question re: cylinder honing

Post by **Sveedy** » 29 Jul 2021, 10:50

So this is kind of theoretical, but I was wondering about how much honing and re-ringing a cylinder might bring up its compression
( obviously depends on condition of cyl. ) And also which is more important; higher compression or having all cylinders similar in compression.
I'm asking because I'm starting the rebuild of Sveedys engine next week. I currently have 100 psi on cylinders #1 & 2. Cylinders 3,4,&5 all have 150+. Leak down points to rings. So if I re-ring all cylinders, would they all come up in compression and there by still leave me with two cylinders lower than the other three, and out of balance so to speak ? Or should I re-ring just the two low cylinders, and leave the other three alone, there by bringing all the cylinders closer to balance, but with lower compression overall ?
Two schools of thought tugging against each other: " I'm in there anyway so I should just do it " - or - " If it ain't broke, don't fix it."
So which is more important in this case; the compression or the balance ?

Post by **volvolugnut** » 29 Jul 2021, 12:04

These answers are from my general understanding of engine rebuild and performance building of engines. I do not have sources for these ideas.
The low compression cylinders likely have broken or stuck rings (or leaking valves) or they have some cylinder wall damage. You will not know until you get into the engine.
If you are doing a rebuild, hone and rering all cylinders for best results. Doing only the low cylinders will likely give unsatisfactory long term results.
Performance engines will have very accurate size and round cylinder walls. The rings will be carefully sized with close end gaps.
While you have the engine apart, replace valve seals and grind or lap the valves.
In my opinion, uneven compression will cause rough idle and purportional loss of overall performance.

volvolugnut

Post by **Sveedy** » 29 Jul 2021, 15:00

Thanks lugnut. I have a shop that is going to do the head. They are very experienced with these white blocks, or so I'm told.
I'm just wondering about how important the spread between piston compression is. I thought the cylinders should be within 10% or so of each other. Just wonder about whether an engine that has all 5 cylinders close in numbers but a little low over all would run better than an engine that had a mix of 3 with really good comp and 2 with a little low comp. Maybe firing order comes into the equation.
I have no idle or running issues. I hadn't started it in two weeks but did so yesterday. Fired right up and idled like I just parked it minutes ago. No smoke of any kind. But the blow-by is just too much to deal with. If I'm on the interstate for 2 hours going avg 80mph, I can expect to drain about 4oz out of my catch can. Not bad like that if I'm puttering around town at 30. Problem is I didn't get it to putter around town

I'm not committing to anything till I get it apart and see what's going on in there.

Post by **volvolugnut** » 29 Jul 2021, 18:55

I think that the 100 and 150 PSI compression differences will cause a ripple in the torque output. I don't know the firing order off hand, but I bet cylinder 1 and 2 do not fire one after the other. And cylinder 1 and 2 would have roughly 2/3 power and torque output of the good cylinders. I think the results would be hard to notice, unlike a dead cylinder miss.
Overall we can assume your power output (and efficiency) is off 1/3 X 2/5 = 13 %. This assumes 150 PSI is good condition.
Does this sound right?
volvolugnut

Post by **abscate** » 30 Jul 2021, 02:13

Your new ringed cylinder might go to 180, but running 180/150 mix will be fine

Torque does not scale with compression pressure.

Post by **Sveedy** » 30 Jul 2021, 08:06

^ Thanks ! Good to know that I'm just worrying about nothing. Looking forward to getting started next week

befarrer · Post by **befarrer** » 30 Jul 2021, 09:12

I'm in the same boat. My car sat for upwards of 10 years, and I am pretty sure I also have sticky rings on 1 and 2 as compression was lower with those, and jumped up with oil down the cylinder. I would guess that if the compression loss is from a damaged or scored cylinder, oil consumption would be quite high as the oil rings would also be not sealing. I want to pull my engine one day to check the rear main seal, I've had both cam seals pushed out when I bought the car, and also still get lots of blowby, where the weep hole on my intercooler leaks oil, but stops if I route the PCV pipe so it vents to atmosphere instead of into the intake.