increasing compression in w302

Sure, its possible, shaving the heads is probably the least expensive way.
A combination of of heads & pistons, would probably be best. Your question is rather generalized to accurately, determine out put increases.
Are you looking to optimize an engine for Propane use?? Or what goal do you have??

Well it may take a little more research on your part to find out what optimum compression for a porpane powered engine is. I'd start with several googles.
Once you have the parameters, you want to use, then adapt your engine to that.
Most stock heads were not designed to support compression ratios like your asking for. Shaving a stock head wont give you more than a couple of points ratio gain, without doing much more than I feel your wanting to do.
If you really want more ratio, pistons is probably what you want.
You might be best served to just adapt your engine to propane as it sits, and/or be satisfied with minor mods.
For instance; I'm in the process of replacing a leaking headgasket, while I'm at it, I have done a little reshaping of the exhaust ports(& remove EGR bump) of my E7 heads. Nothing real extensive, & it only took about an hour to do both heads. This could give me 15-25 free HP. Possibly this is the type of thing you need to do.
First I'd find out more about Propane as fuel.

You could look for a set of used 65-67 289-2v heads. That would bump your CR to from 9.0 to 10.0, the cost to rebuild a good set of 289 heads would be comperable or cheaper than shaving the 302 heads since to achieve your goals with 302 heads you'd also have to shave some material from your intake.

The octane ratings I have seen for propane vary somewhat, but most sources put it around the 108-112 range. You could probably get away with 13:1 compression running strictly propane.

You'll be limited by needing to run on pump gas with iron heads. I wouldn't shoot for much more than 10.5:1 in that case. Also keep in mind that propane is going to kill your volumetric efficiency by about 15% because it displaces much more of the intake air than gasoline.

A few points and questions:

What year 302?
They dropped compression about a half a point per year from 71 to 73.
Was 9.0:1 in 71, then 8.0:1 by 73!
That will affect an accurate answer to what you are looking for.
I don't quite understand your rationale, but understand your basic questions.

With iron heads, you will be limited to about 9.5:1 on pump gas.
They detonate much easier than aluminum heads. You could squeek out 10.5:1 with aluminum.
The 9.5:1 is probably too low for propane.
Remember that too much octane is a bad thing if you don't need it!
You should never run more octane than what you truely need.
It makes your engine less effiecient, lowers MPG, and can carbon up the works.
Higher octane is less volitile, and if you don't have the compression, it won't burn as completely. So it's a big waste. The big secret that the gas station commercials don't want you to know when they talk about using premium fuel with fancy names like 'techron' and such. Most cars do not need premium. I'll get of the soapbox now and back to the subject....

Depending on the year of the engine, I would think you have 58cc heads.
The only way to do better than that is get some earlier 289 heads as proposed in another post.

You can gain by shaving your heads a little and seeking out the thinnest head gasket you can find.
You can also shave the block if you have it apart.
The thing about shaving either the block or the heads is that you also need to shave the intake if you do either one, or a total of more than .010".

Another thing you can do is use a cam with 'pressure building' characteristics.
Most RV cams are pressure builders. They use valve timing to increase the dynamic compression seen by the engine.
If you get a cam with alot of overlap, it will bleed off your static compression, effectively lowering what the engine actually sees when running.

I see nobody's asked him yet what pistons and year he's got. The 70-71's had a slight dish, the 72-76 motors got pistons with huge dished tops(8 to 1 ratio). With these, you'll never get anywhere close to a 10 to 1 ratio. The easiest way, is to use flattops with a 1.610 comp height, and combine that with 54 cc heads (66-67 289) That will yeild a 10.5 or better ratio. Even with the 70-71 pistons the only way to 10 to 1 is to use the 289 heads. If you're shooting for 12 or better, the only route is the old 12.5 to 1 pop up pistons, I think these are still made. I don't see the reasoning in using Propane, unless you haul it and get it free. It cost's nearly the same as gasoline, but it yeilds less BTU's so you lose MPG.

With a piston replacement, you can get any compression you want.
There are almost unlimited pistons to choose from, and even if you don't find one that is perfect, you can fine tune them with reshaping.

That way you don't need to change/modify/shave anything at all.

The only thing you should be careful with is the heads.
Make sure the pistons are compatible with whatever heads you intend to use long term.
If you think you may change heads in the near future, wait to get your pistons until you get the heads. Often the dome on a piston requires it be used with a specific combustion chamber size or shape.
That is about all you need worry about.

Rods will not increase compression at all.
Bore and stroke will...

I think the idea behind longer rods was to pop the piston up higher. It will work in theory, but not in practice. SBFs don't typically have the pistons down the bore any more than ~.020". Rather than change the rods or mill the block, pop-up pistons like was mentioned by ratio411 will be easier and just as effective. Another option is leaving the low compression and super/turbocharging the engine.

Usually engines are built around a special fuel or the fuel is picked based on what the engine needs.

pazur said:

giving it second thought you are right (411, 65shelby, d.hearne); don't need connecting rods. Most likely will try these (or similar):
http://store.summitracing.com/partdetail.asp?part=UEM-KB115&autoview=sku

These are flat top pistons. Is there any easy way to figure out what piston's compression distance is needed (whatever this distance means) at stock 58 ccm chambers and .5" max valve lift (will be less but to be safe), to get CR as high as possible (but not beyond 11:1)?

Click to expand...

The ones you linked will get you to 9.76 with 58 cc heads. Once you have everything on hand, assemble one rod/piston on the crank, then verify the deck clearance. If there's room, you can always mill the deck to get to a zero deck. Actually milling should be done anyway to insure the decks are square and level with the crank centerline. Then have the heads milled to 54 cc, that will get it to 10.28 to 1.

pazur said:

Thanks everybody for the feedback. Can you also tell me what's the piston's compression distance and deck clearance on stock '73 302 2V engine?
Is there any web site with detailed specs of this engine? (google does not show anything)

Click to expand...

Since 73 was 8:1 compression, I am wondering if that was one of the years with the tall deck blocks.

Rather than change heads or pistons, Ford played with the head gaskets and cylinder blocks some years.
For a time, the 302 block was raised .020" taller to gain deck clearance and therefore lessen compression.

If 73 was one of those years, you know right away that the piston is .035" or so in the hole, vs the usual .015"+/-.

Sorry I don't have the details handy!
Thought I would throw that out though, so you could be on the lookout.

The idea behind longer connecting rods is to keep the piston near the top of the bore for a longer period of time during the cycle. It is supposed to increase efficiency during combustion because the volume is smaller during the time it takes for all the fuel to combust.