Good Leak Down but Bad Compression after rebuild

[Walt Fricke]

Polezei

Henry is being very diplomatic here. I will present a rather pessimistic analysis.

The compression ratio has an effect on measured compression. Isn't that obvious? Compression is how much you squeeze the air in the cylinder. With low compressiion pistons you don't squeeze the air as much, so your compression is lower.

The fact that a 3.3 cylinder is a slip fit into the case has nothing to do with whether it is a good idea to use 7.0:1 3.3 turbo pistons in those cylinders without also turbo or super charging. Your 3.2 had a CR of 9.5:1 to start with. Even if 3.3 turbo pistons will produce 8.0:1 CR using a stock 3.2 head, you are going backward, aren't you? I don't think the extra 100 CCs will make up for a substantially lower CR.

And since you did not measure the compression ratio as you were building the motor (it is actually pretty easy if you can get ahold of a burette), you are stuck with suppositions as to what the CR is. But everything you say points to it being quite low for your motor, which is precisely in line with the compression being low. You built a nice, tight motor, though.

And that ain't all. You twin plugged the heads. That alone will lower the CR, though not by much. More to the point, I don't think there is much if any advantage to twin plugging a normally aspirated low compression motor. Twin plugging allows early small motors with high domes and high CRs to live. It allows 964/993 motors to run CRs which would have killed earlier motors. But I don't see how it will contribute a thing to a low CR motor. Maybe it would allow running pump regular instead of intermediate grade fuel?

Your options as I see them are:

1) live with a low compression engine with a slightly larger displacement than what you had beforfe. You have effectively detuned this motor. Am I right that you had these tests done at least partly because the motor wasn't as peppy as it was before you had the work done? You noticed this right away, but were hoping it would improve when the rings seated?

2) add a super or turbo charger, since you have an appropriately low CR - and the twin plugging. There might be problems, though. Didn't the turbos use stouter rods, to start with? Rod bolts are the weak point of stock 3.2s. Home brewing a blown motor is not for the faint of heart or pocketbook.

3) Take it partly apart, and replace the pistons with aftermarket pistons cut for a suitable CR. That may be your best bet if your old Ps and Cs are worn out.

4) Put your stock stuff, with new rings, back in. Sell the 3.3 stuff to someone who is converting to a turbo. Least cost alternative.

I hardly have a Mahle catalog compendium of every piston they ever made in my head or within arm's reach, but the only 3.3 piston one hears about is the turbo one made for Porsche. I can't imagine that needing valve reliefs. Don't they have a sort of reverse dome to achieve that 7.0:1? Maybe they made one for the aftermarket for normally aspirated motors as well? But then why would anyone need to cut valve reliefs? That would all be built-into the casting/forging. But no matter.

Valve timing has nothing to do with your low compression figures. The gauges and tests aren't off. Believe me, I know all about denial and wishful thinking when a motor I have built doesn't live up to what I had hoped.

[Walt Fricke]

Polezei

Henry is being very diplomatic here. I will present a rather pessimistic analysis.

The compression ratio has an effect on measured compression. Isn't that obvious? Compression is how much you squeeze the air in the cylinder. With low compressiion pistons you don't squeeze the air as much, so your compression is lower.

The fact that a 3.3 cylinder is a slip fit into the case has nothing to do with whether it is a good idea to use 7.0:1 3.3 turbo pistons in those cylinders without also turbo or super charging. Your 3.2 had a CR of 9.5:1 to start with. Even if 3.3 turbo pistons will produce 8.0:1 CR using a stock 3.2 head, you are going backward, aren't you? I don't think the extra 100 CCs will make up for a substantially lower CR.

Using Alfonso's formula (which is for sea level, but Norristown isn't very high, is it?), I get about 88 psi for 7.0:1. That's getting within range.

And since you did not measure the compression ratio as you were building the motor (it is actually pretty easy if you can get ahold of a burette), you are stuck with suppositions as to what the CR is. But everything you say points to it being quite low for your motor, which is precisely in line with the compression being low. A nice, tight motor, though.

And that ain't all. You twin plugged the heads. That alone will lower the CR, though not by much. More to the point, I don't think there is much if any advantage to twin plugging a normally aspirated low compression motor. Twin plugging allows early small motors with high domes and high CRs to live. It allows 964/993 motors to run CRs which would have killed earlier motors. But I don't see how it will contribute a thing to a low CR motor. Maybe it would allow running pump regular instead of intermediate grade fuel?

