turbo: blow thru vs. draw thru

Discussion in 'Forced Induction Mopar' started by kittypancake, May 3, 2012.

  1. kittypancake

    kittypancake Well-Known Member

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    What are the pluses and minuses of each system?
     
  2. slimjim

    slimjim Well-Known Member

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    Draw thru:
    Pro-
    You can just bolt on the carb and go (no mods like solid floats, etc)
    basically its as easy as a simple roots type blower. Bolt it all on and play (minus all the little things)
    Atomizes fuel better
    you can use a mechanical fuel pump for low boost.
    Cons-
    Cannot use an intercooler
    Gasoline will eventually wash out the bearings within the turbo (from word of mouth)
    Turbos are normally located on top of the intake.
    cannot use bov

    Blow thru:
    Pro-
    Most people use blow thru so there are lost of resources for advice/tips
    Turbos can be mounted anywhere
    Can use higher boost due to the intercooler
    Cons-
    Pain in the As* to tune
    Must use electric fuel pump
    The carb needs quite a few mods

    Im sure I missed a BUNCH of stuff but thats just what came to mind at the moment
     
  3. bohica2xo

    bohica2xo Well-Known Member

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    I built & delivered hundreds of draw through V8 systems before 1980.

    Don't spread BS about turbocharger bearings. A Draw through system will NOT damage a turbocharger bearing. We only had to warranty one turbocharger, and that turned out to be a blocked oil drain from a bad customer install.

    The turbocharger can be located in any location on a draw through system. Any type of system needs to locate the turbo above the liquid level in the oil pan for gravity drain back of the oiling system. Some people have used scavenge pumps, but this is usually not necessary with a little planning.

    A "BOV" is not necessary on draw through systems. When you close the throttle the compressor runs into vacuum & keeps spinning with very little load. No need for the stupid duck call dumping turbine energy.

    True, you can't run an intercooler on a draw through system. We never found it necessary for street systems with 1.8:1 pressure ratios & pump gas.


    I never had a blow through system I was happy with. All of them had metering issues, especially in transition. Slobbering rich, bowls overfilling when the pressure regulator could not reduce pressure, leaking fuel like a lawn sprinkler, etc. I had an AK Miller install that was a turd on the street - his response was that it was "great at WOT". Yeah, ok. I hear people say they have a blow through that runs great. I have yet to see one you could drive in traffic.

    B.
     
  4. kittypancake

    kittypancake Well-Known Member

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    Can you explain 1.8:1 pressure ratio for me?
     
  5. HOTMOPR

    HOTMOPR Mr Horsepower

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    Ive never seen a draw through support more than 600 hp..
     
  6. kittypancake

    kittypancake Well-Known Member

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    Which is more streetable? I don't need anywhere near 600hp!
     
  7. CUUDAK

    CUUDAK Well-Known Member

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    Gale Banks did it for years. My buddy has a 440 out of an old motorhome with his setup on it. Due to the size of the turbo it is very limited on power. You would have to cut a hole in your hood too.
     
  8. bohica2xo

    bohica2xo Well-Known Member

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    There are two things you need to know when you are looking at a compressor map, pressure ratio & flow.

    The pressure ratio is the ratio between atmospheric pressure and your desired manifold pressure. To calculate it you would take atmospheric pressure (usually 14.7 psi unless you live in Denver) and add the desired boost pressure. Then divide by atmospheric pressure. So 8 pounds of boost + 14.7 = 22.7 Divided by 14.7 = 1.544 That would be a 1.544 pressure ratio.

    Turbocharger compressors are rated in mass flow. Usually pounds of air per minute. There is a fairly close relationship between mass flow & horsepower. Plenty of calculators out there to help with that.

    For an arbitrary target of 400 horsepower, you will need 44 pounds of air per minute. Boost & engine displacement can change, but the mass flow will still be 44 lbs per minute. A 318 with 9psi of boost or a 225 /6 with 19psi - both 44 lbs per minute mass flow.

    When you look at a compressor map, there will be pressure ratios on one axis, and flow in pounds per minute on the other. Where those lines cross is where you wind up on the map.

    B.
     
  9. bohica2xo

    bohica2xo Well-Known Member

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    That was done for a reason. We did 440 motorhome systems as well. The single turbo setup was sized for about 7psi, which kept the 440 alive. We could have made more power, but in a vehicle like that you can hold the pedal on the floor for long periods of time. Even a 440 has it's limits.

