Matching engine and impeller

[wsm9808]

OK, I guess I'm confused on how to build and engine power curve for a jet boat. Are we trying to match the impeller to get the engine to turn at peak horse power or at peak torque or somewhere in between.
The reason I ask is,I've decided to build a new motor for the blower I just bought and was trying to order the cam today for it.
Well, Comp, Crane, and BDS all sell a "blower" cam for a jet boat that is split duration and wide lobe centers. Then I talk to George at Clay Smith cams and he recomends a very large cam with single pattern lift and duration and narwow lobe centers.
I was sold on the Crane, but everyone on the board says Clay Smith is real sharp on the jet boat tuning(cams). I'm just not sure I want a cam as big as they recomended.

[hack job]

Originally posted by wsm9808:
OK, I guess I'm confused on how to build and engine power curve for a jet boat. Are we trying to match the impeller to get the engine to turn at peak horse power or at peak torque or somewhere in between.
The reason I ask is,I've decided to build a new motor for the blower I just bought and was trying to order the cam today for it.
Well, Comp, Crane, and BDS all sell a "blower" cam for a jet boat that is split duration and wide lobe centers. Then I talk to George at Clay Smith cams and he recomends a very large cam with single pattern lift and duration and narwow lobe centers.
I was sold on the Crane, but everyone on the board says Clay Smith is real sharp on the jet boat tuning(cams). I'm just not sure I want a cam as big as they recomended.
wsm,
hey bud i have talked to george at clay smith and he used to race i have never ran yet to run any of his cams but they have always come highly recomended. i have heard soem bad things about crane hard ware and cams but i have never seen it go bad, i would go with the slay smith , george is a resonalbe person and will talk to you if you are serious about sucess ( itt or devry) http://free.***boat.net/ubb/biggrin.gif
and will tell you what you need to do what your looking for! good luck and go down to clay smith and talk to him. its a great experiance.
Tyson www.plumbersassracing.com (http://www.plumbersassracing.com)

[LVjetboy]

Ok, since no one's jumping in here...
If you want killer acceleration, match a big impeller to your peak torque and hang on. Torque = ability to accelerate. If you want top speed...and most of us like big numbers http://free.***boat.net/ubb/smile.gif match your impeller to peak horsepower. Hp = work over time.
To a point.
Trade-offs kick in when a pump is operated at other than it's most efficient design speed or with blades cut shorter. As rpm's climb, pump efficiency drops. As blades are cut, pump efficiency drops. Which means trade-offs. For someone with an already built engine, trying to match impeller size...will climbing the hp curve outweigh the loss in pump efficiency? I think so in many cases...but do they have a dyno sheet to know if they're really climbing??
Unfortunately, pump efficiency and how it relates to impeller size and rpm is not well known, as far as I can tell. And the effects of one size step change can be difficult to measure. That's why you have some say go big and others say go small. I haven't seen any testing or much discussion on these trade-off's. But I'd guess going to a C or D size takes a hit. And turning pumps above 7k takes a hit. Just my opinion of course.
But the general trends are still there. From an engine building perspective, I think the closer you can match your peak hp to a big impeller (lower rpm), without sacrificing total hp, the more efficient your pump will be and the higher your top speed.
Someone else can comment on cams better than me.
jer
[This message has been edited by LVjetboy (edited May 10, 2002).]

[wsm9808]

If you want killer acceleration, match a big impeller to your peak torque and hang on.
My boat accelerates extremely hard right now with a relativly mild 468 and I love it.Its an A impeller set up fairly tight for a lake boat an turns 5200 rpm now. I was wanting to keep rpm 6000 or under so I thought a mild cam with a blower would keep the torque high enough to still get good speed without having to go to a small impeller and winding the $h!t out of it. I was sort of thinking that an AA might even get good top end speed with a low rpm, high torque, blown motor in a light tunnel hull like mine. But I would trade a couple of mph for neck snapping acceleration anyday. Thats why I love the jet boats.

[HYDROMOBILE]

You must give more info , engine size, comp, boat make size, blower size, boast, how many rpm you want to turn engine, I use a clay smith in my blown motor, Jim

[Bense468]

I am running a 468 that is pretty healthy for a lake boat. I am in the process of getting it all put back together right now. Here is what I know though. I had a clay smith cam it seemed to work fine. It was a decent sized cam. I noticed the hardening on the lifter and the lobes wearing off so I decided it was time for a new cam. I also bought Merlin heads. So I worked with world and comp cams together. We all talked 3 way. They knew everything about what I had going on and Comp cams built a special cam for me. It is bigger then my last. I was spinning a AA at about 5200 Hoping for about 5500-5700 on a AA. Oh this is on a tight pump. It accelerates hard. Real hard.

[Bense468]

So if you are really get involved with comp cams they will really be helpful and they know there shit. Good luck

[Banshee]

Interesting topic. I would someday like to build a better motor for my rig and will have the same questions on matching impeller and cam sizes. What I was thinking of doing is buying Desktop Dyno and making runs with a several different cams and then use that data to see which cam matches best with an impeller chart from my pumps manufacture for the type of boating that I do. Just my very inexperienced two cents.

[wsm9808]

Is there such a program that will match impeller charts to engine power curves? That would be very good imput to conceder if someone had one.
Anyway, the cams comp and crane says to use have a rpm range of 3500 to 6500. I dont know about boats, but in my experiance with street/drag cars, the big cam companys tend to recomend cams on the very small side.
Clay Smiths cam rpms from 4500 to 7500. I just wasn't sure if my motor turning 6000 would get high enough into the rpm band to use that big of a cam.
The cam in the current non blown motor(which turns 5200 at WOT) rpms at 2000 to 5500. It feels a little small top end, but accelerates very hard.
New engine is a 454, .005 over JE pistons, heavily ported oval port iron heads with 2.19/1.88 valves,steel crank, H beam rods, ? cam, and a B$M 250 blower with single carb top, which will have a 780 Holley on it untill I get around to buying a Dominator 1250, also have a 250 nitrous plate system. Boat is a 19' tunnel hull pickle fork with dominator pump,droop,shoe ,ride plate,place diverter. http://www.liquidaddiction.net/image...aracothumb.jpg
Oh yea, anyone have a scoop for sale simular to the one I have now that will fit a single Dominator.
[This message has been edited by wsm9808 (edited May 11, 2002).]
[This message has been edited by wsm9808 (edited May 11, 2002).]
[This message has been edited by wsm9808 (edited May 11, 2002).]

[DEL51]

Wsm 9808,I would like to know if you are going with a roller and if so would it be hydraulic or solid. I had an 871 blown 468 that ran well with 2 dominators.I am not familiar with the 250 blower.With your set up, I would think the double A impeller would be better cause your motor seems more torque oriented.I started out with a large single pattern solid roller that required a bronze distributor gear. This cam had 714 lift and 268 deg dur @ .50 which was way too big. I had a different smaller cam installed with differnt gear to allowed me to run a steel distributor gear.I gained 7 mph and had fewer valve train adjustmets.I turned a precision jet billet A impeller at 6400 rpm.With your planned set up I think you could use more carburator. It may be cheaper to use 2 carbs instead of one big dom.