[67gtospud]

I'm just curious here, when putting parts together to build a solid aggressive street engine to run on pump gas, do you figure the 9.5:1 off static or dynamic compression ratio?

I'm just wondering because if you have a 10.5:1 SCR and a large cam it could result in a 9.25:1 DCR. would this still be 91-93 octane pump gas friendly?

This is all assuming cast Pontiac block, cast iron heads ect.

[steve25]

It's the dynamic pressure that any given Cam will allow at cranking that will effect the Octane needs of the motor far more then the SCR!
These days with the fuel we have if you are not every experienced with timing and fuel curve / jetting needs you would do well to stay clear of topping a cranking compression of 175 psi in a street motor that will see a lot of driving / miles ( i.e) fuel going thru it.

If you want to push it / max things out you can get a MSD remote timing adjuster and then add a Snow performance Alky/ water injector which will then add even more power and keep the connecting rods in the motor where they should be , of course this will yank some 700 bucks out of your wallet, but once you hit around the the 450 hp level with any motor that's about what it will always cost to gain 50 hp on any motor unless you can just crank up the boost.

[tempest1964]

Here is a link to some informative stuff about dynamic and static compression. It is written by a guy named Dave Miranda.
http://www.thepontiactransampage.com...blueprint.html

[gtofreek]

You need to be careful when dealing with dynamic compression. They say for pump gas to stay under 8-8.5:1 dynamic, but this can be deceiving. I assume these numbers off of normal hydraulic flat tappet cams. However, some cam lobe profiles do a better job filling the cylinders than others. Two cams can have the same intake closing point, thus giving the same dynamic compression ratio, but one can fill the cylinder much better, due to things like lobe profile, more duration at .200", overlap, etc. The cam that fills the cylinder better, will have a higher dynamic compression, even though the calculators say they are the same. This especially holds true with roller cams. They fill the cylinder much better than a flat tappet cam, so their dynamic compression would be much higher[when compared to a flat tappet cam], even though the intake valve closes at the same point.

Basically, cam the engine for the static compression ratio you are running, and forget about dynamic compression. There is just no pump gas standard for this yet. Nor can there be, IMO.

[HWYSTR455]

There's truth there Paul, cam the engine for the static, but keep in mind that the higher the static, the more cam you should be putting in it. So you have to keep the intended use of the engine a large factor when you target an SCR. Like you wouldn't make a daily driver low-po application with an 11:1 SCR. (theoretically)

Personal experience, with iron heads, and pump gas, the DCR range I found suitable was more like 7.25 - 7.75. I had tried 8.65 and 8.15, neither worked with pump gas unless I 'did the dance' with timing and carb tuning. And even then, it was a compromise of performance.

.

[67gtospud]

Ok, I figured it'd be a lot more complicated than a simple number not to surpass lol

For example my current engine:
SCR: 9.78:1
DCR: 8.59:1 according to Wallace calculator

[HWYSTR455]

Originally Posted by 67gtospud

Ok, I figured it'd be a lot more complicated than a simple number not to surpass lol

For example my current engine:
SCR: 9.78:1
DCR: 8.59:1 according to Wallace calculator

Iron heads? What's your total timing, 28-30 degrees? Does it ping on pump gas?

.

[67gtospud]

Yes iron heads, total timing is 34-35 iirc. Doesn't ping on 91 pump gas. That's why I was curious if there was a safe dcr number for pump gas.

[Steve C.]

WHAT IS THE "RIGHT" COMPRESSION RATIO FOR PUMP GAS?

But these sorta "what CR ?" threads rarely ever go anywhere worthwhile or productive.

One side says do this, other side says do that, neither side will budge an inch off their position, the battle lines are drawn, warning shots are fired back & forth.

And eventually it ends up becoming an all-out p!ss!ng contest...


http://forums.maxperformanceinc.com/...ighlight=bleed




.

[HWYSTR455]

I hear you Steve, which why I was specific by saying 'personal experience'. I basically only provided examples of what I found with my particular combos. Reason I even commented on this thread was because it was more a 'DCR' thread than an SCR thread.

