Logo
FULL SITE MENU

3.7lx 470 best running rpm

F

fishy1

New member
Hi guys, I have a 80' searay sd/fb 29' with twin 3.7lx engines. All improvements have been done, including the electronic ignitions. I am wondering, the valve cover says max rpm 4300-4600 . What is the optimal running rpm? The speed I would like to maintain puts me at a constant 3800-3900 and I run for an hour to an hour and half depending what Island we want to go to. Am I running them hard or is that exceptable for long life? Port was repowered with a reman. and starboard was rebuilt 20 hours ago. So I am past my break-in and want to know best way to run for max life. This has been a 7 month project that is finally ready to bear fruit. And yes, I have read almost every post on the 470. And I have made every improvement necessary except changing head to aluminum. I dont feel that is necessary if I maintain properly.
 
Generally engine life improves the slower (within reason) you run it. Good engine life @ 80% of your wide open throttle RPM with "normal" load, assuming your max RPM is within the range specified on your flame arrestor.
Life improves at 75%, and more so @ 70%. I've no info for slower than that but most boats don't perform satisfactorily much below 70%.
 
Got a good friend whose's not a 'motorhead'. He runs his OBs at full throttle from the dock to where he drops the hook, and back. How his engines are still running beats the hell out of me!

"You gotta breath the motors now and then," I told him, but...

Running at 3,200 rpms is very common with the usual inboard V-8, so you're a might high. I suggest you install a FloScan device (it will soon pay for itself) to determine which speed gives your best MPG. Then run at that speed.

On my old, over-powered 32 footer (that was deliberately over propped) it was 2,500 rpms.

Jeff
 
Thanks guys,

Jeff These are 4cyl if that matters. I have been running it at 2900-3100rpm. But at the 3800 mark I get literally double the speed. At my current speeds it takes me 2-3 hours to get to fishing locations. It would be great to cut that in half. But not if I will burn up the engines too fast. I would really like to get a minimum of 500 hours outa them. So at 75-80% WOT I should be running 3100-3200 max for longer life. Does that sound about right?
 
There is NO magic speed or rpms for lengthening a motors life other than less than wide open throttle.

Most opinions are just that..........opinions..........


what is true is harder use will typically shorten a motors life span. What that life span is, is anyone guess.


If running them at 3800 rpms gives you what you want then run them at 3800 rpms.........

The difference between 3100 and 3800 will not shorten the life span of your motors much if anything.

Ask yourself this..........why is it that in the auto world a car/truck which has been driven mostly highway miles vs a car/truck which has been driven in city traffic such as stop and go and the highway vehicle last so much longer? My opinion is run it the way you want to just keep it below 4000 and life will be good and you will get to where you want when you want.

Besides........Most new parts for those motors are no longer available..........The best thing you could do it a repower with dual V6's or V8's......

No matter what those motors will fail it is only a matter of time............it is inherent in the design........
 
If your speed truly ( as verified by GPS or speedo) doubles between 3100 and 3800 then you have something else going on. Is the boat on a plane with bow "level" @ 3100 ??? what is attitude of hull @ 3100 vs 3800???
 
Well, I am talking on the ocean with a constant swell and wind swell 2 foot or more. at 3100 rpm it will not getup on plane unless it is flat calm conditions. And thats happened zero times in all our trips out. runs at 10mph average on a choppy sea at 3100. On a lake it would take less to get on and maintain speed. Its pushing 8k in weight, so it takes some speed to get on plane. And once on plane it needs to hold at 3600 to stay on plane in a choppy sea, or it drops back down. Of course this would be different in a lake, but this will never leave the ocean. Its not a speed boat, its top speed is 25 for the size and weight. And at WOT I can reach 25 no problem.
 
Last edited:
Yikes! (That reminds me of my buddy's old 27 foot Sea Ray; its 454 needed almost full throttle to stay on plane.)

Your boat is seriously under-powered, period. The best thing you can do for it is to buy a four bladed prop with a lot less pitch. That will get you up on plane a bit easier and, in the long run, save you gas.

Jeff
 
fastjeff said:
Yikes! (That reminds me of my buddy's old 27 foot Sea Ray; its 454 needed almost full throttle to stay on plane.)

Your boat is seriously under-powered, period. The best thing you can do for it is to buy a four bladed prop with a lot less pitch. That will get you up on plane a bit easier and, in the long run, save you gas.

Jeff
Click to expand...

Now thats what I want to hear, thats something that can be easily done. I will definitely give this a go.

Thanks Jeff!
 
fastjeff said:
Yikes! (That reminds me of my buddy's old 27 foot Sea Ray; its 454 needed almost full throttle to stay on plane.)

Your boat is seriously under-powered, period. The best thing you can do for it is to buy a four bladed prop with a lot less pitch. That will get you up on plane a bit easier and, in the long run, save you gas.

Jeff
Click to expand...

So I got the new props on. And I am able to get up on plane and the speed I want at 3400 rpms. So It dropped by 400 rpms.
Thanks for the advice Jeff.
But now I have a new problem, at 3600 rpm it bogs down. And I either have to drop the throttle or if I increase to 3800 rpms it runs fine. So, basically it bogs down between 3600-3700 and runs fine below or above that speed. I have new plugs, cables, timing seams ok, upgraded to electronic ignition, carb was rebuilt a month ago. Fuel pump and filter replaced. Twin engines, only happening on one engine so its not in the fuel tank or lines. I also replaced the distributor at the start of the season as it had a lot of play in it.
 
fastjeff said:
What kind of carbs are these? It almost sounds like the secondaries aren't opening properly.

Jeff
Click to expand...


These have the rochester 4bbl/w electronic choke.
The carb is a reman. that was just installed recently. I still have the old one, I was planning on using it to practice rebuilding since the kits are dirt cheap.
Not a mechanic, but I figured it was something to do when it opens up the 4bbl. So what your saying would be the same thing just with technical terms correct? Is it an easy fix/adjustment. Or will it involve rebuilding? I believe it should be under warranty if that would be the case.
 
Ah-hah! Sounds like those big barrels suddenly opening is causing the bog/ surge. Seeing you have two of them, see if the mechanism to open the secondaries are operating the same--or just live with it. That's what I would do.

Jeff
 
Jeff may be right here.........the secondary large top butterfly is controlled by a spring and is adjusted to control when it opens.

It is normally done with a gram scale when rebuilding...........this is a tricky adjustment when winging it so be careful...It may be your issue or may not, but if the secondary butterfly is opening too soon or too late and that is the issue then this is where and how to adjust

see pic below
 
That solved the carb problem. It was the secondaries opening too soon.
With the new props, my WOT rpm is 4500 well in the 4400-4800 range stated on valve cover. And to boot my max speed is now 34mph at 4500rpm up from 27mph at 4400rpm

at 3600 rpm I am getting 23mph thats up from 17mph at 3800rpm old props.

We managed to squeeze in 90 engine hours this season. With over 240 fishing hours and a record number of yellowtail, I couldnt be any happier!

So, Thanks for all the help guys!
 
Back
Top