Hopefully this is technical enough for this section. I'm planning my 396 LT1 build right now and got to thinking about static compression ratios. Given a fresh build, there are a few ways to make the same compression ratio. In your experience, is there a "best" way to configure compression? For example:

Option 1:

4.030 Bore
3.875 Stroke
4.040 Gasket bore
.029 compressed gasket thickness
64 CC chamber
zero deck
flat top piston(assuming P2V clearance)

Gives 12.55:1 SCR

Option 2:

Change the combustion chamber to 58 CC and give the piston a 6 CC dish, and you still get 12.55:1

Does one setup theoretically or actually provide any benefits over the other? Affect quench or proper combustion? Would one be noticably better for a nitrous car vs N/A?

 

A flat top piston design will give you BOTH a larger quench area, and a smoother combustion flame front travel ...... than either a "dished" or "domed" piston.

As an aside, in your "Option 1" example cited above, your ZERO decked block and .029" compressed head gasket thickness ....... will give you piston to cylinder head clearance problems ...... regardless of what your piston to valve clearance issues might be.

 

I would leave the pistons down a little .005ish and mill the heads a little.

 

I haven't been keeping up with the latest trends in engine building for a while, but the trend after the first few years of the Engine Masters' Challenge was to use tight chambers in the head & a reverse dome piston that was matched to the chamber. Those engines were limited by pump gas, so most had very reasonable static compression, but valve timing was pushing the dynamic compression up quite a bit.

If you use the Search here then you will probably be able to back that up as it was a pretty hot topic.

 

Yeah some builders thoughts are put all the volume into the chamber, meaning flat top pistons and larger head cc's to give desired compression. Others have said smaller chambers and dish that mirrors combustion chamber works best. Most pistons however do not mirror the head chambers so unless you want to get custom made pistons for your heads, i'd get your compression by using flat top piston if possible.

I think leaving most of the volume in the head chamber will be better for pump gas...you'll quench the combustion charge better, as all the heat extraction comes from the head. Leave the burn in the cylinder and it will hold the heat in...it will make power but be more detonation prone..likely require less timing and colder spark plug perhaps.

 

The comments on the cooling being in the chambers is a good one. a funky custom expensive dish in a piston also adds surface area to it to soak up more combustion heat and for real use as opposed to engine master stuff I have to think piston oiling becomes a good idea for cooling. Benchracing what one off race or dyno queen engines have can spiral into thousands of extra dollars for a single digit or maybe no gain on a "normal" engine build.

You buy a gen 1 head bare unmachined so you can put a small chamber in it convert it to gen 2 then get highly custom pistons, then add piston oiling. You have added a lot of expense and hassle.

Trying new things is one thing, trying wacky custom race/dyno queen stuff on a mild street engine is silly.

 

AdioSS, you've confused me with that last little bit of your post here, (highlighted above). What did you mean by "valve timing" pushing UP the dynamic compression ratio? By definition, an engines dynamic compression ratio can ONLY be less than or, (at MOST - i.e. with IVC @ BDC), equal to it's static compression ratio.

You really didn't mean to imply that anything to do with "valve timing" could ever increase dynamic compression ratio above static compression ratio, right???

 

Remember that when the cylinder is filling at the torque peak when VE is really working for the engine where the cam was designed to make all its power, the intake charge is rushing into the cylinder as the piston is going down and the exhaust valve is still open for a while, and the headers, if they are designed correctly are scavenging the cylinder helping to pull the intake charge in. And when the exhaust valve closes the intake charge keeps filling the cylinder even when the piston is going back up, cramming more in there because it has momentum. It is also why these higher duration cams make the dynamic compression crappy down low because this isn't happening and some intake charge goes out the exhaust and maybe back out the intake as well since it just doesn't flow as well at slow speeds so your hotrod gets 6 mpg around town and shakes the fenders off at idle. it might be different with an EFI car since it is only air we are talking about for the most part. But one of the reasons you can run a ridiculous static compression on a high duration racing cam is because you need to do something to get the dynamic compression up or it might be a real slug if you mix low compression with a high duration cam, but at ramming speed, it all works like it is designed to.

 

I dont think he is saying dynamic will be higher than static, but you can have a high dynamic ratio FOR the given static ratio because of valve timing. Say 10.5 to 1 static with a big 230deg cam may sit around 7.8-8.0 dynamic. You could have the same 10.5 to 1 static motor with a small 212 deg cam and have dynamic up higher at 8.6-8.8 to 1. The valve timing could also be changed to allow that 230 deg cam to come in higher on dynamic comp, higher than the previous 7.8-8.0 values.

There are quite a few compression limited racing classes out there that can still make high hp and high rpms with low 9 to 1 compression. There are tricks cam guys can use to build cylinder pressures even with 9 to 1 comp which is low by most standards for aluminum heads and even a little low for iron heads with good chambers.

But dynamic compressions arent the end all say all in pump gas driveability theory. Its a starting point to consider but there are alot of other factors as to what compression you can get away with on a set fuel

An interesting point was made above... VE, volumetric efficiency. Some well built race motors are acheiving well over 100% volumetric eff, which means they are compressing more volume than actual displacement of the motor per cylinder. This would effectively raise compression, much like how forced induction does because in reality, the intake inertia ramming effect into the cylinder to give more than 100% ve IS forced induction

 

I know this post is now several months old, and we've never heard back from the OP (12sec97Z28), but, in the Q & A section of this months new ( i.e. the December 2012 issue of) Chevy High Performance magazine on page 14 in a question titled "Flat or Dished" ......... this very same question is addressed by the CHP staff.

I don't have a scanner and hence can't post CHP's actual response to this question, but, briefly summerized flat top pistons were preferred because of:

1) Better quench

2) Better cylinder evacuation

3) Better valve unshrouding and finally,

4) Better fuel suspension

Again, please refer to the cited article for complete info.

 

I never got notification that anyone had responded until tonight! Glad I did, though. This is some good info, and I'll have to read over it when I have more time to research and really understand it. Most of my question was just answered by necessity. Turns out, the heads I'm using are going to be 71cc combustion chambers and Lloyd said that was the most we should mill! That being the case, I'm going to have to run around a 13cc dome to get the 12.5:1 SCR I'm looking for and will rely on cam/valve train events to keep my dynamic in check. My builder also recommended leaving the piston in the hole .030 or so.

As a side note, this is my first big build and I learned a valuable lesson. When buying parts unfamiliar to you, make sure you know every detail possible about them or else you may get a set of pre eliminator AFR 210s that were promised to be eliminator 227s with a "ported" LT4 intake that was little more than very badly gasket matched. Luckily Lloyd is excellent at his job and fixed me up right for a great price! Feel free to bag on me a little about my mistake, but not too hard. LOL.