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Author Topic:   Compression ratios and pump gas....
'69Stang
Gearhead

Posts: 205
From: Detroit, MI USA
Registered: Jan 2002

posted 02-07-2002 06:24 PM     Click Here to See the Profile for '69Stang   Click Here to Email '69Stang     Edit/Delete Message   Reply w/Quote
Building a 351 cleveland for primarily street, light strip duty. It will be in a '69 Mustang fastback with a 4 speed, engine will (hopefully) put out around 450-475 crank horsepower (375-400RWHP). With the quench 4-v heads what can I get away with in terms of compression with 93-94 octane gas? Thanks guys.

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Moneymaker
Administrator

Posts: 27499
From: Lyons, IL, USA
Registered: May 99

posted 02-08-2002 10:57 AM     Click Here to See the Profile for Moneymaker   Click Here to Email Moneymaker     Edit/Delete Message   Reply w/Quote
All depends on the camshaft. A short centerline cam will dictate a max of about 10 to 1. A wide c/l will lend itself to a higher static ratio.

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Alex Denysenko
Co-Administrator and Moderator
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NHRA and IHRA SS/LA National Record Holder '00, '01, & '02
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steve'66
Gearhead

Posts: 9533
From: Sonoma,CA,USA
Registered: Mar 2000

posted 02-10-2002 07:34 PM     Click Here to See the Profile for steve'66   Click Here to Email steve'66     Edit/Delete Message   Reply w/Quote
I borrowed this text from www.badasscars.com

It answers your question in detail,

Q: How much compression can I run and still use pump gas?

A: This is a good one with no single answer. The key here is cylinder pressure, not compression! Cylinder pressure is controlled by valve timing (overlap), not the piston. Although bore, stroke, compression height, deck height and combustion chamber size, among others, have a great deal to do with the whole ball of wax, they don't dictate the actual working compression or cylinder pressure of an engine. Example: two identical engines with 9:1 compression. The only difference is in the cam profiles. One has no overlap and the other has a lot. Overlap is the time in which both valves are open. When the exhaust valve is closing and the intake valve begins to open, there is a time (on high performance and race cams) where both valves are actually open at the same time. A piston can not make cylinder pressure if a valve (or valves) are open. As the piston comes up and starts to squeeze the air and fuel in the cylinder, some of the air and fuel (charge) is being pushed out of the open valves, thus creating that "rumpity bump" sound everyone likes so much, but by letting some of the charge escape, there is less there to squeeze thus creating less pressure. It also decreases the velocity of air feeding the engine as well as decreases manifold vacuum. When you have too much overlap, you end-up with a dog for an engine that has no throttle response. This is what's known as being over-cammed.


If you did a compression test on these two engines, the engine with no overlap would probably have about 140 -150psi or so in the cylinders. The engine with the overlap may only have 100 psi or so, depending on how much overlap the cam has. Some racing engines with 13:1 compression only have 125psi or so of cylinder pressure when a compression test is done. It just means that the cam has a ton of overlap. Once the engine gets spinning, the cylinder pressure will greatly increase and this is why hot cams "come-in" or "start breathing" at higher RPM's. So, you can run 92 octane pump gas on 11:1 compression with the right cam but 10:1 could ping (detonate) with the wrong cam. Way back in the early 80's, Crower had a fuel economy kit that was designed to run on pump gas (for gas mileage) yet had a compression ratio of 14:1!! Really! It's all in the cam profile. It worked but never caught-on so they dumped the idea. Bottom line, cylinder pressure, not compression dictates what kind of fuel you'll need to run.

SteveW

p.s. The common rule is 10-1 w/iron heads and 10.5-1 w/aluminum heads. But the cam is the key.

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n2oMike
Gearhead

Posts: 2831
From: Spencer, WV
Registered: Jan 2001

posted 02-10-2002 08:28 PM     Click Here to See the Profile for n2oMike   Click Here to Email n2oMike     Edit/Delete Message   Reply w/Quote
What you copied is bogus... They published their information without getting it proof-read.

Tightening the lobe seperation increases the cranking (effective) compression. The piston can only compress the air it traps in the cylinder. Long durations and wide lobe seperations push the intake closing event into the "compression" stroke. If air is escaping out the intake valve as the piston travels upward, it's not getting compressed, and cylinder pressure goes out the window! Narrowing the lobe seperation helps build more compression by making the intake valve close sooner, enabling the compression stroke to capture and compress more air. Advancing the camshaft will do this as well.

Some people mistakenly believe that compression is lost during the OVERLAP period... WRONG! Overlap occurs when the exhaust valve is closing and the intake is just starting to open... Nothing is being compressed yet! NARROWING the lobe seperation pulls the intake closing event out of the compression stroke. Advancing the cam will make the intake valve close sooner as well.

