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Old 12-19-2011, 01:35 PM
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Pontiac 400 comp ratio comfort level...

Hi all

Spent a good deal of time talking to the machinist who is going to do my engine. He recommended using flat top Icon forged pistons (4.5cc single relief, 1.72 comp height) instead of the D-cup dished Icons I was originally looking at in my build with my 72cc heads.

He said at our particular elevation (5300ft), he routinely builds street engines that have between 10.5 and 11.0:1 static compression ratios, and they run just fine on pump gas, provided the camshaft is the right one and the engine is tuned properly...

I checked what he said against 2 different dynamic compression ratio calculators, and sure enough, even though those pistons give me a static 10.6:1 ratio, the dynamic compression ratio corrected for my elevation is right in the middle of the sweet spot at 7.7:1....pump gas should run fine...if you trust the calculator...

He added that the dynamic compression ratio that the flat tops will provide will help me realize the potential of the cam (lunati voodoo 262/268), and vice versa....he said it's an ideal match between cam and pistons running at this elevation...

In addition, he said that even though the static compression ratio is higher, the flat top would actually be less prone to detonation, because its design allows it to better take advantage of a proper quench setting...it will move more air than the d-cup and cool the piston better....it makes sense...

He came highly recommended by others in the area, he's been at this a long time and definately has the most experience with Pontiacs in the area...what he says jives with the calculators and makes sense....

Would you all be comfortable with running that piston if it was you (at this elevation of course)?
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Old 12-19-2011, 02:18 PM
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i dont know man that sounds really high.....10-1 maybe but 11????/

im using the icons you recomended ......with the number 13 heads i just got ported ferrera valves and hardend seats...
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Old 12-19-2011, 03:07 PM
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Yeah, it's defiantely high, but we are at 5300 feet above sea level, and altitude has a very direct impact on an engine's dynamic compression ratio, and dynamic compression ratio, not static, is the important one to look at as far as detonation, power generated, etc....

The calculators jive exactly with what he said...camshaft profile plays into it as well...at my elevation and with my camshaft (intake closes at 59 deg abdc), I can go up tp 11.2:1 static ratio and still be just below the 8.2 dynamic compression ratio that is supposedly the max (8.2-8.5) that can be run on pump gas. At 10.6 static, I'll be right in the middle of the 7.5-8.0 dynamic, at 7.7:1....in theroy it should be just about perfect....

The catch is that if I go significantly lower in elevation, I'll likely suffer detonation with pump gas...but odds are I won't be taking that vehicle anywhere like that....
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Old 12-19-2011, 03:58 PM
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Quote:
Originally Posted by MJS69
The catch is that if I go significantly lower in elevation, I'll likely suffer detonation with pump gas...but odds are I won't be taking that vehicle anywhere like that....
You might want to consider also that the quality of fuels could change overnight. Remember the Arab Oil Embargo and subsequent junk fuel??? Well, maybe you don't, but I do. I'm always amazed at fellows who want to build right up against the wall and leave themselves no margin. Just like with fuel pressure on a carbureted motor, right up against the wall with no margin.
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Old 12-19-2011, 04:29 PM
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Quote:
Originally Posted by techinspector1
You might want to consider also that the quality of fuels could change overnight. Remember the Arab Oil Embargo and subsequent junk fuel??? Well, maybe you don't, but I do. I'm always amazed at fellows who want to build right up against the wall and leave themselves no margin. Just like with fuel pressure on a carbureted motor, right up against the wall with no margin.

Yeah, I remember the embargo...I wasn't quite driving age then, but I remember the huge price increase, and waiting in line for gas when my folks would fill up the car...

You do make a very good point. And I certainly like to leave some room for error. At a 7.7 dynamic, I feel that does give me some room for changing fuel quality. I certainly wouldn't build it up to an 8.2 dynamic comp ratio, even though in theory it would run fine on pump gas.

However, I think it can go too far in the other direction as well...I'm building this car as a fun weekend fair weather cruiser, it certainly won't be a daily driver. Probably less than 3000 miles per year, if that, will get put on this car.....If I'm going to build it way back behind it's potential just becasue they might change the fuel quality sometime in the future, than what's the point? If they do change it, I think I would just have to find another alternative, like AV or racing fuel...

or maybe just swap out pistons if and when that actually happens...

