Relationship between comp ratio and comp test

[SteveEdmonton]

Hi all;
I got my rebuilt L20B running last week and am super pleased with it so far-- 80 miles in.

IMG_6973 new motor 26Jul2015 comp.jpg (205.49 KiB) Viewed 3876 times

The goal of the whole project was a "full rebuild" with all parts either brand-new or machined to spec, and I'm quite confident that the shop which did the work did a proper job of it. Once I got the block and head back from them, I assembled everything else myself.

Details: The motor has an .020 overbore, stock pistons, rods, and crank, and an A87 peanut head with new valves, guides, and rockers. The head was shaved in this rebuild but I don't have any documentation on how much. I think it had had a few thou off it once before too because the cam-towers were already shimmed. I was told that those existing shims were still thick enough to let the lash be properly set. The cam is the stock L20B (with U20 markings).

Other info about the engine that might be interesting but is, I think, irrelevant to where my question is going: I'm running rebuild 38mm SUs with a matchbox EI dizzy. Timing is set for 10 deg BTDC with no vacuum advance hooked up since the mechanical already advances it to about 28 degrees (according to my timing light, which has an adjustable "back-set").

Getting nearer to my question: According to advice I've gleaned over time from various posts on this site, I understand that this combination of A87 head and stock pistons (without the overbore and shaved head) should give me an 8.9:1 compression ratio. In my case of course, the .020 overbore enlarges the combustion chamber while the shaved head makes it smaller-- though all of that is so general as to be pretty much useless, I know, because I haven't actually done the math. (I've got the formula but to be honest I'm not sure I can measure everything accurately enough to bother!)

Anyway, 8.9:1 was about what I was aiming for with this rebuild. And now that the motor is running, with about 80 miles on it, I checked the compression: 182-182-184-182. To me those numbers seem pretty high! From what I can tell on this site, standard pressures on stock L20Bs that are well broken in, but still in good shape, run between 160-170 lbs-- that is, with the standard CR of 8.4:1. When my MG's motor was brand new (120 miles on it) the compression values were between 165 and 175 with an 8.5:1 CR.

So here's the question: Do those actual compression values-- 182-184 psi-- on a new motor seem like they would correlate with a comp ratio of 8.9:1? Or does it sound to you guys like I might have accidentally ended up with a CR that's higher than that?

[okayfine]

I don't think there's a strict correlation. You can have high static compression but test out low if your cam has a lot of duration, for instance. Mainly what you're looking for is consistency between cylinders, which you have.

[Three B's Racing]

"Other info about the engine that might be interesting but is, I think, irrelevant to where my question is going: I'm running rebuild 38mm SUs with a matchbox EI dizzy. Timing is set for 10 deg BTDC with no vacuum advance hooked up since the mechanical already advances it to about 28 degrees (according to my timing light, which has an adjustable "back-set").

I'd hook up the vacuum advance because one your forward carb has a provision for it, two you'll have better throttle response taking off, rounding corners and such, it's there for a reason use it.

[bertvorgon]

I agree with Ok, your camshaft setting and type has a huge affect on static cranking pressure, I have lived that.

I have a known 10:1 static. With my milder overlap solo cam ( turbo grind ), at one point I would crank 210 PSI.

With my huge, top end monster cam, I would only crank about 160-165. This cam had very large overlap and the engine did not respond down low very well at all.

What WILL be a good indicator, is if you get any pinging or detonation ultimately from the fuel you run. That is what you need to pay attention to. Regardless of cranking pressure, if it will run with no detonation, you have won!

Your setup looks really nice!

[SteveEdmonton]

You learn something every day, right? At first Julian's response didn't quite compute. How could cam duration affect the compression numbers? I assumed that at some point on the compression stroke, both valves would be closed and "that would be that"-- you'd have your maximum pressure. But obviously there's more to it than that. It's valve overlap that's the issue, right? Aha!

When I get the exhaust issue dealt with, yes I'll indeed be able to get a handle on pinging / detonation. Right now the header and downpipe are cobbled together due to the taller height of the L20B. I used a couple of short sections of steel conduit as "spacers" between the two pipes so I could connect things up temporarily at least. The seal is pretty poor though so there's too much noise to hear whether it's pinging or not.

IMG_6856 comp.jpg (139.9 KiB) Viewed 3802 times

As soon as I'm back from holidays I'll get this properly fixed-- and be able to actually hear things properly!

[bertvorgon]

Keep an eye on your plugs too. Don't run too hot a plug, and after a full throttle pull or two, pull them out and look at the "colouration".

For sure you do not want to see ANY little silver looking balls on the plug porcelin.

[okayfine]

Yes. I wrote duration, but I meant overlap - related, but not strictly the same thing. Overlap will bleed off some compression.

What are your plans for the exhaust? What condition and size are your current pipes? A mild L20B has been happy with 2" pipe out the back, and the 2-1 collector extended as far back as possible (to the trans mount at least, further if you have room) will enhance torque.

[datzenmike]

If your intake valve closed at BDC you might get the maximum compression but the stock L20B intake closes around 52 degrees after BDC. At cranking speeds the compression only begins after the valve closes. Be thankful. This makes life easier for the starter.

