160 psi Compression / Octane

**DangerNewb** · 06-26-2018, 10:08 AM

As discussed in another thread, I recently had some heads cut for my Mariner 225 Super Mag. They were cut further than the shop and I wanted. A simple mistake was made and I'm not rewinding history. At this point, I have 31 - 32 cc heads (I haven't measured, but the shop said 31). Would make sense they may be 32 based on starting at 38 (stock promax heads) and cutting 90 thousandths (I think). I used 1.2mm gaskets rather than the 1mm promax gaskets.

Bottom line is I warmed the motor up using 100 Octane fuel and checked compression. I didn't let it run very long. Only to about 125 degrees. I'm now at 160 psi. I was trying for 145 or so originally.

I'm working on getting another set of stock heads to either leave alone or have cut to 34-35 cc range.

In the meantime, I'd like to go for a boat ride sometime soon!!! Looking for opinions on fuel needed for 160 psi. Would a 50/50 mix of 93 and 100 be sufficient? Thinking about getting some of the Aces fuel additive for future use since the local 100 octane is $9 a gallon, OUCH. Know the Aces is another can of worms with folks either thinking it's voodoo and some folks swearing by its effectiveness.

I know a 20 year old motor can burn itself up at any moment. I'd rather be safe than sorry. Any opinions on longevity with that sort of compression? Looking for folks who have run an engine for years with no issues. Or first hand experience with not being able to keep an engine alive at that compression level.

I bought the motor a couple months ago and it appeared to be low hour and completely stock other than oil injection removed and the idle stabilizer module removed. All bolts looked like a tool has never touched them beyond original assembly. The stock reeds were in perfect condition. Not even discolored. So it appears it was a lightly used engine.

At this point, I've left the timing stock. I had my injectors checked out with new filters, orings etc. Installed new reeds and a spacer plate. Also had my ecu checked out and was richened some and was told I had some sort of error or problem that was fixed. Who knows?? Anyhow, it cranks up on the hose and idles good so that's a good start.

So let's hear some experience. Can I run this motor with these heads without worry? Or should I wait on a new set of heads? Also worried that it will run so strong that I won't want to go back. Definitely sounds different than it did before. Thanks.

**bullet123** · 06-26-2018, 12:20 PM

Too many variables to give a proper answer. timing, squish, etc etc. I wouldn't run less than 100 octane at 160 psi unless timing is retarded. Kinda defeats purpose.

**FLATBTM** · 06-26-2018, 01:05 PM

Isn't 93 octane good for 145-150psi? Generally speaking, as I know there are a ton of variables.

Would like to see the answer myself. Trying to turn up the juice on a Mercury and taking all the info I can.

**90 5.0** · 06-26-2018, 02:05 PM

There are some guys that run 160+ psi(32cc heads) on pump gas. If they turn the timing down some and fatten them up a little just for riding.

Sonoco gtx (98 octane) would be plenty I would imagine 93/100 mixed 70/30 would do it.

Me, I would get another set of heads cut to 34cc instead of messing with fuel

**Steve Pope** · 06-26-2018, 05:49 PM

Put the 1 mm head gasket on. You need a tight squish clearance when you run high compression to lower chance of detonation. If the pistons are below the block deck at tdc I would even go to .75 mm gaskets. I have seen so many detonated pistons from tight heads and high squish clearances. Cheers, Steve

**FMP** · 06-26-2018, 06:10 PM

Even tighten the squish and relieve the chamber to fall back into psi range

**dwilfong** · 06-26-2018, 06:11 PM

Originally Posted by Steve Pope

Put the 1 mm head gasket on. You need a tight squish clearance when you run high compression to lower chance of detonation. If the pistons are below the block deck at tdc I would even go to .75 mm gaskets. I have seen so many detonated pistons from tight heads and high squish clearances. Cheers, Steve

.75mm gasket for a 2.5 Merc? You have a part # or where I can get a set?

**Steve Pope** · 06-26-2018, 06:33 PM

Pro marine have the .75 mm gasket on their web site. Cheers

**DangerNewb** · 06-26-2018, 07:37 PM

Originally Posted by dwilfong

.75mm gasket for a 2.5 Merc? You have a part # or where I can get a set?

http://parts.promarineusa.com/produc...M50/1/1/844R75

**STV_Keith** · 06-27-2018, 01:04 PM

I ran 91 octane (highest we get here) at 140# on my 2.5/260. Tightened the heads to 160# and ran 91 + Aces 4 at 1oz to 10 gals with no issues. My brother ran 195# drag motor on 93 (FL gas) + Aces 4 at 1oz to 5 gals.

**FORBESAUTO** · 06-27-2018, 01:43 PM

Originally Posted by STV_Keith

I ran 91 octane (highest we get here) at 140# on my 2.5/260. Tightened the heads to 160# and ran 91 + Aces 4 at 1oz to 10 gals with no issues. My brother ran 195# drag motor on 93 (FL gas) + Aces 4 at 1oz to 5 gals.

Wait a minute, that can’t be

. I thought that aces don’t do anything and is just a hoax

.

**DangerNewb** · 06-27-2018, 03:25 PM

Originally Posted by STV_Keith

I ran 91 octane (highest we get here) at 140# on my 2.5/260. Tightened the heads to 160# and ran 91 + Aces 4 at 1oz to 10 gals with no issues. My brother ran 195# drag motor on 93 (FL gas) + Aces 4 at 1oz to 5 gals.

Did you guys run these motors long term or just a couple seasons?

**STV_Keith** · 06-27-2018, 04:07 PM

I put about 75 hours on my 260 that way. My brother only ran that drag motor about 20 hours before selling the boat. If it's gonna blow up because of not enough octane/knock issues, it's going to do it right away.

**Motv18** · 06-27-2018, 09:32 PM

160 is about 10.9:1

91aki should be fine. 93aki great.

Your more likely to ping or pre ignition than knock due to unknown heat sources.

Vp the actual fuel is pricey and it might be worth just running 100ll but it’s 94-5
a bottle of chainsaw gas is about 8 but should boost a small tank up a point. But I will say mixing is tricky. Actually you oil mix itself is a booster.

**eli** · 06-28-2018, 10:39 PM

Originally Posted by STV_Keith

I ran 91 octane (highest we get here) at 140# on my 2.5/260. Tightened the heads to 160# and ran 91 + Aces 4 at 1oz to 10 gals with no issues. My brother ran 195# drag motor on 93 (FL gas) + Aces 4 at 1oz to 5 gals.

so 1 oz of aces to 6 gal 91 is overkill for 34.5 heads