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Thread: Best 351 performance upgrade?

  1. #1
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    Default Best 351 performance upgrade?

    I have a Excel with a carbed 351 pro boss. As I understand the EFI adds about 25 horsepower (285 vs. 310).

    With my V hull and additional weight I need all the power I can get in hard turns. I'm wondering how much of an improvement I will see/feel by adding a new intake manifold such as the edelbrach performer. I'm going to be switching out the Pro-Tec ingniton soon and it will be a natural time to replace the intake as well.

    Anyone with this engine switched to the new intake? How much of a difference did it make?

    I've also seen a lot of guys upgrading thier exhaust manifold. If I was to upgrade either the intake or the exhaut which would produce the most noticible performance improvement?

    Thanks, Tim

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    Default RE: Best 351 performance upgrade?

    ICBW but i dont think EFI adds 25HP. I think it just makes for eaiser cold starts and more expensive/complicated trouble shooting, repairs. What prop are you running? Probably the biggest bang for the buck is a good prop.
    TRBenj has done all the upgrades you were wondering about plus dual exhaust...He as well as others im sure will offer their help.
    Do a search for on here and correctcraftfan for detailed info. Hiteks are the exhaust manifolds.

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    Default RE: Best 351 performance upgrade?

    Thanks Darrel409...I'm not claiming to be 100% on this but if i asked the question correctly to Woody at SECC the two engies are identical except for the fuel system, GT-40 heads, cam, higher compression pistons, etc. Again, I may be off on that and don't want to missquote the man!

    I'm definitely going to change the prop, most likely to an ACME 856. I expect that to help holeshot, will it help power in turns too? If so that would be huge for me.

    TRBenj..if i'm correctl you added the intake first and the Hitecks later? What kind of incremental performance change did you see from the new intake (Edelbrach or Wieland?)

    Anyone else with results from a new intake?

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    Default RE: Best 351 performance upgrade?

    The HP quotes are different depending on who supplies the info and if its measured at crank or prop...PCM or Correct Craft or a magazine advertisement... I have the same carbed pro boss in my 95 SN. My original owners manual (IIRC) has it at 295 at the crank and 285 at the prop. I also have a copy of PCM's 1993 newsletter with their president stating that the new Pro Boss with Pro Tec is "in excess of 300HP".
    Im pretty sure TRBenj is at the Green Lake reunion .

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    Before you start to buy parts and pieces for your current engine get a compression test at the very least, and preferably a leak down test. If your rings are bad, that needs to be fixed, and if the valves are bad, but the rings good, I would suggest getting heads for your engine.
    When I first got my 1997 ski, I was concerned about it not having a carb, and being an multiport EFI engine. 1875 hours later, I am so glad that it is a fuel injection engine. I would like to suggest that a fuel injection engine will have better performance because a carbureted engine has to have a slight restriction (a venturi) to create the vacuum to draw the fuel into the air. A multiport fuel injected engine supplies fuel directly to each cylinder and the fuel distribution is also better on a MEFI engine. Because there are no fuel distribution problems with a MEFI engine, the intake port runners can be tuned better than a carb sitting on a manifold can.
    Long story short, it could be more economical to get a new GT-40 engine from SkiDim, than to have your current engine rebuilt, and add high performance parts to it. Another option, if you want a lot of performance, is to get the motor mount frame for a big block GM motor, I think the 454 was an option in that hull, but I am not sure. If I am wrong about, I am sure somebody else will correct me. If you can get the motor mount frame for the 454, that means a 502 (Python) motor is an option. Just like cars, speed costs money, how fast do you want to go?

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    Daniel C...Good point, I think I wil do a compression test. I don't suspect there will be a problem but you never know.

    As I recall they should all be within 20 psi of each other. If there is a greater difference then put a little oil in the low cylender and run it again. If compression improves its a ring problem. If it doesn't its a head problem. Is that right? . I don't know how to do a leak-down test.

