What would be the nominal combustion chamber cc's for this head? I believe it's a 3931063.
Thanks.
Greg Linton
#45455
Thanks.
Greg Linton
#45455
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Re: 69 L36/L68 combustion chamber size
My 1995 Speed Pro catalog lists the 3961063 as 96.4 cc.
Also google "NHRA" They have tech specs for every OE engine made back to the fifties, but you have to be cautious because what they list are minimum or maximum specs to be considered "stock", so head cc and deck clearances are less than OE nominal, but piston volumes are probably nominal, so these specs are a good source of piston volume data to use for computing as built CR.
Duke -
Re: 69 L36/L68 combustion chamber size
Greg-----
If the head is actually a 3931063, the combustion chamber volume is 101 cc, nominal.In Appreciation of John HinckleyComment
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Typo - I meant 3931063 *NM*
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Re: 69 L36/L68 combustion chamber size
Interesting! What's the source of the 101 cc? I always assumed that the Speed Pro catalog cylinder head volume specs were nominal as built, not NHRA minimum. At least that seems to be the case for the chamber volumes it lists for SB heads.
DkeComment
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Re: 69 L36/L68 combustion chamber size
Thanks guys, and Joe, I rechecked my head and it is a 3931063. I'm still not much farther ahead though...
What I'm trying to do is figure out which replacement pistons to use without making a big mistake in the process and figured that getting the chamber size would be an important part of the equation. I haven't search the archives yet except for the post that I started a couple weeks back to refresh my memory and it actually created more questions/concerns for me!
I'll check the archives and look a little more online first but I'm pretty sure that I'll need someones help on this before too long.Comment
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Re: 69 L36/L68 combustion chamber size
What's your objective? Do you want to rebuild it as basically OE with only "adjustments" (if required) to operate on today's slightly lower octane fuel, or something else?
As a rule, OE replacement pistons (like Sealed Power) are fine, and the Keith Black hypereutectic line is very good, but you must pay strict attention to their wider than OE recommended top ring gap due to the high located top ring. They both have web sites that have have all the piston data you need.
Combining the above with a slightly thicker than OE gasket will drop actual CR (if required) on most OE engine configurations into an acceptable range for current unleaded premium fuel if the engine detonated in its prior life.
In all cases it is a VERY GOOD idea to measure head chamber volume, and you also need accurate deck clearance measurements. The more accurate your data for the individual engine, the closer you will be to achieving an acceptable target CR range without exceeding it. The higher the CR the higher the torque curve across the entire operating range and the better the fuel economy, so you probably don't want to leave anything on the table. You also don't want detonation, and with some effort you can achieve the optimum.
Accurate deck clearance measurements, head chamber volumes, piston volume, and head gasket compressed thickness and bore opening are what you need to compute the actual "as built" CR of your engine using online CR calculators that have been referenced here many times when the subject has come up.
Deck clearance should be measured BEFORE the block is disassembled, so you can compute the CR of the engine as it was prior to disassembly and combine this with operating experience to target a CR for the rebuilt engine and manage the process to achieve the target.
Block deck height is the biggest single variable. Many are machined higher than nominal blueprint dimension, and it's not uncommon to see five to ten thou difference side to side. As a result, actual "as-built" OE CRs are typically a quarter to half point lower than advertised.
If you have operating experience with the engine before teardown, you should have some idea of it's propensity to detonate. If it did and you have done all the homework and compute it's actual pre-teardown CR, a slightly lower target can be established (depending on the degree of detonation) and subequently managed. If it didn't detonate before teardown, there is no reason to assemble it with a lower compression ratio than pre-teardown.
A lot of guys (or their "engine builders') blindly install "low compression pistons" and thick head gaskets, without measuring ANYTHING, which is okay if your objective is to run regular unleaded instead of premium, but you will drop the torque curve across the entire operating range as much as ten percent and will end up with both lower performance and poorer fuel econonmy than OE.
DukeComment
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Re: 69 L36/L68 combustion chamber size
Duke-----
Several sources, including GM specs, all show it as 101 cc.In Appreciation of John HinckleyComment
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Re: 69 L36/L68 combustion chamber size
I might add that the 3931063 head is a closed chamber, small port design that use long reach gasketed spark plugs.
Here is a good link for numbers and specifications.
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Re: 69 L36/L68 combustion chamber size
What's your objective? Do you want to rebuild it as basically OE with only "adjustments" (if required) to operate on today's slightly lower octane fuel,?
Duke, that's exactly want I want. I'm using your advice and others from the archives and it will be pretty much bone stock when I'm done including the original grind from Crane Cams blueprint series.
I've never had any problem rebuilding engines from farm tractors to VW aircooled to turbine aircraft engines as long as I have the correct manual AND factory replacement parts. I'd have no problem with my 427 either if I could go to GM or a jobber and get stock replacement pistons in say .020 over size but I can't. This leaves me with having to choose a suitable replacement and to be honest I'm a little out of my league because a lot of the aftermarket references mention head cc, dome size, c/r, etc and I really don't care about all this because this is still all stock. The deck height is original, the heads have never been shaved and it's still standard bore. All I want is something to fill the holes but about .020 larger and maintain the orignal compression ratio.
I find it interesting that you asked if I had any "operating experience with the engine before teardown" Well yes I drove it about 15k miles in the past 17 years with no pre-ignition issues what so ever even on regular fuel, believe it or not, but... This was with a higher lift and duration cam so I don't know if that made a difference. I remember doing a compression check on it shortly after I bought it and the readings were quite high like around 150 or 160. After I rebuilt the engine and changed to the bigger cam though the reading dropped to about 125 psi across the board so maybe going back to the stock cam will cause me problems with pre-ignition...I don't know. I don't think so though because a friend of mine races a 91 Mustang and he says that there are a lot of guys running around Calgary here with as much as 11:1 compression ratios with no problems on 91 octane and the reason being that we are at 3500'.
I'll check a few of the manufactures sites that you and others have mentioned and hopefully I will be able to pick the right set, the first time!Comment
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