[rockin jester]

So, I bought a dual gauge leak down tester from Jegs. Both gauges are identical. They just show the psi. The instructions are a little blunt. " Regulate the air pressure atleast 10 psi lower than the source. Subtract the right gauge from the left gauge to calculate the percentage."
The motor is a 10.5-1 compression ratio, 400 ci sbf.
Bore 4.1268, Ross blower pistons 4.1209 top ring gap .026 bottom ring gap .027
Compression is 90 psi on 1 stroke and 130 psi after 4 strokes consistant on all 8 cylinders.
When I tried to do a leak down test at the (per instructions)reccomended psi of 75 psi at tdc it shoves the piston down. I put all the plugs back in except the cylinder tested to aide in keeping the piston at tdc.
Did the test at 75 psi and at 40 psi with a consistant reading on all cyninders with a loss of 10 psi. All the air is going into the crank case. No leakage into the coolant, intake or exhaust. Is this acceptable for a F1r blower motor and how do I calculate this percentage?

 

[jester]

10/75 x 100=13.33%
10/40 x 100=25%

You can block the engine for rotation and sneak up to the required psi as to not "bang" it hard. Once at TDC you can rock the piston back and forth a minute amount before beginning the leak down.

There are some video on youtube also if its of any interest to you.

 

[blownzoom440]

I used a breaker bar to hole the crank from turning wedged against the frame or rad core support.I did orient the test at BDC and moved to TDC but be carefull near TDC so you don't go over center and flip the wrench off the crank.I used 90# and just looked at the general condition of things was enough for me.

 

[Torment]

If you have 100psi on the first gauge and, for example... 80psi on the 2nd... you have 20% leak down. If you use less than 100psi than you just do the math for whatever psi you use. I've often used 50psi for the simple x's 2 math. If you get the piston very close to TDC it won't rotate around on you - no mechanical leverage.

I can't say what is acceptable or not for your engine - if the rings are keeping oil out of the cylinders you're probably fine to keep running them. Boosted engines will keep running hard despite small inefficiencies... The fact that it takes 4 hits to acheive max psi on your cranking compression test indicates the rings are worn. I like to see all or very close to all cranking psi on the first hit.

Basically to me... it sounds like your engine is mechanically sound, but the rings have seen bettter days.

 

[rockin jester]

If you have 100psi on the first gauge and, for example... 80psi on the 2nd... you have 20% leak down. If you use less than 100psi than you just do the math for whatever psi you use. I've often used 50psi for the simple x's 2 math. If you get the piston very close to TDC it won't rotate around on you - no mechanical leverage.

I can't say what is acceptable or not for your engine - if the rings are keeping oil out of the cylinders you're probably fine to keep running them. Boosted engines will keep running hard despite small inefficiencies... The fact that it takes 4 hits to acheive max psi on your cranking compression test indicates the rings are worn. I like to see all or very close to all cranking psi on the first hit.

Basically to me... it sounds like your engine is mechanically sound, but the rings have seen bettter days.

The motor went thru a shit load of mis haps for over a year before I finally got it running right. I'm wondering if I didn't wash the cylinders down and damage the rings over the build time. Like I mentioned in my post I have only 90 psi on the compression after 1 stroke but it is consistant with all cylinders. As is the 130 psi after 4 strokes. I'm not seeing and indication of oil on the plugs.

 

[Torment]

I understand... been there, done that too. Unseated rings can do that, overheated rings too - or excessive gap. Anything that compromises the ring seal. If you flow your leak down tester through a fixed orifice - a jet for example - you can get a good feel for the size leak various leak down percentages represent. Do you have an earlier leak/compression test when the engine was fresh to gauge the recent result against? That would tell you if the condition has always been there, or if it is the result of wear over time - the mishaps you mention.

Either way I think the leak down and compression test gave you some good feedback on what to look at when/if you do a teardown and inspection. You know all cylinders appear the same - that's a good thing.

