Can't get rpm's up?? A TUFF! one here.
 

Ok, I have two 468 Mk IV big block that make over 590 hp on the dyno & ran at 6700 rpm with no problems. Using MSD ignition on the dyno stand on both engines. In my 30SS Sonic with 26" props, I can only get 3000rpms. I am using Mercruiser HP500 ignition (complete factory) with no rev limiters on them. With over 1000 hp available, why can I not get any rpms out of it. The props cannot be too big with that hp, so what do ya think???
The only changes from the dyno were ignition & headers. I am using 2" Lightning headers.
Fuel pressure stays at 7.5# w/1/2" fuel lines & with Aeromotive pumps & filters.

What's up here??? We are mystified!!


:mad: :(

Blown or normally aspirated? What cams are you using? If they peak at 6700 rpm, they probably don't have much bottom end and you would need smaller props to get the rpm's up.
Post the engine specs and dyno numbers.

bl

Well, all I can say is the cam is a "secret" gring designed by my mechanic & ground by Crower Cams. Roller cam & hydraulic rollers. He has the same cams in his 29 Fountain & runs 30" props on his. He has no trouble running over 90 mph & comes on plane like a rocket! He does have standoff boxes though,
468 ci
roller cam
roller/hydraulic
Brodix heads,
single plane intakes
950 cfm (new carbs)

594 hp- I will have to find the dyno sheets again.... been awhile.
Bravo 1's 1.50 ratio
Bravo 1 26" props.......

I am running the same props on my 312 Formula with 454 Mag EFI's & it has no trouble pulling 4800-5000.... that with 385hp x2.... ???

Have you double checked the carb lingage to make sure the secondaries are opening all the way??????

Also ck the timing and make sure its at 32 to 34 degrees at total timing. As stated, ck linkage, ign. coil and pick up in distributor.

Keep us up dated.

john

Can't help with the problem your having but your 312 Formula spins 30 pitch props becasue it has a 1.65 ratio drives. I had one and everyone of those I've ever seen/heard of had 1.65's.

Ok, from the top...

The ml as1.5 drives, the ch ppsar on a 29 Fountain not the Formula.

The timing is set to 32 degrees. The carb linkage is opening all the way also. The plugs come out wet & black after running & idling. So it appears I am getting too much fuel BUT it is set as it was on the dyno... where it idled under load very well & plugs were dry.

Remember.... this is TWINS..... and both do the same thing. It's got us bewildered!!

and the fuel pressure is a constant 7.5#

My motors were built and dyno'd. Engine builder said I was good to go and no need to check timing since using MSD while dyno'd.
WRONG!!!!!!!!!!!!!!!!
He used a "7" series box from MSD when on the dyno and I am using a "6" series marine box. Called MSD and they said the boxs act differently using the same distributor. Timing was was off!

My user name is Blown Formula because my 312 Fastech Formula & my prior ride a 252 Formula were & are blown by Prochargers.

My Sonic about which we are speaking is normally aspirated, single 950cfm single plane intake. The timing has been checked more than once, the engine was "degreed" & indexed on the dampener so I know the marks are correct.
I am using late model Mercruiser HP500 ignition distributors & modules with the rev limiters removed. The engines pull nicely up to 3000 rpm (47-49mph on gps) & then no more!!
Frustrating to say the least!

check float level on the carbs while in the water??? may be right on the dyno but the boat does set different in the water....

Float levels were reset in the water. No flooding from the jets.

Seems like everyone lost interest here! BUT, I found the answer finally.
Nothing was wrong with the engines, timing, floats, etc.....
changed props to a 23" from a 26" and WOW!! 80MPH @ 5100rpm!! These will sping to 6000 real easy so I don't know what the top is going to be. Seems my lower unit gear ratio must be different from a 1.50 which is normal.


Any math whiz out there to figure out the lower unit gear ratio??
1.50 upper , unknown lower unit+ 5100rpm@80mph (GPS)- 23" pitch prop???????

Check out this link:
http://www.go-fast.com/Prop_Slip_Calculator.htm

It seems like you have 1.36 gears, but your slip is only 3% at 5100rpm with a 23" prop. Typical slip would be in the 10% range, your tach might be off.

bl

That is about what I figured the ratio was.... but the uppers are SUPPOSED to be 1.50's!! Could the lower ratio be different than the "standard" 1.50 for lowers?? That could explain the low slip figure. 3% is next to impossible, but then @ 80 mph I am watching more things than the tach! Maybe I was 100 or 200 rpm off... it is a dual tach with marks every 500 rpm, not 100's. Still, I feel go about the 80mph figure to start with another possible 800 safe rpm to go.
Here is the boat- soon to be called " WILDFIRE".
The last picture is as purchased from "Too Old" aka Trick today.

you could put it in gear, rotate the crank , and confirm your gear ratio by counting or measuring the prop rotation.


maybe rotate 5 or ten revs on the engine to get accurate prop rotation, maybe use starter with coil wire pulled?

then you would know

my drives are 1.36 with speed at 72 at 5000 rpm
true 24 pitch (merc racing labbed props)

Ramb

Blown Formula
do you mind sending me your painters contact info, I'm wanting to paint mine this winter-- starting to check around now.
thanks in advance!

I think your ratio info is so close I do not need to check my ratio. We match almost perfectly......

My painter was in Tulsa, Okla....... "STREET DREAMS". They did pretty good work but don't normally do larger things like a boat, usually custom cars..... It took them 18 months!! I had to stay on their asses all the time to work on it.

The Dash grafics & pinstriping were done by AWESOME ARTWORKS Collin Cauley 918-342-3703 he does airbrush graphics, cad graphics, etc. Good guy & works fast too.
He can probably recommend a good painter too you or he may do it himself, who know?? Painters can be a strange lot.....