I'm hopping up a 292 Y Block and have some questions. Stock Iron Heads. Ported.

2.02 Intake and 1.60 Exhaust. I'm looking at static compression ratio,

and dynamic compression ratio's and these are the numbers that I've calculated.

Let me know what you think:



Engine Bore Size: 3.830

Piston Stroke Length: 3.610

Head Gasket Bore Diameter: 3.937

Gasket Compressed Thickness: .047

Combustion Chamber CC's: 65

Piston Dome: 0 (I'd like to run the domed pistons, but I can't!) Piston Deck Clearance: 0

Intake valve closes at ABDC: 70 degrees

Altitude: 4500ft



Static Compression Ratio : 10.167

Dynamic Compression Ratio: 7.826



Obviously there are still a lot of things that I can do to tweak those numbers however I need to. But my concern is

that I'd like to get the dynamic compression ratio up a little bit

from that number. I know that detonation is a big concern. But, from

what I've read, detonation won't really come into play unless your

DCR is closer to 8.5:1 on iron heads. My target would be close to an 8.25:1 DCR. So, that being said, what are your thoughts on where I'm at.



I also am leaning more towards decreasing the squish area- just to enhance the detonation resistance of the motor. But, I'm still a novice and appreciate any of your insights.

Heber City, UT (15 mins outside of Park City- basically it's in the mountains)

55 Effie

The only reason that I brought up the quench area is because it's also a new topic for me. If I was to machine the piston -.010 out of the whole, it would take those same numbers up about .2. But the net benifit of detonation resistance might be HUGE! Anyone have some experience here?

Heber City, UT (15 mins outside of Park City- basically it's in the mountains)

55 Effie

Iam not sure what the commpresion is in our motor. It is a 292 bored to 3.80 (standard bore 312) it has flat tops pistons in the hole about .020th Felpro head gaskets. It has cranking compression of 190 psi I was affraid to have it that high. But even with cheap gas it does not ping.

WTB 1955 Courier emblems    Mark, Jefferson City Missouri

I have two different ways at looking at dynamic compression ratio on a solid lifter cammed engine.  One is to simply look at the intake closing as it relates only to the ramp on the camshaft and the other is to look at it from the valve actually closing at the cylinder.  With the later, the dynamic compression ratio is typically about a ½ point higher when taking the rocker arm ratio into account.  When looking at the camshaft closing event only, 8.0:1 is my own limit for 91-93 octane fuel.  But when looking at the dcr with the rocker arm ratio taken into acount and what is actually happening at the intake valve, the dynamic ratio limit is 8.5:1 on the same engine without changing anything else.  It’s simply easier on my end to use the camshaft only value and use the lower dcr value limit than it is to go through the math with the rocker arm ratio and get the ABDC closing number at the valve in which to use the higher dcr limit value.  I hope that made sense.

 

As far as quench goes, I don’t see a major difference in performance attributes where the piston is 0.065” away from the head versus the piston being 0.035” away from the head if the compression ratio is being maintained the same in both instances.  Where the quench area is being reduced from say 0.065” to 0.035” by raising the piston (or lowering the deck) and the compression ratio increases as a result, there will be a power increase but this will be a from the compression ratio increase rather than from the change in quench.

 

If the piston is more than 0.065” from the head, then the propensity for detonation is increased and that’s where a loss in power will come into play.  The reduction in quench area by keeping the piston to head clearance nominally close is what helps to push the fuel air mixture back towards the spark plug and at the same time keeps the mixture lively in what would be normally dead or stagnant areas of the combustion chamber.  That’s the simple explanation.

Lorena, Texas (South of Waco)

Two easy ways to increase the DCR is to advance the camshaft or increase the intake valve tappet clearance.  Also, anything done to increase the static compression ratio will also raise the dynamic compression ratio a similar amount.

Lorena, Texas (South of Waco)

What's also interesting is that every calculator gives me slightly different numbers. Theoretically measuring the same things. For instance United Engine's Calculator shows these numbers:



Static compression of 10.167:1

Dynamic compression of 7.760:1



Wallace Racing gives me these numbers:



Static compression ratio of 10.167:1.

Effective stroke is 2.66 inches.

Your dynamic compression ratio is 6.85:1 .

Your dynamic cranking pressure is 129.74 PSI.

Your dynamic boost compression ratio, reflecting static c.r., cam timing, altitude, and boost of 0 PSI is 6.85 :1.

V/P (Volume to Pressure Index) is 95

(Wallace Racing does take my altitude into account- 4500ft)



Further, if I look at what United Engine refers to as "effective" ratio I get this number (they add 15 more degrees of rotation to account for actual valve closing event- 15 more degrees from .050" to .000" lift):



Effective compression ration: 6.668.



My question is this: 7.7 DCR seems to leave a little room for compression (ie more power) on the table. Am I treading a thin line here?

Heber City, UT (15 mins outside of Park City- basically it's in the mountains)

55 Effie

Altitude will definitely affect it. So then the question is will you ever take it significantly below your altitude. If not, then increase compression. If so, then play it safe and build it for sea level.


Lawrenceville, GA

Rather than altitude, knowing where the top of the piston is exactly within the bore at a given point ABDC is what I use for calculating the dynamic compression ratio.  As the DCR calculation relies on the intake valve closing event, then the connecting rod length and its subsequent angle within the bore becomes a more accurate determination for the DCR.  Because the rod angle changes as the rod length changes, the length of the connecting rod becomes a necessary part of the equation.  As the rod length gets longer without changing anything else, the DCR does decrease even if only slightly.

 

Regarding head gasket volumes, you must keep in mind that Y-Block head gaskets are not circular at the fire ring and are subsequently much larger than the bore size as a result.  I use 10.12cc for the Best Gasket big bore head gaskets.

 

Not knowing what your rod length was, I just used an available & generic aftermarket scrub length in the following calculation.  This calculation is simply done on an Excel spreadsheet.

 

Lorena, Texas (South of Waco)

That's the rub! If I do travel to sea lever I'd need to retard the %$#* out of it, or add octane booster, or both. I don't know that I'd really take the road trip in my truck. I'm sure even a 5 hour trip to vegas would cost an arm and a leg! Sure would be fun driving up the PCH though!

Heber City, UT (15 mins outside of Park City- basically it's in the mountains)

55 Effie

The length of the rods are 6.125.







I didn't take that into account!



But, my question is this? If I want a durable, STRONG street fighter what should I do? Leave everything as it is? Or cheat a little on the pistons with a dome? What would you reccomend? I'm not a genius at timing, tuning, or what have you, but I am patient and would get it right!

Heber City, UT (15 mins outside of Park City- basically it's in the mountains)

55 Effie