|
By DANIEL TINDER - 15 Years Ago
|
When I started building them, they were intended for a .030"-over 292, with flat top pistons. Accepted theory then was, as Y-Block compression is so hard to achieve, an upper limit was never considered.
I now find myself with 318" displacement, and pop-up pistons. When originally chamber-matching the heads, I estimated final volume at 68cc's. With block zero decked, calculated CR is 9.5:1. However, due to abject laziness & wrongheaded assumptions (don't ask), I have ended up with 66.2 cc (smallest chamber). If CR variations due to volume are indeed linear (?), that comes out to 9.8:1. CR.
So, do I bite the bullet and disassemble/grind/clean & CC the heads again multiple times in trial & error attempts to achieve my original target volume (this IS a mild street motor), or will other variables with this particular application that might allow a higher CR (limited breathing, tight piston-head clearance, above average ignition curve/limit control, mixture effects of piston domes, etc.) give me enough leeway? (I would almost rather drag around a case of octane booster, that have to start over again on the heads). Of course, if more chamber grinding IS absolutely needed, better now than after the engine is installed.
|
|
By DANIEL TINDER - 15 Years Ago
|
P.S. (rushed the last post so I wouldn't time-out and lose the text).
I should add: cam is Isky E4 (just slightly warmer than '57 OEM), degreed straight-up. Likely a negative factor re: CR limit.
There is also the complication of some pistons measuring .004-.005" out-of-the-hole. But (luckily), not on the smallest chambers.
I know that FoMoCo was encouraged to start posting the heads due to failures encountered after supercharger use boosted dynamic compression, but since no Y-Block had static CR as high (9.8:1), this adds the complication of gasket failure to likely pinging experienced on 93 octane.
|
|
By Pete 55Tbird - 15 Years Ago
|
|
Daniel If it was my engine I would 1 Switch to a 12 volt system as it will not start hot on 6 volts 2 Run it as is 3 Get the thickest head gaskets and run it 4 Put bolts trrough the top of the cylinder heads to support the combustion chambers from the top 5 Get a cam with more overlap to lower the dynamic compression Of all these I like run it as is best. Pete
|
|
By DANIEL TINDER - 15 Years Ago
|
|
Pete 55Tbird (5/22/2010)
will not start hot on 6 volts
Pete,
Don't recall anyone complaining back in '55 that their poked & stroked hotrods with milled heads were hard to start? BTW, my previously mentioned hot/slow-cranking problem seems solved (fingers crossed) with fresh battery. Only time will tell, but HAVE noticed much improved cold starting. I guess carbon pile/BCI load test is only way to truly determine if it is overdue for replacement.
I will likely have to face up to reality, roll up my sleeves and get out the grinder. It occurred to me I might avoid disassembly/seal damage/cleaning hassle by fabbing a protective metal shield for the valves and wrapping the head in plastic. Two CCs of iron could easily be removed by just un-shrouding the intakes some more.
|
|
By Ted - 15 Years Ago
|
DANIEL TINDER (5/22/2010)
When I started building them, they were intended for a .030"-over 292 .... I now find myself with 318" displacement, and pop-up pistons .... With block zero decked, calculated CR is 9.5:1. However, due to abject laziness & wrongheaded assumptions (don't ask), I have ended up with 66.2 cc (smallest chamber). If CR variations due to volume are indeed linear (?), that comes out to 9.8:1. CR. So, do I bite the bullet and disassemble/grind/clean & CC the heads again multiple times in trial & error attempts to achieve my original target volume (this IS a mild street motor), or will other variables with this particular application that might allow a higher CR (limited breathing, tight piston-head clearance, above average ignition curve/limit control, mixture effects of piston domes, etc.) give me enough leeway? The accepted rule of thumb for max static compression ratio for iron headed engines is 9.5:1 but this can vary somewhat depending upon the camshaft. The later the closing of the intake valve, the more tolerant the engine will be for using a fuel with lower octane numbers. Rather than use the static compression ratio calculation, using the dynamic compression ratio can be a better indicator of what an engines tolerance for fuel octane will be. I use 8.0:1 dynamic cr for 91 octane fuel while the use of 93 octane fuel can raise the max dynamic cr to 8.25:1. Here are the numbers I used to calculate your dynamic compression ratio. 292 crankshaft? You didn't say so that's what I used for the calculations. The 60° ABDC closing for the intake valve is a rough estimate based on some Isky E4 numbers I had handy. You didn’t mention what your piston dome cc’s so I simply based my value on what the typical Y domed pistons are. I did come up with a higher static compression ratio than you did which also drives dynamic compression ratio up as a result. 
