compression test question
Woah - I wonder now after reading this thread if my newly-built /6 has too much compression. I decked the block 0.050", and the heads 0.030", aiming for about 9.5:1 compression. Pistons are +0.030", too.
All six cylinders are within 3.5% of each other, with an average of 194 psi. Wet. Throttle tied wide-open. High-torque starter from a late-model Dodge V8 motor. All spark plugs removed for the testing.
It's only been driven for 10 miles, and I know the stock distributor is having problems that I suspect are tied to the centripetal weights seeming to have worn springs. Even with the adjustable bracket at the bottom of the distributor bottomed-out in its slot, I couldn't get base timing at idle to be less than about 8 degrees BTDC. Was aiming to get 0 or 2 degrees (with vaccuum line disconnected).
And it's got fairly bad 'clatter' / pre-ignition at wide-open throttle. With 89 octane fuel only, so far.
Wondering now if I did the math wrong and ended up with a 10.5:1 motor??
I have a rebuilt distributor I'm installing in a few days. Be curious to see if it helps.
Yeah that's a lot! But the up-side is there's plenty of room for a much bigger cam, to reduce your Dcr,to help with the low-speed and part-throttle operation. Unfortunately, WOT will still be timing sensitive.And for sure, it will require a top-grade fuel.
For a streeter,I actually prefer an engine built this way.Mind you,not as drastic as you seem to have come up with; but around 160psi, or so. Once you get the tune into it, it will have plenty of low speed grunt,and make gangbusters mpg, unfortunately, the engine may not accept "full" timing under full load, so,being knock-limited,the horsepower numbers might not be there. No biggie to me, if it pulls harder under part throttle, and gets great mpgs. When I need my HP fix, I have a different car for that. And If you really wanna drag-race it, theres always race-fuel.
I built my slanty similarly, with plenty of compression, for to cruise with very small throttle opening, and saddled it with really tall gears, and a wide-ratio,loc-up tranny.After much tuning, it paid off. I have been driving that 1980 Volare almost continually since the early 1990s; going on 25 years.The body is done now, soI kinda think I will retire it soon.Powertrain is still going strong.
But yeah;pressure as high as yours is, or may be, difficult to tune for.On the other hand, a full wet test can be misleading.Especially with high compression design; 2 or 3 ccs of oil will change things dramatically;perhaps .3 to .5 of a compression point. And with just 4 or 5 compression cycles required to build peak pressure, the oil might not all have a chance to escape.I would redo the test after it has run a bit and burned off the oil.This time DRY.And be sure the valves are properly lashed.
FWIW A stock bore slanty has a swept volume of about 614cc. If the Scr is 10.5 then the Total chamber volume will need to be 64.7cc. If 3cc of oil does not escape, during the test, the total chamber volume will be just 61.7 and the actual Scr will be 10.96;nearly half a point, to .46 of a point
But if the Scr was 9.5, then the Total chamber volume would need to be 72.3cc. And again if 3cc of oil got stuck in there, the total chamber volume would effectively be reduced to 69.3cc, and the Scr would be increased to 9.87; this time just .37 of a point.
These are probably extreme examples, designed to make you aware of the presence of the oil in there. How this translates to cylinder pressure is anybodies guess at this point.
