Building a 350: newbie finer points?

Discussion in 'Gas Engines' started by curtis73, Oct 3, 2006.

  1. stonebreaker
    Joined: May 2006
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    stonebreaker Senior Member

    Ah. It came out higher with my heads on there - I have the iron heads, but I've got bigger valves so I get more low lift flow than stock. This graph shows the engine with its present cam, the crane 227 (pn 109227), and with the GM 395 cam. Not sure about the 395 cam since it's theoretical, but the 227 numbers are dead nuts on - my car made nearly these exact numbers at the rear wheels the last time I had it on the dyno. Of course, I ran it with headers and open exhaust, so it's really just a convenient coincidence that the rear wheel numbers with open headers match up with Desktop Drag's HP manifolds and muffler numbers.
     

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  2. gonzo
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    gonzo Senior Member

    Does any of that software provide low end torque predictions? On engines with wet exhaust too much overlap in the cam can cause water to flow backwards. The 8.1 and some 7.4 had that problem. The fix was to run a bypass pipe between the exhaust pipes. In a higer performance setup it would be good insurance to install one just in case.
     
  3. julius750
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    julius750 Junior Member

    We can supply hand fabricated stainless steel watercooled headers for through prop or through transom exhaust which overcome the water reversion common with big block engines or engines with modified cam. Chev, Ford, Chrysler, Olds, etc
     
  4. curtis73
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    curtis73 Junior Member


    No on the RPM thing. The software uses VE predictions to create BMEP, IMEP, and MAP, and FMEP. Using these it can rather accurately predict the torque curve and therefore extrapolate the HP curve. The problem is those values are way too variable below 2000 rpm. The way the math works, a miniscule change in predicted numbers becomes exponentially incorrect at lower RPMs. Its pretty easy for the software to assume a proper ignition curve and fuel mixture above 2000, but they can't pinpoint any accuracy below it.

    Forgive my newbie-ism, but won't the prop "stall" up to 2000 when I open it up? I'm used to smaller-propped 2-stroke outboards, so it might be a whole different can of worms, but I assumed a 600-rpm idle, and I just assumed that since my 2-stroke flares to about 2300, maybe this one would flare to 2000 under full throttle.

    I'm hoping that water reversion shouldn't be a problem. Of the two cams I'm considering, one is Mercruiser's own cam, and the other is that roller shown first with even less overlap than the mercruiser cam
     
  5. gonzo
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    gonzo Senior Member

    Props don't stall but either cavitate or slip. Cavitation is bad because the prop may end up spining in a ball of steam producing no thrust. Slip is good because it produces thrust. The blades on a prop act like a revolving wing. The slip stream produces lift perpendicular to it = thrust. The torque between 1000 and 2000 RPM is crucial to get up on a plane. Some high performance engines have very slow acceleration but good top speed. I am curious, if you are using a Mercruiser or look-alike cam and Vortec heads like a Mercruiser, where does the extra power come from? Are you milling the heads for more compression?
     
  6. curtis73
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    curtis73 Junior Member

    I don't understand your question about the "extra" power. Mercruiser uses the same cam for everything from the 230-hp 305 up to the 300-hp 350 mag with inferior 124 heads. 300 hp with superior vortec heads in a carbed 350 is super simple with nearly any mild cam.

    I have two cams in mind:

    1) the roller like outlined above or:
    2) the mercruiser flat cam

    The cam that I actually have in my possession is Melling's 22124 grind, which is the exact cam that Mercruiser uses for its 230 hp 305 engine and for the 260hp carbed 350 and the 300hp mag 350 efi. So, in answer to your question, no more compression (at least not more than mercruiser's 9:1) but the main difference is the (very mildly ported) vortec heads and 1.6:1 lifters.

    Thanks for the explaination about the prop "stall". Most of my experience is with tuning drag rides, so I base everything on a TC stall. I thought maybe the prop would act like a TC and "stall" helping holeshot.
     
  7. Jango
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    Jango Senior Enthusiast

    Curtus, Diffently use the Roller. I just completed a stroked 302 Ford, 327 cu in. using a very mild Ford Explorer, "High Torque" Roller cam, 256/266 duration, .422/.448 lift. Because of the "steep" ramps posible with rollers, enabling longer open times you should get tremendous low RPM torque. I am using ProComp 210 heads, similar to Edlebrock Victor Jr heads which also adds to the good numbers - 375 lb ft @2000 to 450 lb ft @ 4500. The torque curve is very gradual and I would not be supprised if it were more than 350 @ 1000.
    There is no such thing as too much torque. John
     
  8. gonzo
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    gonzo Senior Member

    I see, you are increasing flow at high RPM by porting and increasing valve lift. Unfortunately it may lower low end torque. As you know, hotrodded engines usually idle pretty high. Detuned engines have a flatter torque curve that carries into lower RPM's. This is not to say your setup has not enough torque, but that normally engines are set for a target RPM when they are highly tuned. On the other hand de-tuned engines perform adequately over a wider range. The main difference between boat and car engines is that boats usually have a single speed transmission. You can't use a low gear for acceleration and a high one for speed. This forces you to have an engine that will hopefully perform at both ends.
     