Your options as I see them are:

1) live with a low compression engine with a slightly larger displacement than what you had beforfe. You have effectively detuned this motor. Am I right that you had these tests done at least partly because the motor wasn't as peppy as it was before you had the work done? You noticed this right away, but were hoping it would improve when the rings seated?

2) add a super or turbo charger, since you have an appropriately low CR - and the twin plugging. There might be problems, though. Didn't the turbos use stouter rods, to start with? Rod bolts are the weak point of stock 3.2s. Home brewing a blown motor is not for the faint of heart or pocketbook.

3) Take it partly apart, and replace the pistons with aftermarket pistons cut for a suitable CR. That may be your best bet if your old Ps and Cs are worn out.

4) Put your stock stuff, with new rings, back in. Sell the 3.3 stuff to someone who is converting to a turbo. Least cost alternative.

I hardly have a Mahle catalog compendium of every piston they ever made in my head or within arm's reach, but the only 3.3 piston one hears about is the turbo one made for Porsche. I can't imagine that needing valve reliefs. Don't they have a sort of reverse dome to achieve that 7.0:1? Maybe they made one for the aftermarket for normally aspirated motors as well? But then why would anyone need to cut valve reliefs? That would all be built-into the casting/forging. But no matter.

Valve timing has nothing to do with your low compression figures. The gauges and tests aren't off. Believe me, I know all about denial and wishful thinking when a motor I have built doesn't live up to what I had hoped.

[polizei]

I was chatting with a fellow Pelican last night regarding my engine woes, and he asked me this question:

"When you installed the 97mm turbo pistons and cylinders, you measured the deck height, right?"

I'm embarrassed to say that not only did I not measure the deck height, but I didn't even understand at the time that that is standard operating procedure!

So, it's likely that there's nothing wrong with any of my parts. But I must tear down the engine to fix it. And while I'm in there, I'm going to swap P&Cs with the intention of building a NA engine.

Thanks guys!

[PFM]

Polizei,

FWIW I ran a quick simulation on a 3.2 with 95mm cylinders and 9.5 compression 7000 RPM rev limit, came out 220 HP about right, changed the bore to 97 mm with 7.0 compression 201 HP.

In this case displacement was trumped by compression.

I had CP build pistons to match my otherwise good 97mm cylinders, the verdict on how it all works out is not yet in.

[gsxrken]

Originally Posted by polizei

I'm embarrassed to say that not only did I not measure the deck height, but I didn't even understand at the time that that is standard operating procedure!

Quick question- I'm familiar with measuring deck height on V8 engines; I would describe it as how close to the top of the block the piston rises. Zero deck height would be that the piston is flush to the block at TDC.

And of course, regardless of deck height, valve reliefs and the volume of the combustion chamber would play a huge role in the compression ratio.

So while this is important to check, what is the spec for a Porsche 3.3, and how would you change it if it didn't check out?

[Walt Fricke]

QSX - for measuring compression ratios, deck height is used as you say if you are using the usual multiple measurements method. I found that a trial assembly and measureing how much fluid the chamber would hold at TDC removed the need to do that measuremnt.

I think that for 911 engine building the figure most checked is piston to head clearance. Too little and things can hit. Too much, and you don't have a suitable quench zone around the circumference. I like to use hollow core solder in preference to clay to measure this. But at least one of the highly experienced engine builders who contributes here doesn't trust this and always uses careful measurements. I don't quite trust measuring small distances down to a sloped surface, but if I were a machinist who does this for a living no doubt I would.

I'd have to hunt up what recommendations are for this, as I tend to forget details like that once I have applied them. As in my profession (law), it is usually enough to know something must be checked, and to go check it rather then relying on memory for precise details unless you deal with it all the time. Do a search. How you get your clearance and the CR you want can be done various ways.

A super trick motor would have the exact same CR for each cylinder. But, like having the exact same air flow in and out, that last ounce of extra power and balance is not usually done because you have to futz with the piston domes or the combustion chamber shapes, and it tends to lead to a least common denominator because removing material is more practical than adding it.