Negative Deck Height for 200

The deck height of our stock 1965 200 is 0.031 inches (measured) and our OZ head combustion chamber volume after milling is 54 cc (measured), the cyl are 0.040 over. The pistons are stock and have a 7.3 cc (measured) dish. Thus accoriding to the For I-6 Performance Handbook the resultant static compression ratio with a Fel-Pro composite head gasket (0.055 inch compressed thickness) is 8.2.

Going to a zero deck height would make the deck clearance 0.055 and a compresion ratio of 8.8.

We would like to decrease the deck clearance from its present 0.086 to 0.040 to improve the "quish" (sp?) and move the compression up to 9.1.

This would require decking the block0.046 inches resulting in a deck heigh 0.015 inches BELOW zero.

At present the cam and rockers are stock. To compliment our Performance headers from Mike and Holley 8007 (390 flow) we are thinking about going to 1.6 rockers and/or 272 high lift cam. According the the Performance Handbook valve clearance should be ok even with this negative 0.15 decking.

THE QUESTION: Is it ok to deck below zero. If so, what are the limits.

Thanks, Dave

Hi Dave,

By my calculation a flat top piston, the Ford 2.3L HSC, would give you a CR of 9.0 with your .031 deck. Since you would have the engine apart to do the decking I think to add the flat top would give you a lot of options, i.e., with a .015 deck and HSC's your CR would be 9.4. I have also read that a.045 gasket is available. At any rate, I guess I'm suggesting that a negative deck is not something I would consider. If you do it, be sure you don't let the top ring get too close to the top of the bore. As for clearance, I believe I read somewhere that the minimum deck for stock cam valve clearance is .025. So, in theory, with a .055 gasket you could negative deck by .030. For my money, go with the HSC's and use any cam you want.

Steve

Dave,

I am not certain if your goal is better quench or higher compression. So, I'll give you two answers.

Part of the reason for deck height is so the piston doesn't hit the head (or its valves). But the MAIN reason Ford (and any other manufacture)uses such big clearance numbers is to allow for sloppy work.

"Blueprinting" is using all of the factory tolerances to your best advantage. If you are careful you can go beyond factory tolerances.

If you are after better quench, you can run the piston above zero deck height. I have run my 200 at plus .006 for about two years. I did it to see if it could be done and what effect the more optimal deck height would have on spark "ping". I used .006 with a .041 compressed gasket. Thus, leaving .035 for connecting rod stretch. I use a Clifford 264H cam with .444 lift. I think the compression turned out to be 9.7 (D8 Heads are milled a "bit").

Just don't get carried away. This is not an area where if a little is good then a lot is better. There are NO known limits! When some one goes too far and bends all of their connecting rods and breaks all of their pistons, we'll all know the limits. Don't be the one to tell us. Give your self some leaway.


If you are after the compression and have not bought the dished pistons for the .040 overbore, then I'd suggest you use the 2.3 HSC flat-topped pistons as Steve suggests.

Good Luck

I know it is done a lot of the time on 302s. I have heard of some race motor 302s running as much as .040 out of the block. With all the talk of quench I wondered how long it would take for this question to be asked.
Jim

Steve,

I agree with your compression ratio figures with flat top pistons.

There are theoretical and possibly real reasons to keep the dish and reduce the deck clearance. If I am missing something here please let me know. I am looking for guidance. Here is what my thinking is based on. Forgive the following core dump.

The problem is the resultant "squench" or deck height (0.070 with .015 deck, and .055 with Zero deck) is far from the "ideal" of .040 inches listed on page 13 of the Ford Six Performance handbook.

The benifits of small squench height with our wedge shapped combustion chambers has been discussed elceware on the forum. In short it is supposed to introduce turbulence in the combustion chamber that allow running higher compression without predetonation.

Dish pistons are supposed to offer some benifits over flat top ones. On page 51 of the Ford six performance book: dish pistons "... offer some advantages in helping to control combustion and pre-ignitiion. ...all things being equal, dished pistons, with a wedge type head, make more horse-power than flat top or dome type pistons."

On a practical note, we have two engines, and are going to build-up one block, so decking the block is a matter of how much instead of if at all. We are also running adjustable rocker so milling/decking changes will not cause pushrod problems.

Thanks, Dave

Dennis,

Thanks for the info.

The question about limits includes if there will be enought metal thickness left on the top of the block after decking. The question follows from the 0.090 milling limit on the head. Does a similiar limit exist for the block? Your book provides very good guidance on avoiding valves hitting the top of the pistons, and the pistons hitting the head. Providing a margin for rod streach is very thought provoking, and I guess why a 0.040 deck clearance is the ideal instead of zero. Point very well taken. Thanks.