    As far as how much power I want my goals are pretty meager, a little better holeshot and better power in a hard turn. Translation...I don't want to throw that much money at it. I will be buying a new prop and replacing the protech ignition (which since its acting up replacing it may make a significant improvement). I would also change the manifold IF in the experience of others it genuinely made a difference. When it comes time to replace the exhaust anyway I'll look at the Hightecks but that's probably as far as I would go.

    You're right, the 454 was an option for the Excel. I wouldnt replace my 351 unless it died or was on life support but if I ever need to replace it a 454 is what I would like to use.

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    The check with oil will help you tell if the rings are bad. the compression should be within 20% of each other, and the lower percentage difference, the better. If possible, do compression with a warm engine, throttle wide open, and ignition coil grounded. You do not want any sparks, a carb engine will still move fuel. Have a fully charged battery, remove all the spark plugs.

    On a leak down test, you put each cylinder at top dead center on the firing stroke, and apply air pressure into each cylinder, through the spark plug hole. You then can listen in the carb for an air leak, the exhaust for an air leak, and at the oil filler hole for an air leak. Air leaking in the carb indicates a bad intake valve on that cylinder, in the exhaust, a bad exhaust valve, and air leaking into the oil filler hole bad rings. It is normal for some air to leak past the rings. On an automotive engine, air leaking into the cooling system, from a bad head gasket, will be indicated by air bubbles in the radiator, and this will apply if you have a closed cooling system on your boat. You probably do not. Air will probably just push some water out of the engine cooling system into the exhaust. This might mimic the sound of a bad exhaust valve

    You can buy a leak down tester, but it you are using it on only one engine, you can sometimes just use the hose from your compression tester to make an adapter to apply compressed air into each cylinder.
    Your compression tester probably has a schrader valve that will stop air from going through it, remove the schrader valve with a tire stem tool. Put the schrader valve back in the compression tester hose when you are done.

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    DanielC...Thanks for the detailed post. Sounds like a simple test (although not necessarily easy to do) that provides a lot of information. Am I right in assuming the leak down test would only be necessary if the compression test showed a prolem? I think my laziness is showing here.

    As it stands today my plan is to do the folowing;

    Compression test...not suspecting a problem but worth cheking
    Check the fuel filter
    plugs are new, but will eyeball the plug wires
    Use some carb cleaner

    Based on that see if the missing is eliminated, if not then replace the protec ignition.

    Once the missing issue is settled I'll time performance with my existing prop and then do the same with an ACME 856.

    I will probably skip the edelbrock intake unless I hear from someone that it will make a noticble difference in holeshot.

    How does that sound for a plan?

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    Is the "missing" at idle or at speed under load? How old are your wires?

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    Default Re: RE: Best 351 performance upgrade?

    Quote Originally Posted by darrel409
    The HP quotes are different depending on who supplies the info and if its measured at crank or prop...PCM or Correct Craft or a magazine advertisement... I have the same carbed pro boss in my 95 SN. My original owners manual (IIRC) has it at 295 at the crank and 285 at the prop. I also have a copy of PCM's 1993 newsletter with their president stating that the new Pro Boss with Pro Tec is "in excess of 300HP".
    Im pretty sure TRBenj is at the Green Lake reunion .
    Correct on all accounts.

    There was very little (if any) real power difference between the carb'd proboss and the GT40 (MPI). The big advantage to the EFI is the smooth starts and efficiency. Dont even think about adding EFI to a carb motor- if you want EFI, sell your boat and buy another one that has it already.

    The biggest improvement you can make to get your boat performing better is to change the prop. Talk to Acme and see what they recommend. There are gains to be had from an ignition swap- probably 10-20 hp if you do it right. I didnt do the intake alone (cam-heads-intake all at once), but everything Ive read indicates that alone will only get you 10-15 hp. If youre up for it, a good cam makes a big difference. No need to swap out your GT40 heads unless youre planning to go over 350 hp. I also would do the exhaust last (if at all)- its by far the worst bang-for-the-buck upgrade you can do. The gains are there to be sure, but options are limited and pricey.

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