My Road Trip Motor Progress
rick m:
Johl,
Does your spring buddy Don at R/D make the behive springs and retainers for the single groove valves like CB puts in their heads for a stock VW spring height?
Rick M
Zach Gomulka:
Quote from: rick m on August 10, 2012, 09:04:44 am
The HEMI chamber works well with more compression. If they did not work why did the porsche engineers prefer them when building the 356, 912 and 911 motors??? It was not just about putting the larger valves in either. With more compression, you do not need as much timing.
The Porsche (and Chrysler) engineers preferred to have a greater valve angle (for more airflow) than VW. A hemi chamber was a necessity, not a choice.
rick m:
My reasoning for the chamber has nothing to do withe the PORSCHE engineers wanting more room for the more efficient cross-flow design. The more surface area...the more heat. The more heat...what happens? Also, what does the heat in the chamber produce? :-)
RM
rick m:
One of the key advantages of a hemispherical combustion chamber is that its surface area to volume ratio is the least in the category a compared to other designs like flat head, wedge heads, etc. and therefore they have a superior thermal efficiency than other engines. Its advantages are that it has a relatively low surface area for heat absorbtion and it has little in the way of areas where unburnt mixture can lurk, as all the corners of a wedge design are eliminated and the raised roof of the hemi combustion chamber are well clear of the piston. The low surface area means that there is less area to absorb heat from combustion and this helps to keep the head temperature down. This in turn helps to control detonation. Accordingly, a reasonably higher compression ratio can be run to the benefit of performance. (But not the kind of compression ratio that most people associate with a racing motor, because this design has definite limitations at higher compression ratios.)
The elimination of dead spots of unburnt fuel mixture means that fuel economy is better because more of the charge is doing work.
Another benefit of a well done hemi design (not just a cut one-which means proper blending of all surfaces) is the effect of "squishing" the mixture so that it shoots out of the "squish area" and the resultant turbulence mixes the mixture up bettr. This helps to give even and controllable combustion. Two stroke designers adapted these findings and put a concentric squish band around a ( usually) hemispherical combustion chamber with the spark plug in the middle. Remember the SQUISHY discussions that have taken place on these forums. Guess what....SQUISH with hemi heads is part of the secret. The rest of the functionality is found in the compression ratio, timing, jetting and how well the chambers are finished with the hemi design. Look at the head on a 2 cycle motor cycle engine. The 2 cycle engineers have fully understood for years the benefits of a hemi for the squish necessary to get the best burn on a 2 cycle motor. BURN is BURN...and cylinder head design has a great deal to do with it. The HEMI also helps to optimize what you can get out of lower octane fuels. I had a 1914 in my chop top before the 2275 with heads similar to the ones I posted here that I was getting 33mpg with (that had the HEMI design) I was running 89 octane fuel.
Johl Mohr has mentioned a key ingredient in this whole discussion and that is that you can run more compression with the HEMI design as detonation is reduced with the combustion chamber design, as all the sharp corners are eliminated when after the machine cut the critical finish of the chamber of all the blending work contributes to removing detonation issues normally associated with all the corners and edges found with a wedge design. All machined edges must be blended and no sharp cuts or edges left.
Everyone can keep kicking this to death but if done properly, the HEMI design does work. You can't just hemi cut your heads and throw them back on and expect an improvement. It is not just about cutting a hemi chamber and lower compression. It has to do with a lot more things related to better atomization of the fuel, burn, eliminating detonation and being able to run lower octane fuels.
Each person is entitled to do what they want. However, for those of us that have already worked with and run the hemi design with all the corresponding work that goes with them...they do work. However, you don't have to run them. Everyone is free to do what they want but don't bad mouth something you do not understand or simply jump on someone else's bandwagon and try to out theorize people on the forum. Some people talk while others DO. For those who have done this...we an continue to enjoy the benefits associated with them.
Rick Mortensen
rick m:
Consider what you are viewing carefully. We are not talking about combustion chamber volume at this point. We are talking about the design. Look at the diagram I sketched this morning and think carefully about the questions posed. Take your thoughts away from the compression discussion long enough to think about the design only in terms of chamber surface area and the most prone to detonation. We can still get compression with a HEMI design. Just focus on other things happening in a chamber to get the benefit of the principles associated with the HEMI.
Rick M
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