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Old 03-01-2007, 12:54 PM
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chev open chamber heads with closed chamber pistons

How much detonation problem is there likely to be on pump gas on a 3200 lb 68 vette 454 .030 over with old speedpro Model 7053p pistons ( .250 high closed chamber dome) with older New World merlin oval port cast iron heads (118cc I think) It will have a modest hydralic roller cam and gapless piston rings. I have heard this should be 10.8 to 1 but opinions vary. Also might mention it has a holley,headers,zf 6speed and 4:11's For street driving only. Any body had any experience in this? Thanks.

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Old 03-01-2007, 04:42 PM
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you will be deburring all edges on the dome and smoothing the combustion chamber.you will not get away with running cheap gas you will need to run premium the highest octane you can get at the pump 100 would be ideal but i don't know if you have that in tenn.
good luck and 10.8-1 can be done with cast heads you just gotta know how to do it.
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Old 03-01-2007, 04:51 PM
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chev open chamber heads with closed chamber pistons

My engine has a similar setup,except mine has the 118 casting GM rectangular port heads,but the same pistons.I didnt have any problems until I overhauled the motor in '01 and my machinist milled the heads when he found they were slightly warped.I never CC'd the heads so I dont know the exact cr,but its probably between 10.5-11:1.Now I've really got to watch where the timing is or it will detonate,hot weather affects it some too.
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Old 03-01-2007, 05:00 PM
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I run the L2239F closed chamber piston (ZL1/L88) design and run 781X open chamber heads... The domes were lightly dressed prior to balancing and the chambers were smoothed out somewhat but not to a glass finish... I can run pump premium but it is rather picky about it... Timing & temperature are the big factors... I have to run the base timing at 8 degrees and adjust the vac advance to come in rather slow and total timing of 36 degrees to come in at 3k-3.2k but it works... Also have a large Griffin alum radiator and run a 180 degree thermostat... If I see temps over 200 degrees while in traffic then I do get a ping but under that no problems that can be heard or seen on the plugs... Its an art for sure (balancing act it correct term)...
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Old 03-01-2007, 05:01 PM
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I have a 468 with the same pistons. I don't know the actual compression
ratio, heads and block were milled. I ran it on the street in the mid 80s
with a set of oval port iron heads with premium pump gas it was on the
edge of detonation. I had to watch the tune up carefully.

It sat for 15 years during my youth coaching days. About 5 - 6 years
ago I puled it out and freshened it up with no changes and it rattled
like crazy, 100 octane is what it needed then.

I suppose the fuel quality changed during those 15 years. There are
many variables to consider, 10.8:1 is very high for an iron head big
block. Consider lowering the compression to a trouble free level.
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Old 03-03-2007, 08:12 AM
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chev open chamber heads with closed chamber pistons

Those 7053p pistons are SpeedPro/TRW L2439's and have a dome volume of 29.4 cc and a dome shape of .221,thats the specs for the 0.030 over size.You can find the Federal Mogul catalog on line and see all their piston specs.
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Old 03-03-2007, 08:28 PM
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Memphisvette

I really appreciate all the imput! Two Questions: How much will a thicker head gasket lower the cr ? and, How much should I mill the domes on the pistons to get down to 9.5 to 9.8? Thanks.
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Old 03-07-2007, 08:06 PM
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Thanks!

I appreciate all the detail. I have a mill in my shop. This will be a fun project. What would the cr be if the there was no dome? Not that I would cut it all off, just curious. Wonder how thin is too thin on the piston top?
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Old 03-08-2007, 04:08 AM
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chev open chamber heads with closed chamber pistons

You can only take off up to a certain amount of material,you might want to check with Federal Mogul,but if I remember correctly,you can only take 0.050 off of the dome.
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Old 03-08-2007, 08:24 AM
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I'm only calculating 10:1 for your engine

942cc cylinder volume
118 combustion chamber
10cc head gasket
5cc piston below deck
-29.4 cc dome

A manual trans, light weight car, and 4.11's will also help reduce detonation.

However, the cam will have the biggest effect in reducing cylinder pressure (late intake valve closing point).

