|03-22-2013 11:38 AM|
|spinn||Bogie that is right on, instructors don't spell it out that well. Very nice read too.|
|03-22-2013 11:28 AM|
|03-21-2013 09:58 PM|
|Motosledneck 12||Got the motor together. Just have one last question. And that is what plugs should I run? Do I need a colder plug for the raised compression?|
|03-14-2013 12:06 PM|
DCR is kind of a tricky thing the calculation isn't a straight forward in terms of a mental exercise as the SCR wheree the numbers are what the numbers are.
The DCR forces you to think about what's going on with the valve closing event in terms of opening against degrees. A modern cam that opens and closes the valves very quckly reacts to DCR very differently than older cam designs using very long closing ramps where the valves hangs almost but nut quite closed for many, many degrees.
The Summit 1102 is one of these older rampy designs, good for lobe and lifter life being kinder and gentler that cams with names like Extreme Energy, Voodoo and the like. With todays oil additive issues these older designs should last longer in service. The problem arises when try to deduce a resonable DCR as you've got to know when in crankshaft degrees the intake seats. The rising piston puts a lot of pressure into the cylinder that drives a lot of mixture back out even against the almost closed valve than if you assumed reverse flow at a lift value is similar to the flow bench numbers for that head of intake flow. This doesn't work because the pumping piston is putting a lot more force into the out going reversion than a flow bench is capable of generating for the test numbers of the intake flow. So that simple ide for a comparison though logical goes out the window in the real world.
So what do you do with the 1102 as logic will say that even though the intake is really late to seat, the flow has got to get pretty darn small when the valve is really close but not on the seat. So where does that happen, this becomes the squishy world of assumption.
Peeling your cam apart; first let me say that the DCR is a measure of virtual stroke lost moving the piston upward on compression while the intake is still not seated. The math for this is exactly the same stuff used to determine instant piston velocity at specific points along the bore and is the same for calcualting the effects of differing rod lenghts. Same equations exactly you just pulling different pieces of data for different purposes. All we're doing is using the equations to determine where the piston is as it rises up the bore as that relates to crankshaft degrees of rotation. So this opens and closes a bunch of different shape and sizes of triangles formed by the frixed length of the rod, a virtually changing base length of stroke being the center line distance of the crank mains to the rod journal as the jounal rotates, the throd side being a line from the piston pin center through the center of the main journal.
The 1102 cam card shows the intake valve closure at 26 degrees crank with the valve still .050 inch off the seat. So obviously the valve isn't closed and the crank is still turning. Most online calculators tell you to add about 15 degrees to this to get at the actual valve closure point. This is a pretty good assumption with the modern fast acting cams; but! But the 1102 has an intake duration from zero lift of 281 degrees and a .050 inch lift of 204 degrees. Wow, that's 77 degrees! If you assume the lobe is semetrical there's 33.5 degrees of ramp left to go before the valve is seated, not 15 as found with the online calculators. This makes a difference of closing the valve at 29 degrees aafter bottom center or 62.5 degrees ABDC.
When you feed these numbers into the equation 29 degrees generates a virtual stroke of 3.35 inches while 63 degrees gets you 2.81 inches. This makes a different DCR of 9.47 against 8.1. The latter 8.1 to 1 is a very livable DCR for your heads while 9.47 is iffy. Both of these are computed from a SCR of 9.81. This shows the startling effect of DCR related to the intake closure point on the SCR and where if you're not careful in your assumptions as well as execution, you can get into a ton of trouble.
When I play with taking 29 degrees closing at .050 inck lift and add the on-line recommended 15 degrees that closes at 44 degrees which generates a virtual stroke of 3.12 inches with a DCR of 9.8, still pretty high for these heads. When I assume that most of the reversion will stop around 2/3rds of the difference between the .050 number and the zero closed number I drive a much more livable DCR of 8.6.
I going through this expanation to show how touchy the DCR calculation is to the going in assumptions of when reversion actually stops.
