mid '70's 1500 combustion chamber temperature

[monk-monk]

This has been a "burning" question in my mind...since mercury recommends no more than 21 degrees of timing advance, to guard against burning pistons...i want to know if there is anybody that has ideas about how to monitor combustion chamber temperatures in these motors....and from what i have been told, this max timing advance has been suggested because of the octane grade of fuel available today, as compared to the gas we could get in the earlier years. I want answers as to why i can't go more than 21 degrees of timing...i want suggestions on how to monitor cylinder/piston temps etc...from short term experiment, i know there is a ton of more power in these motors if it wasn't limited to the 21 degrees of advance...just to see what it would do, i set my timing to 30 degrees advance...that turned a decent running 17' bowrider into a rocket ship, it had twice the power! My friend has a 1989 V-6 150hp yamaha on a 21' deep V center console Proline fishing boat, if i put his yamaha on my boat in place of my 150hp mercury, my boat would be an airplane....which brings me to another thought, if mercury calls this a 150hp, and yamaha calls their motor a 150hp, then somebody is lying because there is no comparison...it would take 2 of these 1500 mercurys to do what that one yamaha 150 will do...anybody got any reasonable answers/suggestions to all this?

 

[wired247]

Re: mid '70's 1500 combustion chamber temperature

That 150 Merc inline you have was rated at the crank and that rating was maybe a bit optimistic in the fishing motor version. They are small displacement motors and were developed when pump gas was mid 90's or higher octane and they will not tolerate low octane fuel or too much timing for very long no matter how much you'd like them to. The Yamaha is rated at the prop and knowing Yamaha that number is a bit conservative. When Mercury started rating motors at the prop, like that Yamaha is, the 150 HP motor became the 115 with the only change being a bit lower compression. That and horsepower isnt the only factor here. The Yamaha is a 163 cubic inch motor versus the Mercury 99 Cubes. There is no replacement for displacement when it comes to torque. Yes, the 150 Yamaha will blow the 150 Inline Merc out of the water.

If you want to become an expert at rebuilding Inline Mercury motors, keep running them over 21 degrees BTDC.

 

[Faztbullet]

Re: mid '70's 1500 combustion chamber temperature

Back when we raced years ago ,we would replace the main jet with a .082 and jack the timing till we saw aluminum eyebrow on piston start to bubble then back down a 1/2 turn on timing. Also this was on a lite hull and ran chopper props and we turned motor 6800+ rpm on 1/4 mile distance. Then we got smart and we bought a T2x . As to you jacking yours up its not a good idea if your going to use it as a "everyday" motor and the timing will float a couple of degrees +/- anyway due to belt defection,bearing stiffness and wear in cables /box .

 

[Texasmark]

Re: mid '70's 1500 combustion chamber temperature

A word or two on cubes; opinionated of course.

When Merc came out with the 150V engine it was a 2 liter. That engine was a popular bass boat option. Problem with bass boats is that they have a pretty good dead rise at the transom to make their pad work (hard to plane out), use setback so the engine sits back from the transom which reduces lifting advantages (hard to plane out but fast), usually carry 3 large batteries mounted right in front of the engine with a fuel tank of some 20+ gallons and then a 20 or so gallon live well, and add to that a couple of 200+ pound fishermen......all aft of the center of the boat (hard to plane out....aka hole shot sucked).

Well that was a bit much for a 2 liter engine even though in the process they ported the props to allow them to spin up in the hole and get the engine rpms up fast to get the hp up fast.....had to do something as they were shy on cubes and torque (aka cubes) x rpm x constant of proportionality = HP.

So next thing you know in the same model years they added a 2.5 liter 150 to the mix. Got the cubes up and solved the problem.

Other case in point was back in the early '50's. I was an OMC guy back then and a palsey 25 hp Johnnyrude (with like 35 cu in displacement) could plane out and run at a respectable speed, a 16' plywood boat with a couple of 6 gallon cans of gas and a couple of guys and all their fishing gear.

So I wonder how the Yammie 90 hp engine compares to my Merc 90 in performance (on my bass boat) as there is (was last time I checked) a deficiency of cubes in the Yammie (opinion).

Mark

 

[monk-monk]

Re: mid '70's 1500 combustion chamber temperature

Well thanks for all the great insight and info....LOL, but y'all did overlook one thing i stated...I only "jacked" my timing up for a short test blast...i know not to leave it at more than 21 degrees advance...however, i am going to "sneak" some extra timing to it and keep an eye on the piston domes as i experiment with it...Im very educated when it comes to motors and i have a very good ear when it comes to listening to what the engine is going through...as long as it is not struggling, spark knocking etc, use the correct pitch prop etc..Im going to "up" the jets, might even add a "cool can" in the fuel line...i'll keep a watchful eye on piston domes, pull transfer port covers and inspect piston skirts frequently! I have the experience to rebuild these motors all day long, and im going to sneak up on the timing until i find the seet spot! BTW, im in the middle of rebuilding a 1973 Johnson 135HP V4 anybody have idea about how the johnson will run compared to merc1500

 

[out 2 launch]

Re: mid '70's 1500 combustion chamber temperature

i set my timing to 30 degrees advance...that turned a decent running 17' bowrider into a rocket ship, it had twice the power!

Twice the power, really?

 

[wired247]

Re: mid '70's 1500 combustion chamber temperature

Well thanks for all the great insight and info....LOL, but y'all did overlook one thing i stated...I only "jacked" my timing up for a short test blast...i know not to leave it at more than 21 degrees advance...however, i am going to "sneak" some extra timing to it and keep an eye on the piston domes as i experiment with it...Im very educated when it comes to motors and i have a very good ear when it comes to listening to what the engine is going through...as long as it is not struggling, spark knocking etc, use the correct pitch prop etc..Im going to "up" the jets, might even add a "cool can" in the fuel line...i'll keep a watchful eye on piston domes, pull transfer port covers and inspect piston skirts frequently! I have the experience to rebuild these motors all day long, and im going to sneak up on the timing until i find the seet spot! BTW, im in the middle of rebuilding a 1973 Johnson 135HP V4 anybody have idea about how the johnson will run compared to merc1500

Its a lot more fun burning up your own motor than listening to people who have burned up theirs. Plus you get to get rid of some of your excess cash.
I remember back in high school when my buddy Ian warned me about his ex girlfriend that I had just started going out with. He said" Man, you don't want to go out with her. She's never happy with anything. I clearly remember saying ": Ian , Its not her . Its you man. " . Ian was right and that manic depressive nut job later became my ex-wife. Thats when I leaned to listen to people who know what they are talking about when they are giving sage advice.
Mercury specified 21 degrees for a reason. It was 23 degrees when the TOP's came out with the thunderbolt ignition in '73 and even on good gas they were burning up pistons and replacing powerheads. Then Mercury put the word out to set it at 21. You can't hear the top burning off of the piston over that chainsaw TOP sound.

 

[monk-monk]

Re: mid '70's 1500 combustion chamber temperature

OK, again i really appreciate all the replies! Im not a hard-head and i listen to others advice...thats why i posted this question...advice well taken! SO, my conclusion is this: i have great respect for the old merc motors, but now i realize how limited they are! No choice but to sell all of my old merc motors..i have 2 1500's built and running, and 1 unbuilt but all parts to assemble it....hopefully i can sell all that to put the money towards finishing my Johnson V-4 135hp and look for a Yamaha 150 too! Thanks for all the input!