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Discussion Starter · #1 ·
My 347 is coming together in the next few weeks and I was wondering what you think the horse power gain would be by going from 10 to 1 compression up to 11 to 1.

I guess the cam selection might change a little, but in the end what would you use for a ballpark number?

thanks!
 

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I would let the cam determine the compression ratio. What you really want to pay attention to is the Dynamic compression ratio as it is what determines cylinder pressure which is what will be the deciding factor in allowing the engine to run on pump gas. What cam are you using and you will need to find out what the intake valve closing point is in degrees after TDC. The earilier it closes, the higher the DCR wil be.

But to answer your question, I think one point is generally 10-15 hp
 

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Under 20 HP for sure.....

The reason is your decreasing the volume of the chamber, meaning less room for the intake charge. Not the best way to make HP. The engine will also be more fussy to tune and type of gas you can use if your making a street / strip car....

My 9.8:1 is running on 87 octain for street driving. 380 HP

The cam duration is the most important factor for what HP you want to make.
 

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Mikes66, just wondering what your duration on your cam @.050 is that your making 385HP out of a 302?? Also your heads are Windsor Sr.......What would the flow be on those??
 

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238 degrees at .05 intake. I went with the Windsor Sr.s with 60 cc chamber (from plaining the heads) and 2.05 valves. Valves that big are not really needed on a motor this small, but the larger diameter valve is worth about .05 lift. My cam is .500 lift instead of having a cam with .550 lift ( and more valvetrain stress) special springs, clearence issues...ect.... I went with bigger valves instead.

It is an old custom grid I have made for me about 10 years ago Please keep in mind I designed this motor and built it 10 years ago... only recently found a home for it in this 66. Beehives and other goodies did not exist yet.....

Add a 1 inch 4 hole spacer under an annular venturi booster carb ( A 575) and I have superior tourque at low RPM's and the spacer for an extra 20 HP. Only 1 HG at 6500 rpm's so the carb is not too small.....

I have considered going to 1:1.7 rockers, but I do not think I will gain anything with a higher lift... it is all about the duration at this point.

PS: I get about 17 MPG too.....



<font size=-1>[ This Message was edited by: Mikes66 on 12/12/06 9:35pm ]</font>
 

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Actually, that's 10 points, not 1 point. One point would be 9.0 to 9.1. At that level - 3% or so. Changes to the overall combination to take advantage of the increased compression would result in more.
 

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*your decreasing the volume of the chamber*

I didn't see where op said he was changing chamber size but just remember raising compression has deminishing returns. Go from 8 to 10 to 1 and get big returns but go from 10 to 12 to 1 and get much less.
 

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Raising the compression naturally decreases the volume of the combustion chamber...

He asked about the HP gain from raising the compression one full point from 10:1 to 11:1, that action will naturally decrease the combustion chamber size <U>unless</U> they are useing only the cam to raise the compression...

I have no idea how much power can be had from going from 10:1 to 11:1, but you are right that a jump from 8:1 to 9:1 is more productive by it's self than say going from 12:1 to 13:1 because you're changing all of the fuel requirements when going that high in compression...

Just my $.02

FE
 

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It all depends on the cam.

But, increasing the compression ratio (even the static one) will increase thermal efficiency, and net you more power, no matter what. It may just not be worth it. You squeeze any amount of Air/fuel mix harder before you light it, and you're going to get more power out.

And it may decrease the volume of the combustion chamber, but it has no effect on the swept volume of the cylinder (the amount that the piston actually draws into the cylinder.)

You're talking probably 20HP if you have a cam with large overlap and duration, probably 5-10 if its relatively stock.



You'll see diminishing marginal returns with larger increases in volumetric (static) compression ratio (especially, but FE is right, you'll realize a larger gain from say 8-9 than 12-13.

<font size=-1>[ This Message was edited by: thekingofazle on 12/13/06 3:06am ]</font>
 

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Point was chamber volume is determined by the physical size and shape of the chamber. A 75cc combustion chamber will still be 75cc's wheather you use a flattop or dome piston.
 

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David Vizard's chart shows 2, 1.7 and 1.5% when going from 8-9, 9-10 and 10-11:1 compression however dyno testing on a particular 440 engine showed a bit more.

