Gator
Well-Known Member
I'm. Building a 340 with an overbore of.030.trying to figure out what piston to go with.What compression is good for pump gas
Piston compression
- Thread starter Gator
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metallicaman0258
Well-Known Member
anything from 8.0 to 10.0 will run on pump gas. 10.0 on premium to prevent knocking. if i remember correctly, you can get the pistons to 0 deck clearance with some machine work and get a set of flat top pistons and with a set of heart shaped 360 TBI heads (302 casting I think) and get like 10.5:1 and still run pump gas. double check my thoughts for exact numbers. I think Hot Rod Magazine did a 318 build using those components and got a pump gas 400HP engine with inexpensive parts.
Be very careful if you're trying to do a similar buildup to what a magazine has done. They tend to make things look much easier than they really are..
metallicaman0258
Well-Known Member
Agreed. but i have seen locals perform similar builds. Realistically though with a zero deck, flat tops, 4bbl, air gap intake and TBI closed cc heads without porting you can easily achieve 300-325
F body Deconstructor Jim
Well-Known Member
Keith Black 243's should give you about 10-1 cr.
http://www.campbellenterprises.com/...3-030-Keith-Black-Pistons-KB-Pistons_5922.prt
Or you could go with .070 over KB107's with a 360 crank :icon_razz:
http://www.campbellenterprises.com/...3-030-Keith-Black-Pistons-KB-Pistons_5922.prt
Or you could go with .070 over KB107's with a 360 crank :icon_razz:
Gator
Well-Known Member
Thanks for the info I'm going with Keith black pistons. I have 360 j heads with big valves .purple cam.
Aspen500
Well-Known Member
As you probably know, compression limit on pump gas has a lot to do with combustion chamber design and cam specs also.
What I mean is, some new cars are 12.5:1 but run on pump premium (for example). Of course that's variable cam timing, knock sensors, etc, etc, blah, blah, blah.
I'm running 10.5 on 91 octane (ethanol free) premium with no problems with 34 degrees total, -.005" deck height pistons and .039 compressed thickness headgaskets, closed chamber 413 heads and what's basically an Edelbrock Performer RPM cam (same specs).
What I mean is, some new cars are 12.5:1 but run on pump premium (for example). Of course that's variable cam timing, knock sensors, etc, etc, blah, blah, blah.
I'm running 10.5 on 91 octane (ethanol free) premium with no problems with 34 degrees total, -.005" deck height pistons and .039 compressed thickness headgaskets, closed chamber 413 heads and what's basically an Edelbrock Performer RPM cam (same specs).
Justwondering
Well-Known Member
okay - i've been lurking around and reading some of the postings you guys are doing on serious mechanicals... like this one.
I have no idea what you are talking about and I'm not sure where to go to get more info.
Could one of you recommend a book or site I could spend time with so I'm not as clueless?
I have no idea what you are talking about and I'm not sure where to go to get more info.
Could one of you recommend a book or site I could spend time with so I'm not as clueless?
metallicaman0258
Well-Known Member
Check amazon.com for books on building performance engines if you like print. As far as internet, Google search engine builds or specific information for what you want to know. You can find info for all engines from an antique Briggs and Stratton to a brand new hellcat. Personally I went to car school and that's where I got most of my knowlege.
Justwondering
Well-Known Member
I came to the car scene late. Dad was always willing to do car repair with my brothers but not so much with his daughter. Course he's from a different generation.
I've got plenty of education, just not in cars.
So, what you guys are discussing is boosting performance by increasing the diameter of the piston, the quality of the piston, and combustion in order to generate a higher compression?
I've got plenty of education, just not in cars.
