GT-40 power vs. 351W carb??

[coloradonautique]

Hey guys, I finally got the first pull behind our new boat and was less than impressed with the hole shot. Our 91 SN with a carbuerated engine seemed to get me out of the water faster. The new '01 AN with a GT-40 seemed to drag out of the hole. I am not sure if this is a tuning or prop choice problem or just the effects of a much bigger and I assume much heavier boat (tower, tanks, bow seat, trunk).

Any thoughts or comparisons? I expected the same or better hole shot.
Here were my thoughts for this assumption:

'91 SN with 260HP carb engine at sea level. At Denver altitude (5300 ft) HP should be much less.

'01 AN with 310HP EFI at sea level. At Denver altitude HP should be slightly less but not as big a drop as carb. EFI should adjust better?

50HP is a lot of extra power.

The engine seemed to be fine otherwise. Boat topped out at 41mph which is what I expected. BTW, we hammer the throttle every time so it's max power out of the hole.

Any comments?

Thanks,

Lorin

[Rick]

RE: GT-40 power vs. 351W carb??

Your altitude takes away 20% of your power. When we run at Lake Tahoe 5000' we usually go down 2" in oitch to make up for the power loss. With our 2000SN we don't reprop but you can still feel the loss of power. I would see what props you have and call ACME for their suggestions.

[CRAZY4BOATS]

RE: GT-40 power vs. 351W carb??

I FELL THE SAME I HAD A 2001 196 WITH THE SAME PROBLEM IN FLORIDA

[NCH2oSki]

RE: GT-40 power vs. 351W carb??

No problems with mine, it has pulled everyone up without even coming close to hamering the throttle. I had a PS 190 with GT-40 (285 HP) and Powerslot, and my current CC with either the factory 4 Blade OJ or Acme is more then enough power to pull anyone. I would do as the Rick said and check your prop.

[M3Fan]

RE: GT-40 power vs. 351W carb??

The GT40 is known for two main attributes- 1) reliability and 2) outstanding hole shot. I'm thinking prop as well. Is this with ballast at all?

[coloradonautique]

RE: GT-40 power vs. 351W carb??

Rick, Yup. I know there is a huge power loss for altitude (my vehicles suffer tremendously) but I am comparing two boats on the same lake in the same conditions. However, I could believe that the SN had a prop the compensated better for the loss of power and maybe with the AN, we are well out of the sweet spot of the prop with the 20% power loss and that is what is making the difference. Boat was originally from CA at sea level.

M3Fan, We were running without the ballast filled and with the minimum person load (driver, spotter, skier in the water). So this was probably the best possible scenario. It will only get worse when we load the boat more. Yellow_Flash_Colorz:

It currently has a four blade prop (probably stock?) but I have no idea on the diameter or pitch. I will take a look and post the results. I assume it will be stamped on the prop somewhere?

Nobody is thinking the engine is a problem right? If it is topping out at 41mph then I assume it is putting out the proper power and the only thing left is maybe some strange load problem or torque loss? Is their anyway I can verify the engine is running strong? I will certainly do a compression test on the cylinders but cannot think of anything else. Does the EFI computer track any stats that I can pull out and look at?

Thanks to all,

Lorin

[jhiestand]

I'd say prop and weight differences are likely the main contributors. According to the respective brochures, a 98 Ski is 2340 lbs and your 01 Air is 2890, a 550lb difference. That, plus I'd be suspect the Air may benefit from a different prop for holeshot.

Generally speaking, there are very few boats PERIOD that'll yank you out of the water like a tourny skier direct drive. I don't think you need to be worried about the GT40.

[Rick]

My guess is the stock prop is a 12.5X15.5 1" shaft. What I would try is a 13X13 or a 12.5X13. The nice thing about ACME is it is kind of try til you buy. They will send you a prop. If it doesn't do what you think it should they will swap it out til it does. Good Luck

[lukeduke95]

We just swapped out our carb'd 351W for a GT40. The GT40 is definitley more powerful. It seems to get up better, holds speed better and will do 50+ with the best holeshot prop we could get. (imagine what it would do with a high speed prop!!)

