chewy1955

Can you briefly explain the proceedure to do a leak-down test? Do you need to rotate the crank and do test at different positions or do you leave the piston at TDC?

tomas_wallin

You leave the piston at TDC - bring on the pressure and read the gauge, then rotate the crank to the next pistons TDC and repeat the procedure.

chewy1955

Thanks

Strip Poker 388

Keep us posted Smitty,take some picks also.


Also When I tore mine down I had the crank wet magufluxed.I am just scared of the factory crank at 850hp.

cobra marty

Nice job. Now do away with that MPI intake. How about converting to a 4 barrel single plane intake and throttlebody? I know you talked about it before.

articfriends

In a stupid,stubborn kinda way I'm actually putting more time and money into my mpi top end. My head guy,Rick Lapouttre of lapouttre racing,a real anal kinda guy when it comes to heads and port work is re-working my top end and there is some small gains I may find. I bought my afr 315's bare then bought there valves seperately 2 years ago when I built this motor. I didn't want to wait for lapouttre to set up my heads so I had a local machine shop assemble them which was a mistake. They were supposed to "touch up" the valve job using wider seat margins due to being supercharged,they re-cut the valve job and left some nasty ridges in the valve seats in the heads. Rick feels the ridges were detracting from the heads ability to flow to there full potential and is re-doing the valve job to perfection. He is also going to finish full porting my mpi lower intake manifold and is welding up some of the ports I opened up a little un-even then re-working them. I'm having the heads flowed in the condition they were ran before (where the motor made 944hp/962 ft lbs of tq) then after the valve job and any clean up work he does. I am also having heads flowed with the mpi intake in its as ran condition to see how much flow drop there was then afterwards to see what gains there may be. I am also having a untouched intake flowed bolted to the heads for a comparison before and after too. If I can get bigger throttle blades he is going to bore out a spare throttle body and I'm going to do back to back testing on the dyno too. There might be 2 or 3 hp left to be found (or more ) still and I'm going to try and find it,Smitty
Dart big m came today,its a nice piece,not going to worry about head gaskets with its extra head bolt bosses!!!

NoQuarter

What is the difference between a compression test, and a leak down test? What does each test, specifically?

articfriends

Compression test-you take all your spark plugs out,fully charged battery ,crank motor over with throttle w/o with a compression gauge in each cylinder one at a time until you get the highest reading. In my case my readings were 155-159 psi,the weakest cylinder of course was the one with 155. Then start on one cyl,put motor at tdc,screw in a leakdown gauge. It blows compressed air in the cylinder and you set it to 100% then measure the percentage of air leaking into the cylinder,you do this on each cylinder. I used zero gap rings when I built my motor initially so my readings are lower than what you'd typically see. A good cylinder on my motor was at 4-5%,my cylinders that were at 155/156 were at 14-15%. While you have the leakdown tool screwed into the cylinder you try to determine were the air is going (out exhaust,out intake,into crankcase etc). In my case the worst cylinder started out at 20%,air was leaking oyut exhaust port,I pulled the valve cover and removed the rocker arm then tapped on the exhaust valve stem with a brass hammer. The leakage dropped to 15% or so and air almost completely quit hissing out exhaust port,the reamainder of the air was very clearly hissing into the crankcase,you could put your hand over the valve cover breather tube and feel it blowing gently thru the motor or you could put your ear to it. I was tearing the motor down to freshen it anyways but did it just to see what my numbers were compared to how everything looked. I also HIGHLY reccomend doing these test on a fresh motor after your sure the rings are seated for future reference which I did 2 years ago after dynoing my motor. Back then all my cylinders were between 4-5% and compression was 155-156,i think it went slightly higher from deposits on the piston crowns raising compression slightly. A normal motor with regular rings will be closer to 15 to 20% leakdown,its the cylinder that is at 40% in that case that you'll want to worry about. On most marine enigines its the valve job/top end that goes first so if you do a leak down test and numbers are high look hard for leakage thru the valves,if you didn't have alot of air blowing in the crankcase you can often just pull the heads and freshen them,Smitty