    That 440 with no intercooler & stock pistons / smog cam from 1978 would make 500 horsepower @ 7psi. Push a stock 440 to 500 hp on the road for half an hour and it sort of heats the oil up. The 727 is not real happy about it either. We would beat on them that hard on the Baker grade - 32 foot coach with 24 feet of trailer behind it.

    B.
     
  10. CUUDAK

    CUUDAK Well-Known Member

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    Pretty impressive for such a small turbo and a draw through setup.
     
  11. bohica2xo

    bohica2xo Well-Known Member

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    The only turbochargers we used were Rajay units back then. An E flow Rajay on a 350 cid engine would push 10psi plus with a 1.0 A/R ratio on the hot side.

    We ran a looser 1.3 A/R on the big blocks. Back then it was common to size a turbocharger to max out with the engine to build a system that would work without an exhaust wastegate.

    Gale used a dogleg intake with a pad for the carb that put the top of the air horn below the centerline of the compressor on some truck installs. That was ok for a smallblock, but on big block it was restrictive with that short turn in to the compressor inlet. we saw icing on those manifolds, especially the one with the carb near the passenger side headlight.

    Most motorhome kits had a higher carb mount. Which setup does your friend have? Which turbocharger, Garrett TO4B or Rajay E series?

    B.
     
  12. CUUDAK

    CUUDAK Well-Known Member

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    He has the Rayjay setup. His also has the piece under the carb that bolts directly to the top of the intake. If I remember correctly the turbo mounts to the top of the passenger side exhaust manifold and a crossover from the driverside manny comes under the engine into the bottom of the passenger side manny. That was a mouth full lol.

    So this setup produced those kind of numbers?
     
  13. bohica2xo

    bohica2xo Well-Known Member

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    So he has the split box with the angled floor under the butterflies? Vacuum to the compressor inlet out the front, compressor discharge in the back under the secondary butterflies & into the intake?

    That part was ok on a smallblock. It was very close to the secondary butterflies on a spread bore carb at WOT, so it was a bit restrictive.

    Add a 1" spacer under the carb, and make sure you have a big carb. An 800 cfm TQ or a QJet from a big block.

    You can probably get to 460+ hp with that wafer adapter. they were only about 2 inches high. Adding a spacer will help, along with a big carb. The good news is that one kept the stock linkage & cables.

    We used a boxy manifold with a 3" nipple to couple the carb right to the end of the compressor inlet with a hose. That flows better, but means using a new throttle cable & kickdown cable.

    That exhaust manifold was an off the shelf piece, we all used that part. The crossover under the pan can be wrapped for a little gain, but they burn out quicker when you do that. The shop motorhome had one made from stainless schedule 40 pipe & wrapped. I never had big enough balls to put a thermocouple under the wrap.

    Do not run ported spark with that setup. You need to run manifold to the vac on the distributor. We usually set timing with the line disconnected at factory (smogger) numbers. Gave it plenty of advance at idle & dumped it all before boost came in.

    What is the 440 in now?

    B.
     
  14. CUUDAK

    CUUDAK Well-Known Member

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    Thats the one! It is on a engine stand now.
     
  15. bohica2xo

    bohica2xo Well-Known Member

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    When you get a chance, write down the numbers on the turbo tag. Should be riveted down on the side of the compressor discharge nipple.

    It should have a couple of lines, the top line will have both numbers an letters like :

    3 A1 EE _ _ _ B1

    There may be dashes for blank spaces, or info in those underscored lines above.

    The second line will be a number, usually six digits with a dash & two more digits like this:

    600586-62

    That will tell me a little more about the compressor.

    I would be a liar if I said that 500 hp number was easy for that turbocharger. It took a special trim on the compressor wheel, and was basically all that unit had. It was out in the 55% part of the island near the choke line to get the job done. Back then we just did not have the turbocharger selection that is available today.

    A lot of Gale's motorhome units were 4psi setups. The ran in Vail, CO like a stocker did at the beach. A huge improvement to be sure.

    B
     
  16. slimjim

    slimjim Well-Known Member

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    I was not 100% sure about it washing the bearings out so that is why I put next to it "( by word of mouth)" just tryin to help the guy with his question.
     
  17. Mopars#1

    Mopars#1 Active Member

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    Guys I just picked up a rotomaster kit off ebay...it is the Draw through with butterflies...not trying to hi jack thread... i have a broken right hand at the moment...i just picked up a 66 cuda a few months ago and primed it in my back yard..threw wilwoods on it all the way around...now i just need to rebuild this old turbo or purchase a new one with the carbon seal in it...i really want to get this to work... as I have a .30 over 318 with a little purple shaftcam in it...
     

