350 or 400?
#22
Le Mans Master
Originally Posted by Durango_boy
At what compression does the octane level in the gas need to go up? I thought 10.0:1 was the cut off for regular 87. Am I wrong?
Static compression ratio means nothing because when the intake valve is still open as the piston rises, there is no compressing happening. A better metric is "dynamic compression ratio". This takes intake closing event into account and explain how you can have an 11:1 engine with a big cam running on pump gas. Google "dynamic compression ratio" and expand your knowledge.
#23
Le Mans Master
Originally Posted by 427V8
68cc might be a bit small for a 400 without big dishes in the pistons.
My 434 with 76cc heads needed 9cc dishes to keep the compression down to 10.5:1
Get a good aluminum head
My 434 with 76cc heads needed 9cc dishes to keep the compression down to 10.5:1
Get a good aluminum head
Eddie
#24
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Originally Posted by SteveG75
Yes, you are wrong. Actually, 10.0:1 would require premium fuel for a typical small block with a cam specing about 220@.050" lift.
Static compression ratio means nothing because when the intake valve is still open as the piston rises, there is no compressing happening. A better metric is "dynamic compression ratio". This takes intake closing event into account and explain how you can have an 11:1 engine with a big cam running on pump gas. Google "dynamic compression ratio" and expand your knowledge.
Static compression ratio means nothing because when the intake valve is still open as the piston rises, there is no compressing happening. A better metric is "dynamic compression ratio". This takes intake closing event into account and explain how you can have an 11:1 engine with a big cam running on pump gas. Google "dynamic compression ratio" and expand your knowledge.
Has anyone written a paper here at the forum about static and dynamic compression and how different ends of the motor affect each? I'm interested to learn more from a point of view of the guys with the motors.
#25
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Originally Posted by 427V8
68cc might be a bit small for a 400 without big dishes in the pistons.
My 434 with 76cc heads needed 9cc dishes to keep the compression down to 10.5:1
Get a good aluminum head
My 434 with 76cc heads needed 9cc dishes to keep the compression down to 10.5:1
Get a good aluminum head
#26
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Originally Posted by Demonic Possession
Would aluminum heads work on a metal block?
It's all metal mate. Aluminum is just softer and lighter. Iron, like the block and stock heads, is just heavier and harder. Yes, aluminum heads are put on iron blocks all the time to save weight. It's been said though that the iron heads perform better.
#27
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OK, took some pictures of the heads here they are:
http://s9.photobucket.com/albums/a86...u/202%20Heads/
GM 8991492
F196
http://s9.photobucket.com/albums/a86...u/202%20Heads/
GM 8991492
F196
#28
Originally Posted by Durango_boy
It's all metal mate. Aluminum is just softer and lighter. Iron, like the block and stock heads, is just heavier and harder. Yes, aluminum heads are put on iron blocks all the time to save weight. It's been said though that the iron heads perform better.
In addition to shaving the pounds, aluminum also has better heat transfer characteristics versus iron. And since most of the cooling is done in the upper cyclider/head area, this is where aluminum really shines-less heat=less chance of predetonation=higher CR possibilities.
As far as the choice of a 350 or a 400? well MY vote would be towards a built up 350 since IMO they have more options than a 400 does, and by that i mean less complicated when doing the math and figuring out what to buy so on and so forth. Also the 350 becuase personally my driving style is owning the street and holding my own at the track, whereas your style may be owning the track and holding ur own on the street. So i guess it really comes down to preference.
#29
Race Director
Originally Posted by Demonic Possession
OK, took some pictures of the heads here they are:
http://s9.photobucket.com/albums/a86...u/202%20Heads/
GM 8991492
F196
http://s9.photobucket.com/albums/a86...u/202%20Heads/
GM 8991492
F196
Last edited by 63mako; 10-11-2006 at 09:07 PM.
