Baffling problem with marine use BBC
Thread Starter
Joined: Jan 2002
Posts: 1,802
Likes: 36
From: Dutchess, New York
Baffling problem with marine use BBC I'm posting this for a buddy I know on another board. He has a boat running a pair of blown BBCs, and is having engine problems that no less than 3 builders have been unable to diagnose. Wondering if anyone on here has some good advice?
"So, here’s the situation… guess why my boat engine sucks!
The story:
I’ve got a pair of supercharged big block Chevy (Gen VI) motors (510 cubic inches) that are both built to identical specs:
J&E Forged flat top pistons (4.500” bore – 8.3 to 1 compression)
Forged Crank
Forged Rods
Rectangular Port Iron Heads
1.7 ratio stamped steel roller tip rockers
Hydraulic Roller Camshaft
Intake / Exhaust Lift with 1.7 rocker: ~0.490”
Intake / Exhaust Duration at 0.050”: 210˚
…or some **** like that. I forgot, it’s a pretty mild camshaft and is the stock 415hp MerCruiser 502 EFI camshaft.
Many years ago, both engines supposedly made 650hp @ 5300 rpm (770 fl*lbs. @ 3000 rpm) with 5 psi of boost and 705hp @ 5400 rpm (800 fl*lbs. @ 3000 rpm) with 7 psi of boost.
However, the engines both immediately blew head gaskets once they saw use in the boat. After initial inspection, it was determined that the block and heads needed to be decked (straightened) and the cylinder walls over bored to clean up the shitty hone job left by engine builder #1. Once the engines were decked and bored, they were reassembled and a leak down test was performed. All cylinders checked out between 5% and 8% leak down rate (very good).
On the next sea trail, the two motors did not behave the same. One engine (the good one) was hitting the 5500 rpm rev limiter and seemed to be making all sorts of horsepower. The other engine (the bad one) was struggling to get past 4000 rpm.
The bad engine was removed from the boat and run on the dyno… many times. Each dyno pulled yielded lower and lower power (note - the dyno runs were spaced a month a part, they didn't take place in the same session). The final dyno pull yielded a whopping 525hp at 4600 rpm. While this number might sound impressive to some, a supercharged 510 should be making A LOT more than this. sadyes.gif
In the last dyno tests, it was noticed that the torque in the bad engine started taking a HUGE nose dive around 3800 rpm. However, before that, it was making gobs of torque (750 – 770 ft* lbs.) This appears to be why the bad engine isn’t able to spin the prop past 4000 in the boat.
Since this problem has reared its ugly head, I’ve replaced the timing chain, valve springs, distributor, and (hydraulic) roller lifters. Each one of these items has had Z-E-R-O effect on the (shitty) power output.
All of the basic operating parameters seem to be spot on:
A/F ratio = 11 to 1
Timing at WFO = 30˚
Boost = 5 – 6 psi
However, we did notice one strange thing… after 4000 rpm, while the power (torque) was DROPPING, the boost in the intake manifold was INCREASING. This is usually indicative of valve train problems. I have no idea where this is coming from though. All the valves (intake and exhaust) appear to be seating correctly, hence the good leak down numbers. Also, the engine idles great and has AWESOME throttle response."
He's swapped the blowers (2.3L whipple screwchargers) between the healthy and bad motor, and it had no effect, so they can probably be ruled out. I'm about out of ideas myself.
"So, here’s the situation… guess why my boat engine sucks!
The story:
I’ve got a pair of supercharged big block Chevy (Gen VI) motors (510 cubic inches) that are both built to identical specs:
J&E Forged flat top pistons (4.500” bore – 8.3 to 1 compression)
Forged Crank
Forged Rods
Rectangular Port Iron Heads
1.7 ratio stamped steel roller tip rockers
Hydraulic Roller Camshaft
Intake / Exhaust Lift with 1.7 rocker: ~0.490”
Intake / Exhaust Duration at 0.050”: 210˚
…or some **** like that. I forgot, it’s a pretty mild camshaft and is the stock 415hp MerCruiser 502 EFI camshaft.
Many years ago, both engines supposedly made 650hp @ 5300 rpm (770 fl*lbs. @ 3000 rpm) with 5 psi of boost and 705hp @ 5400 rpm (800 fl*lbs. @ 3000 rpm) with 7 psi of boost.
However, the engines both immediately blew head gaskets once they saw use in the boat. After initial inspection, it was determined that the block and heads needed to be decked (straightened) and the cylinder walls over bored to clean up the shitty hone job left by engine builder #1. Once the engines were decked and bored, they were reassembled and a leak down test was performed. All cylinders checked out between 5% and 8% leak down rate (very good).
On the next sea trail, the two motors did not behave the same. One engine (the good one) was hitting the 5500 rpm rev limiter and seemed to be making all sorts of horsepower. The other engine (the bad one) was struggling to get past 4000 rpm.
The bad engine was removed from the boat and run on the dyno… many times. Each dyno pulled yielded lower and lower power (note - the dyno runs were spaced a month a part, they didn't take place in the same session). The final dyno pull yielded a whopping 525hp at 4600 rpm. While this number might sound impressive to some, a supercharged 510 should be making A LOT more than this. sadyes.gif
In the last dyno tests, it was noticed that the torque in the bad engine started taking a HUGE nose dive around 3800 rpm. However, before that, it was making gobs of torque (750 – 770 ft* lbs.) This appears to be why the bad engine isn’t able to spin the prop past 4000 in the boat.
Since this problem has reared its ugly head, I’ve replaced the timing chain, valve springs, distributor, and (hydraulic) roller lifters. Each one of these items has had Z-E-R-O effect on the (shitty) power output.
All of the basic operating parameters seem to be spot on:
A/F ratio = 11 to 1
Timing at WFO = 30˚
Boost = 5 – 6 psi
However, we did notice one strange thing… after 4000 rpm, while the power (torque) was DROPPING, the boost in the intake manifold was INCREASING. This is usually indicative of valve train problems. I have no idea where this is coming from though. All the valves (intake and exhaust) appear to be seating correctly, hence the good leak down numbers. Also, the engine idles great and has AWESOME throttle response."
He's swapped the blowers (2.3L whipple screwchargers) between the healthy and bad motor, and it had no effect, so they can probably be ruled out. I'm about out of ideas myself.
Jay Rod's diagnosis sounds good to me. Building boost with no power typically means that the air has no where to escape (out the exhaust), so I think looking at the exhaust valves is a good idea. If it stays closed or doesn't open all the way it will give you a good leak down test result. If anything swap the heads of the healthy motor and see what happens.
Did you pull the cam and inspect it when changing the chain and lifters?? Sounds like you might have a wiped cam as well if your power keeps going down as you run it?? just a thought, i had that happen once, WOT just got weaker and weaker?? soon it barley ran then didnt start.. broke the motor down to find a rounded lobes on the cam,
Thread Starter
Joined: Jan 2002
Posts: 1,802
Likes: 36
From: Dutchess, New York
Thanks for the ideas guys. We were leaning towards some sort of valvetrain issues, either flat cam or possibly the steel rockers deflecting under the power.
Keep 'em coming
Keep 'em coming
I think the funny thing is that you have two identical motors and only one has issues, so I don't think it is part related per say unless it's a broken part. If the rockers were deflecting under power it would do it on both set-ups. I think checking the cam and valves are good ideas. Have you gone back and checked your valve lash to make sure it's correct? Too tight and it will hang the valves open and you won't get a good seal when running, bleeding off power, but still give you a good compression and leakdown results.
