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Thread: compression

  1. #1
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    compression

    I read many old threads in archive's but all was bout the same and wasnt what im lookin for.
    1 thing I've never asked yall is
    When u build a new motor and start compression off at 145 150. When do u look to rering?

    Usually a motor has bout 120 or so and about 100 or so u would wanna do something

    So am I good down to 110 or 100? Not that I'd let it get that low

    My motor still has the high compression but its a thought I've had for awhile but never brought up. Thanks

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    I go by leakdown, it's a better indicator than compression. Also, with leakdown, you don't have to know anything about the history of the motor. Fresh builds usually have 3-5% after the rings have seated properly. Once you're at 10% it's tired. Anything over 10% should be rebuilt.

    Balzy's leakdown tester is awesome, IDK if he's still selling those. You can get a good one from any of the tool suppliers (Mac, Matco, Snap On, etc) or HF has a cheap one.
    Josh Peterson

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    Ya i gotta a hf one here. I really don't like using it if I think its gonna give false #s. I do need a balzy for sure.
    But motor with 15% leak down and still has 120lbs on stock then what lol. Just reading sum trex old stuff seem like he didn't care for leak down #s. Which we know he does things his way. He dont like the written book/rules ha. Just curious as how other people do it also but I guess when u start at 150lbs the leak down b only good way to tell if u wanna catch it in time

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    Merc, I use both compression and leak down usually, but your numbers seem out. Your saying every cylinder is at 120psi, and 15%?


    www.InjectorService.com
    Call/Text - 204-326-0390



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    i would like to hear someone's report of a motor/hull they ran and documented it the entire time it took for compression numbers to fall, and what were the performance numbers along the way..........top speed.........time to plane....etc. We would then see just how compression affects power......overall, out of the hole, top speed etc.

    Anyone ever done that?

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    If you don't know the history of the motor and what compression it had stock there's no way to judge health by compression alone. I've seen a cylinder 12 psi off the other 5 and have 40% leakdown.

    Customer brought me a bass boat with a "FF" block 200 efi. Said it wouldn't plane. I took it to the ramp, planed fine, blew through the limiter to 6300 RPM's. I checked link and sync, timing, told him to come get it. He brings it back, says same problem. He checked compression and said they're all within 10. I repeat the check and the low one is 12 off the highest. Take it back to the ramp, planes fine, 6300 rpm, no problem. I tell him to come get it and meet me at the ramp with all the stuff he usually takes with him. Shows up with the boat loaded and a combined weight of about 700 lbs between him and the wife. I tell them to go out and see if it does it wile I watch from the dock. Sure enough won't plane. I hop on with them and it planes once I hang out over the bow. It's suddenly a dog. I take it back to the shop and start checking everything again. 12 psi difference between best and worst cylinder. Break out the Balzy. The 12 psi low cylinder has 40% leakdown. I recommend not to run it anymore and a rebuild. He runs it fishin a few more weeks and slings a rod through the block...

    I do leakdown on EVERYTHING now...
    Josh Peterson

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    Quote Originally Posted by InjectorService View Post
    Merc, I use both compression and leak down usually, but your numbers seem out. Your saying every cylinder is at 120psi, and 15%?
    it was just a example but I have seen em with good compression and leak down very questionable. Actually my motor had roughly 5 or 6% on the cyls I checked with harbor freight guage , couple months ago. But cant say it was accurate tryin do by myself with cheap guage. It was just a random question. I guess when u start at 150lbs leak down only real way to know. Im like keefallan tho. I'd like to try record the drop over the yrs but have to have same load, prop etc. Great idea tho

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    Old trick is to do a compression test and then another with oil down the holes. If theres a bad one it'll flag more obviously this way. Leakdowns a great test though its just a pita anywhere other than at the shop. Sometimes you gotta make the call far from any real tools especially buying something. Older I get the more I come to realize its best to simply tear down every new motor I buy and spec everything all out and reassemble. Usually find one or two little issues and a lot of QC issues that i can solve. Then its all new and good to go. Of course theres not always time for this, as I stare at my 200FF broke on the stand across the room here lol
    Hydrostream dreamin

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    With a compression test, I do 6 pumps with a good, strong battery and see where I'm at....do the same number of pumps on all cylinders.... if one is off, you will know in a hurry. But as said, to know the true health of all cylinders, a leak-down test is the way to go.
    "One of the penalties for refusing to participate in politics is that you end up being governed by your inferiors". Plato .