#30
Le Mans Master
Originally Posted by Durango_boy
Has anyone written a paper here at the forum about static and dynamic compression and how different ends of the motor affect each? I'm interested to learn more from a point of view of the guys with the motors.
http://www.kennedysdynotune.com/Dyna...ion%20Tech.htm
"Dynamic Compression Ratio Explained"
In our attempt to help our customers understand performance and what makes an engine produce power we are going to explain the concept of dynamic compression ratio (DCR). While seemingly esoteric, this is an essential concept in designing an engine for performance use.
The first thing to understand is that "compression ratio" (CR) as it is usually talked about is best termed "static compression ratio". This is a simple concept and represents the ratio of the swept volume of the cylinder (displacement) to the volume above the piston at top dead center (TDC). For example, if a hypothetical cylinder had a displacement of 450cc and a 50cc combustion chamber (plus volume over the piston crown to the head) the CR would be 500/50, or 10:1. If we were to mill the head so that the volume above the piston crown was decreased to 40cc, the CR would now be 490/40, or 12.25:1. Conversely, if we hogged the chamber out to 60cc, the CR would now be 510/60, or 8.5:1.
Everyone knows that high performance engines typically have higher compression ratios. Simply put, higher compression makes more hp. Higher CR also improves fuel efficiency and throttle response. So why not bump up the CR even further? Once CR exceeds a certain point, detonation will occur. Detonation kills power and it kills engine. The amount of compression a given engine can handle is determined by many factors. These include combustion chamber design, head material, use of combustion chamber coatings, etc. Once these mechanical aspects of the engine have been fixed, the main variable is fuel octane. Higher octane = more resistance to detonation and the ability to tolerate more compression.
The above brings up the question that is often on the mind of performance enthusiasts and engine builders: how high should my CR be? Even if you know all about your engine and have decided what fuel you are going to use, the question cannot be answered as phrased. Why? Because without reference to the camshaft specs, talking about (static) CR is next to meaningless!
How is this so? Well, think about the Otto cycle and how a four stroke engine works. The power stroke has been completed and the piston is heading up in the bore. The intake valve is closed and the exhaust valve is open. As the piston rises it is helping to push the spent combustion gasses out the exhaust port. The piston reaches TDC and starts back down. The exhaust valve closes and the intake valve opens. Fresh fuel and air are drawn into the cylinder. The piston reaches bottom dead enter (BDC) and starts back up. This is the critical point as far as understanding DCR. At BDC. the intake valve is still open. Consequently, even though the piston is rising up the bore, there is no compression actually occurring because of the open intake valve. Compression does not begin until the intake valve closes (IVC). Once IVC is reached, the air fuel mixture starts to compress. The ratio of the cylinder volume at IVC over the volume above the piston at TDC represents the dynamic compression ratio. The DCR is what the air fuel mixture actually "sees" and is what "counts", not the static CR. Because DCR is dependent upon IVC, cam specs have as much effect on DCR as does the mechanical specifications of the motor.
DCR is much lower than static CR. Most performance street and street/track motors have DCR in the range of 8-8.5:1. With typical cams, this translates into static CR in the 10.0-12.0:1 range. Higher than this, there may be detonation problems with pump gas. Engines with "small" cams will need a lower static CR to avoid detonation. Engines with "big" cams have a later IVC point and can tolerate a higher static CR. When race fuel is used, much higher DCR (and static CR) may be used because of the detonation resistance of the fuel. Of course, race motors also have much larger camshafts which is another reason they can get away with such high static CR, often in the 13-15:1 range.
Note: there is some confusion about use of the term "Dynamic Compression Ratio". Some people use it to refer to the characteristics of an engine combo running at high speed. In that case, the engines volumetric efficiency will have a major effect on cylinder pressure. In this case, a larger cam will increase cylinder pressure when within its' rev range. Thus, more power and more cylinder pressure will be created. We prefer to think of this concept as "cylinder pressure" to avoid confusion.
#33
Race Director
Originally Posted by Vesa
stay away from 350
400 or even better 540
400 or even better 540
Last edited by 63mako; 10-13-2006 at 04:01 PM.