Compression Ratio

TrueBlue95GT

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Jun 17, 2005

#1

• Jun 17, 2005

• #1

What's the highest compression ratio you would recommend while running on pump gas (93 octane)? Oh and by the way, its a naturally aspirated 347.

 

Joes95GT

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Jan 23, 2003

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Jun 17, 2005

#2

• Jun 17, 2005

• #2

I wouldn't go past 10.5:1.

One bad tank of gas, a bad tune, a clogged injector, wrong camshaft, etc. could mean destruction...

Joe

 

S/CBlack95GT

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Jun 17, 2005

#3

• Jun 17, 2005

• #3

11.0:1 isnt uncommon on 93 octane if you tune it right.

 

TrueBlue95GT

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#4

• Jun 17, 2005

• #4

Well I'm getting a custom cam and I'm getting it tuned the day the motor is first started up.

 

Joes95GT

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#5

• Jun 17, 2005

• #5

TrueBlue95GT said:

Well I'm getting a custom cam and I'm getting it tuned the day the motor is first started up.

Click to expand...

The former would be a great idea. The latter would not.

The rings need to seat before you start revving the **** out of the motor. Some will say 3-5 thousand miles. I don't think that's a common practice anymore now that technology has gotten much more advanced. I would say 500 is sufficient.

11:1 will live. I've heard of 12:1 living on 94 Sunoco fuel, but I'm assuming the dynamic compression ratio (created by camshaft timing) was not optimized.

The reason I suggested 10.5:1 is because it leaves plenty of room for error without sacrificing tons of power.

Believe it or not, there are minimal power gains to be made by increasing compression by one-half of a point, however, there is lots of "room for error" taken up by up-ing the compression by this respective amount, in this situation.

Joe

 

final5-0

Mustang Master

Apr 6, 2003

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Jun 17, 2005

#6

• Jun 17, 2005

• #6

Compression equals power and everybody knows it. For a street car and even more so if it is a d d street car, high compression can bring with it many things you might not want to deal with which some have pointed out.

Consider many of the stock block combos that are in the 300rwhp range. A lot of em have elevated compression levels with smaller chambered heads, milled decks, thin gaskets, and the like but your not gonna up that compression by a whole lot.

With the extra cubes you'll have extra power as it is, so I think you can make a good bit of power for a street car without mega compression as Joe pointed out above.

Later
Grady

 

TrueBlue95GT

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Jun 17, 2005

#7

• Jun 17, 2005

• #7

Well I'm asking because I have 58cc Twisted Wedge heads and with the Scat kit I wanna get, I will be up to about 11.5:1 compression. If I go with the domed pistons, I will be down to 9.5:1 which I dont want. The domed pistons are an extra 25 bucks and I will have less power. Is 11.5:1 compression ok running 93 octane?

 

super302

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May 29, 2004

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Jun 17, 2005

#8

• Jun 17, 2005

• #8

TrueBlue95GT said:

Well I'm asking because I have 58cc Twisted Wedge heads and with the Scat kit I wanna get, I will be up to about 11.5:1 compression. If I go with the domed pistons, I will be down to 9.5:1 which I dont want. The domed pistons are an extra 25 bucks and I will have less power. Is 11.5:1 compression ok running 93 octane?

Click to expand...

Get the domed pistons+boost

anyways, didn't the old musclecars have like 12:1 compression

 

Grn92LX

Fidanza Man!

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Jun 17, 2005

#9

• Jun 17, 2005

• #9

TrueBlue95GT said:

Well I'm asking because I have 58cc Twisted Wedge heads and with the Scat kit I wanna get, I will be up to about 11.5:1 compression. If I go with the domed pistons, I will be down to 9.5:1 which I dont want. The domed pistons are an extra 25 bucks and I will have less power. Is 11.5:1 compression ok running 93 octane?

Click to expand...

