Logo
FULL SITE MENU

Compression

R

redsnapper386

Regular Contributor
"on a 1986 350 chevy,what numb

"on a 1986 350 chevy,what numbers are considered low,when testing.or when would a rebuild be recomended.thanks."
 
"Larry...

There may not be

"Larry...

There may not be an actual "normal" number of compression for the motor. Alot of factors come into play when looking at compression on an engine. Year, combustion chamber cc's, has it been rebuilt and bored, has the deck been machined, have the heads been straightened......

A rule of thumb is that there should be no more than 10% difference between the highest and lowest numbers of compression between all the cyls. Numbers should normally be around 110 to 130.

Wrench"
 
you want to do a leak down tes

you want to do a leak down test on any 4 sroke
a comp test is really a fly on the wall for 4 strokes
 
"Compression test is a good fi

"Compression test is a good first step to checking the overall health for a 4 stroke. If you get a low reading in a cylinder, go on to a leakdown test.
110 is to low!
180 is to high!
140-150 for a good running used engine
Chris"
 
180 is not to high for a '

180 is not to high for a '86 engine.

That year they still sold leaded fuel.

The important issue is as stated the % difference. no more than 10% between all cyclinders.
 
"[b]"That year they still

""That year they still sold leaded fuel."

Ayuh,... You're Off by about 10 Years....

Leaded Gasoline disappeared in the early-mid 70s...."
 
Wrong

I moved to Kansas Cit

Wrong

I moved to Kansas City MO in december of 1989

regular gas was $.75 per gallon if I remember (price only). I used to drive my '72 Nova just to burn fuel due the price and the fact that New England the price was over a dollar.
Also if I remember correctly the '87 was one of the first year of the hardened exhaust vavle seats and improved valve materials to handle unleaded fuelalong with the one piece rear main seal.. I could be off a few years on this but I know leaded fuel was still available into the late '80's.
 
Merc / Chevy engines are sold

Merc / Chevy engines are sold all over the world and here in Australia we didn't switch to unleaded gas until 1990/1. I worked in the oil industry fo 40 years and we were building a new reformer in 1989 to handle the switch to unleaded gas.
Chris
 
"Ayuh,.. I guess you're Ri

"Ayuh,.. I guess you're Right....

You can Still buy leaded gasoline today,... I just don't know where from...

I had to google it up to see,....
This link says,...
"However, with the discovery of the environmental and health damage caused by the lead, and the incompatibility of lead with catalytic converters found on virtually all newly sold US automobiles since 1975, this practice began to wane (encouraged by many governments introducing differential tax rates) in the 1980s. Most countries are phasing out leaded fuel; different additives have replaced the lead compounds. The most popular additives include aromatic hydrocarbons, ethers and alcohol (usually ethanol or methanol). In the US, where lead had been blended with gasoline (primarily to boost octane levels) since the early 1920s, standards to phase out leaded gasoline were first implemented in 1973 - due in great part to studies conducted by Philip J. Landrigan. In 1995, leaded fuel accounted for only 0.6% of total gasoline sales"

My Apologies....
happy.gif
"
 
"Hey Kghost, you need to put d

"Hey Kghost, you need to put down the EggNog.
An '86 350 is a Low Compression engine compared to the V8's of the late 60's, early 70's.
You are right about the leaded gas being available at the time , but that was for those that still had the older,high compression motors and needed the high octane, leaded fuel. By 1975, just about all of Detroit engines were bogged down with low compression, emissions, retarded timing, highway gears etc. It was a sad time for performance.
180 is TOO high a reading for an 86, mercruiser.
The book says so.
And so does my rebuilt 260.
Chris"
 
"lets see what larry gets befo

"lets see what larry gets before we agree to disagree.............

marine and auto are not the same. There were very few if any 260 -280 hp motors in autos in the mid '80's. Only vets and camaros and such and they came with specific octane requirements due to compression needed to aquire the desired hp output.

I said 160 - 180 and I do not really care what your book says, I know what readings I used to get from 1980 - 1995 engines when I would do compression checks.

Just my opinion. I could be wrong though and yes the eggnog was dam good..............LOL

in the days of high compression say 10 to 1 and such you were probably looking at 190-220 psi

Also octane ratings and specs were somewhat specific for years especially factory performance motors."
 
"When I rebuilt my 87 Mercruis

"When I rebuilt my 87 Mercruiser 260, I had cracked heads. I cross referenced the head #'s and it matched a head used on a late 80's Camaro/IROC, Corvette 350. I got the heads from a junk yard and they are an exact match.The engine has dished pistons with closed chamber heads. I figured the compression is close to 9 to 1. That would match the required 89 octane gas.
So, I would say Mercruiser DID use the best that GM had to offer.

Kghost, I was going to wager a bottle of EggNog on his numbers, but I don't think we are going hear back from Larry!
Chris"
 
"hey guys,i really do thank al

"hey guys,i really do thank all of you for your input.when i do the test i will post it.happy new year to all.larry."
 
"To be accurate, the compressi

"To be accurate, the compression test should be done at wide open throttle and with a fully charged battery"
 
Back
Top