cardo0

Wow, sorry to wake u up! Thought Denmark is what 10 hours earlier than out west coast here?

Well the problem is the factory published compression numbers can be significantly off. Myself i wouldnt assume 11:1.

And the intake closes After Bottom Dead Center (ABDC) on the induction stroke. This closing point is where the intake charge of fuel and air become trapped inside the cyl and is used to determine the "dynamic compression ratio" with respect to the amount of trapped charge. This all is to resolve for the octane fuel u use.

There are plenty of dynamic compression ratio calculator on the internet and i suggest to do a search on this forum itself and try a few different calculators (spreed sheets) for 1 that u feel comfortable with.

Good luck c3 and dont become discouraged as once u become familiar with all this number crunching it will narrow your cam choice considerably and give u ideas whether to change pistons and or heads or not.

cardo0

OBTW i didnt see your block is 0.030" over so may not be stock and not stock pistons. U have to resolve this yourself as u have that block in your possession.

boat196

L-46 cams intake valve closing is 75 degrees ABDC

scottyp99

Here's a great article that explains Dynamic Compression pretty well. It also has a pretty easy to use compression ratio calculator you can download down at the bottom of the page.

http://cochise.uia.net/pkelley2/DynamicCR.html

Scott

c3_dk

Hi,

Yes it is 10 hour, it is a long way :- )
Thanks for the info, I will look into this "challenge" for me

Sorry if I didn't explane my self correct, the 0.030" is the engine in my car. The extra engine is 100% factory, even the carbutator ec ec.

Have a great day in USA

/John

c3_dk

THX Scott

c3_dk

Pictures of pistons/heads (186) from the extra engine.

cardo0

Wow those are difficult pistons to identify. Im reading suggestions that the 3942545 piston is an casting/forging number for the 3942541 piston and even the over bore sizes 3942542 (+0.001") and the 3942543 (+0.030") versions of the 3942541 piston for the '69-'70 corvette making 11:1 compression. From here:http://forums.corvetteforum.com/c3-g...on-or-pic.html.

And the Speed Pro cross reference equivalent sealed power (aka TRW) catalog shows 3942541 interchanges with SP p/n L-2304F = + 2.4cc dome. Well that dome looks like more than +2.4cc in those pix but pix can lie.

Can u measure the piston diameter with calipers in inches? And then measure the cylinder bore in inches too? This may help verify correct application.

cardo0

BTW u will need to put the vlvs back in the heads to measure chamber vol.

63mako

I came up with actual compression ratio of 10.84 to 1 in an L46 using factory components. It needs an intake closing point of 74 to run on premium 93 octane fuel or your equivalent. Stock cam has an intake closing point of 75. Should work fine if properly tuned.

v2racing

Mako is 100% correct here. Years back I did a lot of R&D on Quench, also called squish in some circles. I found that the tighter you run the quench, the less problems there was with detonation. The .060" to .070" quench zone actually suffered way more from detonation than .035" to .040" quench even though the compression was down.

I would get flat top pistons, set the quench between .035" and .040". That will have you close to 10 to 1, then select the appropriate cam for your combo/needs. It is still going to need a somewhat late closing intake with the stock iron heads and there old style chambers. IMHO!

c3_dk

Piston = 100,6mm = 3.96"
Bore = 101,6mm =4"

I will measure the chambers, I've just grinded the valves, next is a wash of the cylinders heads and block. After that will assemble and measure the chambers.

Here is my valve grinder, not my valve, valve from a 71 SBC Corvette.

c3_dk

Thx 63mako, a new cam and lifters will be bought.

c3_dk

[QUOTE=v2racing;1586559423]Mako is 100% correct here. Years back I did a lot of R&D on Quench, also called squish in some circles. I found that the tighter you run the quench, the less problems there was with detonation. The .060" to .070" quench zone actually suffered way more from detonation than .035" to .040" quench even though the compression was down.

I would get flat top pistons, set the quench between .035" and .040". That will have you close to 10 to 1, then select the appropriate cam for your combo/needs. It is still going to need a somewhat late closing intake with the stock iron heads and there old style chambers. IMHO![/QUOTE]

Thanks for the input, the extra engine will be build to factory spec. (as good as I can:- )
But I will make my L46 engine in my car to 10:1, so I will go for flat top in that engine.

thx,
John

scottyp99

As you can see by now, the camshaft choice cannot be made without taking quite a few other variables into consideration. Everything has to be arranged in such a way that it all works together. Make one change, and everything else has to be re-evaluated, too. It's kinda zen, if you think about it.

