piston height at TDC?
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From: Nebraska
piston height at TDC?
I got the pistons installed in my 400 today. I'm wondering how much space there should be between the top of the piston and the top of the block at TDC. A friend commented that it looked like I have too much, which will give me a low compression ratio. It measures about .090". These are just standard pistons that the machine shop ordered for me with the rebuild kit.
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From: Delaware
The stock deck clearance of a 400 is 0.0565". Replacement pistons usually have more deck clearance, since the piston manufacturer assumes that some head milling or decking was done during the rebuild. In the pistons that I am most familiar with, the deck clearance is an additional 0.010", but some may have more.
Ask the machine shop to check the compression height of the replacement pistons. It should be 1.65". If less than that, you will have more deck clearance than stock and lower compression ratio.
Ask the machine shop to check the compression height of the replacement pistons. It should be 1.65". If less than that, you will have more deck clearance than stock and lower compression ratio.
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From: Nebraska
My measurements though probably not as accurate as a machine shop, show it to be 1.88" for the old pistons, and 1.16" for the new ones. The machine shop did resurface the heads, but nothing to the block. Is my compression ratio going to be seriously low? Different pistons are probably about the only way to fix this too?
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From: Delaware
The old pistons should be 1.65"CH. If your new pistons are 1.16" you have a serious problem, that is 1/2" difference from what it shoud be and 0.72 less than you say the old pistons are. You are not measuring them corrrectly. The measurement should be from the center of the piston pin to the top of the piston.
You should be able to ask the people who sold them to you.
What is the manufacturer and part number? Maybe we can look them up online.
You should be able to ask the people who sold them to you.
What is the manufacturer and part number? Maybe we can look them up online.
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From: Nebraska
My measurements were pretty crude, so I wouldn't factor that in too much. I talked to the machine shop today and he said it's normal for these Fords to be down that far. They ordered the pistons, which are Sealed Power .040 over. He said I should have around 8:1 compression. I still don't know though, and I don't really want to go any farther until I know it's alright.
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From: Nebraska
No, not at all. I was hoping for much higher. How badly will my power output be hurt with it this low?
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From: Delaware
A 0.040" over 400 with 8.0:1 CR the piston will be down the bore about 0.088".
You will have no trouble with 87 Octane fuel, but it won't do very well in burn out contests.
What kind of vehicle is it?
What do you plan to use it for?
What kind of cam did you use?
What is the rear end ratio?
You will have no trouble with 87 Octane fuel, but it won't do very well in burn out contests.
What kind of vehicle is it?
What do you plan to use it for?
What kind of cam did you use?
What is the rear end ratio?
Last edited by danlee; Feb 8, 2005 at 06:39 PM.
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From: Nebraska
It's going in my 79 F-150 4x4 with 3.50 gears, a semi-daily driver. I'm hoping for plenty of low and mid-range torque. The cam is a Crane Powermax 272-2. I also have a Comp Cams double roller timing set with the three keyways, and plan on running it 4 degrees advanced.
I've driven a similar truck with a reman 400 plus this cam, timing set, and an Edelbrock 600 and was very impressed. I don't know what it has for compression, but being a reman I would bet it's not too much more than stock. BTW, what was the stock compression ratio?
I've driven a similar truck with a reman 400 plus this cam, timing set, and an Edelbrock 600 and was very impressed. I don't know what it has for compression, but being a reman I would bet it's not too much more than stock. BTW, what was the stock compression ratio?
Last edited by brown 4x4; Feb 8, 2005 at 07:23 PM.
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From: FLOWER MOUND,TX
8.4:1 if i'm reading my book right...i'm not a mechanic...was thinking if those are 400 pistons and you have a 400 you should be alright...but double check...the one thing i read in this book that was interesting .say's a 351m piston installed in a 400 would stick out the top of its bore at tdc...and a 400 piston installed in a 351m would stop far short of the top of its bore at tdc and would hit the crankshaft at the bottom of the stroke....
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From: Eastern Pa
I've got the same truck and gearing as you and the same cam I used badgers and was almost .080 in the hole and I wish I would have decked the block before I put it together. I'm running about 8.7:1 with my set up which is at the very bottom of the recommeded range for that cam. My guess is you have the wrong pistons. I would try to switch to the Badgers and get the block decked if the bores are different by more that a couple of thousandths. Mine runs real good but if I could have gotten it up to 9 or 9.5 it would have been much better. I get pinging on 87 octane up at 12 degrees so I run 89 and it seems to be ok. I also took a couple of cc's out of my heads to unshroud the exhuast valves a little and had a about .010 taken off them.
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I degreed the cam today and changed my mind, I'm going to set it straight up. My heads were also shaved off .010", plus I have new hardened exhaust valve seats, so that may gain some, though maybe not enough to matter. The machine shop told me I'm losing maybe 10 or 15 horsepower because of my low compression.