With the proper diameter intake pipe size the intake air speed can be so great that the air can't stop fast enough and will keep flowing in even after the piston reaches BDC. It's possible to actually cram 130 cu in of air into a 122 cu in engine. This is a volumetric efficiency of 106%

Then there is ram tuning....

[SteveEdmonton]

Exhaust plans are pretty much what you suggest, Julian. I've read up on this and am planning to keep the existing twin pipes of the downpipe, which I inherited from the PO. They're each 1 3/4" OD and run back to the trans mount. The only issue is that they're not tall enough to reach the header anyore with the L20B.... From that point back I'm figuring to use a 2 1/4" pipe, with resonator before the crossmember and a Flowmaster turbo-style muffler at the back.

[okayfine]

2.25" is just going to make noise on a street L20B. I ran 2.25" on my KA24DE and didn't see any butt-dyno gains going to 2.5", just added noise.

If you have future engine embiggening plans, building the exhaust for it can make sense.

[SteveEdmonton]

Keith: I hadn't even thought about "little silver balls on the plug's porcelain," but will definitely keep an eye out for those. You obviously remember my concerns with another motor that I was considering buying until I found about about its detonation issues. No way I want to go through that.

As for plugs, I'm away from home right now and can't check for sure but I'm pretty sure I'm running NGK BPR6ES. I've been happy with those plugs in different motors over the years. But what do you think: too hot? about right?

Julian: Speaking of noise, yeah I hear you... In fact I'm still back-and-forth between 2" and 2.25". I'm not a real big fan of real big noise, but at the same time I most certainly do want some rasp and rumble!

So: would it make sense to use the smaller pipe in order to limit the amount of overall drone, and rely on the muffler to let the appropriate amount of sound through when I'm tromping on it?

[bertvorgon]

Morning Steve,

For the most part, depending of course on what type of driving you are doing, those plugs should be ok. Obviously checking them is the main deal. If 90% of your drives are all at lower speed and rpm, where you are are not seeing much full throttle, where heat can build, again that would be fine. My bit of criteria is how much idling and low speed traffic will I be in on a drive. This is where if you have too cold of a plug, it will slowly foul over time, and then when you do want to get on it, you can get a miss as the cylinder pressure increases at full throttle. The odd blast with a "hot" plug should not be an issue, as there is no TIME for heat to build in the insulator of the plug.

On my last drive to that Fort Casey, I knew that we would not be doing a lot of higher rpm, steady state running, just a lot of on and off the throttle through the twistys, so I chose to run a BPR7ES plug. For my setup it coloured well and I knew I had some latitude for the odd blast, like when killing the Subaru, which did not take a lot of boost or time to do that. Nice thing about NGK plugs is that their heat ranges have a wide latitude of temperature they seem to be able to deal with.
Basically you want to run as hot a plug as you can, to avoid fouling, yet have a good high end temp range should you be driving hard.

There are times, when I know our drives will be a lot of 4,000 RPM+ cruising, coupled with mountain climbs (load), where I will just run a nice cold plug just to be kind to the motor should we really open up the throttle, and just deal with the fact that maybe over time they will get kinda crappy at low speed. For the matter, if I"m going at the type of traffic transit, there is not going to be any full throttle blasts anyhow, so it is kind of a moot point.

I actually carry, in my trunk, a full range of plugs, from BP5ES to BPR9ES. I use the 5's if I'm just warming up the engine to maybe just adjust the valves, or warming up the motor over the winter, just to keep everything lubed. 6's if I'm just going to a local car show.

I have the added issue that my Avgas still has a bit of lead in it, so I do try to maximize the life of my higher speed plugs.

Hope that helps.

[okayfine]

So: would it make sense to use the smaller pipe in order to limit the amount of overall drone, and rely on the muffler to let the appropriate amount of sound through when I'm tromping on it?

Sounds like a good plan. The particulars will be influenced by the construction of the resonator and muffler. Typically a "turbo" style muffler is going to be louder because they were originally designed to be after a turbocharger, which would soften up the sound somewhat itself.

I've run perf-core, straight-through resonators and mufflers on the last few cars. My blue KA car was LOUD when on it, but reasonable when not. The white car wasn't ever loud, but it was still noticeably 510ish. The blue car didn't have a resonator, just a Borla XS muffler in the stock location. The white car has a Silverline (nee: Thrush) stainless 24" resonator and Magnaflow 4" round muffler in the stock location.

If you're building an exhaust front-to-rear, think about serviceability down the road. If you need to drop the diff or rear suspension or transmission out, will you be able to with what you build?

[Dave Patten]

Exhaust drone seems to happen more often with larger pipe diameter. I had a 3" exhaust on my El Camino, it had terrible drone issues.

One trick the V8 guys use is a Helmholtz resonator in the exhaust. It is a dead ended exhaust tube tee'd into the exhaust. The length is tuned to counter act the drne at the RPM it is occurring. Helmholtz resonators can be used on any car it is just a matter of tuning the length.

Example of a Mustang H pipe with Helmholtz resonators

[funwithmonkeys]

I have a 2.5" from the header back with no resonator and a turbo muffler. You heard how loud that was when I saw you in Vancouver, I couldn't hear either of the 2 cars on the drive to Fort Casey even when Keith was 40' in front of me at full throttle. I don't find that it drones until about 4500rpm. It is stupid loud though.