Here's another site with some lobe seperation info.

http://www.fordmuscle.com/archives/2001/04/camtech/

Good Luck!

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Mike Burch
66 mustang real street
302 4-speed 289 heads
10.63 @ 129.3
http://www.geocities.com/carbedstangs/cmml_mburch.html
http://www.fortunecity.com/silverstone/healey/367
http://www.mustangworks.com/cgi-bin/moi-display.cgi?220

[This message has been edited by n2oMike (edited 02-10-2002).]

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steve'66
Gearhead

Posts: 9533
From: Sonoma,CA,USA
Registered: Mar 2000

posted 02-11-2002 08:37 PM     Click Here to See the Profile for steve'66   Click Here to Email steve'66     Edit/Delete Message   Reply w/Quote
Mike,

I won't dispute what you said, but I'll invite Arron to explain if you'd like. He's a little busy 'cause he's building 15 race engines a week now days. Maybe he does need a proof reader, but the "jest" of it is that the cam is a major contributor.


SteveW

Could it be that we are confusing lsa with overlap? shortening the lsa increases overlap. Now I'm getting confused Mike

[This message has been edited by steve'66 (edited 02-11-2002).]

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n2oMike
Gearhead

Posts: 2831
From: Spencer, WV
Registered: Jan 2001

posted 02-11-2002 09:10 PM     Click Here to See the Profile for n2oMike   Click Here to Email n2oMike     Edit/Delete Message   Reply w/Quote
The cam is definitely relavent... He just has his facts wrong. His explaination, as well as his conclusion are completely off base.

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Mike Burch
66 mustang real street
302 4-speed 289 heads
10.63 @ 129.3
http://www.geocities.com/carbedstangs/cmml_mburch.html
http://www.fortunecity.com/silverstone/healey/367
http://www.mustangworks.com/cgi-bin/moi-display.cgi?220

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n2oMike
Gearhead

Posts: 2831
From: Spencer, WV
Registered: Jan 2001

posted 02-11-2002 09:40 PM     Click Here to See the Profile for n2oMike   Click Here to Email n2oMike     Edit/Delete Message   Reply w/Quote
Here's an image that will help explain how increasing lobe seperation DECREASES cranking cylinder pressure. You can see how spreading the lobes further apart puts the intake event further into the compression stoke. This causes the valve to be open longer into the compression stroke... bleeding off cylinder pressure.

Good Luck!

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Mike Burch
66 mustang real street
302 4-speed 289 heads
10.63 @ 129.3
http://www.geocities.com/carbedstangs/cmml_mburch.html
http://www.fortunecity.com/silverstone/healey/367
http://www.mustangworks.com/cgi-bin/moi-display.cgi?220

[This message has been edited by n2oMike (edited 02-11-2002).]

[This message has been edited by SteveLaRiviere (edited 02-12-2002).]

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n2oMike
Gearhead

Posts: 2831
From: Spencer, WV
Registered: Jan 2001

posted 02-12-2002 02:07 AM     Click Here to See the Profile for n2oMike   Click Here to Email n2oMike     Edit/Delete Message   Reply w/Quote
My wife tried to post the picture where I could link the image, but it didn't seem to work. I emailed it to the webmasters... maybe they can post it. Sorry.

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Mike Burch
66 mustang real street
302 4-speed 289 heads
10.63 @ 129.3
http://www.geocities.com/carbedstangs/cmml_mburch.html
http://www.fortunecity.com/silverstone/healey/367
http://www.mustangworks.com/cgi-bin/moi-display.cgi?220

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mvierow
Gearhead

Posts: 209
From: Bay Area, Ca
Registered: Apr 2001

posted 02-12-2002 02:45 AM     Click Here to See the Profile for mvierow   Click Here to Email mvierow     Edit/Delete Message   Reply w/Quote
Actually, that makes perfect sense Mike, the only time the intake and exhaust valves would be open would be between the exhaust and intake strokes. The exhaust valve would be long since closed by the time the compression stroke comes along. However, the intake valve may not be closed by then, as you mentioned, leading to some compression leaking back out into the intake manifold.

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n2oMike
Gearhead

Posts: 2831
From: Spencer, WV
Registered: Jan 2001

posted 02-12-2002 03:57 PM     Click Here to See the Profile for n2oMike   Click Here to Email n2oMike     Edit/Delete Message   Reply w/Quote
Thanks for uploading the picture, Steve! It works great now.

If you study the graph, you can see how large cam lobes and wide lobe seperation push the intake closing event into the compression stroke... bleeding off valuable cylinder pressure.

Good Luck!

------------------
Mike Burch
66 mustang real street
302 4-speed 289 heads
10.63 @ 129.3
http://www.geocities.com/carbedstangs/cmml_mburch.html
http://www.fortunecity.com/silverstone/healey/367
http://www.mustangworks.com/cgi-bin/moi-display.cgi?220

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