rpm/hp increase with 5mph loss

2679 Views 43 Replies 18 Participants Last post by 23scarab, Jun 19, 2008

23scarab

Discussion Starter · Jun 5, 2008

just joined, have a 1988 Scarab Concept, 454. Last season boat ran 58 on gps with stock motor, 23 mirage prop. After season I pulled the motor and a very reputible local engine builder upped the compresion, added a different cam, roller rockers, performer intake. My total timing is right, jetting seems good. The boat come out of the hole much harder, the rpm went from 4600 to 4800 WOT. Running the same 23 the boat now runs 52 on the same gps. I'm baffled, the engine builder is confused, the prop guy says that this ammount of slip is nearly imposible and suggests an engine problem, but the boat runs so good, what gives! The engine builder says that the cam is right, no doubt. Any help would be apreciated I'm running out of good ideas!

1 - 20 of 44 Posts

cfm

· #2 · Jun 5, 2008

How close are you to the Speed Limit Lake in NH ?
That would be Winni.

23scarab

Discussion Starter · #3 · Jun 5, 2008

about 3hrs away, don't have any intentions of going to Winni, there is no speed limit on the lake that I keep the boat on

Horse power

· #4 · Jun 5, 2008

I had the same thing happen to me this year. If you learn any thing let me know. I ran 63mph on gps last year and now can only do 60 mph gps.

cfm

· #5 · Jun 5, 2008 (Edited)

23scarab said:
about 3hrs away, don't have any intentions of going to Winni, there is no speed limit on the lake that I keep the boat on

That's where I'm at, so was just asking in case (I knew slight chance) your close I could've helped on the water after asking a bunch of questions here. :winker:

List all engine parameters and parts here: yes, including cams seat duration, at .050", LSA, Lift
Carb used including tuning parameters
Intake (Performer or Performer RPM )
Cam
Cylinder heads - what type and if any work
Valve springs used and how set up (installed height)
Compression
Who's Exhaust system including if thru prop or thru transom and if any mufflers and what kind
What is your total timing spec ?

Do a compression test ! Yes, especially if just rebuilt.
Check your fuel psi !
Check timing and ignition. OH, Make sure a wire didn't fall off.

Have you checked/removed your anti-siphon valve ? It is typically a ball/spring inside a fuel line fitting that is screwed to the tank.
=====================

BTW:
I have a 23" Mirage with solid hub that you could borrow. Should be cheap to ship since I'm close by and just 1 day aways using UPS ground. If interested give me a ring 603-238-6721 8am-6pm.

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cfm

· #6 · Jun 5, 2008

Horse power said:
I had the same thing happen to me this year. If you learn any thing let me know. I ran 63mph on gps last year and now can only do 60 mph gps.

I was trying to help you but you stopped giving me info.

hump

· #7 · Jun 5, 2008

Could you have bent a blade on the prop causing the high slip #'s?

cfm

· #8 · Jun 5, 2008

cfm said:
BTW:
I have a 23" Mirage with solid hub that you could borrow. Should be cheap to ship since I'm close by and just 1 day aways using UPS ground. If interested give me a ring 603-238-6721 8am-6pm.

I just checked it. It's a solid hub 23" Mirage Plus in perfect shape.

Also: how old is the fuel ? Have you dumped the water seperator contents into clear bottle to see if a bunch of water seprates ?

Super24

· #9 · Jun 5, 2008

I'm not a motor guy but I did notice you said the cam was changed. Maybe you are not hitting the RPM range where the cam is making the power?

Did the builder tell you what RPM range you should be in for best results?

tmthunder

· #10 · Jun 5, 2008

Where are you in Maine? Good to see a few people on here!

cfm

· #11 · Jun 5, 2008

After I get a speeding ticket or two I might have to visit my neighbors. :laugher: :laugher:

Black Wishbone

· #12 · Jun 5, 2008

I am going to throw my 2cents out there. If your RPM went from 4600 to 4800 you should gain speed it would have nothing to do with your motor. So that would leave your tach being wrong or your slipping to much.

