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Tech / General EngineIs your car making a strange sound or won't start? Thinking of adding power with a new combination? Need other technical information or engine specific advice? Don't see another board for your problem? Post it here!
I’m doin some tinkering on my car over the winter. Currently mocking up a T trim. After mocked up I’m pullin the engine to upgrade the heads, key the crank, etc. I have my current piston spec, and head cc. Only thing I’m unsure of is head gasket thickness, but I think it’s .038 compressed thickness. Byno means positive though. Heads are GM fastburns that were 60cc chambers. (Factory were 62, theses were cleaned up some).
piston spec: Bore = 4.040 Stroke = 3.480 Rod length = 5.850 Compression height = 1.410
Spec is according To Ross pistons from part number on bottom of piston.
My calculation brings that to 11.53:1, but I don’t see that as possible. I feel like when it was built we specified a piston to be at 10.25:1, which would make more sense. I’ve not experienced any noticeable detonation on 93 octane. It runs strong, never dynoed though. At 11.53:1 I’m pretty positive it’s beyond safe for any boost, but at 10.25:1 I may be able to get it low enough through the top end change.
Currently pullied for roughly 14 psi, but obviously that’s just on paper, who knows in reality. If plan to switch to some AFR 195s which have a 5cc bigger chamber, plus I could run a thicker gasket, but I don’t think that would even get to a comfortable spot. Will be intercooled, not opposed to cuttin the 93 with race cars, but I wouldn’t know where to begin with octane. What’s any suggestions for ideal compression and best way to get there?
a little long winded, but thanks for the input
Last edited by 2slow5.0; Dec 30, 2020 at 09:48 PM.
You need to account for how far the piston is below or above the block deck.
And you need to account for the bore diameter of the gasket itself (will be larger than the cylinder).
You need to account for how far the piston is below or above the block deck.
And you need to account for the bore diameter of the gasket itself (will be larger than the cylinder).
I believe it was zero deck. Been searching for the original build paperwork, I think gone forever though
Generally speaking, that's far too much compression for a boosted engine.
For boost - you don't rely on compression for power. You use the supercharger to cram air into the engine instead. Target compression is typically around 8.5 for boost in the neighborhood of 1 bar.
Yes. I know its far too high for boost, i just dont feel like its actually that high. I would have thought I would have detonation issues on pump gas at that point, especially early on with a chitty tune. Im starting to wonder if Ross gave me specs that were off some.
Stock, the deck height of a block (distance from crank CL to deck) is usually around 9.025 - 9.035". The stock "height" of the rotating assy is 9.000". Deck clearance is the difference between the 2. "Spec" for deck clearance is .025" but is usually more, normally .005 - .010" more, but sometimes ALOT more; and highly variable, either front to rear on one side (deck not perfectly parallel to crank CL), from intake side to exhaust side (deck not perfectly tangential to the rotation of the crank), driver's side to passenger's side (decks not the same on the 2 sides), in pairs (cyls on opposite banks sharing the same rod throw affected by variation in stroke length from journal to journal), random (rods or pistons not all the same), and so on. 70s motors were UNBELIEVABLY sloppy in this area: I've seen as much as .025" of variation within a single motor.
3.48" ÷ 2, plus rod length 5.850", plus piston height 1.410", equals exactly 9.00". If the block isn't "zero decked", your deck clearance is probably in the range of .025" or grater.
A sure sign of someone that hasn't measured ANYTHING but instead goes by "spec" and "catalog", thus leading to any number of potential surprises, is quoting compression ratios out to 2 decimal places. You clearly have NO IDEA how much potential there is for variation and uncertainty in what you're doing.
A "standard" composition head gasket like a 7733PT is about .039" compressed. Steel-shim (which I would NEVER use in a NA application let alone with boost) can be as low as around .015". The old red GM gaskets, which I think you can still get, were about .028".
If you want to know your CR even to a precision of only ONE decimal place, you need AHELLUVALOT BETTER measurements than what you have. Specifically, deck height, chamber volume, and piston volume (which includes the VRs, the space between the top ring and the crown, and any/all other voids).