It may also help that we swapped the 1.23:1 tranny with it. Now the running gear in our Ski2001 is the same as an 02 210

[DanielC]

What rpm are you running at WOT? My GT-40 in a ski will turn just a little over 5000, no, I do not hold it there very long, but it will get there fairly quickly. This is in Oregon, about 90 feet above sea level.
A carb engine has to have a slight pressure drop at the carb, or it could not flow fuel into the engine. An efi engine measures the amount of air going into the engine, and the injectors push in the right amount of fuel. The longer, and even length intake runners also give an EFI GT-40 an advantage.

[coloradonautique]

That is the thing I find really funny. I cannot remember what the old boat turned for RPMs but I do remember the feel. Solid acceleration at start then I would HAVE to trim back pretty aggressively about three seconds later to keep it from overshooting the target speed (32mph).

With the new boat, I watched the RPMs shoot to 3000 in about two seconds then get stuck there for a long time and eventually ease up after five or six seconds after dropping the hammer. I don't think it ever hits 5000 until I get to 35-40mph. I don't feel that same big acceleration the old boat had no trouble putting out. I also don't have to pull back on the throttle like I did before. I can almost wait to adjust right when it hits speed instead of 10mph beforehand. It felt like the engine did not have enough power or the prop was cavitating and trying to play catch up. Very very strange. As someone mentioned, prop and/or almost 600 extra pounds of boat may be the source of the problem. I will know more when I check out the prop specs.

Thanks,

Lorin

[boysrus]

Lorin,

Not sure I have a solution - but I do have the same boat/motor and live just on the other side of the mountains from you (NW Colorado). My boat seems to pull out of the hole fine - and I'm still running the factory prop (actually switched to Acme, but it performs about the same). I don't have an older SN to compare to, but my boat does pull me up as quickly as my friend's MC's - even a new 197.

Mine only hits about 4300 RPM at 41 MPH - so I could definitely stand to drop an inch or 2 of pitch, but it hasn't bothered me. I could definitely tell a difference when I skied in Florida this past summer - similar boats came up much quicker.

With your old boat, you might have been feeling that kick when the carb secondaries opened. The GT 40 certainly has more linear acceleration. It might just be a different feeling that you're getting used to.

At any rate, if you're ever lost in this part of the state, I'd be happy to give you a pull for comparison!

Jeff

[TRBenj]

I don't have an older SN to compare to, but my boat does pull me up as quickly as my friend's MC's - even a new 197.

I havent been in one personally, but from what Ive heard the newer MC's (190/197) are not very quick out of the hole. I remember when Waterski published acceleration numbers a few years ago that the MC was quite a bit slower to 36 than the 196 or RLXi.

If it performs similarly, your Sport/Air might be performing at its full potential and simply isnt as quick as your old Ski Nautique.

Either way, I would expect you to see an increase in performance by going to the Acme 470. It outperforms the 422 in every way, and with 1/2" less pitch it will help you pull some more RPM's.

[boysrus]

I havent been in one personally, but from what Ive heard the newer MC's (190/197) are not very quick out of the hole. I remember when Waterski published acceleration numbers a few years ago that the MC was quite a bit slower to 36 than the 196 or RLXi.

I was really surprised that it didn't come up quicker - being all brand new, shiny and all over the magazines!

Jeff

[coloradonautique]

Hey Jeff,
When you get a chance, could you pull some numbers from your boat for me? I tried the recommended change to a ACME 470 12.5 x 15 from the stock 13x16 OJ Legend, but did not feel much difference. WOT gets me 39mph @ 4400 rpm and it takes FOREVER to get there. I was running 3500 rpm @ 30mph.

I am curious what ACME prop you are running, what boat you have, what rpm numbers you are getting at different speeds and what the altitude is at your location.

I may try to step down to a ACME 13x13 but want to get some solid numbers for baseline and comparison to rule out the engine as the problem. I will be running a more definitive test this weekend and will record more numbers.

Thanks,

Lorin