See full version: 1985 Evinrude 90 HP Compression readings
How do you ID a 'commercial motor'? I have a V6 which I was told was a commercial motor with only 100 psi compression across the board. It was given to me on a boat. I haven't used it as of yet simply since I felt that the 140 V4 was a better choice as far as it's hp to weight ratio. The added weight didn't justify the additional 10 hp on a smaller boat.
Re: 1985 Evinrude 90 HP Compression readings? more
What should I expect to see on these? do some V4's have more compression than others? It seems that the lower hp motors all seam to have less compression. One local dealer said that the 85 through 115 motors always read lower than the 135/140 motors. For comparison sakes, I checked a buddies motors, a pair of 1984 115s and got between 121 and 125 on all cylinders, both of those motors have two fresh power heads last fall.
All those look good to me, even the numbers on the 85 115 are aceptable.
All those look good to me, even the numbers on the 85 115 are aceptable. more
On most Johnson/Evinrudes you want compression numbers that are within 10% of each other and preferably over 100lbs. Some smaller rope start motors will only give readings of around 80psi, but as long as the reading are even across the cylinders they run well. I hope this answers your question. [links]
Re: 1985 Evinrude 90 HP Compression readings? [links]
All the numbers are pretty normal for each type/year OMC engine you have checked. They all are pretty healthy blocks, with solid readings. The 85 and 115 are exactly the same short block, with same crank, pistons and rods. These typically read in the 125 range-brand new. Your numbers look just fine for the two. The older (high performance) 125/135 and early 140 blocks all had the same pistons, rods, cranks, but had higher compression heads and higher port timing. Numbers in the 135-145 range are normal for these engines. The piston and rod was redesigned in the 1973 timeframe, but as were later pistons/rods were used commonly across all V4 engines. OMC changed hp, by using different port timing, changing heads, special tuned exhaust, intake blocks, and by varying the carb throat sizes. These 2 strokes make high hp numbers with small displacements (99.6 cubic inches) because they fire every rpm. Actually, the larger OMC V6 crossflow engines actually run considerably lower compression than these V4 engines. Commercial engines have lower timing, and lower compression so that they can run on low-octane fuels in foreign countries, may have different lower unit ratios. These generally don't make good recreational engines. [links]
Re: 1985 Evinrude 90 HP Compression readings?
All those look good to me, even the numbers on the 85 115 are aceptable.
25hp Evinrude = great compression would be > 125lb, (but run and idle ok at 100psi) [links]
6hp Evinrude = great compression would be >85 to 90psi [links]
You can not compare compression ratios from 25hp motor to that of 6hp motor. [links]
So when somebody says 125# is not that good, I feel the same way.
Post by cajuncook1 » 07 Jul 2011, 17:21
Put really simply: an internal combustion engine, such as the one in your car, works entirely on the principle of compression. Fuel injectors mix gasoline and air and spray this combination into a combustion chamber, where it is ignited, creating combustion which compresses in the cylinders of the engine to power the car and make it move. It’s easy to see how low compression could create a problem for a motor vehicle.
For another example, the valve spring may get broken. The valve spring allows the intake or exhaust valve to close again after the camshaft opens them. If one breaks, the valve cannot fully close and the compression leaks out. more
If you don’t have the time or budget for a physical (hard part) repair, your next step should be adding an engine compression restoration treatment product such as Rislone Compression Repair with RingSeal to your engine. While a product like this does not guarantee you will never have a compression issue (some compression issues are too severe and a hardware fix is the only repair option), it can make it much less likely by unsticking rings and filling cylinder wall gaps which renew worn engines, rebuild compression, repair blow-by and restore power.