So, what you guys are discussing is boosting performance by increasing the diameter of the piston, the quality of the piston, and combustion in order to generate a higher compression?
kkritsilas
Well-Known Member
They are talking about increasing the size of the cylinder bore (diameter) in the engine block, then getting pistons to fit the enlarged bores. The "quality" of the piston is a matter of choosing the correct piston for the proposed use and desired reliability (Eutectic pistons (a type of cast piston) are cheaper, usually weighs less, and are well suited/reliabie to less extreme engines. Forged pistons are more expensive, weigh somewhat more, and will be better suited/more reliable in more extreme engines, or those using forced induction (turbocharging, supercharging) or nitrous (nitrous oxide, sort of a chemical form of forced induction). The choice of piston really doesn't do anything in terms of performance unless it raises the compression ratio.
There are a few ways to increase compression ratio (static compression ratio). Special pistons could be used, the combustion chamber size can be reduced (using closed chamber heads vs. open chamber heads, and shaving either the cylinder heads or the engine block). Our cars generally have a rather low compression ratio at about 8:1; raising it to 9:1 or 10:1 will produce more power and most likely, better fuel economy. Again, specific to our engines, the cylinder heads don't flow very well in comparison to more modern head designs, which further reduces their power output.
In general, there are a number of ways to get the power up, cylinder head porting, decking the cylinder block (the pistons on our engines can be as far as .080" below the deck surface (deck surface is the surface that the heads mate to)), and using the high compression pistons. The better ways to do this, and to address a number of issues, is to get better cylinder heads with closed combustion chambers, which addressed the flow issues, and helps increase compression ratio at the same time (camshaft will need to be changed at the same time). This is what Chrysler did with the Magnum line of engines. For the most part, the Magnum and LA series engines are the same. Chrysler did completely redesign the cylinder heads, increasing compression ratio and vastly improving flow. The 318-4BBL V8 was rated at about 160 HP in our cars, the Magnum 318 was rated at 220-230 HP, although it had moved to fuel injection as well, so it may have gained some of the power increase from that change. The LA 360-4BBL was rated at 180-190 HP, the Magnum 360 245-250 HP. Much of that is improved cylinder head flow, the remainder is increased compression ratio due to the closed chamber heads.
All the ways of increasting compression ratio and flow have costs associated with them. Decking a block is a machining operation that costs money, as does boring (along with the cost of the new pistons). Putting Magnum or Magnum type heads is the cost of the cylinder heads, plus a new camshaft and associated parts (lifters, pushrods, etc.). Ideally, this would all be done together while the engine is being rebuilt, to maximize power, and to keep costs down (it costs more to try and do this in piecemeal fashion than to do it while the engine is apart for a rebuild anyway).
There are a few ways to increase compression ratio (static compression ratio). Special pistons could be used, the combustion chamber size can be reduced (using closed chamber heads vs. open chamber heads, and shaving either the cylinder heads or the engine block). Our cars generally have a rather low compression ratio at about 8:1; raising it to 9:1 or 10:1 will produce more power and most likely, better fuel economy. Again, specific to our engines, the cylinder heads don't flow very well in comparison to more modern head designs, which further reduces their power output.
In general, there are a number of ways to get the power up, cylinder head porting, decking the cylinder block (the pistons on our engines can be as far as .080" below the deck surface (deck surface is the surface that the heads mate to)), and using the high compression pistons. The better ways to do this, and to address a number of issues, is to get better cylinder heads with closed combustion chambers, which addressed the flow issues, and helps increase compression ratio at the same time (camshaft will need to be changed at the same time). This is what Chrysler did with the Magnum line of engines. For the most part, the Magnum and LA series engines are the same. Chrysler did completely redesign the cylinder heads, increasing compression ratio and vastly improving flow. The 318-4BBL V8 was rated at about 160 HP in our cars, the Magnum 318 was rated at 220-230 HP, although it had moved to fuel injection as well, so it may have gained some of the power increase from that change. The LA 360-4BBL was rated at 180-190 HP, the Magnum 360 245-250 HP. Much of that is improved cylinder head flow, the remainder is increased compression ratio due to the closed chamber heads.
