Jump to content

Open vs Closed chamber head


Recommended Posts

I have a good short block I want to build for a backup engine. I built it a couple years ago with OC heads, but one of the heads was bad. At the time I had the block zero decked for the KB hypertech pistons. I'm afraid if I run the CC heads the compression may be too high. I don't remember all the specs for the pistons but they are the standard .030 KB flat top pistons from summit with single valve relief. My goal is to have a weekend cruiser with about 325-350hp. Fairly mild. Any opinions on which way to go? I would like something with really good street manners. Also, do the OC heads really have issues with detonation/pinging if they are built fairly mild? I have never experienced it with the 2 sets I've run? But then again I'm no expert. I have no cam in the block right now, I just figured I would wait and choose a cam after I settle on which heads I'm going to build. My OC heads are the 2.19-1.71 1971 castings.

Link to comment
Share on other sites

73 Grande H Code. Headman long tube headers, T-5 Transmission, 3.70 Traclok, Lowered 1" all around, Aussie 2v heads w/ 2.19 intake, 1.71 exhaust, screw in studs, full roller cam 608/612 lift 280/281 duration LSA 112, Quick Fuel 750 CFM double pumper, AirGap intake.

 

- Jason

 

 

082-hot-rod-power-tour-2017-1970-1970s.jpg

 

Link to comment
Share on other sites

Open chamber heads are good heads that can produce plenty of power and are probably more efficient at burning all of the fuel than closed chamber heads. However, you are correct, because they are not quench heads (the quench is what makes closed chamber heads inefficient) they can be subject to detonation if the proper cam is not selected. You will be able to easily make your 350 horsepower goal with them and not have detonation. Select a cam that will get your dynamic compression ratio into the desirable range.

 

 

“If you can't explain it simply, you don't understand it well enough.”

--Albert Einstein

 

Link to comment
Share on other sites

As I have no real experience with open chamber heads, despite having two Mustangs in the past with them, I thought I'd re-hash my issue with spark knock on my  '71 351C 4v CC heads. I too had zero deck and KB 177 flat top pistons at .030" over as you describe. With the heads planed as well and using .040 gaskets and a mild Melling cam, I got lots of pinging. The best non ethanol gas I can get is 91 Shell. Octane boosters did nothing, waste of money. I eventually changed my pistons to KB 148 13cc dish tops, still zero deck, but still had some pinging under load. The answer was to learn how to build and set up a distributor and then get the Holley 670 set up and tuned right. I am convinced that a good reman. Motorcraft stock distributor with the correct mechanical advance built into it and the right amount of initial timing, pinging can be eliminated no matter what the heads. From my own experience, 30 degrees of crank advance is too much even for an automatic trans. Mine is a manual trans, so there may be differences. The reman. distributor I bought had slot width equal to 30*. I welded that up on the right hand side looking down on it and cut it to .410" which equals 20* on the crank. That's from a chart.

With 14* initial, vacuum disconnected, it's all in at about 2800rpm. I also cut the vacuum advance down to only 3-4* ( adjustable vacuum canister) Now it runs sweet and strong, pulls like a train and no pinging. BUT, I still think I can get a bit more out of it. I'm going to add a plastic sleeve to the peg, stop, post or whatever it's called to cut the mechanical down to about 18*, then bump up the initial to 16* and see what happens. Playing with springs will also be part of the experiment. It did not like 20* +16* so that's why I'm thinking 18* +16*. I also run a Pertonix Ignitor II and coil on a well adjusted and lubed points base plate.

To me it's not just heads, cam and compression ratio, it's about making it all work together and I'm learning every day and now finally having fun with it.

Geoff.

  • Like 1

Geoff.

 I learn something new every day!

Link to comment
Share on other sites

I made a cc tester for about $20 but I had lexan laying around for some reason. I actually still have some if you want a plate. I bought a plastic 50cc buret for like $5 shipped on amazon, a bottle of weak rubbing alcohol and some vasoline. You can use food coloring to make it easier to see but I could do it without it and didn't have any. You can really dial it in based off of the measured values. The calculator 73pony posted is all you would need, even helps with the cam.

Link to comment
Share on other sites

Accurately cc the heads, then use the Diamond pistons calculator to calculate static compression ratio. Then use the Wallace Racing Dynamic Compression Ratio (DCR) calculator to see how cam specs change DCR. DCR is not an end all indicator of resistance to ping but it is an indicator. A normally safe DCR is about 7.8-8.0 with pump 91. As previously posted, getting the distributor set up right is also critical as are carb calibration and spark plug heat range. Good luck. Chuck

Link to comment
Share on other sites

  • 2 weeks later...

I assume this is a 351c your talking about. If so, I would spring for a set of 2v Aussie Heads. Best of both worlds, smaller intake runners, closed chamber heads will give you a crap ton of power and be very streetable.

 

check the link

 

main site

http://ausfordparts.com/

 

Original Aussie heads.

http://ausfordparts.com/ausheads.html

 

 

 

aussie_2v_heads1.jpg

 

aussie_2v_heads2.jpg

"I drank what?" - Socrates

Link to comment
Share on other sites

  • 2 weeks later...

If you have 4Vs use them. Especially given what you guys pay for our heads. 2V build will hit your target easy but so will your heads for no extra cost

He has all the vices I admire and none of the virtues I despise

Link to comment
Share on other sites

Join the conversation

You can post now and register later. If you have an account, sign in now to post with your account.

Guest
Reply to this topic...

×   Pasted as rich text.   Paste as plain text instead

  Only 75 emoji are allowed.

×   Your link has been automatically embedded.   Display as a link instead

×   Your previous content has been restored.   Clear editor

×   You cannot paste images directly. Upload or insert images from URL.

 Share

  • Recently Browsing   0 members

    • No registered users viewing this page.
×
×
  • Create New...