If you find the compression to be too high to where you've got to remove the heads you would be far better off to slightly enlarge and polish the chambers, perhaps even lengthen or connect the valve reliefs in the piston than you would to go to a thicker head gasket. Usually these pistons have a band across the inside center of the piston where the relief is machined, carefully enlarging the relief along the length of this band is OK, just don't go deeper. There are several brands of pistons that do this, they just ditch the entire width of the piston under the valve pocket.
|03-14-2013 06:08 AM|
|454C10||Piston Head Clearance Guide - Tech Article - Chevy High Performance Magazine|
|03-13-2013 08:15 PM|
|Motosledneck 12||Wow that is some very useful information. I will use the thinner gasket. And the fuel I will be running will be bp 93 octain. It is the best fuel I have found for pump gas from my experience with running my 348whp cobaltss on a completely stock engine with 120000 miles and 38mpg, there fuel out performed holiday and sa fuel by far from tuning on the dyno. Here is a picture of the cam card also. And thank you for your information. I'm new to the old school v8s and need all the info I can get especially when it comes to tuning a carberated engine. And please, no crap about having a 4 banger haha.|
|03-13-2013 05:27 PM|
With a rebuilder piston .020 short on the crown mixed with a stock deck of .025 with flat top pistons having 4 valve reliefs under 64 cc chambers you get a Static Compression Ratio (SCR) of 9.56 to 1 with a .026 inch gasket. The compression ratio that came with the pistons assumes a block deck at zero height between it and the piston crown. This is because these are mass rebuilder pistons and the mass rebuilders don't screw with measuring the deck for flat and warp; they just whack .025 off every block passing through the mill and they all pass through the mill. From the data published at Summit on your cam; there really isn't any way to determine when the intake valve closes. That point determines the Dynamic Compression Ratio (DCR). The DCR is what everybody that contributed the words "bleed off" is talking about. The is a bottom end feature that relates to the ability of the intake flow velocity to develop enough inertia to overcome the reversion pressure forming inside the cylinder from the rising piston. This takes place from Bottom Dead Center (BDC) till the intake valve seats in crankshaft degrees. In essence this shortens the stroke of the engine which greatly reduces compression ratio. Remember air and evaporated fuel are gasses and gasses are compressible which means their density and pressure are independent functions. So the actual pressure in the cylinder is only vaguely related to the compression ratio. Since a late closing cam allows the piston below a critical RPM to push mixture back out, the engine reacts like the stroke was shortened and thus with the same volume above the piston when it gets to Top Dead Center (TDC) the compression ratio is lower as well as is the density (number of molecules of mixture). This is what causes the ragged idle and loss of bottom end power in a big cammed engine. This is why the SCR is increased with big cams as it is a means of forcing the thermodyamic side of the equation (density) to work harder to recover some of this loss which is the expression of the DCR. The DCR should be from 8 or 9 to1 depending on material composition of the head, size and shape of the combustion chamber, octane of fuel plus a few other considerations. Large chamber heads with the spark plug far from the bore center need lower compression, cast iron needs lower compression, wide squish/quench needs lower compression. Heads that move the plug close to the bore center, tight and energic chambers like the L31 Vortec, tight squish/quench clearance of .040 or a bit less, can take higher DCR's and certainly aluminum needs more compression because of its faster heat transfer rate to the cooling system than iron.
The good side of long duration cams is from super filling the cylinders at high RPMs. This comes from the engine's RPMs which also increase the mixture velocity inside the intake system such that at some critical RPM the mixture has so much inertia that it overcomes the reverse pumping of the rising piston and overfills the cylinder which increases the mixture density so power goes up because there's more material to burn. So while the stroke still gets shorter and the DCR (SCR for that matter) doesn't change, but the actual number of molecules in the cylinder goes up, therefore, the actual compression pressure rises even though the dimensional compression ratios do not change.
The 193 head is a bottom end torque head also designed for excellent gas mileage. It's not what you'd think of as a performance head even if it has larger valves than your other head. But your other head has large chambers so don't switch them. The 193's ports cannot feed the larger valves beyond about 4500 RPM on a 350. This is a case where more lift is not very helpful either; although you can pick up some flow early and late in the lift cycle. More lift over the top of the lobe is of limited usefulness because the port cannot support feeding the greater valve curtain area so the extra size in the valve or the maximum lift is of no real value with these heads. Actually porting out the vane is damnned useful to flow. Keep in mind that anything that causes the intake flow to slow makes the engine more cammy, so bigger valves, bigger ports while capable of feeding higher RPMs only push the suboptimal lower RPM performance into higher RPMs. Of course knowing this you can play a lot of games by changing around parts other than the camshaft or rocker ratio.
Since the piston is .045 in the hole already, putting on a thicker head gasket will only reduce squish and quench will make the engine more prone to detonation even-though the compression ratio came down. Since your compression is not out of line (but I'm guessing at the cam closure point for that statement) using the .026 gasket gives you a .071 inch squish/quench which is a bit wide but really is pretty close to what the production Swirl Port with 193 heads has with its dished piston. You can probably get away with this. you actaully might be able to get away with a .015 or .019 shim gasket which while increasing the compression ratios will also increase the squish/quench function. Think of this as tunning, a major thing to mess with but tuning none-the-less.
