Fuel/compression

[Mark75H]

about 7k

[FMP]

about 7k

And what is this monster of 275psi , what exhaust configuration?

[VkingMike]

What kinda fuel or real octane booster , or mix of fuels do I need to run 170lbs compression? This will b a 1st for me and don't wanna burn her up

Check out Boostane marine. It is not your auto parts store octane gimmick, its the real deal octane additive. I'm a performance calibrator for a living and we use it to take 93 to over 100 on a weekly basis on the dyno.

[Instigator]

Apples to oranges but,

When we lived in Fla I ran Av gas in all my hi comp motors.

In the areas popular w/the airboats, you can get it at local gas stations.

If not, local airports.

I was running OMCs though that are much larger bore than a 2.4/2.5 Merc so our junk won’t put up w/what your Mercs will.

At 170 lbs I was mixing 100 oct av w/93 pump w/no problem.
Lightly loaded.
As in, drag set up.

W/no other changes other than adding:
A passenger
20 gls fuel
A 30” lake wheel

It ran a 1/2 mile before detonating itself to pc’s!

On the smaller Merc bores, I’m sure it would have been fine.

Just remember to figure in loading, to your equations ; )

Remember too, compression on a 2 stroke is way different than a 4 stroke.
Due to firing twice as often, the combustion chamber builds/retains so much more heat than a 4 that you hit a point of diminishing returns way quicker.

For drag you see the positives way longer.
For lake, I think that 170 is as high as I would attempt for an OMC Looper.

For a Merc you can probably add at least +10 to this.

[Merc 2.5]

My thinking is I run 145/150 as norm setup but on weekend I just wanna ride , I can pull weight out of it , swap heads to 170lb ,throw some fuel in ,and let her ride. Is it a good idea to do a jet change goin from 93 to mix of 93 and 100 or 110 just less of it. ?

[Mark75H]

No jet change needed unless you were fat just to be fat.

[TripleA]

I have been running 93-91 ethanol free fuel due to supply issues in my area mixed with ACES for years. I do run lighter weight, low pitch props but here is some better details on it.

https://www.screamandfly.com/showthr...highlight=ACES

[gmorgan]

Yesterday I ran my warmed up 2.5 225 hp mariner with 164 pounds of compression on 93 octane pump gas and 1 ounce of aces per 6 gallons of fuel. No problems whatsoever with this concoction.

[Merc 2.5]

Maybe I should start new thread but figured I'd try it here 1st

I put a holley red and regulator on my carbed 2.5 , only concern I have is long idle periods , after awhile of idle will it flood out or start leaking in places ? Mite b a dumb question, but I've always used a orileys 70$ ele holley pump. Like 3.5lbs to feed the pulse pump on motor , now that pulse pump gone , I'll have a steady 4.5 ,5lbs at all times. Thanks

[Merc 2.5]

Also. What is the small nipple for on front of the pump ? Is that a vent or.........

[merc1250bayliner]

On the inboard 3.0 volvo penta I worked on it had a regular car mechanical fuel pump but with a nipple like that. It attached to a hose going somewhere, carb I think. In the book it said it's there incase the diaphragm fails. Not 100% sure though.