Compression Ratio Question

[1986GLH]

So my brother and I are currently putting together an engine to put in my 87 Horizon. We are planning on keeping it NA with MPFI for a mileage car. My question is would it be possible to use a swirl head with g head pistons? How much would this change the compression ratio? Or just stick with the g head and get a custom cal?

[1986GLH]

Anyone know?

[bakes]

Use NA pistons or flat top and the swirl head , i like to use the 2.5 tall block and crank with 2.2 na pistons and a mild ported swirl head . then mpfi it.

[1986GLH]

Use NA pistons or flat top and the swirl head , i like to use the 2.5 tall block and crank with 2.2 na pistons and a mild ported swirl head . then mpfi it.

Well what I am wondering is since there are two different types of pistons. The g head has flat top pistons and the swirl has decent sized valve releifs in them. So would it increase the compression ratio alot using the smaller combustion chambered swirl head with the flat top pistons that came with the g head?

[bakes]

it wil

[1986GLH]

it wil

Any idea how much it would increase it?

[jory]

I had the same Q because I'm running 782 head on a 84 turbo block. To quote somebody else:

a stock uncut swirl is 49-50cc. A stock uncut g-head is 55-56cc. Figure 6cc. 4daytons has it correct you gain about half a point to around 8.6:1 if everything else is stock.

[1986GLH]

I had the same Q because I'm running 782 head on a 84 turbo block. To quote somebody else:

So basically since a stock 84 carb engine is 9.0:1 it would raise it to 9.5:1 with the swirl head on with the flat top pistons. I can deal with that, should be a nice little bump in power. Thanks guys for the help.

[amoparacer]

But if you have your timimg belt break you bend valves so why do it. TURN UP THE BOOST AND USE THE G head. Sorry for yelling.

[thedon809]

He's staying n/a.

[1986GLH]

But if you have your timimg belt break you bend valves so why do it. TURN UP THE BOOST AND USE THE G head. Sorry for yelling.

The car is staying NA for now. I have enough turbo vehicles and I want a MPFI daily driver with good fuel mileage. I think with MPFI and the 3.05 FD 525 it should do pretty decent for mileage.

[4 l-bodies]

So basically since a stock 84 carb engine is 9.0:1 it would raise it to 9.5:1 with the swirl head on with the flat top pistons. I can deal with that, should be a nice little bump in power. Thanks guys for the help.

Check piston to valve with this combo, because without valve reliefs you might have an interference motor if you get into a belt breakage situation with the swirl head. If you guys can figure out true cc dish of piston, I can direct you to a website that has compression ratio calculator to figure out exactly what compression ratio will be with any combo.
By my calculations if it is a true flat top piston, with .020 overbore, .065 headgasket, zero piston deck height, & 50 cc chamber, compression would be 9.43:1. Factory ratios are usually a little optimistic.
Todd

[1986GLH]

Check piston to valve with this combo, because without valve reliefs you might have an interference motor if you get into a belt breakage situation with the swirl head. If you guys can figure out true cc dish of piston, I can direct you to a website that has compression ratio calculator to figure out exactly what compression ratio will be with any combo.
By my calculations if it is a true flat top piston, with .020 overbore, .065 headgasket, zero piston deck height, & 50 cc chamber, compression would be 9.43:1. Factory ratios are usually a little optimistic.
Todd

Thanks Todd,
Not too worried about it being an interference motor. Just have to check the timing belt more frequently. If the timing belt breaks then I have a reason to put that turbo engine in it Just want to see what kind of mileage I can get out of it for now. I'm hoping for mid 30's with it.
The pistons are a true flat top and it is a stock bore and will stay that way just going to rering it with the original pistons. So I would say 9.43:1 would be pretty close. I can deal with that.
Justin

[turbovanman²]

I'd agree with Todd, I'd say 9.5 to almost 10:1 compression. If you search google, you can find a compression calculator and you can figure it out exactly,

[rbryant]

My calculations show the 6cc decrease in volume increases the 9.0 compression ratio to 9.7 on a 2.2.

Using the tall deck with 2.2 pistons and a swirl head is around 10.8:1.

Of course it is probably easier to just mill the head than mess with all of this...

-Rich

[Dominic]

My calculations show the 6cc decrease in volume increases the 9.0 compression ratio to 9.7 on a 2.2.

Using the tall deck with 2.2 pistons and a G head is around 10.8:1.

Of course it is probably easier to just mill the head than mess with all of this...

-Rich

Quick question on this last little calculation, these '2.2 pistons', are we talking flat tops or dished...Which the later N/A's had.

[Vigo]

If you use older (carbed/ g head) flat top 2.2 pistons in a 2.5 tall block with a 782 head, you get the 10.8 Rbryant mentioned. I considered this for a long time but never did it because my tall block is .020 over.. then i had a std bore tall block and foolishly recycled it one day without remembering that i had wanted to try that..

[135sohc]

Quick question on this last little calculation, these '2.2 pistons', are we talking flat tops or dished...Which the later N/A's had.

later 2.2 efi swirl head pistons are also flat top. tiny valve reliefs but no dish like the 2.5 variety.

[Vigo]

I guess that's true, but it seems like i rarely find 2.2 tbi's. May just be my area though.

[rbryant]

Quick question on this last little calculation, these '2.2 pistons', are we talking flat tops or dished...Which the later N/A's had.

This assumes flat top 85 or older 2.2l NA pistons.

If you go with the newer 2.2 pistons it is about 10.55:1.

I have found that calculating the compression ratio doesn't always match the stock specs for our cars and it tends to be a little lower than the spec from time to time.


Also I said the wrong head in the 2.2 piston tall deck. The swirl head is actually the smaller combustion champer 50cc head where the G head is 56cc. To get the 10.8:1 you need the swirl head and the early 2.2 pistons.

There is nothing really magical about the tall block here. The same thing can be done with a normal 2.5 if you have custom flat top pistons for it. The only advantage here is that you can use factory components which may be cheaper if you can find a std bore tall block and a set of good 2.2 pistons.

Overall a high compression engine like this should probably have forged pistons though so it isn't a big win and it would be easier to just use a common block 2.5. Plus if you overbore the 2.5 by .020" with flat tops it goes to 11:0.

Here is a spreadsheet that calculates things. It can easily be changed with whatever combination of parts you might want.

compression ratio dodge 22 or 25.zip

The only black magic number in it is the deck clearance (column k) which I fudged to make the numbers work correctly so it wasn't measured. Either way it should be pretty accurate though. If you wonder about the 12.87 for converting to cc it is a combination of pi and the inch to mm conversion.

-Rich