You will NOT be 11.5:1 with the scat kit. That kit uses a probe srs forged piston. Is your block zero deck? If not, what is it? I'm doing a 347 now with a scat cast crank, probe ultra light 5.315" rods and probe pistons.

My TW heads are 55.6cc and with a zero deck block and the probe -4cc pistons. It comes out to a tick over 11:1. I do not see this as being a problem since my cam was spec'd knowing my compression was about 11:1.

 

TrueBlue95GT

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Jun 18, 2005

#10

• Jun 18, 2005

• #10

http://www.adperformance.com/index.php?main_page=product_info&products_id=212

it says on the site that with a 58cc head u will get 11.5:1 compression. I don't know if the site is right, but that's what it says.

 

Joes95GT

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Jun 19, 2005

#11

• Jun 19, 2005

• #11

I wouldn't go by that.

We know the following:
1) Crank size - 3.400"
2) Rod Length - 5.400"

We still need to know the following:
1) True deck height
2) Piston compression height
3) Piston design and measurement
4) Gasket thickness
5) True head cc's.

Joe

 

18mustangs

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Jun 19, 2005

#12

• Jun 19, 2005

• #12

super302 said:

anyways, didn't the old musclecars have like 12:1 compression

Click to expand...

The highest Mustang only went to 11.4:1 compression in 1970 on the 351 Cleveland 4V, the others went between 10.5-11.0:1 compression (until 1972 when big brother enacted the smog laws) and that was with good high octane leaded gas.
--Tim

 

TrueBlue95GT

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Jun 19, 2005

#13

• Jun 19, 2005

• #13

Well how would I measure all of these things without having the rotating assembly already?

 

bjl95mustang

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Jun 20, 2005

#14

• Jun 20, 2005

• #14

Ok, Here are some ideas.

3.4 stroke
4.03 bore
58cc heads
.039 gasket
0 deck
0 piston

11.74:1

------------------------

3.4 stroke
4.03 bore
58cc heads
.06 gasket
0 deck
0 piston

11.07:1

------------------------

3.4 stroke
4.03 bore
58cc heads
.039 gasket
0 deck
-5cc piston

11:1

----------------

3.4 stroke
4.03 bore
58cc heads
.06 gasket
0 deck
-5cc piston

10.4:1


If you get a .06 head gasket and a -5cc piston you get a 10.4:1 cr. The best thing is to call adperformance and get the piston specs to get an idea on what cr you can get. your cr will also drop if your not at 0 deck. For every .01 of deck you will lose about .25 of a cr point. The .25 is a close guess in my opinion.

 

TrueBlue95GT

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Jun 21, 2005

#15

• Jun 21, 2005

• #15

The guy from AD performance said that the flat top pistons were 4cc and the dome pistons are 9cc. What would my compression ratios be then?

 

Grn92LX

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Jun 21, 2005

#16

• Jun 21, 2005

• #16

You will be appx 10.9:1. I had my builder do the math for my combo (same pistons/deck/cubes as you) and itcame out to about 11:1 with a 57cc chamber, .039" gasket and zero deck. I ended up having my heads clean up milled so their 55.6cc now.

 

go-stang5.0

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Jun 21, 2005

#17

• Jun 21, 2005

• #17

I run a 10.78:1 comp ratio on my 331 according to DSS who did my whole long block using their graphite head gaskets. So far I havent really had any problems with detonation (only once it happend when my fuel filter got clogged and fuel pressure dropped to like 30-33 psi at WOT, and I'm only running 24 lb. injectors so anyone knows thats too low) but that was not anyone but the gas stations fault. I also drive daily and it is still untuned. Even when I changed my plugs recently (mind you the lean condition happend during auto crossing so were talking hard driving on a 85 deg. day) the plugs were browner (not the light grey/tan normal color) but they had no appreciable damage to them so the detonation had to be pretty light. Anyways you can get by on pump gas...now if you were to ask me to run 91 octane ... I'd really have to think about it.