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    Apples to apples, same 96ci same porting, same carbs hair bigger jet, couple ° ign , psi range from 135 to 155 gave 10-15 hp . 105-115-120hp range

    Not including 78-81 115 which was the 99 ci big carb 135 from 77 .

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    I had an old edger with a 2 1/2 Briggs on it. I threw the blade part away and mounted a one wire Delco alternator on it. I threw it in the trailer and let people use it to charge a dead battery ... Think I might have needed it a time or two myself ..
    It got to the point it would stall when you hooked it up. The exhaust seat receded so badly you could see at least .060 to .080 thou of daylight thru it .. Popped both valves out , clipped the stem on one and lapped them both in . That POS is probably still running. After tuning it up it took more than a pinky pull to start it .. but man .. what 4-stroke torque ..

    I have two leak down testers . One is a Snap-on and the other came from Powerhouse Products , back when they were in Ft. Liquordale. I had to put new Ashcroft gauges in it about 20 years ago when someone dropped it.
    My compression gauge in a NAPA , off of the rotating board. I put a new 4 1/2 inch , 1% resolution , Wika gauge in it about ten years ago.

    Now had I checked the compression on the Briggs .. It might have had 10 - 20 lbs. But the gauge would have shown me what it could muster through out the "entire" compression stroke.

    Had I leaked it down , it would have had 80 - 90% leak. The best part of a leak tester is that you can listen for where the hemorrhage is at. In this case it would be the exhaust. But since we are "frozen" in time , you can listen to the intake , water ports (radiator) and crank case.
    The down side is it can only tell you what the ring seal is at TDC. And sometimes the bore is worn so bad that the ring will snug up against the ridge and the land and show good numbers , when in reality .. it could be wore slap out.
    They both are just another 9/16 wrench in the tool box. Both provide info that can tell you a story you need to hear .. as well as lead you out , and get ya lost in left field somewhere .
    When I was nitrous racing , sometimes it took two pretty healthy kits to get into some T/Dragster fields . We ran crankcase vacuum pumps . On mule Big Blocks will pull 14 -16 inch's if its sealed up good. After a run , if the data logger had a dip in the curve .. it was detonating and you could bet that if it lost half , you knocked the radial tension out of the rings , lipped a piston or shoved the head gasket out into the water jackets. Sometimes it would overheat and tulip {stretch) the exhaust valve.

    On Nitro-rats obese customers boat .. it obviously detonated from repeated attempts to climb the hill . Pizz gas , normal timing , low ridin , and a Texas mile wheel , instead of one that would yank a couple skiers out the water ..
    It picked on the worse hole .. transferred some aluminum to the wall , then next time out .. five said go , one said no . Stuck and sent a bone out the side.
    Five even , one down by 12 numbers .. it was tellin a story .

    After runnin WFO from the FPL wires that cross the ditch .. to the Jensen causeway, I slowed to an idle to go under the bridge .. the port motor was idling 100 - 150 RPM's low . Even stalled once. A smart man would have tied up to the docks , called the Mrs. to park at our ramp and bring the truck and trailer to come get me ..
    But Noooooooooooooo , it had to dirty up a plug .. Pppfftt , if it wasn't warning enough ... I got the hint , when it sawed itself in half five minutes later ..
    Look , listen .. use the best tools you can afford . Take your motor's word for it when it starts tellin you a story ..

    How long do they stay "crisp" . Depends on the tune-up .. generally speaking , 10 - 20 hours .. is the best its ever gonna run .. after that it goes downhill so slow you don't notice it. It's like that niece or nephew ya only see once a year. Holy cow did they get big .. they'll tell ya they ain't changed a bit ..

    I know some Comp eliminator guys that would flog a motor on the pump for hours , lookin for crumbs. Then pull it down , hand sand the high (shiny) spots , touch the bores with a brush, put a new top ring in and go lookin for an event to go bomb the record book .. How fanatical do you wanna be .. an obsessed lunatic , or just let the old Maytag .. err .. Briggs .. err .. Murkery just chuff along ..

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  16. #12
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    It really depends on the rpm you are spinning. The higher the rpm the more stress is put on the pistons and rings. On my OMC V6 spinning it 8850rpm every pass I rebuilt the engine and replaced the pistons and rings every 100 hours. No way you are going to get 100 hours out of a merc drag motor spinning 10500 every pass. RPM is key in my book.

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