was I believe 144 cubic inches for mildly boosted 350 sized engines and the other was a 177 incher for mild big block sized engines. Both of those were of smaller capacity than even a 4-53 GMC blower (212 CI) and they were of a nice small size easy to fit in most applications.

In theory, a blower providing 305 cubic inches of air per revolution directly driven at crankshaft speed would make 15 pounds of boost on a 305 cubic inch 4 cycle engine. In reality, the charge heating would probably reduce intake charge density to the equivalent of about 8 pounds of boost. If I recall, the maximum efficiency of the GMC roots blower is about 45 percent.

Between your engine size and boost desired, for any given blower size, you'll be able to easily calculate the needed drive ratio. I'm sure there is plenty of information and probably graphs of the various outputs of the many blowers out there, including the GMC's.

[Cuttyman9]

Thanks turbobill

The harder part seems to be finding info for non 71 style blowers in terms of cfm and displacement.
I don't believe the 8v71 and 8v92 are different blower wise but are from the diesel they came off of but I'm having trouble finding anything on the blower itself. As for what I should use, the 8v92 is already on its way and is new so I'm sticking with it.

Something to note, I may have a handle on 2 478's and a 401 so I may be stepping up the cubes in that way.
JE said they can produce a 5.125" piston so then I'd just need to machine the crank to fit a rod that matches a decent compression height.

[turbobill]

If the 8V92 and 8V71 blowers turn out to be the same, in your search for info, you will probably find that it is driven faster on the 8V92 engine. I'm sure there is a Detroit Diesel forum out there for the guys that love 'em. Such a forum may also be linked to an page for old GMC trucks and buses. The Antique Historical Truck Society forum has a few guys that know the old Detroits well, so maybe they can tell you.

Scour ebay for old Detroit service manuals for both the 71 series and the 92 series. They may also have the tech info you're looking for. I have one from my military days, but it is buried somewhere lol, and it was on the 71 series. That was before the 92's came out.

I have a 478 that I'm going to boost with a turbo. That particular truck currently has a turbocharged 468 Chevy big block, but I've always wanted a 478 for it. Took well over 10 years to find a nice one and I've had it now 17 years so I need to get started. Retirement is less than two years away fortunately.

Also laying around is a 478 Toro-Flow and a few 305's. At least one of those will eventually wear a turbo too. (maybe both) Don't know what I'll use them in as I already have over 20 vehicles and really don't need any more lol. Guess I could always put them on display and run 'em now and then!

[Cuttyman9]

Haha nice!

I'm curious the differences in block and internals on the diesel 478's.

I'm only stating that as that's all I've found differentiating the -92 vs -71.... some say it's bigger bearings etc.
I would agree there's probably a bump in speed that the blower is spun at.
I haven't dug through the diesel forums because differentiating between the blower they use and the engine itself is difficult based on the blowers not having a name/number themselves.