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Old 08-02-2018, 10:27 PM
lake weir skier lake weir skier is offline
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Compression test on 350 TBI

I bought my 94 PS 190 with 350 TBI in 2013 and have found in doing several compression tests since then that No. 7 pretty consistently tests a little lower than the other cylinders. How significant is the difference? I've seen differing guidance for how much variation between cylinders should trigger further investigation with the low cylinder being anywhere from 10 to 30 percent below the maximum cylinder being offered as a trigger. No. 7 generally falls within that range of suggested triggers for further action.

My most recent test with a nice OTC compression gauge that I've validated through parallel measurement against a Grade A Valley gauge from Northern Tool yielded the following results:

No. 1 180 psi
No. 3 185
No. 5 190
No. 7 168
No. 2 194
No. 4 200
No. 6 198
No. 8 192

This puts No. 7 16 percent below the high, which was No. 4. This was a dry test with the engine hot after running it on the lake. I tested four cylinders today at ambient temp and found similar relative results between cylinders compared to the warm/hot test with the pressures running about 5 psi below the test with the engine warm/hot.

I find the plugs look good and similar across cylinders, and I'm generally satisfied with the performance of the engine, but want to squeeze out of it everything I can.

Any thoughts?
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  #2  
Old 08-02-2018, 10:39 PM
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Double D Double D is offline
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Wow, Iíve never seen numbers that big. When I was testing my old 94 350 TBI in 2016 with 1313 hours, the max in 1 was 160 the rest averaged around 120. Since then itís been replaced.

What are normal numbers?
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Old 08-02-2018, 10:45 PM
EricB EricB is offline
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Ok, so the numbers vary. You state you are satisfied with the performance. So the question I have for you is when you punch it from an idle, do you feel a miss-fire? Do you feel a vibration until you back off the throttle?

How does it feel? What it top RPM & speed (prop dependent)?
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Old 08-03-2018, 12:26 AM
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mikeg205 mikeg205 is offline
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200? To 168.. thats quite a swing... hows she running...

Sent from my SM-G955U using Tapatalk
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Old 08-03-2018, 06:38 AM
lake weir skier lake weir skier is offline
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Boat: 1994 MasterCraft ProStar 190, 350 TBI, 1:1
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Quote:
Originally Posted by Double D View Post
Wow, I’ve never seen numbers that big. When I was testing my old 94 350 TBI in 2016 with 1313 hours, the max in 1 was 160 the rest averaged around 120. Since then it’s been replaced.

What are normal numbers?
I have every reason to believe these numbers are accurate measurements of the cylinder pressures. As to what procedure produced the cylinder pressures, I did crank until the pressure stopped rising and did not stop after a pre-determined number of revolutions.

OTC has a good reputation and the OTC and Valley gauges -- both with a range of 0-300 psi -- are tracking closely. For example, on the ambient temperature test yesterday, OTC was 161 psi when Valley was 160 psi and OTC was 193 psi when Valley was 190 psi.

The Valley is a Grade A gauge. Grade A refers to ASME standard B40.1. According to Valley, a Grade A gauge under this standard is 2-1-2. This means it's accurate within 1 percent of the gauge span (span = 300 psi on this particular gauge) over the middle half of the scale (75 psi - 225 psi on this particular gauge) and within 2 percent over the first quarter and the last quarter of the scale. So, since my cylinder pressures are all in the middle half, my readings should be within +/- 3 psi of the actual. They don't give a confidence level, but it's probably 95 percent.

My Clymer manual for inboards with GM V-8 engines 1983-2003 says "Low compression is evident if the compression pressure varies more than ten percent between cylinders or if any cylinder pressure is less than 100 psi." Mercruiser Service Bulletin 97-25 that says on each cylinder a minimum of 100 psi and at least 70 percent of pressure on the cylinder with the highest pressure.

I checked compression on another 94 PS 190 with 350 TBI when I was shopping in 2013. That was with a different compression gauge. As I recall, I got about 175-185 on the cylinders on that one. I don't recall if we did that test before or after we ran the boat on the lake.

