Troubling possibly high compression.
Thread Starter
Team Owner
Joined: Aug 2006
Posts: 24,125
Likes: 15
From: Columbia Missouri
Troubling possibly high compression.
I'll be the first to admit I'm very new to figuring out compression, and while I've done some reading in the area I can say I still feel a bit lost. Anyway, after getting the bottom end of my 400, (405), built, I began worrying a little about it's compression and was hoping someone could help me get a rough estimate of where this engine stands on comp. It's a 400, bored +.020, with a 10/10 stock crank and stock rods with Speed Pro pistons, and an LT4 Hot Cam. The heads being used are basically stock '89 350 heads...I don't have the castings handy so I can't speak for that yet...I'll look in the morning. Do you need any more info to figure this out? Here are the pics of the finished rotating assembly with #1 at TDC. The piston flat is just a hair or two under the deck height.
Race Director
Joined: Sep 1999
Posts: 11,361
Likes: 383
From: Plano TX
You will need some data and we can tell you your exact CR:
1. Find the piston specs and get the valve relief volume. you got 4 of them so it is significant.
2. Measure how far down the piston is at TDC. It may look like a hair but really be 20 thousandts or so.
3. CC the heads. Doesn't take long and should be done. Don't trust published specs for heads.
4. What headgasket? Most manufacturers will publish the gasket volume.
1. Find the piston specs and get the valve relief volume. you got 4 of them so it is significant.
2. Measure how far down the piston is at TDC. It may look like a hair but really be 20 thousandts or so.
3. CC the heads. Doesn't take long and should be done. Don't trust published specs for heads.
4. What headgasket? Most manufacturers will publish the gasket volume.
Thread Starter
Team Owner
Joined: Aug 2006
Posts: 24,125
Likes: 15
From: Columbia Missouri
Originally Posted by zwede
You will need some data and we can tell you your exact CR:
1. Find the piston specs and get the valve relief volume. you got 4 of them so it is significant.
2. Measure how far down the piston is at TDC. It may look like a hair but really be 20 thousandts or so.
3. CC the heads. Doesn't take long and should be done. Don't trust published specs for heads.
4. What headgasket? Most manufacturers will publish the gasket volume.
1. Find the piston specs and get the valve relief volume. you got 4 of them so it is significant.
2. Measure how far down the piston is at TDC. It may look like a hair but really be 20 thousandts or so.
3. CC the heads. Doesn't take long and should be done. Don't trust published specs for heads.
4. What headgasket? Most manufacturers will publish the gasket volume.
Okay, I'll get to work on that tomorrow. Can someone walk me through CCing the heads as I've never done that either. Thanks so much.
Le Mans Master
Joined: Mar 2004
Posts: 7,493
Likes: 1,502
From: Little Rock AR
You will be in the range of 9.7 to 12.5 C/R depending on the details. 
I made some assumptions at the high side and at the low side.
76cc smog heads,.039" composition gasket, .025" deck ht, 5cc valve reliefs = 9.7 C/R on a .020 overbore 400.
64cc heads, .015" steel shim gasket, zero deck height block, 5cc valve reliefs = 12.5 C/R on a .020 overbore 400.
Could be trouble but now is the time to check.
-Mark.
I made some assumptions at the high side and at the low side.76cc smog heads,.039" composition gasket, .025" deck ht, 5cc valve reliefs = 9.7 C/R on a .020 overbore 400.
64cc heads, .015" steel shim gasket, zero deck height block, 5cc valve reliefs = 12.5 C/R on a .020 overbore 400.
Could be trouble but now is the time to check.
-Mark.
Melting Slicks
Joined: Feb 2006
Posts: 2,209
Likes: 0
From: Mo
Unless you have some really small combustion chambers,I doubt you will have 12.1,I am with out knowing all the details,I am going to say around 10.1 give or take a 1/2.
I doubt you like that hot cam in a 405,It wasn't that great in a 350.
Good luck.
I doubt you like that hot cam in a 405,It wasn't that great in a 350.
Good luck.
Race Director
Joined: Sep 1999
Posts: 11,361
Likes: 383
From: Plano TX
Originally Posted by Durango_boy
Okay, I'll get to work on that tomorrow. Can someone walk me through CCing the heads as I've never done that either. Thanks so much.
Place head upside down and perfectly level. Get a 10 cc burette from a hobby shop. Water with a drop of dishwashing liquid to remove surface tension. Add water until the chamber is full. Many like placing plexiglass on top so they know when the chamber is full and add water through a hole in the plexi.