  9. curtis73
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    curtis73 Junior Member

    Gonzo, I appreciate that you're trying to help, but in this case you're just wrong about the engine's powerband.

    I didn't say ported, I said mildly pocket ported. Basically a bowl blending to help airflow in the valve transition bowl. I added exactly 1.4 cc to the port volume. Porters know that intake VELOCITY is what makes torque, and these vortecs make tons of velocity while still providing nice flow. That's why everyone loves them. The heads have a miniscule 170cc intake port, trust me there will be plenty of velocity for low end torque.

    This is by NO means a hotrodded engine. For cripes sake they're stock iron heads, the cam is 202/213 duration with a lobe lift of .273"!!! That's hardly hot rodded territory. The 1.6 rockers are there to help match the cam's events to WHERE the heads flow, not as an attempt to get more peak flow. Vortecs show excellent mid-lift flow, and since you want the lift curve to exist about 15% above the second turn in the flow plot, using 1.6 rockers is simply tuning the cam to the heads. Adding lift in this case is ADDING low end torque because I'm making more efficient use of WHERE the heads flow.

    Gonzo, I really appreciate that you're trying to help, but so far all you've done is disagree with everyone, put them down, and make insulting remarks. Again, you've chimed in telling me I have to add more torque. Three times now I've asked for your suggestions on how you would make more torque and you haven't said anything. I'm an engine builder. Its part of what I do for a living, so I'll understand it if you just make some suggestions!
     
  10. curtis73
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    curtis73 Junior Member

    I like how you think :)

    Is that statement still true with an Alpha in a 2600-lb boat?
     
  11. Jango
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    Jango Senior Enthusiast

    Since your Boat is "only" 2600 lbs, in my opinion, you can Add alot more torque than if it were 3600 lbs and still have reliability with the Alpha.
    In addition,If you are like me, you probably will not nail it each time you leave the Dock. It's nice to know the power is there if you need it. Go For It

    John
     
  12. curtis73
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    curtis73 Junior Member

    OK, good to know, thanks. I might nail it a few times a year pulling the odd skier, but thanks for the confidence.
     
  13. mtnrat
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    mtnrat Junior Member

    Been watching this with interest as I am also researching to build my 350.
    curtis73, I am coming to about the same conclusions as you with respect to my build. This cam, http://www.compcams.com/Technical/Search/CamDetails.asp?PartNumber=12-416-8
    12-416-8: HYDRAULIC ROLLER: Good for inboard/outboard pleasure boat, skiing and good economy.

    Duration at .050: 212
    Intake Valve Lift: 0.488
    Exhaust Valve Lift: 0.495
    Lobe Seperation: 112°
    ,roller retrofited to my 1980 4 bolt block. About 9 to 1 compression. Using the vortec heads. I currently have sealed power cast dished pistons with 8 to 1 compression with stock heads. To get proper quench I may change the pistons to a low compression quench style. Undecided yet.
    Anyone know what springs come on a stock vortec head. I may have to change to something like GM # 3927142(110lbs@1.70 & 285lbs@.490) or # 10134358 to work with the cam.
    As well I will have to make sure the heads can take the lift. IIRC vortec heads are only good to about .450 gross valve lift. Correct me if I am wrong.
     
  14. stonebreaker
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    stonebreaker Senior Member

    Those springs are a good choice. Not too expensive, either. Only thing is, the vortec heads' valve guides are kind of thick - .832" off the top of my head, although I'm not sure about that - that might be the minimum inside spring diameter, not the valve guide. Anyway, Comp Cams sells a beehive valve spring designed for the LS1 that fits the vortec heads just fine - part number 26918. It doesn't require a damper because of the beehive design, but is quite a bit more expensive than the normal spring designs. Another advantage of the beehive design is it has a very small retainer - the steel retainer actually weighs less than a titanium retainer for the regular design springs.
     

  15. gonzo
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    gonzo Senior Member

    Your original question was about tuning the engine for 5000 RPM. If you do that the low RPM torque will decrease. If porting, mild or otherwise, just shifts the whole curve upwards it is a good thing. However, the software you are using isn't able to calculate that. With the data you provided, it seemed as if the low end torque would drop dramatically. I apologize if you feel insulted or put down, but rereading what I wrote I am not sure what is it that offends you. Volvo makes, or used to, a high rise intake manifold in aluminum that has really long runners. It looks similar to an Edelbrok but I believe it performs better at the low end. It has the brass inserts on the water bypass to prevent corrosion.
     
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