My engine has a bit over 500 hours right now. I validated the hour indicator number on the dash in 2013 during purchase by having the dealer mechanic read the hours directly off the engine with diagnostic software. I read the hours on the screen myself.
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Last edited by lake weir skier; 08-03-2018 at 02:14 PM.
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Old 08-03-2018, 06:54 AM
lake weir skier lake weir skier is offline
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Boat: 1994 MasterCraft ProStar 190, 350 TBI, 1:1
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Quote:
Originally Posted by EricB View Post
Ok, so the numbers vary. You state you are satisfied with the performance. So the question I have for you is when you punch it from an idle, do you feel a miss-fire? Do you feel a vibration until you back off the throttle?

How does it feel? What it top RPM & speed (prop dependent)?
I don't observe any of those problems. I think it runs smoothly and strongly.

I did a long thread on here in 2015 about the RPMs on this boat at WOT because they were lower than specs I'd seen but inline with the experience of some other owners. I finally concluded, as some others had, that what I was seeing was normal for a 94 PS 190 with 350 TBI. Larry Englebert, Director of Customer Service at Indmar, eventually admitted these 94s may have left the factory a bit overpropped and not running at the published RPM spec.

In that thread, I reported "WOT with two adult occupants, a half tank of gas, no ballast, and nothing towed: 4,320 RPM (measured with tach in new Innova digital timing light) and 43 mph (measured with Garmin GPS)." That was with a brand new Acme 541.

More recently, with the same prop and similar other conditions, it's topping out at 41. I'm not sure at what RPM.

I'm searching for historical compression readings for context.

94 PS 190 350 TBI RPMs at wide open throttle
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Last edited by lake weir skier; 08-03-2018 at 11:41 AM.
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  #7  
Old 08-03-2018, 07:02 AM
lake weir skier lake weir skier is offline
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Boat: 1994 MasterCraft ProStar 190, 350 TBI, 1:1
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Quote:
Originally Posted by mikeg205 View Post
200? To 168.. thats quite a swing... hows she running...

Sent from my SM-G955U using Tapatalk
I just addressed the evaluation criteria I've seen and how it's running in two previous posts. I've only been in three 190s -- two 94s and a 96 or 97 (I think), but I think it runs very well. There is no grossly evident problem in the performance that I can detect. Maybe it's lost a smidge on the top end since 2015. That's a close call. Could be measuring with more fuel weight than in 2015.
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Old 08-03-2018, 08:20 AM
EricB EricB is offline
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What type of skiing do you do?
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  #9  
Old 08-03-2018, 08:48 AM
lake weir skier lake weir skier is offline
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Boat: 1994 MasterCraft ProStar 190, 350 TBI, 1:1
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We've used the boat mainly for open slalom skiing. May use it next year for barefooting. Have also had people behind it on two skis and in a single person towable.
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  #10  
Old 08-03-2018, 08:52 AM
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JimN JimN is offline
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Quote:
Originally Posted by lake weir skier View Post
I bought my 94 PS 190 with 350 TBI in 2013 and have found in doing several compression tests since then that No. 7 pretty consistently tests a little lower than the other cylinders. How significant is the difference? I've seen differing guidance for how much variation between cylinders should trigger further investigation with the low cylinder being anywhere from 10 to 30 percent below the maximum cylinder being offered as a trigger. No. 7 generally falls within that range of suggested triggers for further action.

My most recent test with a nice OTC compression gauge that I've validated through parallel measurement against a Grade A Valley gauge from Northern Tool yielded the following results:

No. 1 180 psi
No. 3 185
No. 5 190
No. 7 168
No. 2 194
No. 4 200
No. 6 198
No. 8 192

This puts No. 7 16 percent below the high, which was No. 4. This was a dry test with the engine hot after running it on the lake. I tested four cylinders today at ambient temp and found similar relative results between cylinders compared to the warm/hot test with the pressures running about 5 psi below the test with the engine warm/hot.

I find the plugs look good and similar across cylinders, and I'm generally satisfied with the performance of the engine, but want to squeeze out of it everything I can.

Any thoughts?
Are you testing it wet, or dry? Should be dry. Remove the fuel pump fuse if you're doing this test wet.
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