Larger air intake for more top end???

[thriller]

Back a year or so there was an article in Hot Boat about adding a larger air scoop and larger flame arrester on top of a single carb Holley to increase top speed.I have never bought this recipe as I have found out that to increase the top speed of a jet boat you must first increase the rpm!! Any comments??

[ck7684]

Well...no matter what you shouldnt have any kind of restrictive intake...to add top speed to anything you must add RPM someplace

[Cas]

yes, it's possible. A few years ago, 1977, to be exact, I had a little 18' Tahiti with a 455 Olds and a Berk JC pump. With the stock flame arrestor, that engine would turn about 4400 rpms. With a less air restirctive scoop, it went up to 4550 to 4600.
I also experienced a larger increase with a friend of mine when he switched out an 1-1/2" flame arrestor to a 4"......went from about 68 mph to 75, increased the rpm by 500.
Given all that, if your engine can't flow and exhaust the air, it really won't make that much of a difference.

[Jetoholic]

As Cas said...the engine has to be able to breathe. Think of the engine as a fire driven air/fuel pump. The more air it can get into the cylinders, the more fuel it can burn. More fuel burned=bigger boom=more power. Also, the better it can clear the cylinder of spent exhaust gasses, the more air/fuel it can fit into the cylinder...again more power.

[Cas]

to increase the top speed of a jet boat you must first increase the rpm!! Any comments??
that also isn't necessary true as there are a lot of factors that come into play. A couple of examples are hull weight and impeller size.
You can take a engine/pump combo out of an 18' hull that does 80mph at 5400 rpms and put it into a 21' hull. The engine will probably still turn the 5400 rpms but very doubtful the hull would get to 80.
Going a little further, you can have that same 18' hull, engine and a pump that has an A impeller doing 80 at 5400. If you take that A impeller out and replace it with a C cut, the rpms will increase but there's no guarantee the speed will. There's a good chance it will decrease in speed.
Speed is not a direct result of RPM.

[ck7684]

Well...no matter what you shouldnt have any kind of restrictive intake...to add top speed to anything you must add RPM someplace
On second thought, I take that back. I was thinking all else being equal
However, if you can lessen the drag on the hull or increase the efficiency of the jet, your speed may increase without an increase in RPM...

[Jetoholic]

On second thought, I take that back. I was thinking all else being equal
However, if you can lessen the drag on the hull or increase the efficiency of the jet, your speed may increase without an increase in RPM...
BINGO! Pump and hull efficiency!
However once you have maximized both of those things, you will have no choice but to add power to go faster.

[TIMINATOR]

I just bought another boat and it has an aluminum scoop and flame arrestor. I will post CFM flows and pics, while on the flowbench, within a few days. P.S. I am familiar with that ***boat article you referred to. TIMINATOR