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Old 03-01-2007, 11:54 AM
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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, 03:42 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, 03:51 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, 04:00 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, 04:01 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 03-02-2007, 05:00 PM
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For trouble free, every day street use on 92 octane you want a true 9.6 to 9.8:1 compression on this one. That will allow full timing. BBC's seem to like 38deg. Don't know the details of those pistons ( lent out my Federal Mogul catalog and didn;t get it back) but if they have a solid dome you could knock them down to get the cr down. You can still make huge power at a modest 9.6:1 on a BBC without being "on the edge" all the time.
need to find out the dome volume and deck clearance . Can be measured in the block if nessessary. Is the motor already assembled?

Last edited by F-BIRD'88; 03-02-2007 at 05:18 PM.
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Old 03-03-2007, 07:12 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, 12:33 PM
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if I was building this. Based on the info ya got, i would remove
6cc's from the dome. and use the GM "LS-6" .022" head gasket.
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Old 03-03-2007, 07:28 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 03-04-2007, 01:54 PM
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Quote:
Originally Posted by 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.
Ummmm remove enough metal till ya get a 6cc pile of aluminum shavings for each piston. Use the LS-6 .022" gasket (proper quench clearance) your 454 needs all the help it can get. Use the right gasket. A .040" felpro gasket will not eliminate the need to reduce the piston dome volume.
The LS6 head gasket combined with the piston deck clearance will give you the right net quench clearance without decking the block. You have to got to a GM dealer to buy it.

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.

Barring that you'd have to calculate the area of the piston dome surface. You can do this using MM graduated graph
paper. (most school supply stores carry it) Graph paper is lined horizontaly and vertically making 1mm squares. Hold the graph paper on the piston dome and draw the outline of the dome edge with a pencil. No you have the dome surface shape transfered to the graph paper. Now add up all the whole MM squares , and all the half squares on the paper within the penciled in area. This is the piston dome surface area in square Milimeters. Now with a little grade 10 math you can calculate how much thickness equals 1Cubic centimeter. 1CC. multiply that thickness by 6. Remove that amount from each dome.
All this assumes the piston is .025" down in the bore at TDC and the head is 120cc . Just adjust the math if you combustion chambers and or piston deck clearance are something different. Measure twice, cut once.

4.28"bore 4.00" stroke 29.4cc dome .025" deck clearance
4.44x .022" head gasket (Ls6) 120cc head = 10.23:1

reduce the piston dome by 6cc to 23.4cc =9.72:1
if the heads are actually 118cc finished then make the piston dome 21.4cc by removing 8cc of them. The cr will again be 9.72:1.
Just right for you buildup using 92 octane gas.

Last edited by F-BIRD'88; 03-04-2007 at 02:09 PM.
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Old 03-07-2007, 07:06 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, 03:08 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, 07:24 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, 09:31 AM
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Quote:
Originally Posted by memphisvette
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?
You can calculate the cr using whatever piston dome or other varables yourself Click here
The typical bbc head gasket is 4.44" diameter, not bore size
The flat top BBC piston valve relief volume is -5cc
Most BBC forged piston s are either .022 or .025" in the bore at TDC you have to measure yours.
You need to know your actual finished chamber size before cutting the piston dome. Flip the piston over and look at the under side. if the roof above the pin is flat accross the dome is a "solid dome" You can mill it all off if you want.
If the roof area of the piston underside above the pin is convoluted with a step, that is a "hollow dome piston" there is a limit to how much you can mill off the dome. usually the dome is quite thick reguardless.

Dynamic compression ratio theory is just that, therory.
It does not take into account many important factors that determine wethere a motor will detonation with a particular dynamic cr or not. It does not facor in combustion chamber quality, actual specific fuel octane performance, or volumetric effeciency of that motor. it will not predict the right compression ratio for your BBC based on camshaft intake valve closeing point.
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Old 03-08-2007, 09:42 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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