'86 Mercruiser 3.7 / 470 ---> max rpm range?

[Happypappy50]

1986 Chris Craft SeaHawk w/3.7 {470} I-4 over Alpha One Drive.

What is the max rpm range? I generally run 3,800-3,900 rpm's. Has more throttle but don't feel comfortable in turning higher rpm's. At above rpm's, seems to be smooth at about 25-mph. -----> considering a little more prop, not sure whats on it now. Suggestions: max rpm's, more prop?

And yes ..... no trim tabs yet but definately considering tabs {mainly to correct list}.

 

[Don S]

Re: '86 Mercruiser 3.7 / 470 ---> max rpm range?

You should be seeing somewhere in the 42 to 4600 rpm range with a 2bbl carbed motor.
Also, here is a list of possible causes of low WOT.

Engine Won't Reach Operating RPM. Check

• Fuel condition. Type and Octane possibly old fuel
• Propeller pitch or diameter, damaged blades
• Restricted fuel pickup tube or anti siphon valve Fuel System Test
• Crankcase oil volume, high oil level can cause aerated oil and lifter collapse
• Marine growth on hull and outdrive
• Wrong gear ratio in outdrive
• Restricted carburetor air intake (clogged flame arrestor)
• Restricted exhaust system (broken exhaust shutters/flappers) in engine transom shield or drive
• Poor cylinder compression Compression Test
• Carburetor defective, or wrong type.
• Fuel pump pressure and vacuum
• Boat overloaded, improperly loaded, or improperly trimmed.
• Engine Overheating
• Engine timing and ignition system operation
• Remote control cables and linkage for proper travel to open throttle plates fully.

 

[Haut Medoc]

Re: '86 Mercruiser 3.7 / 470 ---> max rpm range?

Welcome to iboats....
4200-4600 rpms......
What size prop are you running?....​

 

[Happypappy50]

Re: '86 Mercruiser 3.7 / 470 ---> max rpm range?

Thanks for the rpm info. Current engine/prop has no problem going about the 3,900 I turn .... it will tack out in the mid 4's -----> just wasn't sure if that was too high.

{guess I'm too comfortable with larger slower running diesels in land-based toys}

Do you feel 1 or 2 more on the prop will load engine too much. As I said ... it'll turn mid 4's if I give it the juice!

 

[Don S]

Re: '86 Mercruiser 3.7 / 470 ---> max rpm range?

You have to run at WOT (Wide Open Throttle) at best trim and normal load with a good running engine and have the drive trimmed properly. If you don't trim the drive up, you will be plowing the bow and never get the rpms or speed you should.

 

[Bondo]

Re: '86 Mercruiser 3.7 / 470 ---> max rpm range?

As I said ... it'll turn mid 4's if I give it the juice!

Ayuh,.......

Mid 4s is Much to Vague,.... You need to know Exactly the rpms the motor is capable of at WOT, before you can adjust the Prop Pitch.....
If you increase the Pitch,+ the motor can Not reach it's rated rpms at WOT,.....
You'll Smoke the motor, whether you run it at wot, or slower.....

 

[bushar]

Re: '86 Mercruiser 3.7 / 470 ---> max rpm range?

I have a merc 3.0 and I WOT @ around 5200 rpms @ 36 mph. Is that too much RPM for not enough speed? It's on a Bayliner 1952.

 

[Don S]

Re: '86 Mercruiser 3.7 / 470 ---> max rpm range?

Start a new thread with your question, don't hijack someone elses post with a completely different engine.

 

[bushar]

Re: '86 Mercruiser 3.7 / 470 ---> max rpm range?

Start a new thread with your question, don't hijack someone elses post with a completely different engine.

Sorry. I didn't realize we were ONLY on 3.7's here. I thought it was a general "what's your rpm range" question. My bad.

And thanks for reaming my ***** on my very first post on this board.

 

[Bondo]

Re: '86 Mercruiser 3.7 / 470 ---> max rpm range?

Sorry. I didn't realize we were ONLY on 3.7's here. I thought it was a general "what's your rpm range" question. My bad.

And thanks for reaming my ***** on my very first post on this board.

Ayuh,...... Welcome Aboard Bushar,......

It's just normal forum edicate to start a New thread when asking a question about Your boat, rather than bringing up an old thread from somebodyelse....

Don spends Many hours a day here posting answers,+ We sometimes tend to give short answers that sometimes sounds harsh,....
We don't alwats look to see how many posts a poster has,....
Being a Newbie,.... You're forgiven........

In answer to your Question,........
You should be seeing 4400/ 4800rpms,..... Put a taller pitched prop on it to bring it down.....

 

[Haut Medoc]

Re: '86 Mercruiser 3.7 / 470 ---> max rpm range?

Hmmmm......
What could you ream that has five asterisks?......
I guess "Deliverance" comes to mind!......​

 

[bishop1976]

Re: '86 Mercruiser 3.7 / 470 ---> max rpm range?

Welcome aboard Bushar. Not only is Don full of good information but he is also a little abrasive. Stick around and enjoy the forums.....

Thank god this wasn't your first post and you did have a 470....you would have really gotten your ***** reamed...lol.