Flat-top pistons and a .015" head gasket? Good compression, good quench. Using a .030 overbore, .015" head gasket with a 4.1" hole (1094 felpro), .025" piston-to-deck clearance, 64cc combustion chamber, 5cc for the piston face valve clearance eyebrows, my calculator spits out 10.4:1 SCR, and 8.6:1 DCR using a Comp Cams 268HE cam, which is a little on the high side, I guess, but the L46 cam has more duration and less overlap, so it would probably run just fine. It's hard to find comparable numbers for advertised duration on the L46/L82 camshaft, as GM measured it right at lash, while aftermarket cam companies very commonly measure it at .004" or .006" of lift. Also, bear in mind that most of the data I used is just a guesstimate. The actual measurement of deck clearance, combustion chamber volume, piston volume, etc., may be different.

Scott

cardo0

Here is my valve grinder, not my valve, valve from a 71 SBC Corvette.

Wow i can see u like do as much by yourself as u can. Congrats. Make sure u hand lap those vlvs and check the seal (pour some gas inside the runners) before your finished.

Thanks for the input, the extra engine will be build to factory spec. (as good as I can:- )
But I will make my L46 engine in my car to 10:1, so I will go for flat top in that engine.

If your current rebuild is to factory specs then u may not be currently interested in changing pistons or cam let alone heads.

Well regardless, if u really want to nail your compression u can measure the above piston vol by direct measurement even with a dome. Since u will be reusing the original domed pistons or an over sized domed piston it becomes difficult but not impossible to measure. And with the engine out of the car being assembled on the engine stand it would take more patience than effort to measure the above piston volume. U can do this before selecting a camshaft for your desired compression ratio and fuel octane.

So heres my idea: once pistons and crank are assembled in the block u need a degree wheel to find your chosen cyl top dead center position. Once u have TDC then bottom dead center will be 180 degrees crank rotation away from TDC. Move the piston to BDC and apply a little grease to the piston crevice - just enough to seal the crevice. Now with the cyl pointing straight up - use a level - u can add measured liquid (oil) to top off the cyl. Thats it, by subtracting the swept vol (we talked about before and it dosent change) now u know both the deck to piston vol, the dome vol, and if u were careful the piston crevice vol all as one vol.
All that remains is your measured head chamber vol and the gasket compressed vol.

cardo0

63mako

Right, GM still makes the factory cam. It is really your best bet for your high compression engine due to the really slow ramps. Late intake closing point, real wide LSA and a fairly mild 222 degrees @ .050 for a decent operating range. There is not a modern equivalent available unless it is a blueprint copy of the factory cam.

v2racing

You could do an updated version of the factory cam with a custom grind using Comp lobe #5212 on the intake and #5213 for an exhaust lobe. If you put it on a 114 LSA it would be very close to the factory cam but with a little more exhaust duration which those stock exhaust ports could use. Final specs would be: 268 degrees @ .006", 222 @ .050" with .464" lift, installed at 110 LS on the intake and 276 @ .006", 226 @ .050" with .464" lift exhaust.

You could use the 5201 lobe on the intake also. 270 @ .006, 224 @ .050" and .470" lift. Would help a little more with the DCR.

As Mako said, GM used .000" as the opening point, in other words, they would record the duration when the needle would just start to move off the base, so the advertised duration looks much bigger than the comp lobe.

Custom grinds aren't that much more.

7t9l82

years ago i took apart a 1970 LT-1 and calculated the actual compression at 10.6-1. chevrolet was optomistic on compression on most models. a set of aluminum heads would probably be fine at that level. i agree with mako on temperature and timing etc.

c3_dk

Yes I can see that, and the good part is for me, that it is funny to find out about all this. But it is also a bit "hard" :- )

But for sure there is a huge gain to find out the right parts for the project. And to find out, what is it I really want from my engine/car?

But that is why this forum for me is perfect.

c3_dk

Yes I like to do "things" by my self, if I can. And if I can't, then find out how to do.
I prefer to buy the tools, instead of paying someone to do the job for me.

That is a good way to find out, thanks cardo. I think I will do it this way. Then I will be sure of the cc of the piston. But it is funny that I can't find it on the internet. Everything else is on the internet, besides the cc on my L46 pistons

c3_dk

This looks very good, for sure this will give me something to think about
Thanks v2racing.