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Author Topic: What's my compression ratio  (Read 3155 times)

Steve

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What's my compression ratio
« Reply #45 on: February 10, 2010, 11:09:10 AM »

Stan
 
Ed has me beat on all this fine tuning stuff. . . . .  I've been a stocker all these years.  But I can curve a distributor.
 
Don't let the old geezer get off cheap Ed.   
 
He'll ship it to you and forget he sent it.
 
Really Ed, Stan is a cool guy.  Good man and friend.  Even if he can't figure out how to answer his cell phone.
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What's my compression ratio
« Reply #46 on: February 10, 2010, 11:41:47 AM »

Yeah, yeah, yeah, I'm just a bucket of information.  If Stan knows what a distributor is, how to remove it from the block, (and mark it so the rotor points the same way as it is running right now) pull the cap off and wire connection, he can then pull the rotor off, and undo the two screws on the side of the case where the clamp attaches (two phillips screws), and gently lift the plate off the inside (might need to remove the vacuum advance (again, two phillips screws) and once the two are loose, it will unhook from the vacuum advance plate.
 
Underneath you will see two small springs. If you hold the top of the distributor shaft and twist the bottom of the shaft, you will see these two springs allow a certain amount of twist for mechanical advance. This is what we need to slow down from occuring. We still want it to advance, just not as fast as it is doing it. The auto parts stores have these springs as an advance kit (Mopar, SBC, SBF, doesn't matter they are all the same springs), and you want the heavy one that is on it now (a stock one, looks like it is stiff enough to lift your garage door), and in the kit you want one that looks to be the next step below this stiff one, and this will slow down the advance curve, allow cheaper gas octane without harming anything.
 
Put her back together and then play with the timing to see how much you can put into the harmonic initial advance, thinking if you can run 12degrees advance and 91-93octane keeps her from pinging, 85octane should be starting with 6degrees advance and then advance her two degrees at a time after driving her around a bit to make sure different circumstances don't kick in and make her ping.
 
You can do it, it isn't that difficult.
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Steve

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What's my compression ratio
« Reply #47 on: February 10, 2010, 01:35:38 PM »

We have to first change his Depends.  Then a gallon of Geritol, then a 6 of that Iron Gut he drinks. . . .
 
As you can see Ed, we can relax over here.  But don't get too relaxed.  
 
Stan is a moderator. . . .
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What's my compression ratio
« Reply #48 on: February 10, 2010, 02:39:13 PM »

Aaaaaa crap, gimme a few days, see if I can raise some hell or piss someone off.
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Leaburn Patey

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What's my compression ratio
« Reply #49 on: February 10, 2010, 03:15:05 PM »

I agree with Ed that a recurve and fine tuning is key.
I did that with my 440 in the  68 C Barge.
Just a reminder that whenever anything is changed (i.e. cam,pistons,etc) you can toss  the factory timing specs out the window.
Test and tune,trial and error.
Originally,my 440 called for 8 degrees initial with factory cam,points,single exhaust,etc.
After a rebuild,cam swap,452 heads that were massaged,dual exhaust,electronic ignition and a recurve,10.5 initial.She comes in at full advance around 2300 with a total of 41 degrees.My dizzy had a parts jobber vacuum advance pod with 9.5 on it (18 degrees).
Swapped it out to 11.5 (23 degrees)
I get almost 21 inches vacuum and she idles smooth.
I re-used the factory flat top pistons so it does ping only when I run cheap low grade gas.
I never get ping when running Ultra94.
My best MPG is 16.28 MPG on my trip to Joisey in 2008. 
Just ask Steve,my 4600 pounder moves quite well!! LOL!
 
