Cylinder leakage test

[Kirk Morgan]

Here is some detail to my findings. This is a Nitrous engine which was chirping out of the left bank after a run. Iam not a Nitrous person but a N/A engine person. Ran the valves all was good. Ran a compression test not good. The compression was a roller coaster from 200 PSI to 50 PSI. Un adjusted all the valves and removed the rocker arms. The 50 PSI was a exhaust valve leak with 80% leakage and the 200 PSI is 20% leakage. I have a cylinder leakage tester and they way i use it i turn the pressure on one side to 100PSI and read the other gauge to see how much leakage there is. To me the 20% leakage on the 200 PSI cylinder is to much i was always trying to get in the 10% or less leakage range on engines i have built. I have never built a engine with gas ported pistons. BTW i always spin the engine over 5 revolutions when checking static compression.
Thanks

[Jeff Stout]

Exhaust valves leaking I wonder why, they run away from each other. Low valve spring pressure or broken spring.Any of the low

cylinders measurements next to each other? What did plugs look
like? Asking as I have had all these problems on nitrous plus more.

[Henrys Toy]

Originally Posted by Jeff Stout

Exhaust valves leaking I wonder why, they run away from each other. Low valve spring pressure or broken spring.Any of the low

cylinders measurements next to each other? What did plugs look
like? Asking as I have had all these problems on nitrous plus more.

Good morning Jeff, Kirk and all,
I'm not a nitrous user, but I'm thinking the EGT's might be on the extreme high end and could cause exhaust valve face and seat erosion. Exhaust valve guide wear or loose clearances could be contributing factors. Like a " stacking of clearances " and the possibility of the valve springs fatigue and pressure loss causing the exhaust valves to bounce once they are trying to seat I would think should all be taken into account.
What condition was the engine in when the customer started using N20?
If it was " seasoned / used " there may have been underlining conditions that were exasperated once the "bottle" was used. The quality of the exhaust valves themselves can also be a factor.
No definitive answer, just food for thought, I hope this helps.

Respectfully,
Henry Kunz 1534 H/SA

[Jeff Stout]

Originally Posted by Henrys Toy

Good morning Jeff, Kirk and all,
I'm not a nitrous user, but I'm thinking the EGT's might be on the extreme high end and could cause exhaust valve face and seat erosion. Exhaust valve guide wear or loose clearances could be contributing factors. Like a " stacking of clearances " and the possibility of the valve springs fatigue and pressure loss causing the exhaust valves to bounce once they are trying to seat I would think should all be taken into account.
What condition was the engine in when the customer started using N20?
If it was " seasoned / used " there may have been underlining conditions that were exasperated once the "bottle" was used. The quality of the exhaust valves themselves can also be a factor.
No definitive answer, just food for thought, I hope this helps.

Respectfully,
Henry Kunz 1534 H/SA

I would never read egts, they would naturally show hot. Spark plug reading is your friend. Like you said used motor and maybe had underlying issues. Nitrous always finds the weak link. Hope it's not internally.

[Jeff Stout]

Also if motor would be built for nitrous more ring gap is required and I would expect 10 or more leakage.

[Terry Drinkwater]

I was fortunate to travel with Johnny Pilcher and his family to a lot of Pro Mod races, including ADRL and many local events. There was a lot of teardown in between rounds at the larger events due to excessive cylinder leakage. In all of our mid-race repairs, the problem seemed to be detonation and/or excessive cylinder temps, which would lock up the top ring.

[Adger Smith]

As with most instrumentation the end user is usually looking for a specific number rather than using the instrument as a diagnostic tool.
I have always used a leakdown to diagnose conditions. As said earlier Valve leakage, ring leakage and even head gasket condition.
All engines are not the same due to various clearance diffenences in cylinder wall and ring lands and end gaps for the specific aplication.
I remember when EGT gauges became the vogue.
I had customers telling me that EGT "Had to Be" XXXX number because the instructions sais so.
Put the engine on our dyno and tune it and the number didn't match what they said. Do a re tune to try to achieve their numbers and the power was way off...
I always leaked a fresh engine as I built it.. then re leak after a short run on the Dyno...
For the question about gas ports. I have never done a back to back, but it would seem they might leak a little more. However with ring seal being aided by the air pressure pressing the ring out against the cylinder wall and down against the ring land it might help static ring seal.