7.4 -Bravo 3 Problem RPM-speed

[erick11s]

Hi, I have a cobalt 1994 model 272, with 454 carburetor quadrajet 4 barrel stock, small manifold edelbrock, and pitch 22 dual propeller bravo 3.
I put a new crane cam mod. 133802, with range 2000 to 5600 RPM to have more performance ( the old came is to max range 4400 rpm),BUT the new cam give me very low RPM at top speed only 3000 rpm, and slow speed, I do not what happend ?...Question: I have to change to a propeller low pitch, or a big carburetor like holly 750? ......or the problem is another cause ?, please give me your opinion.
Eric

 

[mkast]

Re: 7.4 -Bravo 3 Problem RPM-speed

please give me your opinion.
Eric

You might try rewriting this post, hard to read/understand.

 

[Bondo]

Re: 7.4 -Bravo 3 Problem RPM-speed

with range 2000 to 5600 RPM to take more performance

Ayuh,... My guess is, the cam is toooo Big...
It can't make good power, down low, to get up to the higher rpms...

You could try a smaller prop, but I believe it'll just continue yer downward spiral...

 

[TilliamWe]

Re: 7.4 -Bravo 3 Problem RPM-speed

Only lower props than what you have are 20 pitch. Ain't gonna solve your problem. You either put in a bad cam, or did something wrong when you put it in. No, you don't "need" more carburetor.

 

[TilliamWe]

Re: 7.4 -Bravo 3 Problem RPM-speed

Eric, are you a drag or cirlce track racer by any chance? If you are, forget almost everything you know about building power in those applications. They don't tranlate well to building power in boat engines.

 

[erick11s]

Re: 7.4 -Bravo 3 Problem RPM-speed

Dear Friends, Thanks for you opinion, see all the history of this case:

I wrote a email to Crane cams:
Hi, I have a Boat with old engine, but in good conditions, all stock, with small Edelbrock Manifold, and Carburetor 4 Barrel Quadrajet OEM.

I open the engine and have to change the CAM, ( hydraulic), but I do not want the original, I prefer from your Brand, and one model in a small up grade performance from the original.

This Boat is for family, but again I want to feel small more performance.

Please see the Factory specifications of this boat:
# engine 4100144472
454, Gasoline
Boat BRAND Cobalt 1993, open 27 pies ( see attachment pictures, for better idea)

ENGINE GM 454 ( 7.4) YEAR MAKED 1992-1993
Spec: 868498
V-8 4 stroke compression de 8:1
7.44 L. de 330 hp.
MAX 4400 rpm.
USA/VPA
TRANSMISION Bravo 3

CRANE CAMS REPLY TO ME:
Eric,
I have attached a spec card for #133802 cam and lifter kit. This camshaft is the next step up in performance for your application.These marine camshafts from Crane provide the power you want for your performance boat. Designed specifically for marine applications, these cams produce a good idle and good horsepower.
Manufacturer's Part Number 132561
Part Type Camshafts
Product Line Crane Marine Camshafts
Summit Racing Part Number CRN-132561
UPC 021174039900
133802
H-278-2 (REPLACES HMV-278-2A)
1967-95 CHEVROLET V8 396 THRU 454 CU.IN.
GOOD IDLE, DAILY PERFORMANCE USAGE
BRACKET RACING, 3200-3800 RPM
Cam Style Hydraulic flat tappet
Basic Operating RPM Range 2,800-5,800
Intake Duration at 050 inch Lift 228
Exhaust Duration at 050 inch Lift 236
Duration at 050 inch Lift 228 int./236 exh.
Advertised Intake Duration 298
Advertised Exhaust Duration 306
Advertised Duration 298 int./306 exh.
Intake Valve Lift with Factory Rocker Arm Ratio 0.530 in.
Exhaust Valve Lift with Factory Rocker Arm Ratio 0.551 in.
Valve Lift with Factory Rocker Arm Ratio 0.530 int./0.551 exh.
Lobe Separation (degrees) 114
Computer-Controlled Compatible No
Grind Number H-228/312-2S-14 T1.2
Valve Springs Required Yes



Thanks,

Greg

Regards

Ayuh,... My guess is, the cam is toooo Big...
It can't make good power, down low, to get up to the higher rpms...

You could try a smaller prop, but I believe it'll just continue yer downward spiral...

 

[erick11s]

Re: 7.4 -Bravo 3 Problem RPM-speed

Thanks I did.....sorry but my bad english, I am from Italy

 

[erick11s]

Re: 7.4 -Bravo 3 Problem RPM-speed

Eric, are you a drag or cirlce track racer by any chance? If you are, forget almost everything you know about building power in those applications. They don't tranlate well to building power in boat engines.

Thanks for your opinion, but See the problem , I put the cam that the factory tell me .

 

[erick11s]

Re: 7.4 -Bravo 3 Problem RPM-speed

Only lower props than what you have are 20 pitch. Ain't gonna solve your problem. You either put in a bad cam, or did something wrong when you put it in. No, you don't "need" more carburetor.

Thanks for your opinion, .....What do you think, I can do it bad, when i instaled the cam?