 

[rockin jester]

I understand... been there, done that too. Unseated rings can do that, overheated rings too - or excessive gap. Anything that compromises the ring seal. If you flow your leak down tester through a fixed orifice - a jet for example - you can get a good feel for the size leak various leak down percentages represent. Do you have an earlier leak/compression test when the engine was fresh to gauge the recent result against? That would tell you if the condition has always been there, or if it is the result of wear over time - the mishaps you mention.

Either way I think the leak down and compression test gave you some good feedback on what to look at when/if you do a teardown and inspection. You know all cylinders appear the same - that's a good thing.

I unfortunatly have nothing to compare the readings to. I only decided to do a leak down test just for an idea what I have. It isn't having any power loss or issues to indicate a problem, but I really don't have much data to compare to considering I have only just recently got it running right and have only had it to the track 3 times. Does 90 psi sound low for this build?

 

[Torment]

I can't say with cranking compression - cam plays a big role in that, and I don't have any experience with an engine like yours. For example my 10:1 BBC has 65-70psi on a cranking test and there's nothing at all wrong with that. You're really at 130psi - it just takes 4 hits to get there. That's part of what's indicating a ring seal issue - that and your comment that the pressure is all going to the crankcase, and 25% leak at the 40psi you measured at. If I spin my current engine over it hits 100% of the cranking psi on the first hit - I can keep spinning it over, but it isn't going to show more. Conversely I've had several engines that act as yours is - it typically indicates the rings are no longer sealing ideally. With a supercharger on top though... they'll still run like stink either way

 

[Torment]

More... In my opinion it's tough for you to be certain the right call to make with only the one test data point, and the fact that all 8 holes are showing you the same thing. It's conceivable that it's been that way from the start. With a fresh engine I'll do a leak on it before I even fire it, then after the initial run in - heat cycle. This gives you a benchmark on the condition so after you head to the track you can check again and see if something has changed. Just like checking your lash before and after - did it get tighter? Did it get looser? Impossible to say if you don't know where it started... Often we miss these steps and get to checking things when we think we might have a problem - and sometimes the problem is very evident. With what you describe though, I think it's a tough call. Keep running it and see if it improves, stays the same, etc... or take it apart and determine 1st hand what's driving the pressure loss.

 

[rockin jester]

Thank for the input Torment. I think I'll keep an eye on it and try to get thru the next race on schedule. After that I'll re test and see if there is any changes. I really don't have enough time for tear down right now but will after the 8th.

 

[allmotor65stang]

i Only have about 105-110 cranking compression and 35% leak down..Runs better then it ever had.. Besides im fighting RFI issue now.

 

[JJXPRESS]

Make sure your carb or throttle body is all the way open when doing a compression check.

 

[rockin jester]

Just completly went back thru and double checked everything again and its all consistant. Compression is 130 psi on all cylinders and the leak down is just as consistant on all cylinders tested at 75 psi leak gauge reads 65 psi.

 

[bjs344]

I've never heard of or seen an engine make full compression on the first hit. Taking 4-5 hits to get your number is normal and is NOT an indicator of bad rings.

 

[allend43]

I always heard it should hit 75% of the highest reading on the first crank?

 

[Torment]

I'm not one to typically argue... But in my experience a fresh engine with the cranking compression of the OP's will show all or nearly all of it's available cranking compression on the first cycle. It if takes 4 cycles the valves or the rings are leaking. Doesn't mean it won't run. As I said in my original post if there's no oil in the chamber it's probably ok. I think the OP's plan is good - I'd run it too and plan to evaluate in the off season.

 

[blownzoom440]

With a boosted engine the oil has less of a chance to get in the chamber with bad rings and you get mostly blow by,NO?

 

[hemidup]

I've never heard of or seen an engine make full compression on the first hit. Taking 4-5 hits to get your number is normal and is NOT an indicator of bad rings.

That's what I was taught. Blades open and a 4 hits.

 

[Torment]

The OP could squirt a little oil in the cylinders and rerun the test. If the number increases it would further indicate ring wear.

 

[bjs344]

Don't get me wrong, first hit is important. If one cylinder is noticeably lazier on the 1st hit, then I'm worried. Between the last leak numbers the OP quoted and the consistent pump cylinder to cylinder, the rings would not even be on my mind.