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Old 26-08-2015, 12:18 AM   #1
Diablo
 
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5.7 sbc

Has anyone had any history with small blocks and their internal components being manufactured wrong? I.e.cam or crank?

In summary we have a new long block. Put it side by side with the old long block and taken off one motor and bolted on to the new one. But now we are 500-600 rpm down. We have checked timing sensors etc etc etc. With no change.

Could the cam or crank just simply be wrong?
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Old 26-08-2015, 01:17 AM   #2
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I'd have assumed that for the crank to be "wrong" it'd either have to be the wrong stroke, inferior quality or both. At a guess its highly unlikely to be a different stroke as the most common derivatives of the Mercruiser SBC all have the same stroke. Quality wise, I'd assume you'd only know that its inferior when it fails!

As for the cam, I suppose it could easily have a 305ci or an older 260hp cam in it. If the old cam is about & in fairly decent condition, is it not worth the swop or at the very least cross ref the part numbers?
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Old 26-08-2015, 08:08 AM   #3
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Thanks for coming back to me. I am assured by the builder that the cam is the same in all 5.0 and 5.7 applications.
I am concerned that the cam had been ground wrong?
Our old cam is not reuse able.

Dc
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Old 26-08-2015, 12:12 PM   #4
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Just had a quick look, it appears tat most (but not all) 5.0L and 5.7L Mercs have the same cam. So I suppose the only thing to do is to make absolutely sure that the correct cam has been installed. I think the identifying marks are stamped timing gear end of the cam.
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Old 27-08-2015, 12:11 AM   #5
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Is it brand new or remanufactured?
Check the simple stuff too from the swap over, leaking gasket, pinched hose, timing off, damaged plug lead, throttle cable adjustment, etc
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Old 27-08-2015, 10:32 PM   #6
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I have built and caused to be built a number of small block motors.
Do you know for sure the origins of the short block which you have just removed and our you certain that the replacement is the same ?
My primary concern would be the compression ratio, different pistons less power, same prop less revs.
As has been said by other,first start with the simple stuff.
Trying to diagnose a problem without knowing the build spec can be a challenge.
However I do have 2 very low hours Marine high performance hydraulic cams for sale plus a number of other small block parts from an abandoned rebuild in favor of a custom new build should you wish to upgrade.
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Old 31-08-2015, 04:03 PM   #7
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Thanks for replies. The remanufactured motor came from Michigan motorz. It is to replace a vp 5.7 320 gxi not merc. They assure me all components are gm and are to better tolerances than a production motor.
We have checked all wiring and sensors now and all are in the parameters. And no wiring faults. Throttle has full opening adjustment. Dizzy has been checked again and again in case. No ht lead faults no plug faults. Is the motor just getting really tight when hot??
I am knocking my head against a brick wall with this one!
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Old 31-08-2015, 08:52 PM   #8
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Fuel pressure ok? Did you swap intake manifold from original motor?
Nothing in the downpipe (bullhorn) causing an ostruction?

Any other symptoms, stalling, backfiling etc?
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Old 31-08-2015, 09:28 PM   #9
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if all the basics are covered and are ok, was it defiantly a 5.7 block and not a 5.0 that was sent out by mistake? (casting mark on back / side of the block)
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Old 02-09-2015, 01:23 AM   #10
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Intake manifold is from original motor. It has straight through pipes the boat is a v24. Which has run generally faultlessly for the last 4 years and a months ago a big end let go. To get racing again we bought a reman 5.7 long block and put all our original parts onto it. It then never ran past 4400 rpm but normally revs to 4900.
Yes it does stall occasionally when you come off the power to idle and then it is hard to restart, like flat battery but then will restart no problem after 10 minutes or so. No backfiring.
The block is stamped 5.7.
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Old 02-09-2015, 01:38 AM   #11
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Thats strange!
Sounds like its overheating or hydrolocking. Is it tight when it stalls?

Could be a crack in the block or the intake manifold..
Which bearing went in old block, look for evidence of water in same cyl in new block.
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Old 02-09-2015, 08:57 PM   #12
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Strange your not kidding. Runs on the gauge at about 175 same as old motor maybe 5 degrees hotter. Can't speak for the block but inlet manifold is spot on has been stripped right down. No evidence of water in the cyls.
Our exhausts are dry systems. And there was steam coming from left bank when we first ran the motor we put it down to it being stored for a while and did not notice it again. However we were not looking either as we are strapped in a canopy boat. Possible lead??
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Old 03-11-2015, 10:23 PM   #13
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Blue Print

No two similar engines will be the same "Period" ! What is advertised by Engine Builders as Hp & Torque is what they guarantee the engine will produce but then some will give a bit more ! The higher the advertised number the closer all those engines will be. Because the assembly care by the builder will be much more precise. Loads of good info above but the fact is what you took out revs 500 higher than the one you have just put in.
There are many causes for this and you have covered most of the obvious but now is the time to index inspect and tear down the new engine, inspect and measure all the internals, correct any mismatch and assemble, indexing the crank, cam, front damper, piston height, deck height, H/Gasket thickness, R/Arm ratio, port match, push rod length, ring leak-by test ( TDC, BDC & Max Throw point ), rotating drag, rotating & reciprocal drag and then the experience sneaks that engine builders do. Which are each builders secrets, never to be disclosed. You just get the edge !
Never trust that the crank keyway is at true TDC or the cam peg is in the right place or the cam drive markings are correct.
I am interested in why you kicked out a rod. You only Rev to 4900/5000. 11/32 Chevy Rod Bolts are good to 5700 (smallest fitted to small block GMs) We use them in Ford Rods, to upgrade them ?
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