 

[Don S]

Re: 7.4 -Bravo 3 Problem RPM-speed

Do a compression test. That may tell the story as to what the problem is.

 

[TilliamWe]

Re: 7.4 -Bravo 3 Problem RPM-speed

Thanks for your opinion, .....What do you think, I can do it bad, when i instaled the cam?

No I meant that the camshafts specs were a bad match for your application. reading the specs from Crane, it sounds like it might have too much lift/duration. But I am not a Big Block Chevy expert.

I agree with Don, compression test is a good place to start.

 

[Bondo]

Re: 7.4 -Bravo 3 Problem RPM-speed

Basic Operating RPM Range 2,800-5,800

No I meant that the camshafts specs were a bad match for your application. reading the specs from Crane, it sounds like it might have too much lift/duration. But I am not a Big Block Chevy expert.

I agree with Don, compression test is a good place to start.

Ayuh,... I Agree, Completely,...

Marine motors live from Idle to 4800rpms, 5200rpms, Tops...

 

[erick11s]

Re: 7.4 -Bravo 3 Problem RPM-speed

Let me see if I understand, The cams is not the correct for my boat?---but remember the problem is : a top speed the boat can make only 3000RPM,....... wait...it is not another problem, bad adjust of the internmedy jet of the carburetor quadrajet?, or another electric problem?

 

[TilliamWe]

Re: 7.4 -Bravo 3 Problem RPM-speed

Ok erick, this may get long-winded, but bear with me.

Unlike a car that has momentum after it starts moving, with "little" resistance from the road and wind, a boat has constant and TREMENDOUS resistance from the water. Think of it as driving your Fiat 500 up the side of Mt Vesuvius with a headwind of 100kmh! It takes LOTS of torque and horsepower to overcome that.

What you've done is installed a camshaft that can't make enough power at 3000rpms to overcome all that resistance, so it won't turn the motor any faster. Sure, if you put a really low pitched prop on there, it would help. But two problems, you would need a 17 pitch prop, probably, and it cause you top speed to be really slow. Second problem, Bravo three propsets only go as low as 20 pitch. Dropping 2" in pitch is not going to magically raise your rpms by 1800.

I really think your camshaft has too much lift and too much duration. The operating range listing of 2800-5800 pretty much confirms that. You need a camshaft that lists its operating range as 1500-5500, tops.

 

[Bondo]

Re: 7.4 -Bravo 3 Problem RPM-speed

it is not another problem, bad adjust of the internmedy jet of the carburetor quadrajet?, or another electric problem?

Ayuh,... The carb Could be Junk,...
You Might have an Igntion problem...
That would be Combined with the Too Big a Cam problem though...

The carb is in No way to small,...
A good runnin' rochester will support Much bigger motors than yours....

 

[HT32BSX115]

Re: 7.4 -Bravo 3 Problem RPM-speed

Ayuh,... The carb Could be Junk,...
You Might have an Igntion problem...
That would be Combined with the Too Big a Cam problem though...

The carb is in No way to small,...
A good runnin' rochester will support Much bigger motors than yours....

I think it would be nice to know the drive ratio also. 22 pitch props are pretty low for that application. I going to assume that you have a 2.00 or 2.20:1 ratio drive.........unless someone put a 1.81:1 drive on it. (which might explain the 22p props)


If that engine didn't have ANY mechanical problems with it (other than possibly the WRONG cam) I would say that the timing is WAY RETARDED!

Retarded timing will ABSOLUTELY limit the MAX RPM. (as would retarded CAM timing)


Regards,


Rick

 

[mkast]

Re: 7.4 -Bravo 3 Problem RPM-speed

Was the cam degreed during installation?
Try a vacuum gauge at normal operating temperature, idle rpm, a low reading will tell you if the cam needs to be advanced.

 

[erick11s]

Re: 7.4 -Bravo 3 Problem RPM-speed

Hello my friends, I'm back, after finishing the home work you gave me:
1 ?I Tested the Compression and all 8 cylinders, and each is 140 PSI.
2 ?I advanced the distributor, and finally the engine took 4800 rpm (the cam is for 5600RPM) top speed to 45 miles, with the propellers 22 ". But I do not agree to go a 20 propellers, it would be absurd. So, what are you Think?

Now, the engine have new small problems, in a medium speed acceleration is unstable, I replaced the spark plugs, wires spark, and put a new coil from MSD IGNITION 45.000 VOLTS, but it does not change..... .
I think the Quadra jet carburetor (jet 94), is very old, we changed all the kit system and made a service from a professional, to add, the distributor is not Marine, it is a GM for car and does NOT comes with a vacuum, the actual fuel pump electric max is 30 psi, but it has not a gas regulator installed, I do not if this is really necessary?

My idea is to change: Carburetor, distributor, wires spark of 8mm or more, and buy a gas regulator............ But before, I want your opinion again, about what is the best purchase Set, for my case, remember I installed the crane cams cam, with small performance and I have an Edelbrock manifold, small from the old generation model C454.

That is very important for me, because after this STEP, IF THIS DOES NOT WORK Well, I will SINK this boat, and the problem will be over... JAJAJA:facepalm:

Thanks in advance ...for your comments.