compression vs. quench question

[Peer81]

Hello everybody,

I'm just goofing around with number and calculating different engine setups. I read a couple of books to understand everything better and on the net are also a lot of thing to find on the subjects but still I have a question on Dynamic compression versus Quench. What would you sacrifice?

I know I can always change the camshaft, gasket thickness but to make it simple for me just this comparison.

350 engine with:

72cc iron heads with 0.045 quench thickness gives me a Static compression 8.92:1 and dynamic 7.55:1

same engine (same cam) but with:

64cc iron heads with 0.067 quench thickness give me a Static compression 9.67:1 and dynamic 8.17:1

So the question is what is more important the 0.6 compression increase or a better quench thickness? Thanks,

Greetings Peter

 

[Hib Halverson]

Take the extra compression.

 

[damoroso]

Agreed. Compression gives you more "Bang" for the buck so to speak.

 

[Peer81]

Thanks,

I also left out me pistons have 4 valve reliefs so the quench area is also minimal I think. But it's very interesting to see what happens when you change one thing and you try to optimize by changing another part (cam or gasket thickness). It isn't as easy as I thought it would be.

Greetings Peter

 

[Tom Bryant]

If you can achieve your desired static cr with your flat top pistons by using a smaller chamber, setting deck height, using the right thickness of gasket and even taking a cut off the heads if necessary, you will be more pump gas friendly than by using a domed piston. There is nothing in the way of the flame travel.

Also if you pay any attention to the pump gas drags these guys are running much higher compression than we would ever think possible on 93 octane. The basic trick seems to be a very small combustion chamber and a D shaped dish in the piston top.

Tom

 

[1963SS]

The "quench pad?" on the piston is pretty important also. You are trying to squeeze as much of the fuel/air mixture as possible towards the combustion chamber. What you are really trying to do is create a bunch of turbulence. I used Mahle pistons and have a .038 quench. I have 11.4 static compression and have never had any knock issues.

The Mahle's have a real nice "quench pad?" and a -16cc dish.

Oops, I just looked at the topic and noticed that your's is a C3. Sorry for butting in but the info may still help. I'll just go kill myself now.....or maybe just get drunk.

 

[Peer81]

Thanks everybody, always nice to see what somebody else thinks and has
The only thing for me is, pistons (H345P) are the one thing that stay in place, if I had to do it all over, yes I would take a better look at the quench area but like Tom says, I think it's still better to have valve reliefs then a dome.
I also see it very much depends on which DCR calculator I use, when I use P. Kelleys calculator DCR is 1:8.37 (with a Dart 64cc platinum head) which would be to high and the 72cc head with a 0.020 gasket would be around 1:8.05. At this moment I hold on to max DCR around 8.25:1 and our regular fuel here is around 90-91.
If I remember correctly the bigger the head is (with the same DCR) the better because there would be less valve shrouding.

Well it looks like Pat Kelley's DCR calculator isn't working properly on my computer, maybe somebody who has it working on his computer?

Groeten Peter

 

[tracksnblades1]

Peer81, Sorry for an untimely response. As I read your query I am worried that I my not be up to speed or on the same page but I'll give it a try.
History. The sbc engine is a wedge type combustion chamber. Sbc
were orginally designed to operate with a quench(piston/head clearance) of 0.035-0.038 (steel rods only). Most all 350s have a piston installed height of 0.020 below deck. Most all early 350 were equipped with steel shim head 0.018 compressed.
Math 0.020 + 0.018 = 0.038
Logic The quench induces turbulence into the a/f mixture enhancing flame problegation(quick and smooth)(no explosions a controlled burn). Proper flame problegation will not radiate excess heat ahead of the flame front and cause detonation.
1963ss photo and clearance is very accurate. I drove a 355 sb 275,000 with kb pistons(flat top, two valve trough,with revised top ring land location) 68cc dart cast iron heads with revised spark plug location cc'd back to 68cc after unshrouding valve and polishing and porting, 0.000 piston/deck, 0.038 felpro blue 4.000 bore(not the 4.130) rod/main/piston to cyl all minimum factory and kb spec. 235* 245*@0.050 camshaft, True blue and square with the world, A true static 11.7:1, 90-93 octane 36*btdc total. 87 octane only to get to the 90+. Rattled on hard accel on 87oct at 36*
sources, GM power and better the legend Smokey Yunick sbc (the original gm performance r/d designer of the gen 3 sb) (way back in the early 60's)

Answer: Both, its the best of both worlds

 

[Peer81]

Thanks for the info Tracksnblades, good info is never to late
I'll still be hunting for a good as possible quench but I'll be up to date on this subject after I remove the heads to see how deep the pistons are down the hole.