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Leaburn Patey

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What's my compression ratio
« Reply #50 on: February 10, 2010, 03:44:20 PM »


Ed,I am in the middle of rebuilding a 68 383 2bbl engine (9.2 CR factory)
I am converting it to a 4bbl HP set up.
Despite having 62,000 miles on it,the block had to be bored 40 over.
Piston selection is very limited and my builder located a set of Sealed Power cast pistons.
They shaved ten thou off the block's deck.
Original 906 heads got hardened valve seats and planed (do not know by how much).
But were cc'd to 87.8 cc's.
 
(Some extra specs that you may want to know,but not directly related;
I will be running the 268H Comp Cams,911-16 springs,Cloyes timing set,factory points dizzy-recurved,with Pertronix conversion.750 Eddy carb stock intake,HP logs with TTi 2.5" duals.Stock convertor, 2.94 gears and 26.69 OD tire.)
As Steve mentioned,I will be doing further massaging on the heads once they come back home,port match the HP logs and intake.
 
1: Any idea what the CR ratio would be?
2; do I need special length pushrods after all the machining??
If so,how can I calculate what lenght I need?
 
 
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Steve

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What's my compression ratio
« Reply #51 on: February 10, 2010, 04:45:16 PM »

Ed
 
Lea tells me they took an extraudinary amount off the heads and deck. I'm inclined to think he's going to need pistons with valve reliefs.  The 383 does not have, what do they call that. . . .  Valve relief.
 
I told Lea to have the intake shaved to match.  Lea said they took something like .25 off the heads and another 10 off the block.  That's nuts!
 
I'm alittle pissed off at that.  I have a feeling those pistons are not going to work.  He'll be bending valves
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Steve

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What's my compression ratio
« Reply #52 on: February 10, 2010, 04:48:48 PM »

Leaburn
 
I think you're going to need something like these
 
http://www.summitracing.com/parts/UEM-KB400-040/
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What's my compression ratio
« Reply #53 on: February 10, 2010, 06:15:53 PM »

And you could run that cheap 87octane if you slowed it down just a little bit more, full in at 2800-3000rpm would probably do it.
 
Here's another way to look at this. Take an engine that runs stock compression at around 9:1. It has a smaller duration cam in it and those valves close sooner and build more compression and yet it doesn't ping does it? The shorter the duration the more pressure built inside the cylinder and in all reason it should ping like all getout at low speeds but they don't do they? Raise the compression and increase the duration, the duration bleeds off the cylinder pressure so it can build at higher rpm, and yet it pings (remember those rules, where it says in the cam directions, a cam this large needs this or that compression minimum? That's the reason for it). Lower compression due to bleed-off, yet there is pinging, something is wrong with this, it is backwards, so it isn't so much the level of compression it is the speed of which the spark happens, so reduce its speed and cheap gas works.  When you get above the 10.5:1 range then you have to work on the combustion chamber itself (kind of like why a Hemi head can have higher compression and not ping on cheap gas, yet a wedge head can't run that high). Also, additionally, the speed of the explosion increases with the higher compression, thus a spark that makes the gas explode and cross the cylinder at (example only) 500fps at 9:1 may travel 600fps at 10:1 compression, so slow it down and it works.
Ed
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Leaburn Patey

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What's my compression ratio
« Reply #54 on: February 11, 2010, 01:46:35 AM »

Hoping to see the pistons today if they showed up.
I will check for valve reliefs
 
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firedome

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What's my compression ratio
« Reply #55 on: February 11, 2010, 03:35:49 AM »

That's a HUGE amount to shave off - were the surfaces really bad or were they warped?

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Steve

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What's my compression ratio
« Reply #56 on: February 11, 2010, 06:00:42 AM »

Hell Yeah. . .If those numbers are accurate. . .  Lea was quoting, but not sure
 
I never heard of anyone taking that much off.  That's way out there
 
Ed
You didn't comment on my thought about the pistons
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Leaburn Patey

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What's my compression ratio
« Reply #57 on: February 11, 2010, 07:01:39 AM »

I could be wrong,guys.
Not really fluent in engine mathematics and geometry.
Still trying to contact Rob at Bullet
CBarge2010-02-11 12:02:27
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What's my compression ratio
« Reply #58 on: February 11, 2010, 08:39:01 AM »

Quote from: CBarge

Ed,I am in the middle of rebuilding a 68 383 2bbl engine (9.2 CR factory)
I am converting it to a 4bbl HP set up.
Despite having 62,000 miles on it,the block had to be bored 40 over.
Piston selection is very limited and my builder located a set of Sealed Power cast pistons.
They shaved ten thou off the block's deck.
Original 906 heads got hardened valve seats and planed (do not know by how much).
But were cc'd to 87.8 cc's.
 
(Some extra specs that you may want to know,but not directly related;
I will be running the 268H Comp Cams,911-16 springs,Cloyes timing set,factory points dizzy-recurved,with Pertronix conversion.750 Eddy carb stock intake,HP logs with TTi 2.5" duals.Stock convertor, 2.94 gears and 26.69 OD tire.)
As Steve mentioned,I will be doing further massaging on the heads once they come back home,port match the HP logs and intake.
 
1: Any idea what the CR ratio would be?
2; do I need special length pushrods after all the machining??
If so,how can I calculate what lenght I need?
 





http://www.rbracing-rsr.com/compstaticcalc.html
 
Here is an easy and explainable compression calculator, just plug six items in, the missing two are the distance of the piston at TDC to the top of the deck, and the head gasket thickness.  The heads didn't get shaved very much because of the combustion chamber size, pretty close to stock still, 906 is an open chamber design and about 88-89cc stock, so they were cleaned maybe .010 at best (big block heads rarely need more than .005-.010 to get them straight). Blocks on the other hand can be as much as .015 out of square from one corner to the other, that's why blueprinting Mopars is such an improvement over flat stock.
 
So, using 4.290 bore, 3.375 stroke, .038 gasket thickness, zero on the deck height to piston clearance, zero on the piston top (valve reliefs are good for 4cc which you don't have), and the combustion chamber of 87.8cc. This information gives you 9.25:1 compression. This is decent compresssion for the street and will run strong.
 
Now, the pushrods, myself, I prefer to have adjustable rockers, especially with heads and block machined. Since you are going four barrel and all that, decent cam, you want to be able to adjust the rockers and use a set of anti-pump-up lifters to take advantage of what you have, otherwise 5300rpm valve float will be a given (built into the engine design). Anti-pump-up lifters prevent the lifters from pumping up too much at high rpm and floats the valves due to the fixed preload with non-adjustable rockers. It is also easier on the cam itself and can save a valve or two when floating. The only thing I don't like about adjustable rockers is the cost, which is right around $250 and above cost. Aluminum are OK, steel is best, they last longer. And of course you need pushrods with cups in the end to go with them, stock length would be fine, but then you know exactly how much preload is on the lifter (I like setting zero lash, none of that take it to zero and then 3/4 turn crap, don't need it, adds a little extra no valve float possibility and some give if a valve touches a piston). Port match the stock intake (no aluminum dual plane to port match?), TTi is great sounding, (I have 3inch on my Charger and HP manifolds, it has been referred to my young daughter by passerbys as "Your dad's Charger sounds like a %ucking monster.") 2.5 is a bit quieter, but still nicer sounding than the  "other" brands, Dynomax is a sweet sounding muffler. I can help with the head porting when you get to it, port match, short side turn, gasket rim, intake manifold ports, couple lumps and bumps, some edges at the carb flange to fix.
 
Distributor sounds good for now, might need some fine tuning, carb is OK for now,fine tuning later, and run a 180 degree thermostat...there.
 
 
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Steve

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What's my compression ratio
« Reply #59 on: February 11, 2010, 09:05:53 AM »

I sure hope Leaburn is wrong about the amount they took off those heads.  383's aren't that forgiving in that department.
CBarge2010-02-11 17:58:21
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