Last edited by MJS69; 12-19-2011 at 04:48 PM.
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Old 12-19-2011, 05:02 PM
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the dynamic would be somewhere around 8.4 to one with that cam not the 7 ish you thought.
The only drawback to that much compression is if you want to go anywhere that isnt a mile high, other than that sounds like fun.
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Old 12-19-2011, 05:36 PM
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Originally Posted by latech
the dynamic would be somewhere around 8.4 to one with that cam not the 7 ish you thought.
The only drawback to that much compression is if you want to go anywhere that isnt a mile high, other than that sounds like fun.

Agreed, I get about the same thing you did, a little higher(8.8 dynamic) when I use the calculator you gave me a while back... http://www.projectpontiac.com/ppsite...tio-calculator

I'm sure that is the correct dynamic compression, but thats at sea level, as that calculator doesn't have an option to correct for elevation (or boost).

Using the one at Wallace Racing which does compensate... http://www.wallaceracing.com/dynamic-cr.php
You have to know your static comp ratio up front to use this one...I get 7.7 out of that one. I've found several other calculators similar to this one, and they all give similar results...within 0.2 of eachother...

here's what I entered...

Number of Cylinders : 8
Bore in Inches : 4.15
Stroke in Inches : 3.75
Rod Length in Inches : 6.625
Static Compression Ratio :10.66 (:1)
Inlet Valve Closes ABDC : 59
Boost Pressure in PSI : 0
Target Altitude : 5300 (Feet)

And here is what it gives me...

Static compression ratio of 10.66:1.
Effective stroke is 3.04 inches.
Your dynamic compression ratio is 7.77:1 .
Your dynamic cranking pressure is 153.06 PSI.
Your dynamic boost compression ratio, reflecting static c.r., cam timing, altitude, and boost of 0 PSI is 7.77 :1.
V/P (Volume to Pressure Index) is 151


If this calculator is accurate, and I feel it is... I'd say that 7.77:1 and a cranking pressure of 153.06 are well within a safe range...wouldn't you agree?

I put in the same values, except I used 0 (sea level) as the value for elevation and I get this...

Static compression ratio of 10.66:1.
Effective stroke is 3.04 inches.
Your dynamic compression ratio is 8.83:1 .
Your dynamic cranking pressure is 180.57 PSI.
Your dynamic boost compression ratio, reflecting static c.r., cam timing, altitude, and boost of 0 PSI is 8.83 :1.
V/P (Volume to Pressure Index) is 178

That jives very closely with the calculator you gave me....

I agree, I'll need to stay at this elevation...but that's ok...I don't see this car ever seening too many trips to places significantly lower in elevation...
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Old 12-19-2011, 06:31 PM
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Looks good, makes perfect sense too. Looks like you have done the homework on your build.
Also I really like the piston choice with the single set of valve reliefs, Giving a large undisturbed flat top half to the quench pad side of the head. That should give the best possible amount of "turbulence" in the chamber you could have.JMHO but it just makes sense.
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Old 12-19-2011, 06:46 PM
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Quote:
Originally Posted by latech
Looks good, makes perfect sense too. Looks like you have done the homework on your build.
Also I really like the piston choice with the single set of valve reliefs, Giving a large undisturbed flat top half to the quench pad side of the head. That should give the best possible amount of "turbulence" in the chamber you could have.JMHO but it just makes sense.

Thanks....

He said exactly the same thing you said about the flat top with 2 valve reliefs...takes full advantage of the quench and makes plenty of turbulence...he did say that even at the higher compression ratio, this piston actually will make the engine less prone to detonation than the lower compression d-cup dished piston, because this one will make much more turbulence and cool much better as a result...

I'm feeling better about it...really the only scary number in the whole equation is the 10.6:1.....

When he mentioned it, I think he saw the somewhat concerned look on my face, and he said "look at it this way...If I recommended a part that was going to grenade your engine with detonation, what would you tell your hot rod buddies about me? Wouldn't be good for my business to design people's engines like time bombs..." lol
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