As far as any "ideal" compression, I'm not the person to ask about that. Can't help you there. I CAN tell you however, that attention to detail will help DRAMATICALLY; particularly, making sure there are no sharp edges anywhere in the chambers. Very thin metal like that will tend to get red-hot and lead to pre-ignition. In particular, chamfer or radius the top of the bores where they meet the deck, and the circumference of the chambers where they meet their deck surface; as well as the margins of the valves, especially exhaust.
Attention to detail is the difference between 2 motors with identical BOMs that run 50 HP or more different. "Engine Masters" vs Billy Bob and Cletus under the shade tree. Edelbrock getting 435 HP out of their "Performer RPM" kit, and Bubba next door not even cracking 275.
Last edited by sofakingdom; Dec 31, 2020 at 09:40 AM.
Spec is according To Ross pistons from part number on bottom of piston.
Care to share that part number?
How much were the heads milled? Average results are about 1cc/.006".
FWIW: Inputting at a minimum a 5cc valve relief you're now at 10.9:1.
...3.48" ÷ 2, plus rod length 5.850", plus piston height 1.410", equals exactly 9.00". If the block isn't "zero decked", your deck clearance is probably in the range of .025" or grater.
This the key though. Without this value there's no real idea what the compression ratio might be.
With an .025" deck, potential CR drops to 10.25.
Last edited by skinny z; Dec 31, 2020 at 11:20 AM.
Sofa. I understand im shootin in tbe dark here. Kinda why im askin the questions. Its an older bowtie block, was never in a car from the factory. I was just trying to get an idea to gather parts for the next step. Obviously ill be disassembling it to get the correct stuff.
130349YH is piston part number.
We measured chamber volume on the heads before they were installed. 60cc.
I am unsure of head gasket dimensions. Simply going from memory. Same goes for deck clearance. Piston info is directly from Ross. I have the head info written down from years ago when it was put together.
Its been a fun engine for a few years (bus fare not needed....yet) and given me 0 issues. Just bored and want to change it up.
Thanks for the info guys. Appreciate it
Last edited by 2slow5.0; Dec 31, 2020 at 05:30 PM.
Heres an original piston I saved from when i bought the engine. I do recall a couple cylinders bein a little scored up, requiring a .010 bore. This not the current piston. Simply wahat was in it when i acquired it.
Seems to me you know exactly what you're up against. The previous comments have been educated guesses.
Once you have the heads off you can measure the deck. Then you'll have all of the data you need.
If it were me this is something I'd do before ordering new heads as there might be a change in combustion chamber volume.
Here's an original piston I saved from when i bought the engine. I do recall a couple cylinders bein a little scored up, requiring a .010 bore. This not the current piston. Simply what was in it when i acquired it.
That's quite the dish.
Last edited by skinny z; Dec 31, 2020 at 12:52 PM.
What does the ross pistons look like? Pure flat top no reliefs?
can you get e85 or buy drums of e98 and 60/40 cut pump gas 60% e98 40% 93 or even 87.
afr might or should still have 75cc chambers
I dont honestly remember what the ross pistons looked liked. No 85 around here. Local racetrack has 112 I believe. Trust me, i was temped to put it together with some bigger injectors, and just run that for the weekends i wanna tear around this summer. But If compression is really that high i dont think its a good idea anyway. But open to suggestions
Ross specs show that 4.040 reverse dome piston at a 9.4cr with 64cc heads. So if your heads have a 60cc, although I suspect they're probably closer to 64, if they were originally 62, but have been "cleaned-up," and your gasket thickness guessed at .038, then you'd be in the 9.2-9.6 range(64-60cc).
Where you're getting confused is the stated dome volume for that piston is -11.80, which is what's been removed to make it a reverse dome piston(a "dish" piston). Using that figure will calculate to an insanely high cr. But that figure is actually adding 11.80 to the chamber, not deducting it. And when you change that figure from -11.80 to +11.80, you're in the 9.4cr range that Ross states.