Drifting
Joined: Oct 2004
Posts: 1,496
Likes: 1
From: Lafayette Louisiana
Originally Posted by zwede
You will need some data and we can tell you your exact CR:
1. Find the piston specs and get the valve relief volume. you got 4 of them so it is significant.
2. Measure how far down the piston is at TDC. It may look like a hair but really be 20 thousandts or so.
3. CC the heads. Doesn't take long and should be done. Don't trust published specs for heads.
4. What headgasket? Most manufacturers will publish the gasket volume.
1. Find the piston specs and get the valve relief volume. you got 4 of them so it is significant.
2. Measure how far down the piston is at TDC. It may look like a hair but really be 20 thousandts or so.
3. CC the heads. Doesn't take long and should be done. Don't trust published specs for heads.
4. What headgasket? Most manufacturers will publish the gasket volume.
The deck height, dish volume, head gasket thickness will add up to enough space that it will make a difference when you plug it all into a calculator.
Thread Starter
Team Owner
Joined: Aug 2006
Posts: 24,125
Likes: 15
From: Columbia Missouri
Originally Posted by l88rocket
I doubt you like that hot cam in a 405,It wasn't that great in a 350.
Good luck.
Good luck.
I'm all for changing to a better cam if I don't like this one, but I really didn't get many suggestions for a good 400 cam for this motor...
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Thread Starter
Team Owner
Joined: Aug 2006
Posts: 24,125
Likes: 15
From: Columbia Missouri
Originally Posted by zwede
The chamber has to be clean, no carbon buildup, and assembled with valves & spark plug.
Place head upside down and perfectly level. Get a 10 cc burette from a hobby shop. Water with a drop of dishwashing liquid to remove surface tension. Add water until the chamber is full. Many like placing plexiglass on top so they know when the chamber is full and add water through a hole in the plexi.
Place head upside down and perfectly level. Get a 10 cc burette from a hobby shop. Water with a drop of dishwashing liquid to remove surface tension. Add water until the chamber is full. Many like placing plexiglass on top so they know when the chamber is full and add water through a hole in the plexi.
Okay, I'll check intot hat, and report back with those numbers and the dish sizes too.
Melting Slicks
Joined: Jan 2006
Posts: 3,028
Likes: 2
From: North NJ
Originally Posted by zwede
The chamber has to be clean, no carbon buildup, and assembled with valves & spark plug.
Place head upside down and perfectly level. Get a 10 cc burette from a hobby shop. Water with a drop of dishwashing liquid to remove surface tension. Add water until the chamber is full. Many like placing plexiglass on top so they know when the chamber is full and add water through a hole in the plexi.
Place head upside down and perfectly level. Get a 10 cc burette from a hobby shop. Water with a drop of dishwashing liquid to remove surface tension. Add water until the chamber is full. Many like placing plexiglass on top so they know when the chamber is full and add water through a hole in the plexi.
awsome advice. I didn't know how to do this either.The plexi seems like the most accurate so you don't get a "bubble" affect which would add more volume then whats really there.
Durango, take some pics of this process and post. I'd like to see how it all works out.
I learned my one thing today....going to rest my head on my desk just for a second....worked on the car all night, only 2 hrs of sleep.
Thread Starter
Team Owner
Joined: Aug 2006
Posts: 24,125
Likes: 15
From: Columbia Missouri
Originally Posted by 68 NJConv 454
....worked on the car all night, only 2 hrs of sleep.
Drifting
Joined: Aug 2004
Posts: 1,883
Likes: 124
From: Lubbock TX
How much trouble did you have getting the factory dogbone lifter setup to work? I gave it a bit of a shot but didn't have the machine tools needed to get the tops of the lifter bores perfectly flat. It doesn't take but a couple of degrees out to tilt the dogbone into a bind.
Thread Starter
Team Owner
Joined: Aug 2006
Posts: 24,125
Likes: 15
From: Columbia Missouri
Originally Posted by L88Plus
How much trouble did you have getting the factory dogbone lifter setup to work? I gave it a bit of a shot but didn't have the machine tools needed to get the tops of the lifter bores perfectly flat. It doesn't take but a couple of degrees out to tilt the dogbone into a bind.
The cast on my 400 block made it a little harder, but all I used was an air compressor, a die grinder, and a could good bits. A little time, and they fit right down flat. Drilling and tapping the holes for the spider bar studs was pretty easy too once you mark the holes. I would help anyone who needs it, and while the startup will determine how great the conversion is I've heard everyone likes it over the more expensive link bar style roller lifters.