67gtospud - How are you getting 8.59 DCR? What intake closing value are you using? I'm getting 7.79 - 7.81 DCR using the Wallace calculator:

Bore - 4.150
Stroke - 3.75
Rod Length - 6.625
SCR - 9.78
IVC - 64
Boost - 0
Altitude - 0

= 7.81

At 100 feet ALT, I get 7.79

Formula I used to calc intake closing:

LSA + (Adv Intake Dur/2) -ground in advance -180

So, with the 60904 cam:

110 + 276/2 (138) -4 -180 = 64

My guess is that hot, under load, you get a slight ping at the 1-2 shift, and maybe at the top end. Unless you're running on the slightly rich side. If you're running a 160 stat, that would help some too. Higher numeric rear gear will help as well.

.

[67gtospud]

Originally Posted by HWYSTR455

I hear you Steve, which why I was specific by saying 'personal experience'. I basically only provided examples of what I found with my particular combos. Reason I even commented on this thread was because it was more a 'DCR' thread than an SCR thread.

67gtospud - How are you getting 8.59 DCR? What intake closing value are you using? I'm getting 7.79 - 7.81 DCR using the Wallace calculator:

Bore - 4.150
Stroke - 3.75
Rod Length - 6.625
SCR - 9.78
IVC - 64
Boost - 0
Altitude - 0

= 7.81

At 100 feet ALT, I get 7.79

Formula I used to calc intake closing:

LSA + (Adv Intake Dur/2) -ground in advance -180

So, with the 60904 cam:

110 + 276/2 (138) -4 -180 = 64

My guess is that hot, under load, you get a slight ping at the 1-2 shift, and maybe at the top end. Unless you're running on the slightly rich side. If you're running a 160 stat, that would help some too. Higher numeric rear gear will help as well.

.

I just 're calculated the dcr from Wallace racing site and got 8.79. I think I had the wrong cam last time. I used the intake closing off the cam card.

My numbers:
4.12 bore
3.75 stroke
6.625 rod length
9.78:1 scr
Intake closing event 42.5 degrees (from cam card)
Altitude : 784' ( I used the altitude in the town I live.)
0 boost

Did I mess up somewhere?

[Formulas]

DCR calculators are just a ball park tool to predict octane tolerance.

Take any engine combination you wish with stock ports, Calculate DCR you will come up with X figure now do nothing but port the stock heads to max effort what have you done ?

You have effected the engines ability to change how much air/fuel/exhaust charge is going to be packed in the same space that your DCR is being calculated from.

LSA also will effect the outcome of octane tolerance all else the same LSA changes will effect the characteristics of the fuel/air/exhaust charge.

Exhaust scavenging will effect octane tolerance by changing how much exhaust is being squeezed in place of a new intake charge

So its a dynamic tool based on fixed events to predict an outcome based on know factors which are influenced by several unknown factors
Kind of a misnamed tool as it doesn't and realistically cant take into account the near infinite number of variables when a engine is actually spinning at 6000 RPM`s

Measure twice with a laser then cut once with a hacksaw.

.

[HWYSTR455]

Originally Posted by 67gtospud

I just 're calculated the dcr from Wallace racing site and got 8.79. I think I had the wrong cam last time. I used the intake closing off the cam card.

My numbers:
4.12 bore
3.75 stroke
6.625 rod length
9.78:1 scr
Intake closing event 42.5 degrees (from cam card)
Altitude : 784' ( I used the altitude in the town I live.)
0 boost

Did I mess up somewhere?

Yeah, you have to use seat timing/advertised timing. Not the .050 numbers. But that to me explains why it doesn't ping.

.

[HWYSTR455]

Originally Posted by Formulas

DCR calculators are just a ball park tool to predict octane tolerance.

Take any engine combination you wish with stock ports, Calculate DCR you will come up with X figure now do nothing but port the stock heads to max effort what have you done ?

You have effected the engines ability to change how much air/fuel/exhaust charge is going to be packed in the same space that your DCR is being calculated from.

LSA also will effect the outcome of octane tolerance all else the same LSA changes will effect the characteristics of the fuel/air/exhaust charge.

Exhaust scavenging will effect octane tolerance by changing how much exhaust is being squeezed in place of a new intake charge

So its a dynamic tool based on fixed events to predict an outcome based on know factors which are influenced by several unknown factors
Kind of a misnamed tool as it doesn't and realistically cant take into account the near infinite number of variables when a engine is actually spinning at 6000 RPM`s

Measure twice with a laser then cut once with a hacksaw.

.

I hear ya, and yeah, it's only a general guide, not a hard 'law' of physics. But just to say, in calculating the advertised seat timing intake closing, LSA is used, so it's kinda factored in.

