|01-29-2013 02:29 PM|
Compression needs to walk with the camshaft this gets into the subject of Dynamic Compression Ratio (DCR) which is a measure of stroke lost up to the point of intake valve closure. Under any circumstance low compression costs power while increasing fuel consumption but especially when combined with any amount of aggressive cam (say over 200 degrees of duration measured at .050 inch lift it really cripples engine power and increases fuel consumption.
For a boat I'd really recommend an aftermarket head, these come with a lot more physical beef which takes the punishment that running with high loadings for long periods of time that a boat requires. From that you taylor the chamber size for the needed compression ratio.
|01-29-2013 02:18 PM|
So do you happen to know what the initial timing and the total timing is, and at what rpm the mechanical timing is all in by? The distributor shouldn't have a vacuum advance, or if it's there it shouldn't be hooked to a vacuum source.
|01-29-2013 11:55 AM|
|01-28-2013 07:34 PM|
hope the machine shop can do a simple fix
|01-28-2013 05:28 PM|
|techinspector1||What's the casting number on the heads?|
|01-28-2013 04:32 PM|
|gfackrell||I checked the cam timing, and it's indeed on. I'm not assuming i have only one issue, which is why I'm going to tear it all down and work with my local machine shop to get it back to where it should be. Thanks for all the help guys. I learned a valuable lesson. Don't buy a cheap long block off the internet....|
|01-28-2013 04:12 PM|
Your compression numbers are low for a rebuilt engine. Rough calculation would show you at about 7.8 to 1. Not the recipe for a 300+ hp engine. I would take a hard look at the cam timing before you go crazy. The timing chain being off a tooth can affect compression. I would expect to see at least 135 compression in each cylinder. This is assuming the combo is setup to produce around 9:1 compression ratio.
If the exhaust valves are a little tight or the valve job was sub par this could also be part of the problem.
Dont assume you only have 1 issue.
|01-28-2013 01:17 PM|
The engine should be brought up to operating temperature before testing. This closes the ring gaps to operating clearance. If you don't do this then a thick engine oil should be introduced before testing a 30, 40 or 50 weight oil so it can seal up the rings without being blown out by the test pressure. Keep in mind that an operating engine is also using cylinder pressure to push the rings outward against the cylinder walls which is what actually makes the seal. Cranking or static testing cannot duplicate this exactly, so the pressure loses appear higher in these tests than they really are. If these things aren't done the leakage rate will look excessive. Yours is high on some cylinders and certainly the differential between cylinders is way out of bed. I look for no more than a 5 percent differential between cylinders. The test should be done with each cylinder at TDC and the crankshaft locked so the pressure cannot rotate the crankshaft, this is both from a safety and unified test process standpoint.
However, I start getting concerned when the loss exceeds 10 percent when following the process I just outlined and the differential between the high and low cylinders exceeds 5 percent.
I'm be less than pleased with leakage past the exhaust valves that you're seeing. This can indicate poor seat conformability between the valve and seat or it shows weak springs. Intakes are more flexible than exhausts simply due to increased size they conform to the seat better. Exhausts need better attention paid to them when doing the seats and may need a bit stouter spring than the intake.
Boat motors work a lot harder than road vehicle motors so extra cautions are in order when it comes to cooling and lubrication as well as timing and mixture. It is mighty easy to toast the exhaust valves and certainly piston and bore wear are greater issues but that said at 20 hours unless the timing and or mixtures are way off you shouldn't see valve damage. Since you are; my list of suspicious charters would immediately turn toward ignition timing against RPM and mixture ratio's. If these things get out of line; the motor becomes toast pretty quickly. Detonation is much harder to hear from a boat motor because of all the other noise, if you're running into this problem it can do a lot of damage before your aware that things are amiss. Sparkplug color is not always a good indicator on boats as they tend to run quite cold when fired up which may carbon the plugs making them appear that the mixture is richer or the timing less advanced than it actually is. Boat motors are lot trickier to diagnose than car/truck motors in these instances.
|01-28-2013 12:34 PM|
|mbossio||You state your are turning 5200 RPM and only going 30 MPH. Might look at the prop pitch.|
|01-27-2013 02:50 PM|
|01-27-2013 02:21 PM|
you should(if needed) build a 360 horse marine engine.
360 x .7 = 250 ish continuous
|01-27-2013 02:11 PM|
I don't know if the exhaust valves have rotators. Sorry for not having much info.
The engine is on the stand at this point, and honestly I'm just looking to see which parts of it aren't completly crap. I am ready to put together an engine myself that doesn't suck. What I'm looking for is ways to quickly diagnose if the bottom end is any good. I'm pretty much planning on new heads, intake, carb, etc. If I need to have the bottom end gone thru, I have a great machine shop that can do it. I would be happy for suggestions of what heads intake, etc for a good 300 hp budget build.
|01-27-2013 01:57 PM|
|vinniekq2||If you are reaching 5200 RPM,thats probably all you will get.You might be running out of prop? and what painted jester posted?|
|01-27-2013 01:54 PM|
|camaro355||sounds like the rings arent seated or the valve springs are weak|
|01-27-2013 01:50 PM|
you stated you adjusted the valves! and only the exhausts are leaking, try backing off the exhaust valves a little. to give them a little more clearance and they might seat better! The intake valves run cooler and the exhaust valve stems expand a little with the combustion temps and might be rising just a hair off the seats! Do the exhaust valves have rotators installed ? Is it a solid cam or hyd.? What did you set the valve clearances at? Is this a Chevy, Volvo, Ford, ETC? Reverse rotation? What r.p.m. did your old engine turn at 30?
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