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Old 11-10-2017, 01:17 PM   #1
broncobuckinaround
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ok i want to start this short thread by saying thx to those who have been giving me the great info.


ok so i know the compression ratio for this 91 460 is 8.5 to 1.
i took apart another 91 460 i had here. just to see whats in side. the head casting are E7TE they arent too bad of heads. i know there better ones out there. but these are good from what ive read.

from what i can see and read. the piston at tdc isnt flush with the block deck.
the velves open very close to 1/2 inch. after tdc the valves start to open. by the time the valves get past the chamber, very little does, the piston is plenty far away.

now heres my question. how far can i go before a stock head gasket can no longer be used. i was told by a machine shop at 50 thousands my compression would be around 9.5 to 1. can i go higher?

we have 91 octane here where i live. would this fuel be sufficient to use at 9.5 to 1. as a daily driver.

i would like it to be at least 10 to 1. but if it isnt safe and 91 octane wont cut it ill stick with what i can get.

thx to all who reply.
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Old 11-12-2017, 10:22 PM   #2
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you'll have to re-calculate the compression ratio. in addition to stroke and bore you'll need the combustion chamber volume, thickness of gasket, volume above piston including any reliefs, and to be real accurate, the volume above the top ring between the piston and wall. that's from the Ford Racing tech notes in the 2013 edition.
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Old 11-13-2017, 12:26 AM   #3
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Quote:
Originally Posted by broncobuckinaround View Post
ok i want to start this short thread by saying thx to those who have been giving me the great info.


ok so i know the compression ratio for this 91 460 is 8.5 to 1.
i took apart another 91 460 i had here. just to see whats in side. the head casting are E7TE they arent too bad of heads. i know there better ones out there. but these are good from what ive read.

from what i can see and read. the piston at tdc isnt flush with the block deck.
the velves open very close to 1/2 inch. after tdc the valves start to open. by the time the valves get past the chamber, very little does, the piston is plenty far away.

now heres my question. how far can i go before a stock head gasket can no longer be used. i was told by a machine shop at 50 thousands my compression would be around 9.5 to 1. can i go higher?

we have 91 octane here where i live. would this fuel be sufficient to use at 9.5 to 1. as a daily driver.

i would like it to be at least 10 to 1. but if it isnt safe and 91 octane wont cut it ill stick with what i can get.

thx to all who reply.

its a boring night, so I decided to run some numbers.

how much the compression ratio changes depends on what shape the combustion chamber is. if it is full round and matches the cylinder you will get a 9.3 CR.
if the chamber is less than full round, maybe heart shaped, you won't get the full effect of a .050 cut. assuming the area of the chamber is only 75% of the full cylinder area drops the CR to 9.09

volume of cylinder, pie R squared, times height.

bore is 4.36, radius is 2.18, squared is 4.7524 times pie is 14.93.

14.93 times height of .050 is about .7465 cubic inches taken off per cylinder if the combustion chamber is full round.

if its only 75% of the area compared to full round the change drops to .56 cubic inches. .7465 x 75% = .559875
for those that like fine details please note I used area, not diameter.

the total volume above the piston calculates to 7.66 cubic inches assuming an 8.5039 compression ratio with cylinder displacement of 57.48 cubic inches.

Last edited by pjtoledo; 11-13-2017 at 12:30 AM. Reason: really bored tonight
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Old 11-13-2017, 03:03 PM   #4
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wow. ill be taking some classes to learn what you just did. thats awesome.

what would be my limit as far as compression. like how far can it safely be decked. i would like to see close to 10 to 1.

im just not educated on this type of work yet.
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Old 11-13-2017, 09:39 PM   #5
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that was just plain 'ol math.
since compression ratio and displaced volume were known I ran the equation backwards to get the volume above the piston at TDC, then subtracted the volume for decking the head to get the new compression ratio.

are you planning on a full rebuild? to get to 10:1 it may be easier with new pistons. that way you can get valve reliefs and stay safe.

I do have a 429SCJ/460 in the basement. it's been many years since I touched it, but I swear that someday the 78 Bronco will get re-assembled with that engine in it.
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Old 11-14-2017, 01:39 AM   #6
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ok so no 10 to 1 if i dont plan on a rebuild. good to know. it still has plenty of life in it. maybe in a few years ill do it.

so just stick with what the machine shop said and go .050 with a stock head gasket. i saw stock gaskets are .042 would that be ok or should i go a little thicker.

i also plan to port match and do a little p&p. not much is needed with these heads as they are opened pretty good. i just want to knock off some factory cast edges.
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Old 11-14-2017, 12:04 PM   #7
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Quote:
Originally Posted by pjtoledo View Post
that was just plain 'ol math.
since compression ratio and displaced volume were known I ran the equation backwards to get the volume above the piston at TDC, then subtracted the volume for decking the head to get the new compression ratio.

are you planning on a full rebuild? to get to 10:1 it may be easier with new pistons. that way you can get valve reliefs and stay safe.

I do have a 429SCJ/460 in the basement. it's been many years since I touched it, but I swear that someday the 78 Bronco will get re-assembled with that engine in it.
Listen to him..... I wouldn't cut anything more from them heads than whatever is needed to make them true...... if u wanna bump the compression that u buy new pistons. U can go ahead and chop away at the heads, however now u are gonna have to start thinking about piston to valve clearances....
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Old 11-14-2017, 09:27 PM   #8
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"I'm not a professional engine builder, but I did type some BS on the internet once"

with the disclaimer out of the way I'll try to add some helpful advice.

with cast iron heads that don't dissipate heat as fast as aluminum, a huge bore of 4.3 inches, and lots of compression, be ready for some pre-ignition. with that big of a bore the flame front has a long way to travel and there will be plenty of time for hot spots to cause problems.
consult with engine builders, do some research, and plan according to what you want to use the engine for.
it's all good provided you are clenching a wrench with a greasy hand.
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Old 11-16-2017, 01:06 AM   #9
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Old 11-16-2017, 01:27 AM   #10
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i appreciate all the helpful advice. ill look at the link provided and see what it says
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