Our goal is the best performance on pump gas, hopefully regular. Thus the static compression ratio goal is between 9.0 and 9.3 and a quench height of 0.040 (per your great book). By the way, when I called Clifford asking about thin head gaskets, to test higher CR before machining, (they did not have any) they stated that they run a 10:1 static CR with performance cam (270?) on regular pump gas. Could the cam's valve overlap effecting actual CR? Interesting. Knowing and comparing compressions in addition to static CR could provide guidance here.

I note that with your deck height 0.006 inches below the pistons and 0.041 head gasket results in deck clearance of 0.035 inches which is of course 0.005 inches less than the 0.040 (-.015 deck heigh + 0.055 Fel-Pro head gasket) we have proposed.

What brand of Head gasket are you using to get 0.041 compessed thickness? NAPA?

By the way what compression reading do you get with your configuration's 9.7 cr? What type of gas is required?

By the way caution is my nature, antiseize is the means.

-Dave

You will need to purchase your pistons first to determine how much you want to take off your deck, I think you will find that when you purchase oversize pistons, some manufacturers reduce the top of piston pin to top of piston height to allow for decking the block. I made the mistake of decking to zero deck height with a .030" over piston and when I determined that it would have to go .040" to clean up the #1 cylinder, the .040" over pistons were .010" longer pin height from the top, proving that different manufacturers make a difference, leaving me with .010" poking out of the deck. I am still running that today after 2 years.

Dave,

Re-read 63 Comet's advice about having your pistons in hand BEFORE you do the cutting. He is RIGHT ON! My "stock" pistons were a different height than the Silvo-lite +.030 replacement pistons.

Clifford is telling you true. The more duration a cam has the less dynamic (as oppopsed to static) compression you have. That's why cam manufactures tell you to keep increasing the compression with each step up in duration. Basically, the intake valve is still open when the piston starts up on the compression stroke thus reducing the effective compression stroke.

So now someone is thinking "doesn't having the valve open with the piston coming up force air out of the cylinder. NOPE! When the piston is sucking down there is such a steam of gas-laden air coming into the cylinder that it doesn't even slow down when the piston hits the bottom and starts up. The cam manufactures know when to close it before it starts back-washing.

I don't remember which brand head gasket I've got. I should - cause I have had the head off twice since the block was built. But I just say, "Gimme a head gasket" to the machinist I get my parts from. He's a "MN to A" machinist. That's March to November and then to Arizona. I'll make it a point to find out as soon as he returns for the circle track racing season.

I use either 89 or 91 octane gas depending on the weather. Add this to your building plans. Every 10 degrees temperature drop in the under hood is equal to something like one octane. Plan for a cold air intake. David uses a LTD air cleaner (on about page 55) and I have an open hood scoop. His is more efficient - mine looks better.

You asked what compression reading with 9.7 - if you mean psi - about 145 to 150.

About the limit to cut the block. Tuff call with an old engine. Has it been rebuilt before? Was it cut then? Dave cut the top of his 250 block (2.3HSC pistons) a big cut but he'd be the one to remember that.

Good Luck

Just for reference:
Ford used .0005" to .0000" (that's 1/2 of 1 thou, down to 0.0") for deck height tolerances on the 390 "H" series engines. These were often decked to be 1/2 of the head gasket height, or about .020" ABOVE the deck in assembled configuration.

And, they run easily over 100,000 miles set up like that, so long as the cam isn't so radical as to get the valves in the way of the piston crown. Wide duration is OK, but high lift is not on those engines. Redlines typically stop at 5800 RPM, which is a little higher than the 200 I6 seems to like to spin.

Hi i am Davels's son. The engine is a freshly rebuit engine. The man who we purchesed it from had it completly rebuilt. From what he said he put about 500 miles on it before he took it out to swap for a v8.

Hot 6t Falcon":ljefflyy said:

You asked what compression reading with 9.7 - if you mean psi - about 145 to 150.

Click to expand...

So, does this mean that there is a correlation between CR and cylinder pressure?

I'm wondering because I never did shave the cylinder head during my rebuild last summer. Before I rebuilt the engine I was averaging 130-150 PSI for each cylinder (except the #2, bad, 30 PSI). Now, I am getting somewhere between 120-130 PSI for each cylinder.

I knew I should have compensated for the thicker head gasket. :roll:


-Chris

Howdy All:

I decked my 250 block .070" with no problems so far, except for another lesson in bottomed head bolts and the value of using waher under the head bolts on assembly. Any more than that and I recommend sonic testing to verify structural rigidity

CR and cylinder pressure have a relationship and it is consistant so long as cam timing does not change. The more cam duration, the less cylinder pressure at idle. This is one of the reasons more cam duration loses bottom end power. The pressure will build with rpms though and more than break even at higher rpms.