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  18. Mopars#1

    Mopars#1 Active Member

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    I would love to know which turbo I should run.....seems like you really know these draw through kits...everyone is telling me to go blow through, but I really think this can work well...The numbers on mine are AR88 (HOT) M76 T-4
    Cold side AR/60 I posted a pic earlier...Looks like I need an open plenum intake...and I need to find a way to get the 3" exhaust pass the torsion bar...or go out thru the fender...This is a Rotomaster kit..any knowledge passed on is much appreciated...
    Thanks,
    Jim
     
  19. bohica2xo

    bohica2xo Well-Known Member

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    Wow. Have not seen that exhaust manifold in about 30 years. That is almost a unicorn. Nice find.

    Rotomaster still sells service parts for that turbocharger. As long as the wheels are not damaged, it can simply be rebuilt. Not really difficult - some carbs are harder to rebuild. Here is the kit:

    http://www.rotomaster.us/index.php?p=products_details&product_id=308&cid=3#

    That intake adaptor should be fine on a dual plane intake. Can you post a picture of the bottom side of it?

    The exhaust dump can be as small as 2.5 inches if it needs to be. You will probably need to make some room for that manifold on the passenger side of the early A body. I look under the hood of my '65 Barracuda and know I will have a hard time getting the exhaust upgraded. Can probably hammer a little clearance for that manifold, and sneak a 2.5 inch pipe out past the torsion.

    Can you post a pic of the actuator on the exhaust?

    The turbocharger needs a good oil drain. I recommend removing the mechanical fuel pump, and using that port to return the oil. Make sure the return line has at least 1/2" inside diameter all the way from the turbo to the block. Install an electric fuel pump at the tank.

    What carb do you plan to use?

    B.
     
  20. Mopars#1

    Mopars#1 Active Member

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    Yes, I was going to get a new 1/2" sending unit for the tank...run an electric pump with a regulator....tee off from the oil pressure gauge tap and run a braided line over to the top of the turbo. Big line out the bottom and run it to a block off plate where the old fuel pump was.

    I guess I want to run as big a carb I can get, after reading your posts... I was thinking of taking a grinder and opening up the plenum a little because it looks a little restrictive.

    It looks like you run a boost line to the waste gate before the turbo....I'm not sure what the electrical wires are on the sensor on the charge tube?

    I have a broken hand at the moment so I am getting ideas..would like to throw this on and get the parts for it...next month:burnout:
     

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  21. bohica2xo

    bohica2xo Well-Known Member

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    Well the compressor wheel looks like it has a trashed blade, but those are not expensive.

    The tips of the turbine wheel are the important part, and they are not visible until you pull the exhaust housing off.

    Did someone find this unit in a swamp? It has been wet for a while.

    I would contact Rotomaster about parts. There is a link on their site to the service manual so you will know how to get it apart without damage.

    That exhaust downpipe needs to go.

    That is an early Rotomaster wastegate. It should be ok, and they probably still sell diaphrams for it.

    That intake adapter is the part that Accel was selling at one time. The two discs were supposed to float on the shoulder bolts and act as a "Priority Valve" Martin did an end run with the reed valves. Can't tell for sure, but it looks like the valves may have been disabled.

    All of it can be brought back to spec. It will all clean up.
    The electrical device in the return pipe is a Hobbs switch. Usually used to activate the water injection under boost.

    B.
     
  22. Mopars#1

    Mopars#1 Active Member

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    Thanks for the info!.....I plan on sandblasting some parts.....and cleaning everything up
     
  23. HOTMOPR

    HOTMOPR Mr Horsepower

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    Another reason why blowthrough is better.. Good luck running one of these on a draw through.. Yes full size mens basketball..

    [​IMG]

    The one on the right is off a 94 dodge ram cummins..

    [​IMG]
     
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    • bohica2xo

      bohica2xo Well-Known Member

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      No problem running that draw through. I can support 1800 cfm easily enough, more if necessary.

      Pretty common on things like an 8-71. In fact I have never seen a roots blower set up as anything but a draw through.

      B.
       
    • Mopars#1

      Mopars#1 Active Member

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      I like challenges...We already have a N/A Small block mopar that runs 9.60's at 137 in the family....street car....pump gas....If I ever switched to blow thru I would build an R3 block....

      I really would like to get this thing to run good....I think this setup is neat...I was looking at billet compressor wheels....I'm wondering if they make one that helps with keeping the fuel atomized...I was thinking of running a dominator carb with some good boosters and a spacer to help atomize the fuel as much as possible...
       
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