compression ratios & octane

nosub4cid66

New Member

Dec 31, 2006

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Cedar Springs, MI

Oct 19, 2007

#1

• Oct 19, 2007

• #1

I'm gonna' build a new engine for my mustang and I'm looking at several different rebuild/stroker kits. I'm gonna build a 347 out of my 5.0 or a 408 out of my '94 351w roller block. I noticed that most of them steer you to the direction of 10.5:1 compression with 64cc combustion chamber heads, but if you specify a dish to the pistons it drops compression to 9.4:1 to 9.7:1ish depending on the size of the dish. I'm going to use the car as a weekend street machine I could care less about paying $2.00 more for a 20 gallon fill up to use 93 octane premium fuel...I'm worried that if I run 10.5:1 compression and the wife or someone else uses 87 octane regular it will suffer detonation. Will I loose significant HP & TQ by droping my compression? I'm looking to get 350ish HP out of a 347 or 425ish out of the 408 and either would live its life under 6,200 RPM...also I never plan on using a power adder should pay more and use forged slugs or would hyper's be fine in a 408 pushing a little over 400hp

 

ninety15.0

New Member

Mar 10, 2004

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Oct 19, 2007

#2

• Oct 19, 2007

• #2

10:1 should be fine for a street car on pump gas....even 10.5:1 is ok. Detonation will be controlled by timing and octane. Im sure you can get a 10.5:1 408 under control with 93 octane. For a N/A car i would say the higher the compression the better.

a 10:1 or 10.5:1 408 with a good HCI setup should be a very healthy engine and will be a ton of fun on the streets. Good luck man

 

samickguy15

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Sep 1, 2005

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Delavan, IL

Oct 19, 2007

#3

• Oct 19, 2007

• #3

Just keep your timing relatively stock for street use. When you bump it up for "performance driving" is when you run into issues with high compression. Either that or just beat it in her head that she has to run premium or she's paying for the new motor

 

Fast63

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Sep 20, 2007

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Oct 19, 2007

#4

• Oct 19, 2007

• #4

I run my 5.0 with 87 octane and it has 10.1:1 compression. Timing is 10 degrees initial, 32 total with mechanical. Add some for vacuum advance which is not added under hard acceleration. It depends on other variables than timing and octane. Good "squish" between the head and the piston and combustion chamber shape play a large role. I've got .041 squish with the heart-shaped combustion chambers of the Roush 180 heads, so it burns very efficiently.

 

nosub4cid66

New Member

Dec 31, 2006

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Cedar Springs, MI

Oct 20, 2007

#5

• Oct 20, 2007

• #5

ok well it sounds like I should keep my compression around 10:1 - 10.5:1, what about my pistons? Would hypereutectic be fine in an engine pushing just a little over 1HP per CID? I see some expensive stroker kits that still use hyper pistons for "street" applications from reputable companies such as eagle then scat has a kit with forged slugs for less money, what gives?

 

Georgie

New Member

Jul 13, 2005

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Montreal, Canada

Oct 20, 2007

#6

• Oct 20, 2007

• #6

sorry for butting in..but what does detonation mean?

thanx

 

samickguy15

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Sep 1, 2005

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Delavan, IL

Oct 20, 2007

#7

• Oct 20, 2007

• #7

Detonation is when the cylinder starts burning before the piston reaches TDC. What it does is tries to compress the expanding gasses(the fuel air expand as they burn) while the gasses are trying to expand. It does really bad things to pistons and rods and generally blows the head gaskets out. It's just bad news.

 

5spd GT

"the 5.0 owns all"

Founding Member

Aug 7, 2002

9,516

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99

Arkansas

Oct 20, 2007

#8

• Oct 20, 2007

• #8

Most engines are going to see detonation from time to time.

350rwhp is quite attainable, and you can go with 9.5-10.5:1 compression and still get those power numbers.

If you decide to go any higher in compression than that, you really need to have a cam designed to not allow so much cylinder pressure.

Higher compression, the hotter the car will run, if you do not have the cooling for it.

I have yet to see anyone with 11.0:1 or greater, and literally daily drive it as their only car.

 

samickguy15

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Sep 1, 2005

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16

Delavan, IL

Oct 21, 2007

#9

• Oct 21, 2007

• #9

5spd GT said:

I have yet to see anyone with 11.0:1 or greater, and literally daily drive it as their only car.

Click to expand...

It appears I have a challenge...mwuha ha ha ha ha...E-85 anybody??? I think that's the only way you could do 11:1 for a DD.

 

5spd GT

"the 5.0 owns all"

Founding Member

Aug 7, 2002

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99

Arkansas

Oct 21, 2007

#10

• Oct 21, 2007

• #10

Do it with corn