All the ways of increasting compression ratio and flow have costs associated with them. Decking a block is a machining operation that costs money, as does boring (along with the cost of the new pistons). Putting Magnum or Magnum type heads is the cost of the cylinder heads, plus a new camshaft and associated parts (lifters, pushrods, etc.). Ideally, this would all be done together while the engine is being rebuilt, to maximize power, and to keep costs down (it costs more to try and do this in piecemeal fashion than to do it while the engine is apart for a rebuild anyway).
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Justwondering
Well-Known Member
. Thank you for taking time to write up that info. It looks like I got it exactly backwards earlier. A few key phrases lead me to this site article http://www.hotrod.com/how-to/engine/mopp-0607-mopar-cylinder-head/
so from my perspective I am way too novice to do anything but watch, read, research, and learn.
The 5th I have runs well for my purposes since my daily drive occurs only once or twice a week. Higher speed where I live presupposes there is no horse trailer or combine on the road in front of you.
Looks like my major learnings will occur when I get this car stabile and begin work on my 61 Pontiac tempest which has a trophy four that I can't turn by hand.
Again, thank you for taking time for me. I really do appreciate it.
so from my perspective I am way too novice to do anything but watch, read, research, and learn.
The 5th I have runs well for my purposes since my daily drive occurs only once or twice a week. Higher speed where I live presupposes there is no horse trailer or combine on the road in front of you.
Looks like my major learnings will occur when I get this car stabile and begin work on my 61 Pontiac tempest which has a trophy four that I can't turn by hand.
Again, thank you for taking time for me. I really do appreciate it.
kkritsilas
Well-Known Member
Read the article, its pretty good, covering a lot of things that are important. It may be a little bit dated though, as they do not discuss the Engine Quest heads at all. The Engine Quest heads are basically an improved Magnum cylinder head, eliminating the cracking that is commonly seen between the valves in factory Magnum cylinder heads, and also improving flow over the factory Magnum heads. These heads are a somewhat new development, and may have come out after the article was published.
80mirada
Well-Known Member
That article included Mopar Commando heads, they haven't been available for years. Anyone wanting to learn more about engine dynamics may want to find a copy of Smokey Yunick's engine blueprinting manual, and a copy of performance engine math.
M_Body_Coupe
Well-Known Member
Talk to the cam grinder...they will be able to give you the recommended static CR to aim for...this is all driven by the cam lobe openning and closing timing, the LSA, etc, etc...ultimately you are trying to build as much dynamic compression (think cylinder PSI) as possible w/o detonating.
Once you have this figure then go shopping for pistons, you literally have 3 options:
1) OEM aluminum pistons (not really suitable for performance use)
2) Hyperutectic (high silicon alloy, finished piston has better material properties giving you a tighter piston to cylinder bore spacing, but is also more brittle, which means if you detonate often things may get damaged)
3) forged (toughest combination, but they will require slightly more piston to bore clearance...not a big deal though)
In my 360 motor I run a 9.8:1 static CR with a hydraulic flat tappet cam, 238/244 @ 0.050", putting me in the 165 PSI cranking compression region. I use 94 octane pump gas, very fast advance curve on the distributor, 21 initial, 38 total mechanical all in my 2000 RPM.
Once you have this figure then go shopping for pistons, you literally have 3 options:
1) OEM aluminum pistons (not really suitable for performance use)
2) Hyperutectic (high silicon alloy, finished piston has better material properties giving you a tighter piston to cylinder bore spacing, but is also more brittle, which means if you detonate often things may get damaged)
3) forged (toughest combination, but they will require slightly more piston to bore clearance...not a big deal though)
In my 360 motor I run a 9.8:1 static CR with a hydraulic flat tappet cam, 238/244 @ 0.050", putting me in the 165 PSI cranking compression region. I use 94 octane pump gas, very fast advance curve on the distributor, 21 initial, 38 total mechanical all in my 2000 RPM.
