What Octane Would You Use?

[sasto]

We are running a 1974 318 Chrysler inboard. What octane would you use?

Another question....The PO ran the boat consistantly at 4000 rpm which keeps her on top a little more but she should, and does, sound better at around 3800. WOT rpm is about 4700. Reconditioned prop. Is it suggested I continue running her at 4000? I wish I knew her speed at different rpm's for imput. Fuel economy is an issue but at this point I am more concerned what would be best to prolong the life of the motor.

Thanks!

 

[choochooharley]

Re: What Octane Would You Use?

I have a 1975 and I use 89 I also add lead additive once in awhile since the motor calls for leaded fuel I dont know if this is nessicary or now. I would imagine that yours calls for leaded fuel as well

 

[ChampionShip]

Re: What Octane Would You Use?

For what it's worth, on our big boat we cruise around 3200 rpm's per flowscan results. We found that bumping up to the upper 3000's ate more fuel than it was worth for the speed we were seeing. I try to find fuel locally with no ethanol, which is actually fairly easy in our region, it's usually of the 89 variety.

 

[sasto]

Re: What Octane Would You Use?

Thanks for the replys! This boat is a pig. I would consider her more of a simi-displacement hull that will get on plane... somewhat. A 1974 Penn Yan Avenger Sportfish. At 3200 she is digging a hole up to about 3600. At 3800 she is somewhat smooth and feels efficient. At 4000 she is somewhat on top and running smooth, but I feel 4000 is high. Anybody run at 4000 for an hour at a time?

 

[sethjon]

Re: What Octane Would You Use?

We are running a 1974 318 Chrysler inboard. What octane would you use?

Another question....The PO ran the boat consistantly at 4000 rpm which keeps her on top a little more but she should, and does, sound better at around 3800. WOT rpm is about 4700. Reconditioned prop. Is it suggested I continue running her at 4000? I wish I knew her speed at different rpm's for imput. Fuel economy is an issue but at this point I am more concerned what would be best to prolong the life of the motor.

Thanks!

Theres no difference from 3800 to 4K. However, both seem a little much unless you're trying to make the engine wear faster. And the n theres fuel economy. Except for having to get some where fast, 3K will save you a lot of gas.

 

[sasto]

Re: What Octane Would You Use?

Theres no difference from 3800 to 4K. However, both seem a little much unless you're trying to make the engine wear faster. And the n theres fuel economy. Except for having to get some where fast, 3K will save you a lot of gas.

Thanks, sethjon. Appreciate your imput.

This boat just digs a hole at 3000rpm and I'm going no faster than at 1500. Not in a hurry at all to get anywhere....or damage this engine, belongs to the Admiral. Is there more harm done at 4000 compaired to 3800....or compaired to 3600, which the pig pushes along and works her butt off?

 

[HT32BSX115]

Re: What Octane Would You Use?

Fuel economy is an issue but at this point I am more concerned what would be best to prolong the life of the motor.

Howdy,


Unless you don't use your boat much at all, Fuel is the most expensive thing you will put in your boat PERIOD.

@ $4/gallon, 5 gal/hr ($20/hr) and say 100hrs per year, you're going to pay $2000/year EVERY YEAR. JUST FOR FUEL

In 10 years, that's $20,000!

My last (1966 model that dad bought new) boat went 39 years and the hour meter broke in the mid 80's at 1500hrs.....I sold it in 2005.

My dad ran the lowest octane unleaded fuel available until I got it and then I ran the boat almost 9 years the same way.....


Your engine will fail from poor maintenance or a freeze-cracked block LONG before the fuel you use does anything bad to it....


Regards,


Rick


I had to edit to add a typical gal/hr for many small boats....

I'll also add that there's NO "LEAD" additive that actually contains Lead. you cannot buy it. It's a lead "substitute". (AKA Snake Oil)

IMHO. you're just pouring additional (unnecessary) money in your fuel tank.

 

[Aloysius]

Re: What Octane Would You Use?

Thanks for the replys! This boat is a pig. I would consider her more of a simi-displacement hull that will get on plane... somewhat. A 1974 Penn Yan Avenger Sportfish. At 3200 she is digging a hole up to about 3600. At 3800 she is somewhat smooth and feels efficient. At 4000 she is somewhat on top and running smooth, but I feel 4000 is high. Anybody run at 4000 for an hour at a time?

NO!

4000 rpms for a stock engine is too busy for sustained operation. The guys who run on lakes seem to do it, but that's why there's so many blown engine questions. Us offshore guys don't do it for long, and expect to get home. Last guy I knew who did it (SeaRay with big blocks) blew up his motor in the Bahamas..REALLY expensive lesson, and only 10 year old motors.

Your engine is a bone stock, single valve spring engine. Essentially a passenger car engine. Not designed for 90% duty cycle, especially if you expect 2000 hours or more life.

I'm afraid you're simply underpowered.

Fuel..the non-ethanol stuff we have here in Stuart is 90 octane. It's available in Manatee pocket, the bait shop next to the OLD West marine, even the station at the top of the hill on Jensen Beach blvd.

 

[Dakota47]

Re: What Octane Would You Use?

4000 rpm`s is to high running for long periods of time on a OLD gas engine, my 23 year old boat has a Chevy engine and i run it a 3200-3500 for crusing speed.

 

[sasto]

Re: What Octane Would You Use?

maintenance under control... running proper fuel is important.... Vessel is under powered from factory. She preforms almost to origional specs. No ocean running in this thing. 4000rpm scares me.........3800 fells good. She is an old vessel that was never meant for speed. 3600 and she digs in a hole. If I have to run 1500, so be it. I don't want to tear up a motor needlessly, but if she needs replaced with a larger more powerful 360... I'm also ok with that.

How about 3800 vs. the 4000? Much difference?

 

[choochooharley]

Re: What Octane Would You Use?

I think every engine is different as is every boat. The difference between 3800 and 4000 everything being the same with her on a plane will b minuscule in speed and fuel economy but if she sounds and feels better at 3800 you better believe that's where id keep her. The way she runs feels and sounds is up to the captain and may very from skipper. To skipper

 

[Aloysius]

Re: What Octane Would You Use?

maintenance under control... running proper fuel is important.... Vessel is under powered from factory. She preforms almost to origional specs. No ocean running in this thing. 4000rpm scares me.........3800 fells good. She is an old vessel that was never meant for speed. 3600 and she digs in a hole. If I have to run 1500, so be it. I don't want to tear up a motor needlessly, but if she needs replaced with a larger more powerful 360... I'm also ok with that.

How about 3800 vs. the 4000? Much difference?

A 360..great idea. In the meantime, run it like ya stole it. You've gotta great back-up plan. Who knows..little bugger may run like that (4000 rpms) for years.

 

[pwpns]

Re: What Octane Would You Use?

Octane rating is decided by the amount of static compression of the engine which for a 318 mopar made after 73 is 8.6-1. Using anything higher than 87 octane is just a waste of money. You might want to add a lead additive to your fuel because you probably dont have hardened valve seats ( unleaded fuel was introduced in 1975 which required the use of catalytic convertors and the birth of the hardend valve seat made from stellite)and you will need something to cool your exhaust valves. If you a pulling a constant 3800-4000 rpm for long time this will beat the seat up in the head lowering your spring pressure, your compression (leaking around your exhaust valve) and reducing your horsepower and the rpm range of the engine.