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wishbone

flathead six with super high compression...???

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I've been working on my 1938 Packard Six and discovered that the head had been milled too much which caused a couple of exhaust valves to hit and bend.  I followed some advice from this site and relieved the area in the head above the valves, put in new exhaust valves (lapped), and re-installed the head with a new head gasket.  I got the engine running pretty quickly and let it come up to temp.  Next morning I re-torqued the head as per usual.  Then I decided to test the compression for kicks.  I'm getting numbers like 140-145 lbs.  Higher than I reported before.  If I'm not mistaken this engine is suppose to be in the 87 lbs range.  This just seems wildly high.  I'm wondering if I actually need to relieve the block just to make it run easier.  I have to admit...it seems a little restrained with the throttle response.

 

Hmmm???  Anyone experience this before?

 

thanks, E

 

 

 

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Buy the hi octane gas.

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If the high 80's is proper numbers, and you now have these high numbers, seems to me such will be putting a lot of strain on the lower bearings, just saying.

Dale in Indy

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I'd retry with a different guage. 147psi ~ 10:1 cr and hard to reach with an L head or just advance the timing about 4 degrees & run 93+ octane. OTOH 87 psi sounds like about 6:1 which is kinda low. Key is that all are about the same.

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If it runs good and shuts off without shaking around for a few seconds just forget you took the readings. If you can't stop being curious, drive it a couple of hundred miles and do the checks again.

Bernie

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Accoring to my  Motors Manual there were two heads avalible for the 1938 Packard Model 1600, 6 cylinder and the compression pressures were 110 and 120 psi at cranking speed based on what head you had. The 120 psi (120/14,7) is about 8.1 compression ratio.  If you have 140 psi I think that equates to (140/14.7) about 9.5 compression ratio. Not sure if that will be a problem with the rod bearings or crank. The only way to get it down some is to use 2 head gaskets as someone mentioned.

Edited by hwellens (see edit history)

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Highest practical compression on a flathead is around 8:1. Anything higher cuts off breathing. You could try relieving the block but a better solution would be a new head, or two head gaskets as was suggested by Mr LaFong.

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Thanks for the thoughts guys.   I'll try testing with another gauge first.  It might take a while though, as the packard takes a smaller than standard size spark plug.  I'll have to either fabricate an adapter, or locate one. 

 

I was reluctant to use two gaskets at first, but if in fact my compression is that high, then it might just be worth it for temporary until I get my new head machined (but not too much!).

 

thanks again, E

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While you wait to verify compression pressure , use the highest octane gasoline you can. I would put off advancing the timing for now. Advancing timing a bit against higher octane than originally designed for (low compression) , is a good idea. But your car may have modern compression , and you might actually have to retard it just a little. Bearing load will go up insignificantly on the compression stroke. I say insignificant because bearing load is MUCH higher on the power stroke. But bearing loads were higher using the much lower octane gasoline of the late '30s. Now , with the higher compression , the power stroke loads may be close to design loads. I don't know how to calculate this , and if it is confusing , you can control loads by the way you drive. High manifold pressure (while climbing a grade or driving into a headwind for example), and high revs while driving old cars too fast , factor into loads. See what you find when you check again. Maybe , you might just luck out. You may have a more efficient , somewhat cleaner running engine. Take it easy on the old thing in any case. It is quite true that flatheads have their limits. I am sure we all are curious to hear the progress here. I wish I could significantly raise the compression on my mid '20s Cadillacs. - Carl

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If my theory is correct the car should have plenty of pep and climb hills well, but have trouble  breathing at hiway speeds.

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Got it running....and it runs very well and smooth.  Slow to accelerate still. I have to "feather" it up gradually.  But very smooth motor now.  I then backed it out of the garage and drove around the driveway a bit....I just had too....it's been three years sitting on blocks!  But not much power in forward gears. Hmmm?

 

I jacked up the rear and the rear brakes are dragging a little.  I can still turn them by hand, but I feel resistance and hear the shoes.  I'm hoping that is the main issue. The front is the same.  New shoes (cylinders, hoses, springs, etc) on all four corners.   They could be just a little stiff.  Tomorrow I'll back them off a bit to see if that helps. 

 

But a step in the right direction nonetheless. 

 

thanks for your comments, E

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I think for all of your efforts, it's time to treat yourself (and your Packard) to a replacement head. Packards, and especially six cylinder Packards, are being street rodded as often as anything else. I've seen numerous Packard six drivelines for sale over the last year. Do whatever it takes to cope with your situation for the time being, but, there are better (less machined) heads out there. I suggest you start searching Craigslist communities around your state.

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