How much positive deck height can safely be run in an engine is determined by many factors. Many have already been mentioned. Another is how close to the top of the cylinder the top/compression ring comes. It is critical to maintain a dimension from the top of the ring landing to the top of the deck of the block. The margin on a stock OEM 200/250 piston, measuring from the top of the 1st ring land to the top of the piston, is about .250" or 1/4" it is clear that one has a margin of error to play with. But, be careful. The higher out of the bore the piston comes, the more heat is directed at the compression ring, with less time to transfer that heat to the piston or cylinder wall. Not good!

It's clear from previous posts that .006", .010", and even .020" have been used sucessfully. How much more do you think you need to achieve your goals, DaveLS?

I'm curious about the details on the pistons you're using with this large of a dish. Most small dish OEM and replacement pistons I've measured were closer to 6 cc? If you have the large dish (Calif Pistons) you are losing in bore to quench ratio. Check page 38 of the FPH for photos. Maintaining bore to quench ratio is as important as the quench dimension of .030" to .040".

Mark P is the "MAN" on head gaskets, but, IIRC, Victor is the brand that is in the .045" thickness.

After all is said and done, I'm on the conservative side of this issue. I'd rather build an engine with zero deck height and live with less than ideal quench height with a .045" to .055" head gasket.

In my next engine I'm going to use a set of HSC flat-top pistons and have a "D" shaped dish machined into the top that reflects the compustion chamber. That will increase/maximize bore to quench ratio, give me a dished piston but, with a smaller dish, and a lighter piston.

Adios, David.

Thanks to everyone for their input. This forum has saved our project on more than one occasion.

Here is a summary of key points:

1) neg deck height is ok but requires care, if you want to do this reading this entire string would be useful
2) Up to 0.070 inches may be removed from a "new" block from a metal thickness standpoint
3) Rod streach must be accounted for with a safety value (.035 inches)
4) Top Compression Ring location must be considered and maybe different with different pistons, so have new pistons and measure before preforming block surgery
5) not all pistons will give the same deck height, so again have new pistons and measure before performing block surgery
6) The relationship between Compression Ratio and Compression (PSI) is complex and while neither provides absolute octane/pre-ignition information Compression Ratio is the better of the two indexes.
7) The maximum compression ratio that can be used with a given grade of gas is strongly effected by valve timing (cam).
8) Difference in head gasket compressed thicknesses can be critical.

So the plan at present: Deck height, compression ratio, and piston type are still open questions. First we will determine the cam and rocker arm rations for the build and then determine the max CR for those components with pump gas. From there the deck height and piston type will follow.

Why the question in the first place:

We have a 54 cc combustion chamber. To achieve our compression ratio goal of between 9.0:1 and 9.3:1 with flat top pistons would result in an excessively tall and therefore inefficient quench zone. Additionally the head gaskets that we have been using have a compressed thickness of 0.055 inches (Fel-Pro) so getting a Deck Clearance of 0.040 is clearly not possible with these gaskets. Finding a 0.040 head gasket would solve a lot of these problems. So it looks like we will be looking for a Victor Head Gasket with 0.045 and maybe .005 negative deck.

David,

You asked about the piston's dish. The pistons came with a recently purchased used rebuilt engine from a private individual and I do not know the rebuild shop or piston brand. When we swapped out the stock head for our OZ head from our old engine, using a venire caliper (±0.001 inch) I measured the dish diameter at the bottom and top (its tapered), and the depth and calculated the dish volume in inches and converted it to cubic centimeters. I took five measurements of each value and used the average of these for the calculations. I have not calculated the margin of error the resultant value and of course the actual value could be different.

avg dish depth 0.1169 inches 0.296926 cm
avg dish diam 2.20545 inches 5.601843 cm
dish vol 0.446579543 inches 7.318127555 cc

You asked about how much negative decking we are thinking about. Assuming nothing changes about 0.015 inches. This give a 0.040 margin for rod stretch with the standard Fel-Pro gasket.

The calculated static compression ratio 8.26.
Kevin (kstang) measured the compressions today and found the following:
#1 cyl 179, #2 176, #3 174, #4 185,#5 189, and #6 181 psi

Interestingly our compressions (stock cam and 1.5 adj rockers) are greater that the 145 to 150 reported with a 9.7 CR and Clifford 264H cam with .444 lift. .

Before changine to the OZ head, the stock head was about 10 psi greater. I have not measured the combustion chamber volume of the original stock head.

Thanks again for the thoughtful insights.

Regards, Dave

Howdy DaveLS:

I did some diggin' and found that Victor brand head gaskets are typically .045" and are distributed through NAPA locally. FelPro are .055" and are available from most parts store outlets.

Please share your final plan details, discoveries and dimensions.

Adios, David