DCR is helpful when considering boost, you can compare at 0 psi to whatever your target boost psi will be. DCR is roughly double when boosted with an intercooler, so as an example, you can run 14-16 DCR on pump gas. Some of the GN guys push that into the 20-24 range, but I wouldn't suggest that without a deep understanding.

.

[gtofreek]

Originally Posted by Formulas

DCR calculators are just a ball park tool to predict octane tolerance.

Take any engine combination you wish with stock ports, Calculate DCR you will come up with X figure now do nothing but port the stock heads to max effort what have you done ?

You have effected the engines ability to change how much air/fuel/exhaust charge is going to be packed in the same space that your DCR is being calculated from.

LSA also will effect the outcome of octane tolerance all else the same LSA changes will effect the characteristics of the fuel/air/exhaust charge.

Exhaust scavenging will effect octane tolerance by changing how much exhaust is being squeezed in place of a new intake charge

So its a dynamic tool based on fixed events to predict an outcome based on know factors which are influenced by several unknown factors
Kind of a misnamed tool as it doesn't and realistically cant take into account the near infinite number of variables when a engine is actually spinning at 6000 RPM`s

Measure twice with a laser then cut once with a hacksaw.

.

Very well said!

[johnta1]

Those factors will affect the octane requirement of the engine but DCR isn't affected by the quality of mixture or the efficiency of the combustion process.


The LSA is basically factored into the DCR equation.
(such as the intake closing point and exhaust open basically)

The intake closing point is the primary data along with the rod/stroke ratio.

I would say the relationship of octane requirement to the DCR is what varies.
It is just a guideline.
(the octane requirement and DCR)

[wheneaglesfly]

Originally Posted by gtofreek

You need to be careful when dealing with dynamic compression. They say for pump gas to stay under 8-8.5:1 dynamic, but this can be deceiving. I assume these numbers off of normal hydraulic flat tappet cams. However, some cam lobe profiles do a better job filling the cylinders than others. Two cams can have the same intake closing point, thus giving the same dynamic compression ratio, but one can fill the cylinder much better, due to things like lobe profile, more duration at .200", overlap, etc. The cam that fills the cylinder better, will have a higher dynamic compression, even though the calculators say they are the same. This especially holds true with roller cams. They fill the cylinder much better than a flat tappet cam, so their dynamic compression would be much higher[when compared to a flat tappet cam], even though the intake valve closes at the same point.

Basically, cam the engine for the static compression ratio you are running, and forget about dynamic compression. There is just no pump gas standard for this yet. Nor can there be, IMO.

Your right GTO Freak.
What your talking about is called EFECTIVE COMPRESSION RATIO.
Taking into account the True Volumetric Efficiency of the engine.
Volumetric Efficiency can only be checked on an engine dyno.
The Carb Airflow measuring bell hat used.
Or if modern EFI With a Mass Air flow sensor used.

I am active on another Hotrod forum & we get into all the race math.
I have determinef the online dynamic compresdion ratio calculators , Algorithm Formulas were baded off a SBC With cst iron heads used.

[dmac]

Not only can't DCR be calculated for any one engine, but it will change with rpms, temp, speed, exhaust back pressure, temp. etc, etc, etc, etc.

[Stan Weiss]

Originally Posted by 67gtospud

I just 're calculated the dcr from Wallace racing site and got 8.79. I think I had the wrong cam last time. I used the intake closing off the cam card.

My numbers:
4.12 bore
3.75 stroke
6.625 rod length
9.78:1 scr
Intake closing event 42.5 degrees (from cam card)
Altitude : 784' ( I used the altitude in the town I live.)
0 boost

Did I mess up somewhere?

What is you cranking compression?

Stan

[Stan Weiss]

Originally Posted by wheneaglesfly

Your right GTO Freak.
What your talking about is called EFECTIVE COMPRESSION RATIO.
Taking into account the True Volumetric Efficiency of the engine.
Volumetric Efficiency can only be checked on an engine dyno.
The Carb Airflow measuring bell hat used.
Or if modern EFI With a Mass Air flow sensor used.

I am active on another Hotrod forum & we get into all the race math.
I have determinef the online dynamic compresdion ratio calculators , Algorithm Formulas were baded off a SBC With cst iron heads used.

I do not have an online DCR calculator. But when I calculate DCR I use intake valve close at the seat. That means if you have a solid lifter cam and change your valve lash your DCR changes, also just like your cranking compression will.

Stan