PDA

View Full Version : Matching engine and impeller



wsm9808
05-09-2002, 08:44 PM
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
05-10-2002, 10:17 AM
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
05-10-2002, 10:21 AM
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
05-10-2002, 11:40 AM
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
05-10-2002, 11:41 AM
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
05-10-2002, 01:49 PM
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
05-10-2002, 01:50 PM
So if you are really get involved with comp cams they will really be helpful and they know there shit. Good luck

Banshee
05-11-2002, 04:57 AM
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
05-11-2002, 07:50 AM
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/images/Karaco/Karacothumb.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
05-11-2002, 01:04 PM
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.

1tricky1
05-11-2002, 10:41 PM
Hey WSM, MPD has a graph that shows how much HP and how much torque it takes to spin an "A" impeller. This might help you. It's under the "tech info" link @ http://home.pacbell.net/jmclure/. BTW love that tunnel of yours!

wsm9808
05-12-2002, 08:08 PM
1tricky1, Thanks for the link, but I cant get it to work.
del51 , I was just wanting to use a flat tappet hyrolic cam. I've got a roller, but it is for a 496 that will go in later. The 454 i'm building right now is just a bunch of spare parts I had setting around that I thought would work good with the blower.

flat broke
05-12-2002, 08:40 PM
http://www.liquidaddiction.net/impeller.html
That might help. These are the standard Legend and Berkeley curves, and not indicative of what the curve might look like for an MPD prepped impeller. From what I have seen and heard, Jacks impellers seem to pull a few less RPM with the same HP as a stock impeller, but somehow flow more water. Hence the reason some people call it the MPD magic.
Hopet that helps,
Chris

Banshee
05-13-2002, 01:26 PM
Hi everyone. What I thought WSM or anybody building a motor could do is take a chart like the one Chris has a link to and make some runs on Desktop Dyno with different cams and see what matches up best. I don't have destop Dyno so it might not be as easy as I'm making it sound. BTW nice boat WSM.

burbanite
05-13-2002, 01:50 PM
On the subject of Desktop Dyno, I understand there are a few different companies making this type of software - who has what and would they recommend it over another version?

wsm9808
05-13-2002, 05:25 PM
The impeller charts are great, and I have several, but unless I'm missing something all they tell you is how many rpm you can expect to turn at a given horse power level, or how much horse power you are absorbing at differant rpms. For example, my A american turbine impeller absorbs 600 hp at 5700 rpm. Which brings me back to my core question. How do I cam this dang motor.
Here is the way I'm thinking.....using the 600hp and 5700rpm example.
Say you have an engine with a peak hp of 600 at 5300rpm and the hp drops off a little after the peak to around 560hp at 5700. So you never quite reach the 5700 because you dont have enough hp left at that rpm, so the boat ends up running a little slower and peaks at around 5500rpm because that is all the usable horse power you have at the intersection where your hp and impeller curve meet. Kinda like putting too low of gears in a drag car, you run out of motor before you cross the big end.
Next example would be a motor making peak 600 hp at 7000 rpm. In this case the motor would never get anywhere near peak hp because the impeller needs 600hp at 5700rpm to turn 5700rpm, so the rpm will stop climbing long before 5700 is reached and the hp/impeller curve intersection will be way down maybe around 4500 because the impeller will not allow the engine to turn more rpm than what hp is avalable at that rpm. Kinda like a truck with a huge cam trying to tow up hill, it is very sluggish and low on power because you cant get the the power range of the motor.
So I'm thinking if you dont have dyno #s to know exactly what your peak hp is , then it would be less detramental to your top speed if you miss guess your cam size slightly on the small side rather than grtting one too big to.
Any one want to run some desktop dyon runs on my combination?
Thanks, Scott

1tricky1
05-13-2002, 10:00 PM
[QUOTE]Originally posted by wsm9808:
[B]1tricky1, Thanks for the link, but I cant get it to work.
Sorry bout that, there's 2 c's in "mcclure". But it looks like flatbroke gave you the same graph. I'm building a 460 BBC with approx. 525hp with a Clay Smith cam. Those guys were a great help in selecting my cam. My cam will be rated to 6500 rpm but I know I will never see it but at 5400-5600 it will be in the "sweet spot" of the cam and still making max torque. I believe that's what they were trying to accomplish in a jet boat application...keeping torque as high as possible near max hp. good luck!

Banshee
05-14-2002, 05:28 PM
WSM, I’m thinking exactly what you said in your previous post about engine HP Vs impeller HP. I just thought that making runs on Desktop Dyno or something similar would be a slick way to get some fairly close numbers on what an engine is making for power before money is spent on a cam that doesn’t work well with the intended impeller or use of a motor. Somebody that has Desktop Dyno or actually knows what they’re talking about (unlike myself) feel free to jump in here.

flat broke
05-14-2002, 09:42 PM
Banshee and Scott are on the money for the concept of matching the cam to the impeller. The one question I have never gotten a straight answer from anyone on is how much of a factor the torque curve plays in the equation. Since I have never gotten solid confirmed info, I always compare off of the HP curves. Just print out a couple impeller charts, then overlay the various HP curves in different colors for each engine/cam combo; then you can see where your engine HP and impeller HP absorbtion intersect and you'll have a pretty good idea of what to expect.
One thing I have to stress is that if your impeller has been detailed by a pro, it may not follow the curves seen in the stock impeller charts. As an example I have a sneaking suspiscion that an MPD prepped impeller varies from the stock curve at an exponential rate rather than a more predictable geometric rate. But thats just theory as Jack seems to keep his methods clandestine. I just hope he's passing it on to someone else so the art/science isn't lost in years to come http://free.***boat.net/ubb/smile.gif
Chris www.liquidaddiction.net (http://www.liquidaddiction.net)