For those of you who don't know, I am a senior level mechanical engineering student and have been assigned a project for my Thermal Systems class. We have to design any kind of thermal system that we choose and I convinced my group to design a turbo system for my 74 dart with a 225 /6 & 904 transmission. We don't have to build it, just design it. However, I do want to be realistic with the design so that, if I choose to, I could actually build the system sometime down the road. Everything will be designed using a stock, carb'd engine using a 4brl intake and carb.
We will also be using an intercooler. I have a really good general idea of how the system should work, but as I dig into the details, I'm sure a lot of questions will come up.
For the people who have done this and have turbo build threads, can you please post links and pictures that you would be willing to share? Anything that we use for our presentation will be given credit. Pictures will be a huge help since we are not actually building anything.
Now for my first round of questions...
1. How much boost can the stock slant take while maintaining reliability?
2. What makes a "blow through" carb different from a regular carb and what would be a good one for a turbo 4bbl slant?
3. How fast can I safely turn the stock motor? Everything will be stock for this design. Cam, crank, pistons, heads, etc...
Thanks in advance! We have to make a 20 - 25 page report and a powerpoint presentation. I will be posting all of it after it is finished.
1. That is the $64,000.00 question. I have no firsthand information about how much boost a stock Slant 6 can stand, and I have spent a goodly amount of time looking for the answer to that exact question. There are some significant factors to consider:
The slant 6 was originally designed as an aluminum engine. Aluminum lacks the strength of cast iron, so the original design parameters reflected that. The aluminum blocks apparently were fraught with warranty problems, so after 60,000 of them, Ma Mopar gave up, and changed the block material over to cast iron. The heads were always cast iron.
When they did that, they were apparently interested in doing it as cheaply as it was feasible to do, so the factory didn't do a lot in the way of reducing the cross-sectional area of structural members, such as the main-bearing webs, and thin-wall casting techniques were't around yet, (to my knowledge,) so the new "iron" block was built with an unusually stiff/strong infrastructure Almost like a Diesel! The head was unchanged, but had a nearly half-inch thick gasket sealing surface (my slant 6 head weighs about 84 pounds.) That stiffness adds to the overall stiffness of the block/head "package."
The forged-steel crankshaft was designed super-stiff, as short as possible, with only 4 main bearings (as opposed to conventional 7-main bearing design for inline 6s.) To further ensure severe crankshaft load-supporting capability, the main bearings were designed with the same dimensions as the 426 Hemi. Yeah...
So, the basic engine has an unusually strong block, crank and head, and is a really good candidate for large amounts of boost.
The cylinder head is an interesting exercize in engineering in this way: The original design for the slant 6 was mandated to be built as short, (front-to-back,) as possible. That meant bore-center spacing needed to be made as close and small, as was feasible. That gave it really small bores. That meant small valves, but the relatively-small valves were plenty big for even high-performance work (such as the original Hyper-Pak engines) given that it was just 170 cubes.
When the one-inch stroker (225) was introduced,
nothing was done to the head to enhance breathing for the 33-percent larger motor.
Bores were the same size... as the 170/225 engines were only different sizes because of the 1" difference in stroke. So, there was not a whole lot that could be done with the stock, factory head. Oversize valves, and a certain amount of porting can alleviate (to an extent) the asthmatic 225 condition, but making a LOT of horsepower, normally aspirated for a 225, is a steep, uphill battle.
Enter forced induction:
There are a couple of turbocharged slant sixes currently running, that make around 500 horsepower. Here is one of them:
[ame="http://www.youtube.com/watch?v=7QzUfV8iTpQ"]Turbo Slant Six 10.74 @ 127 mph 7-19-10 - YouTube[/ame]
Of course, that is a race motor running well over 20 pounds of boost, but shows what is possible with this motor, given some meaningful cylinder pressure.
The prospect of using a forced induction system like this one on a stock motor is not possible, for a number of reasons, but you can build something more low-key, gain significant performance, and still get by with stock components.
Expect markedly less-spectacular results.
Stock (cast) pistons will probably allow you to run up to about 10 pounds of boost with reasonable expectations of reliability. A blow-thru 2 or 4-bbl carb setup and a modified stock exhaust manifold (with a turbocharger mount,) using a stock cam and drivetrain, should produce about 200-250 horsepower, I think. That should put a 3,300-pound Duster through the quarter-mile in around 14-flat at 96mph.
FABO member, Tom Wolfe, put a junkyard Buick turbo on a stock slant 6 that had a 4bbl added, and ran 12.90 at 104 mph, in a 3,300-pound Dart, but his car was running 21 pounds of boost to go that quick. I don't believe that that motor would last long with that kind of boost, given the stock, cast pistons. It would probably grenade, before very long.
Here's that run...:
[ame="http://www.youtube.com/watch?v=pPe_vHwZsF4"]Slant Six Turbo 1970 Dodge Dart 1/4 Mile pass - YouTube[/ame]
To answer your question,, the difference in a blow-thru 4bbl and an off-the-shelf, normally-aspirated unit, has to do with mixture control under boost. The folks who modify these carbs (usually Holley, but not always) modify the internal circuits to pass more fuel under boosted conditions, and the procedure is not a simple one, if done "right." I paid $900.00 to the Carb Shop in Ontario, CA, for the one on my supercharged Valiant, for example.
The odd thing about turbocharged slant 6s, is they don't need a lot of RPM to do their job. Tom's motor (and the one in the video, owned and driven by FABO-member Ryan Peterson,) both have virtually 500 horsepower engines and neither recommends taking such an engine over 5,500 rpm.
Those engines seem to like being heavily-loaded, and both owners report that their drag racing experiences indicate that they both run quicker and faster with a 2.76:1 rear axle ratio than they do with, say, a 3.90:1.
That flies in the face of time-honored tradition, and seems bizarre, but it's a fact.
The turbocharged slant 6 is, in my opinion, the best-kept secret in the Mopar hot-rodding world. Just how much power you can get from one, reliably, with s stock head and short block, will be interesting to see.
The 500+ hp engines have forged pistons (9:1 compression ratio,) 600cfm Holley 4bbl carbs (modified for blow-thru), camshafts that are very mild compared to "racing cams," and aftermarket rods that are supposedly stronger than the stock ones. Cam specs run in the 210/210* range at .050"-lift, with only about .484" total-lift. They are usually ground with about 115-degrees of lobe separation... not really very far from a stock cam, except for lift.
These engines seem to make egregious amounts of mid-range torque, and in addition to an intercooler, seem to benefit from a high-pressure, atomized alcohol injector, such as a "Snowperformance Boost Cooler."
You might consider adding one to the mix, when you do your design/calculations.
Here are some pictures that might help fire your imagination.
:wack:
