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To change pistons, or to not change pistons...

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72BoB View Drop Down
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    Posted: 31-January-2012 at 2:03AM
...that is the question...
 
So yeah, when I bought the engine that I am currently about to put in my car the builder said it should have about 11:1 compression. Me of course being as zealous as I was at the time said "sure, I can work with that." I might have been a little starry-eyed looking at the parts list considering all of the parts used in the build are really good parts, and everything was balanced and blueprinted, and it had worked C9 heads.
 
So now, as the time gets closer and closer for me to drop the engine in the car (which I was planning within the next month or so), I have become more and more nervous about that 11:1 compression with iron heads, considering that this is a street car that sees about 5000 miles per year. I have talked to people and they are all saying that I can run it on premium as long as I back off on the timing, but is that really that safe in the long run, or am I just going to install a ticking timebomb that is going to detonate and blow chunks all over the place? Plus, how is backing off the timing enough to make 11:1 survivable going to affect performance?
 
Should I just risk it and install it and run it? Or should I fork out the dough to have the pistons changed out for a CR around 10:1?


Edited by 72BoB - 31-January-2012 at 2:06AM
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ilyes View Drop Down
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Post Options Post Options   Thanks (0) Thanks(0)   Quote ilyes Quote  Post ReplyReply Direct Link To This Post Posted: 31-January-2012 at 2:21AM
Pistons are not everything, it depends of the cylinder bore, piston relief, gasket thickness and head volume. The cheapest way to go is to buy a thick gasket, it will drop compression a little, some valve notching would help as well
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Rockatansky Quote  Post ReplyReply Direct Link To This Post Posted: 31-January-2012 at 7:53AM
i've run 11.4:1 before, you can't back off the timing enough to make it work
 
you might want to 'pump' a cylinder to find out exactly what you have before doing anything else. this is how the tech guys determine if a race engine is legal or not for class without taking it apart, they fill a cylinder with liquid with the piston at either top or bottom then turn the engine 180* and measure both the pump volume and how much it took to fill the chamber at TDC
 
your cam also has a lot to do with whether or not 11:1 will work, i had a lot of cam and still misery. it sounded like Orville Redenbacher was blowing the lid off my piston tops with ping & detonation... but if you don't mind filling up with LL100 at the local airport... mine loved that LL100, or you can buy it by the can for about $60/5gal, some stations pump 100 octane for about $7/gal
 
possible you can have some of the domes milled off and still use the same pistons 
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Regul8r Quote  Post ReplyReply Direct Link To This Post Posted: 31-January-2012 at 1:45PM
11:1 is EXTREME for s street car.
Going to cost money to run the HIGH Octane gas it wants in order NOT to detonate badly!
Many things you COULD do to lower the compression ratio.
After ALL the work that has been done to it... IF you want a little less compression I would make a deal with the machine shop... they take this short block in exchange for a milder built short block.
About the same money.
OR put the motor up for sale and have a milder one built.
 
No concerns/worries about the heads though.
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Post Options Post Options   Thanks (0) Thanks(0)   Quote madmaxtorino Quote  Post ReplyReply Direct Link To This Post Posted: 31-January-2012 at 1:59PM
my 429/466 was 10.5:1 had no problem on 93 octane. also my 2012 5.0 is 11:1
ENGINE:
466 ci
10.5 comp ratio
TRW forged pistons
Arp Rod Bolts
High Flow Aluminum Water Pump
Pro Comp Harmonic Balancer
Cobra Jet Baffled Oil Pan
DOVE heads ported polished
Cobra Jet titanium valves
procomp full roller rockers
CobraJet/Police Interceptor Exhaust Manifolds
comp cams custom grind camshaft
Cobra Jet valve covers
Weiand stealth intake manifold
fast EZ EFI Fuel Injection
serpentine belt conversion
2.5 inch exhaust with X pipe and Dynomax VT Mufflers and High Flow convertors
all hoses and belts new
Roush plug wires
MSD 6A ignition
MSD Pro Billet Distributor
Aluminum High flow 3 core radiator
Aluminum power steering cooler
140 amp 3G alternator conversion


Edited by madmaxtorino - 31-January-2012 at 2:03PM
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Post Options Post Options   Thanks (0) Thanks(0)   Quote steve Quote  Post ReplyReply Direct Link To This Post Posted: 31-January-2012 at 3:11PM
  Hi, I plann on high octane fuel with my build. Then detonation shouldn't be a problem. The oil should have a zinc additive as well. This is all new to me but I don't plann on any long rides !
72 gts fst bk in 1982 !!
currently building 73 fast back with a dove-a 429.
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Rockatansky Quote  Post ReplyReply Direct Link To This Post Posted: 31-January-2012 at 9:00PM
Quote my 2012 5.0 is 11:1
 
modern computer controlled cars running high CR have knock sensors to pull out timing several times a second to control detonation, no comparison to old school
 
72BoB, do you have the cam card info? if you know the intake closing point you can calculate the Dynamic CR and you'll have an idea if it'll run on pump premium or not, but you'll also need to be accurate on the static CR for the calculation
 
i'd call local speed shops and ask if they have the equipment to 'pump' a cylinder to find the CR 


Edited by Rockatansky - 31-January-2012 at 9:03PM
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Post Options Post Options   Thanks (0) Thanks(0)   Quote bigblocktisdale Quote  Post ReplyReply Direct Link To This Post Posted: 01-February-2012 at 4:26AM
i have found that by doing a compression test, if you stay around 180psi or below 93 octane works fine.
1972 Gran Torino Sport : 10.30 @ 131 mph NA
1991 mustang : 11.38 @ 123 mph sold 91 and bought a 67 mustang
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Post Options Post Options   Thanks (0) Thanks(0)   Quote stuck Quote  Post ReplyReply Direct Link To This Post Posted: 01-February-2012 at 10:43AM
I am finding E85  interesting ,but i,m not sure yet what the minimum CR needed is[12:1?]. you will need a carb and fuel pump for it.+ in the winter they change the formula a little so you might have to reset timing. sorry to hear what a pain in the a. Or maybe with a thicker gasket and cam timing+ ignition timing. i'm not sure about boost in a bottle. It would stink to machine and rebalance $$$ouch.
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Rockatansky Quote  Post ReplyReply Direct Link To This Post Posted: 01-February-2012 at 8:05PM
yeah you could run it on E85 and run all the timing you want!
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Post Options Post Options   Thanks (0) Thanks(0)   Quote 72BoB Quote  Post ReplyReply Direct Link To This Post Posted: 02-February-2012 at 2:13AM

Sorry for the late reply... This is a 460, bored to a 472. I am looking for the ideal solutions, or close to it. I have some money that I can play with, I just wanted to find a good compromise. Options I am not looking into is running aviation fuel, switching the E85, or just trying super thick head gaskets.

First off, the builder didn't have too much information on the specs of the cam, but he said it had .530 lift.

I haven't taken it to a shop to have them check the actual compression, but I'm looking into doing that right now.

I have a set of Kaase Super Cobra Jet heads that I can put on, but they're currently bare and I know that won't be the end-all fix.

Once again, I know I was pretty stupid getting an engine without knowing the full spec details of it, but it was a really good deal. Even if I have to pull the pistons out and change them I will still come out ahead than compared to if I built the engine myself.

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