Am I too concerned about compression numbers?

[roger3645]

I am currently looking for a 115 V4 Evinrude or Johnson. I am currently running a 1989 90hp Evinrude, with 120 psi on all four cylinders. The motor runs very good. However; the boat is rated for a 115 and to be honest, I want to go faster. I am using 120 psi as an indicator of how good the motors are. I ‘ve read on the post on Iboats regarding the 10% rule and compression needs to over 100 psi. So, I am not looking at motors with less than 120psi. I’ve found a 1990 Evinrude 115 hp, with compression between 110 and 115 psi, for $1000.

Am I putting too much value on engine compression?

Thanks

16ft Sprint bass boat

 

[Bosunsmate]

put a different prop on and you will go faster

 

[keith2k455]

Most of the time it is said over 100 psi, I have seen lower numbers run fine. At any rate, 110 to 115 should be fine.

 

[Bosunsmate]

put a different prop on and you will go faster

 

[bonz_d]

The 90 and the 115 are basically the same engine and will turn the same RPM at WOT so the only way you will see more speed at top end is if you then can increase the prop pitch a couple of sizes. Even then it will be a marginal increase. Where you will gain the most is in the hole shot. So is it worth a grand to gain a few MPH?

 

[emdsapmgr]

I am surprised that you get 120 on the compression reading. That engine has low compression bathtub heads. I'd think it would be more in the 105-110 range. Engines do vary compression, usually related to model year. The very early crossflows (mid 70's) ran 125-135+, these later ones run these lower compression numbers. That's normal. Either engine has "good" compression. (are they both measured with the same compression gauge?-these tend to vary greatly) Compression is actually a huge factor for a 2-stroke. Checking to see that it is solid is one of the most critical purchase decision factors. I wouldn't take a seller's word for anything, bring your gauge and test it yourself. A 90 and 115 will on identical boats will both have comparable holeshot. It's when you get on plane towards the mid-high rpm's, the 115 will start to pull away from the 90 and eventually leave it in the dust. It's possible the 115 will run the next size larger prop, which could/should lead to more speed. The 115 is ported better and has larger carbs. Both engines are rated to run 4500-5500 rpm's. So prop your engine to run right at 5500 rpm's. You might be looking at a 4 mph speed difference.

 

[roger3645]

First, I did replace the prop. I replaced the 13X19 Aluminum prop, with a 13X19 SST Ballistic prop and gained 5 mph. I am currently running 5500 rpm at WOT with a GPS of 49 mph. I don't think I can ask for more from a 90 hp.


Second, when I do find and buy a 115, I will have the 90hp to sell.

" I am surprised that you get 120 on the compression reading. That engine has low compression bathtub heads. I'd think it would be more in the 105-110 range. Engines do vary compression, usually related to model year. The very early crossflows (mid 70's) ran 125-135+, these later ones run these lower compression numbers"

MY numbers according to my gauage 120 118 120 120

How is one to find the particular compressions for the associated model years?

I am hoping a 115 will put me at 55 mph, with a 21p prop


Oh well that 1990 115 has been sold, Will keep looking thou

 

[roger3645]

First, I did replace the prop. I replaced the 13X19 Aluminum prop, with a 13X19 SST Ballistic prop and gained 5 mph. I am currently running 5500 rpm at WOT with a GPS of 49 mph. I don't think I can ask for more from a 90 hp.


Second, when I do find and buy a 115, I will have the 90hp to sell.

" I am surprised that you get 120 on the compression reading. That engine has low compression bathtub heads. I'd think it would be more in the 105-110 range. Engines do vary compression, usually related to model year. The very early crossflows (mid 70's) ran 125-135+, these later ones run these lower compression numbers"

MY numbers according to my gauage 120 118 120 120

How is one to find the particular compressions for the associated model years?

I am hoping a 115 will put me at 55 mph, with a 21p prop


Oh well that 1990 115 has been sold, Will keep looking thou


Thanks again

 

[emdsapmgr]

OMC and most outboard manufacturers do not post compression numbers for 2-stroke engines. Their only mention is that the cylinder compression should be within 10% of each of the other cylinders. They have used different heads on similar blocks to make higher hp in the same model year. It would likely just become a point of confusion for the consumer.