The engine had open chambers so quench was not a factor. Camshaft was optimized for 11:1 but remained unchanged.

They varied compression by stacking .060 copper gaskets.
Intake gaskets were also stacked so as not to create a port mismatch.
Power went from 500 with 11:1 down to 440hp with 8:1.
3.3, 4.9 and 4.4% when going down from 11-10, 10-9, and 9-8:1 compression.

I can't think of a more accurate, real world way to test for compression changes alone but the proper way to build an engine is to optimize compression by choosing the right cam too.
 

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Compression makes power!! Put in as much as you can! Look at what my 427 did with 14.7 to 1!!
 

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On 2006-12-12 15:26, FoMoCo wrote:
Point was chamber volume is determined by the physical size and shape of the chamber. A 75cc combustion chamber will still be 75cc's wheather you use a flattop or dome piston.
FoMoCo

A 75 cc combustion chamber will alway's be a 75cc chamber unless the head is milled. BUT, if you fill that 75cc chamber with a dome top piston you no longer have 75cc's of empty space to combust in...

See what I mean?

FE
 

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On 2006-12-12 15:26, FoMoCo wrote:
Point was chamber volume is determined by the physical size and shape of the chamber. A 75cc combustion chamber will still be 75cc's wheather you use a flattop or dome piston.
FoMoCo

A 75 cc combustion chamber will alway's be a 75cc chamber unless the head is milled. BUT, if you fill that 75cc chamber with a dome top piston you no longer have 75cc's of empty space to combust in...

See what I mean?

FE
 

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Discussion Starter · #15 ·
this started because the original plan was to go for about 10.1 in my 347, now its ended up a minimum of 10.8 or higher depending on head gasket.

The cam is spec'ed with the new compersion and I was just wondering what the HP change might be.

If you look at the ford motorsport crate 347 that puts out 450hp, it has 10 to 1, if you raise it to 11 to 1, tweak the cam a little to match the combo, would really get that much more power? Somebody said 20hp, that seems like it would be worth it.

thanks everybody.
 

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I have also seen the 4% per point on dyno graphs, but cam selection can really effect it. Using the same cam at 8:1 vs. 11:1 will not be the optimum cam for one or the other. More compression is usually better unless you have to run a really lame advance curve to avoid detonation.
 

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On 2006-12-12 03:20, Mikes66 wrote:
Under 20 HP for sure.....

The reason is your decreasing the volume of the chamber, meaning less room for the intake charge. Not the best way to make HP. The engine will also be more fussy to tune and type of gas you can use if your making a street / strip car....

My 9.8:1 is running on 87 octain for street driving. 380 HP

The cam duration is the most important factor for what HP you want to make.

i didnt know u could get an annular booster in the under 750cfm size carbs or did u have to convert or custom order?
 

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Under 20 HP for sure.....

The reason is your decreasing the volume of the chamber, meaning less room for the intake charge. Not the best way to make HP. The engine will also be more fussy to tune and type of gas you can use if your making a street / strip car....

My 9.8:1 is running on 87 octain for street driving. 380 HP

The cam duration is the most important factor for what HP you want to make.

380hp @ tires or flywheel? I'm assuming flywheel, if that's at the wheels that is impressive!!!
 

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380hp @ tires or flywheel? I'm assuming flywheel, if that's at the wheels that is impressive!!!
Since Mike66 hasn't stuck has head in here since 2009, I doubt that you will get an answer. I agree that its probably flywheel though.
 

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this started because the original plan was to go for about 10.1 in my 347, now its ended up a minimum of 10.8 or higher depending on head gasket.

The cam is spec'ed with the new compersion and I was just wondering what the HP change might be.

If you look at the ford motorsport crate 347 that puts out 450hp, it has 10 to 1, if you raise it to 11 to 1, tweak the cam a little to match the combo, would really get that much more power? Somebody said 20hp, that seems like it would be worth it.

thanks everybody.
My 347 combo works out to be 10.8 and I was told from the builder of the short block (CHP) that with my heads, intake, carb, etc. and the cam they spec'd I'd be making roughly 475-480HP. The way the car runs I have no reason to doubt their estimate! :eek:

John
 
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