Based on these values, your combination is on the ragged edge of detonation with 93 octane fuel when trying to maintain optimal ignition timing curves.
|
|
By Ted - 15 Years Ago
|
|
If you are dealing with a +040 over 312, then here are the new compression ratio (both static and dynamic) calculations based on the 318 cubic inch you mentioned. 
|
|
By 314 - 15 Years Ago
|
|
i hate to bother you ted with the same question.i have a 314 using g heads.the pistons are flush and i used thick head gasgets so comp, will be more or less stock 57 312.cam is mild 222@050.nice idle with112.heads were not shaved.i dont think i will have a problem.its a 292 with 312 crank.
|
|
By DANIEL TINDER - 15 Years Ago
|
Ted,
Surprised rod length/bore Vs. stroke/etc. affected CR (?). Thought only displacement/volume was determining factor.
Actual specs:
6.200 Rod length
3.820 Bore
3.470 Stroke
.150 cut from Probe Piston domes
Best head gasket
Aside from providing "ragged edge of detonation" cushion, further grinding/chamber equalizing should also improve motor smoothness, justifying the effort.
|
|
By Hoosier Hurricane - 15 Years Ago
|
|
Daniel: Re-read Ted's post. He talks about two compression ratios, static and dynamic. Your assessment of CR is of the static CR. Dynamic is what the engine sees when it's running.
|
|
By DANIEL TINDER - 15 Years Ago
|
John,
Thought as much. Will assume that static CR IS fixed and entirely dependent on displacement/volume only (regardless of valve timing). Goes without saying dynamic CR affected by cam selection. Just wasn't aware bore/stroke/rod length also important factors. Don't want to belabor this point, as much relevant data readily available.
|
|
By Ted - 15 Years Ago
|
314 (5/24/2010)
i hate to bother you ted with the same question. i have a 314 using g heads.the pistons are flush and i used thick head gasgets so comp, will be more or less stock 57 312.cam is mild 222@050.nice idle with112. heads were not shaved. i dont think i will have a problem. its a 292 with 312 crank. The critical part of the dynamic compression ratio calculation is the value used for the intake valve closing. The connecting rod length is a minor player but does make a subtle difference in the dynamic compression ratio calculation. For a solid lifter camshaft, the intake closing value is easily obtained when degreeing in the camshaft but it’s typically not stated as such on the spec card supplied by the manufacturer. Here are the numbers for the 314 combination using out of the box cc’s for the G heads but using an estimate for the intake valve closing. This combination will run well on 89 octane and could possibly get away with using 87 octane fuel if not being flogged heavily.
|
|
By Ted - 15 Years Ago
|
|
DANIEL TINDER (5/24/2010)
Ted,
Surprised rod length/bore Vs. stroke/etc. affected CR (?). Thought only displacement/volume was determining factor.