Trading ignition advance for compression ratio and or squish/quench effectiveness isn't a good idea. Especially where compression is concerned you lose a lot more power by sub optimizing the ignition advance than you do being a little low on compression. If detonation is a problem you can gain on it with a bit richer mixture and/or a little cooler running engine and intake manifold. That includes terminating the heat cross over if there is one on the intake. It is advisable to try different brands of gasoline, you will find that not everybody’s high octane fuel has a consistent anti-detonation response within the engine as it gets heated up as its advertised octane rating would suppose.
Gears and weight are important too as the amount of resistance to crankshaft rotation goes down (stiffer gears and/or less weight) the engine's resistance to detonation goes up. Explaining that is another thesis so I'll skip it for now.
Data off your cam's timing card would be helpful.
|03-12-2013 08:26 AM|
Since the piston is already in the hole 0.040" then might as well run the thick head gasket and drop cr as much as possible. Quench is already gone with the piston so deep.
fyi, 38 inch tall tires and a 4.11 gear is the same as a 28" tall tire with 3.08 gears. So, more gear will help with tires that tall.
Your engine will work the way it is with reduced ignition timing, but not ideal. Power and mpg will be reduced with low timing.
|03-11-2013 04:56 PM|
|Motosledneck 12||I'm hearing about 50/50 from everywhere half say its fine half say it won't work I guess I'm just going to run it and see what happens. But witch head gasket should I use the .026 or the .039?|
|03-11-2013 06:00 AM|
Compression should be 9.45 with a -0.040" deck:
8cc :deck height (-0.040)
6cc :4 relief piston
Still a little too high for that cam. Look at the next size bigger cam from summit racing. Use an intake duration of 210 to 214 at 0.050", 112 LSA, plus 8 to 10 degrees more on the exhaust as compared to the intake.
Double check the deck height. Makes a big difference in compression ratio. If your engine had a zero deck then the cr would be (727+78)/78=10.3.
882 heads flow better than the swirl port heads.
|03-10-2013 05:04 PM|
|Motosledneck 12||The pistons I have are 1.54 instead of the normal 1.56 for hight from the pin to the top of the piston. but they say that they are a 10.2:1 with 64cc heads. The block has never been decked. Also i have metal GM performance head gaskets that are .026 thickness. Would that be better? But then my compression goes up even more with those|
|03-10-2013 04:56 PM|
|cadillacdave1||You would need a thinner head gasket to bring your quench down, ideally to somewhere in the area of .040 to .050 in your case. However, you said the block is decked at .040. That seems kind of high to me. Most stock blocks are .020 to .025, unless the engine has been rebuilt and the pistons are sitting lower.|
|03-10-2013 03:56 PM|
|Motosledneck 12||Is there anything I could do like maybe get a thicker head gasket maybe?|
|03-10-2013 02:38 PM|
Using Pat Kelly's calculator and the folowing variables:
4.030" gasket bore?(minimum)
5.0cc valve reliefs
I come up with a 9.40:1 static compression ratio, and a 0.079" squish/quench. Both are too high for pump gas IMHO.
|03-10-2013 01:07 PM|
Motosledneck 12, I believe your CR will be more in the range of 9.5-1. I am running a 69 3/4t chev 2x with a grandma 4psd, 4.10 dana rear. Have a .010 350 with flattops and 2 eyebrows valve cutouts. A 650 holley, 2 plane performer intake, set of those 190cc, 64cc Proform aluminum heads, Howards cam with 213* @.050 .430 lift. Put some long slot 1.6 stamped rockers on for a lift of .458 Have a Proform HEI with 12* initial and a total of 36* @ 2800rpm. Engine idles @ 650rpm with manifold vacuum at 15.5 inches. With the vac adv. to manif. vac. it runs at 53* total@ 3000rpm. I get no ping and run 87oct. pump gas. Engine is a little lazy below 1100rpm. If I up the intial to around 18* it pulls great off idle with no ping but my total goes to 60*. I am going to find a better adj. vac. can ( this can doesn't adj. it out properly) to bring the total down with vac adv. to 48* and set the initial to 16*. Obviously I have a little more tuning and adj. to timing management to do but the combo works very well. Also I feel a slight off idle, tip in, lean condition. I will be addressing this after I adj. the timing curve to what I need.
Your combo I believe will work fine....your just lacking in the heads flow. You will be fine, just watch your timing. Initial, mech. and total with vac. and make sure you don't run lean anywhere in the range you operate the engine.
|This thread has more than 15 replies. Click here to review the whole thread.|