Were the water conditions near the same. I know if I don't have enough chop I will lose an easy 5mph. Just Sunday I ran over 77 with about 400lb+ more then two weeks ago running at 75 and I had more to go on Sunday.

Steve

RSCHAP1

· #13 · Jun 5, 2008

If you are getting more RPMs, the drive ratio is same, prop pitch is same ( and not damaged ) you would HAVE to be going faster. Either prop is slipping, ot tach is wrong. Turn the same drive and prop more RPMs physically it would have to be a prop issue and slippage or false readings. If the drive is being turned 4800 rpms, it doesn't matter whether a poor running 5 horse Briggs or a well tuned hipo 502 is turning it, the prop will still be spinning at that speed. So the drive isn't turning @ 4800, or the prop changed/gain slippage...Atleast that is what I am gathering from info stated so far.

23scarab

Discussion Starter · #14 · Jun 5, 2008

tmthunder said:
Where are you in Maine? Good to see a few people on here!

on the coast by boothbay

23scarab

Discussion Starter · #15 · Jun 5, 2008

Black Wishbone said:
I am going to throw my 2cents out there. If your RPM went from 4600 to 4800 you should gain speed it would have nothing to do with your motor. So that would leave your tach being wrong or your slipping to much.

Were the water conditions near the same. I know if I don't have enough chop I will lose an easy 5mph. Just Sunday I ran over 77 with about 400lb+ more then two weeks ago running at 75 and I had more to go on Sunday.

Steve

I know! thats what I think too!, I don't get it, I did check the tach and I'm using the same gps

Pantera28

· #16 · Jun 5, 2008

Horse power said:
I had the same thing happen to me this year. If you learn any thing let me know. I ran 63mph on gps last year and now can only do 60 mph gps.

Your problem is easy to answer. You went for a bravo to an XR drive. I'm pretty sure I read a thread about losing speed with an XR.

23scarab

Discussion Starter · #17 · Jun 5, 2008

cfm said:
That's where I'm at, so was just asking in case (I knew slight chance) your close I could've helped on the water after asking a bunch of questions here. :winker:

List all engine parameters and parts here: yes, including cams seat duration, at .050", LSA, Lift
Carb used including tuning parameters
Intake (Performer or Performer RPM )
Cam
Cylinder heads - what type and if any work
Valve springs used and how set up (installed height)
Compression
Who's Exhaust system including if thru prop or thru transom and if any mufflers and what kind
What is your total timing spec ?

Do a compression test ! Yes, especially if just rebuilt.
Check your fuel psi !
Check timing and ignition. OH, Make sure a wire didn't fall off.

Have you checked/removed your anti-siphon valve ? It is typically a ball/spring inside a fuel line fitting that is screwed to the tank.
=====================

BTW:
I have a 23" Mirage with solid hub that you could borrow. Should be cheap to ship since I'm close by and just 1 day aways using UPS ground. If interested give me a ring 603-238-6721 8am-6pm.

Thanks for the input here we go....

performer intake
q jet jetted up one size
compression from 7.7 to 8.2 115cc
comp cams roller rockers
some polishing on heads
cam
duration at .050 in 223 exh231
110 separation
duarion at .006 280 288
gross lift .525 .550

32 degrees total timing
thru prop
fuel is new.....I even pumped out tank and filled with 91

really appreciate the offer about the prop!!

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blackhawk

· #18 · Jun 6, 2008

I agree with others if the rpms went up and speed went down with the same prop the engine hp doesn't matter. UNLESS you are now overtrimming the boat to get the 4800 rpm, causing more slip.

Horse power

· #19 · Jun 6, 2008

cfm said:
I was trying to help you but you stopped giving me info.

I started a new job and had not had time to get that info yet. This weekend i will try to get that. Sorry.

Horse power

· #20 · Jun 6, 2008

Pantera28 said:
Your problem is easy to answer. You went for a bravo to an XR drive. I'm pretty sure I read a thread about losing speed with an XR.

May be I should trade it in for a B1.

:laugher: :laugher:

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