Actual specs:
6.200 Rod length
3.820 Bore
3.470 Stroke
.150 cut from Probe Piston domes
Best head gasketNot knowing the dome volume for the pistons, I simply estimated it at half of normal and used 5cc’s. The problem with using assumed values is the overall accuracy is compromised and the final compression ratio numbers still end up being fuzzy. Here’s what the calculations look like based on what’s supplied.
|
|
By DANIEL TINDER - 15 Years Ago
|
Ted,
Assuming dome volume estimation is in the ballpark, can you calculate what target chamber CCs would bring dynamic CR into the 93 octane safety zone using this formula? It did also occur to me that much improved flow that might result from extensive intake unshrouding on only a few cylinders could be just as detrimental to smooth running as existing CR variations?
|
|
By Ted - 15 Years Ago
|
|
DANIEL TINDER (5/25/2010)
Ted, Assuming dome volume estimation is in the ballpark, can you calculate what target chamber CCs would bring dynamic CR into the 93 octane safety zone using this formula?For a solid lifter camshaft and using the closing event as measured at the camshaft and not at the valve (using the actual valve event will numerically increase the dcr value), I lean towards 8.0:1 dynamic cr being the safe limit for 91-93 octane gasoline so with that in mind, 70cc combustion chambers are required without altering the piston dome further. The 70cc value is based on the assumption that the intake valve is closing at 60° ABDC which appears to be a valid number if the E4 camshaft is installed 2° advanced (106° intake lobe centerline) and the lash is set at 0.018”. Advancing/retarding the camshaft as well as altering the valve lash makes the cr value flexible. Here are the numbers using the same 60° ABDC for the intake closing event.
If the camshaft is indeed installed straight up (108° intake lobe centerline), then the dynamic compression ratio reduces slightly from 8.35:1 to 8.23:1 with the same 0.018” lash adjustment. Decreasing the lash further to 0.015” while staying at 108° intake lobe centerline drops the dc to 7.72:1 so you can see where lash adjusment is a big player. If remachining your pistons into flattops and leaving the heads as they are, then here are the numbers using the same 60° ABDC intake closing value. 
If the rotating assembly is already balanced, then this may not be an option unless ‘overbalance’ has not already been figured into the bobweight calculation for your particular rotating assembly. If only 2-4 grams of oil was used for the bobweight calculation, then machining the pistons is an option which would automatically add ~27 grams of overbalance to the bobweight calculation and would not require rebalancing of the rotating assembly.
|
|
By charliemccraney - 15 Years Ago
|
|
Ted, What is the formula? I'd like to figure mine.
|
|
By DANIEL TINDER - 15 Years Ago
|
Thanks Ted,
Shortblock disassembly not an option at this time. While previous posts would indicate running wider than Isky recommended lash might produce smoother idle, It's reassuring to see my original target chamber volume should be OK with their specs. Can always experiment with increased lash later on, as other setup factors might provide additional latitude. Wish I knew for sure how much iron I can safely remove around the intakes? Will have to study sectioned head photos.
|
|
By Ted - 15 Years Ago
|
|
charliemccraney (5/26/2010)
Ted, What is the formula? I'd like to figure mine.I’m performing these calculations in an excel spread sheet. Here’s the formula for dynamic compression ratio. =((B6*B6*(B7-((((SQRT((B5*B5)-((0.5*B7*SIN(RADIANS(B12)))*(0.5*B7*SIN(RADIANS(B12))))))-(0.5*B7*COS(RADIANS(B12))))+(0.5*B7))-B5))*12.87036997)+B8+B9+(B6*B6*B10*12.87036997)+B11)/(B8+B9+(B6*B6*B10*12.87036997)+B11) B5 = rod length B6 = bore B7 = stroke B8 = head cc’s B9 = piston dish cc’s (use a negative value for dome volume) B10 = deck height B11 = head gasket cc’s B12 = degrees ABDC that the intake valve actually closes.
|
|
By Butch Lawson - 15 Years Ago
|
|
Charlie, Try this: www.wallaceracing.com/dynamic-cr.php
|
|
By 62galxe - 15 Years Ago
|
|
heres another article with a calculator at the bottom. http://members.uia.net/pkelley2/DynamicCR.html
|
|
By charliemccraney - 15 Years Ago
|
|
I know of the calculators. I'm really more interested in how it is calculated. Thanks, Ted.
|