A cam with at least 230 degrees of intake duration should be used. 240 degrees would be better. Besides, 240 degrees intake duration should be good for a 4.11 gear and 6 speed.

Merlin heads flow well on the exhaust port so use the same intake and exhaust duration. And use at least 110 LSA, 112 would smooth the idle and reduce cylinder pressure even more.

Static compression ratio (SCR) doesn't mean much. You need to calculate the dynamic compression ratio (DCR) and keep it under 8.25:1 for a bbc. 7.7 to 8.0 DCR would be ideal for pump gas. DCR uses SCR, rod:stroke ratio, and intake valve closing point to figure the actual cylinder pressure.

Another problem with closed chambers pistons in open chambered heads is the dome slows down the flame travel which makes the engine like a few more degrees of timing.
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Old 03-08-2007, 10:42 AM
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You haven't done any balancing on this setup yet, have you? You'll want to do mods to your pistons before you do that.

You can help your detonation problem by deburring your combustion chambers as well. A little more gear may help you in that department, too.

NO compression ratio calculator is going to be perfect, but having both a static and a dynamic number my assist you in making choices. The more data the better.

K
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Old 03-08-2007, 12:57 PM
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I'm using 10cc for the gasket volume. That is a typical felpro gasket which is very commonly used.

And I assume the dome size accounts for the reliefs

And I assume the piston under the deck around 0.015".


Yes, different engines will detonate at various DCR values. I have a BBC set up around 7.9:1 DCR and I can run 91 octane with full timing. So, I know a BBC can easily handle around a 8:1 DCR. That is with a 4600 pound truck, auto trans, and 3.08 gear and I pull a trailer around once in awhile. A lighter car with 4.11's and a manual trans will not load up the engine as much and therefore could run a little more DCR.

Calculating the DCR is a much better way of predicting detonation than just saying, "sure you can run 10.8:1 cr on pump gas". Trying to predict that without taking into account the intake valve closing event is silly.

A 9:1 BBC with detonate like a pig using 93 octane if a stock cam is used. But install a 250 degree cam and it will run easily or 87 octane. The DCR is an important tool if you know what that particular motor and generally handle. And I found a BBC can handle 8.0:1 dcr under a pretty heavy load.
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Old 03-08-2007, 02:25 PM
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"Ok how do you know when you've removed 6cc's.
Get a 100ML graduated cylinder. Fill it with say 50ML of water.
Clean off the milling table before starting to remove dome material. remove say a .010" cut off the dome. Collect the aluminum shavings that you cut off and put them in the water in the graduated cylinder. the metal shavings will displace the water. Say that ends up being 2cc of metal off the dome for a .010" cut. repeat cutting off.010" 2 more times.
Now collect and add the shavings till the water is at 56ML.
Measure how much shorter the dome is after milling the 6 cc's material off it and do the 7 others the same."

You have to be kidding me.


tom
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Old 03-08-2007, 02:46 PM
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Quote:
Originally Posted by F-BIRD'88

It does not account for volumetric effectiency at speed. (cylinder filling)


Here is an a example for a real world combo that would not run on 87 octane gas without pinging at WOT at max performacne timing (38deg BBC) even thou DCR theory says it will with room to spare.........


the motor was in good overall condidtion and did not run hot or use oil.
it ran for may many years and went very well


It would not run on regular gas at WOT. Driving around town at part throttle was ok.anything past 1/2 throttle or so would ping on 87.
Ran just fine on 92 octane. ran the best times on 92 octane. I've been racing and building cars for over 30 years


.

Legitiment question here.

You seem to be actually meaning WOT not high rpm?

So you are saying it is a volumetric efficiency problem,

and not a problem with lean WOT fuel mixture
or excessive timing?????

Is this one of the engines with locked out ignition timing? Fixed at 38*.
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Old 03-08-2007, 02:50 PM
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I'm not say DCR is perfect, but it is a good place to start.

I understand you know a lot about engines, however your engine seems a bit overly detonation prone. You must of missed something. I can understand it pinging on 87 but requiring 92 seems a bit high, did you try 89 or 91 octane? Were you running points ignition on that engine? I have seen point bounce cause detonation.

Here is a pretty good site on DCR.

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