Groeten Peter

 

[tracksnblades1]

Sorry, 10:7 not 11:7.

 

[Peer81]

The static compression isn't really what I'm worried about but dynamic is. With a relative small cam static isn't that high but dynamic is. And of course they are only numbers! But it's still nice to see what other people think of them.

So here the entire story.
Running on 90-91 regular fuel, but can go up to 93-94 or even higher.
Sbc. 3.484 stroke, 4.00 bore
Hyper speedpro pistons with 4 valve reliefs (-5cc)
Pistons 0.025 below deck (have to check that)
Gasket thickness 0.018 or 0.039
Iron Eagle 64cc "platinum" 180cc's
Connecting rod lenght 5.7
CompCam XE262 with a 57 degrees intake valve closing angle

With a 0.018 head gasket the numbers will be:
0.043 quench thickness gives me a Static compression 10.16:1 and dynamic 8.57:1

With a 0.039 head gasket the numbers will be:
0.064 quench thickness gives me a Static compression 9.7:1 and dynamic 8.19:1

I read that 8.25 - 8.3:1 DCR is the max...
Of course this all depends on how deep the pistons are down the hole (I've read 0.025 and 0.045)
btw, I'm not going bigger with the cam, need good vacuum for my carb and I'm not going to see that much time about 5500 rpm.
Then the next question, will 1.6:1 rockers instead of 1.5 change something to this story?

Greetings Peter

 

[tracksnblades1]

quench and swirl

The static compression isn't really what I'm worried about but dynamic is. With a relative small cam static isn't that high but dynamic is. And of course they are only numbers! But it's still nice to see what other people think of them.

So here the entire story.
Running on 90-91 regular fuel, but can go up to 93-94 or even higher.
Sbc. 3.484 stroke, 4.00 bore
Hyper speedpro pistons with 4 valve reliefs (-5cc)
Pistons 0.025 below deck (have to check that)
Gasket thickness 0.018 or 0.039
Iron Eagle 64cc "platinum" 180cc's
Connecting rod lenght 5.7
CompCam XE262 with a 57 degrees intake valve closing angle

With a 0.018 head gasket the numbers will be:
0.043 quench thickness gives me a Static compression 10.16:1 and dynamic 8.57:1

With a 0.039 head gasket the numbers will be:
0.064 quench thickness gives me a Static compression 9.7:1 and dynamic 8.19:1

I read that 8.25 - 8.3:1 DCR is the max...
Of course this all depends on how deep the pistons are down the hole (I've read 0.025 and 0.045)
btw, I'm not going bigger with the cam, need good vacuum for my carb and I'm not going to see that much time about 5500 rpm.
Then the next question, will 1.6:1 rockers instead of 1.5 change something to this story?

Greetings Peter

You might enjoy Researching some of Smokey Yunicks work in the quench and swirl technology that Detriot is finally using. Best quench occurs at contact (be careful). It appears your set with your heads and pistons. Some believe single trough style and true flat quench areas are most desireable in sbcs. You could experiment with rod lengths to optimize your qoal. Gm has well desighned cams, also, at the 252 Comp Cam performance level. I would rely on in block standardized measurements only. If you have to retard your total ingition timing, from your Dart heads optimum, will you achieve the highest potential of your desired combination? Ignition lead time has been a gauge of some to mesure cylinder head and cumbustion chambers design efficiencies. If you deck your block too acheive your desired quench consider your valve train geometry. You may need shorter push rods all else unchanged. I like to consider current and past productions and experimentals of Detriot and the aviation industry. In some areas the wheel may have already been invented. Good luck on your successful project.

 

[Peer81]

I already read Smokey's Power secrets about a year ago but I'll look it up. He states no minimum for quench but the quality of the builder will determine the quench distance. Well then I think 0.035 - 0.040 minimum will be save for me.

The camshaft needs to be changed so I can take it one step higher to get everything right. And as I read it retarding the ignition will only admit the engine doesn't have the right parts. But for my question changing the cam was the easy way out I'll look up some different camshafts and of course everything will be clear by the time I can measure the piston to deck hight. (and that will not be soon).

Groeten Peter