Ross specs show that 4.040 reverse dome piston at a 9.4cr with 64cc heads. So if your heads have a 60cc, although I suspect they're probably closer to 64, if they were originally 62, but have been "cleaned-up," and your gasket thickness guessed at .038, then you'd be in the 9.2-9.6 range(64-60cc).
Where you're getting confused is the stated dome volume for that piston is -11.80, which is what's been removed to make it a reverse dome piston(a "dish" piston). Using that figure will calculate to an insanely high cr. But that figure is actually adding 11.80 to the chamber, not deducting it. And when you change that figure from -11.80 to +11.80, you're in the 9.4cr range that Ross states.
Confused is an understatement I know the heads are 60cc, i was there when they were measured. I really appreciate your time. If someone wants to pm me an email address Ill fwd the email from Ross. Thank you guys all for the input
Last edited by 2slow5.0; Dec 31, 2020 at 05:35 PM.
I dont honestly remember what the ross pistons looked liked. No 85 around here. Local racetrack has 112 I believe. Trust me, i was temped to put it together with some bigger injectors, and just run that for the weekends i wanna tear around this summer. But If compression is really that high i dont think its a good idea anyway. But open to suggestions
last resort is compression cranking test. If engine is still together get a couple readings. You can roughly estimate compression based on cam events and the cranking compression but its a very rough idea lol ring seal and starter speed makes all the difference in the world lol
As far as piston spec goes, (from the email forwarded) all the info posted above is all that Ross has made available. No mention of piston crown profile. Which doesn't help the CR guesstimates.
I guess at this point pull the heads if you are dead set on boost. Verify piston type, piston to deck and verify gasket with a new gasket and calculate the new spec. Might not have to change heads afterall
I don't see anything said about the Ross piston being a flat top, just that it isn't remembered what it is. And the specs listed should include a +/-cc number from Ross, but I don't see that listed either. And according to Ross' website(and the entire internet), 130349YH doesn't exist. So I was going by the pictured piston, matching it to a Ross piston that's nearly identical, full complete specs. So I guess, nevermind lol.
I don't see anything said about the Ross piston being a flat top, just that it isn't remembered what it is. And the specs listed should include a +/-cc number from Ross, but I don't see that listed either. And according to Ross' website(and the entire internet), 130349YH doesn't exist. So I was going by the pictured piston, matching it to a Ross piston that's nearly identical, full complete specs. So I guess, nevermind lol.
Hey, that wasn't meant as a shot or anything. Just trying to clear things up. I did the same internet deal with the PN. No luck.
Came across the same Ross piston too. I'm thinking 16cc dish?
Not sure how I got the flat top thing in my head.
last resort is compression cranking test. If engine is still together get a couple readings. You can roughly estimate compression based on cam events and the cranking compression but its a very rough idea lol ring seal and starter speed makes all the difference in the world lol
I've done this and had compiled a list of calculated vs actual results from various posters here and there. It was surprisingly close.
This calculator.
Read the text and it'll let you use the IVC if known. Comparing to the several SBCs I've put together along with about a half dozen other contributors, it was found to be a reasonable way to reverse calculate a CR. Especially if you have the cam timing.
Fully charged battery. Engine nice and warm. Crank it till won't increase any further. It's helpful to get the pressure after a couple of strokes too as that data is useful but my best test comparisons came from the crank it till it peaks approach.
Originally Posted by Orr89RocZ
I guess at this point pull the heads if you are dead set on boost. Verify piston type, piston to deck and verify gasket with a new gasket and calculate the new spec. Might not have to change heads afterall
Its not an off the shelf piston. It was a special order, the 5.85 rod isnt super common. Took around a month to get em made back in the day. I can clearly remember some things, others are fuzzy. Too much booze over the years I guess.
You guys are right, I want to have the crank double keyed anyway. I dread special ordering a set of pistons right now if that’s what’s needed
Last edited by 2slow5.0; Dec 31, 2020 at 10:22 PM.
You clearly have NO IDEA how much potential there is for variation and uncertainty in what you're doing.
) and given me 0 issues. Just bored and want to change it up.
