Packard Forum Index » Modification Discussion

Hi guys,
typing this a second time, grrrr.

Anyway, my 327 is slowly going together, and was bored +.020 making it a 331 now, (yay) and I have a 288 AT head which will raise the CR to 8.38:1
The engine builder said the head needs to be milled .020 to clean up the marks in it from the old head gasket(s) which I saw, and are pretty obvious.
So, I did some math, and my new compresion ratio will be about 9.2 or perhaps 9.3:1 (I approximated the reduction of chamber volume as 4 cylinders .020 high and surface are of the piston, 2 valves and a spark plug)

I know this is higher than the max stock CR from packard of 8.5:1, but we do have higher octane gas here in germany, (91,95,98 and 102) which I can run.

Is this CR going to be driveable? I'll be super careful for ping, and the engine builder is cleaning out all cooling passages in the block, so that should be up to snuff...

I got chrome acorn nuts for the head, so it will look all 'hotrod'. Will this CR be hotrod, or ping city? It is going to be 327 with OD in a 2 door with no power options...

Thanks, Mike

No comment on the ratio other than it's pretty high. You might want to verify the valve clearance before milling any off just in case the head has been milled before.

Based on another 327 I'm familiar with that had the head milled excessively (we had to grind the combustion chamber to provide valve clearance) and had compression probably in the same range you anticipate, I'd say that if you have enough valve clearance and can get high-enough octane gasoline to control the preignition without having to retard the timing, you'll probably be OK but I doubt very much that you'll experience any improvement in engine performance (power). Packard found about 8.7 to be the maximum for flat head in-line 8s (as in the 359); GM found much lower CRs to be the maximum, for example in the last Pontiac in-line 8s.

I was surprised when I did the math how much the CR will be raised with milling the head 020...I guess I will use this head. I already told the engine builder to be very careful with valve clearances and the head, and if I have problems with pre-ignition then I can go back to the original head.
As long as the clearances check out OK, I'll start with the 288 head, and for the price of a head gasket can swap it back if I have to.

Flat head engines in general are fairly sensitive to compression but 9.2 is not too high by any means, I would keep your timing fairly mild (stock specs) and put in a higher octane gas but I wouldn't think you should have any detonation issues. Just wondering did you mill the block any? this would also raise your compression ratio. The thing to remember is back when these engines were new the octane rating of gas was around 60 something but if you look at a page from the Ford V8 book you'll find that its very easy to run 9.2:1 with modern tech. the problem with VERY high compression on a flat head over an over head valve engine is you need to get the flame front out over the piston to get a good complete burn, a .020 cut on a head is not really much in refrence to the age of the engine and with inline motors tendency to warp heads. If your really loosing sleep about it there are custom gasket manufacturers out there that could make you a head gasket to take up the difference.

Dear TOBS:

I don't understand "marks from the old head gasket" The only reason to cut the head is if the deck is not flat or if the surface is damaged. So I assume he is talking about an area where the gasket was leaking.

Anyway, I think your compression ratio calculation requires some revision. You need to take a graduated cylinder, and with a piston at TDC, pour gasoline into the cylinder to determine the residual volume R. For accuracy using the weight will eliminate false measures.

Now do the math for the displacement V = PI * ((Bore/2)^2) * stroke.

The compression ratio is now (V + R) / R.

A rule of thumb I've heard is milling .060 raises the compression by one.

Long stroke L head motors are very susceptible to piston noise and carbon so I would avoid to high a compression ratio.

Hope this helps.

Ahhhh,
oo?ps my bad...Glad I opened my mouth to show that I didn't know what I was talking about and somebody corrected me....It's embarrasing for a minute, but then you're smarter afterwards. (I was calculating CR as V/R, not (V+R)/R)
So....My head is a 51 288 AT, which should be 7.5:1CR. This *calculates* to a combustion chamber of 91cc. (I didn't and don't think I will cc the head with a pipet)
The block was bored +.020, but not milled or decked.
A 91cc head with a standard 327 calculates to 8.37:1CR. (matches with the chart posted for head xref)
A 91cc head with a 331 cubic calculates to 8.45:1CR.

Now, if my machinest does indeed remove 0.7mm (.027 Inch) then I esimate/calculate a 84.73cc combustion chamber. This will yield a CR of 9:1, which I feel a little bit better about.

The 'marks' on the head are more like gouges from the head gasket. It looks as if somebody really tightened the head down, and the gasket made some dents into the head. It matches the outline of the combustion chamber.

Thanks for setting me straight8.

Tobs

Dear Tobs:

I didn't think about backing into the residual volume until later. It would be interesting to see how accurate the calculation is given the compression ratio is accurate to one significant digit. And an adjustment also needs to be made for the spark plug insulator.

Anything over 8:1 will have to be monitored for carbon build up in the quench area.

I've seen carbon knock cases even on the pre-war motors with the HC cylinder head and they were well under 8:1. 6.85 to one on the Clipper for example.

I traced the shape of the comustion chamber on a piece of paper, and calculated the area with a couple of circles, half circles and rectangles, and came up wit pretty much the same volume/CR as my estimate calcs, so I'm going to play it safe, and use the original 327 head with 8:1CR.
The 327 head has the 'Thunderbolt' on it, which at least looks cool, and from what I have read, the higher CR cuts off breathing becasue there is less flow area above the valves. The extra compression doesn't really add much power either.

Got my OD Trans yesterday after a wild goose chase in the cargo area of the airport (with a hang over and 95 degree F Temps) ugh...
Slowly a packard comes together! Looking forward to the day when we can cruise again.
Tobs

Dear TOBS:

That is a very good point about flow problems in the L-head as Packard used longer stroke (4 1/2 vs 4 1/4) in the 359 motor to increase compression.

According to Turnquist the limit on milling is .045, but that is on pre-war cars. Elsewhere I have suggested milling the pistons on cars to reduce detonation problems.

The 359 was an 8.7:1 compression motor and I assume that represents the practical limit as to compression in an L-head.

One big problem here is that when you mill the Packard cylinder head you reduce the sealing surface between cylinders and so higher compression will be more stressful on the gasket. Of course head gasket technology in those days was still developing and the L-head might have done better with plain metal spacers as is modern car practice.