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Author Topic: Hemi heads  (Read 3062 times)

Online Geoff Goodworth

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Hemi heads
« on: June 02, 2018, 01:59:05 AM »
There have been numerous examples of engines modified with hemi heads.

Apart from the obvious case where the compression ration is reduced slightly, what is the action that makes this modification successful?
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Offline Chris Wilson

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Re: Hemi heads
« Reply #1 on: June 18, 2018, 05:27:37 PM »
A shaped head 'shapes' the charge  or the way the gas flow loops around the chamber.

The closer the entire combustion chamber is to a sphere with the ignition point as its centre the more the direction of the charge will have an effect on the engine as it reduces stagnation or flow reduction.
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MAAA AUS 73427

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 Nothing in life is so exhilarating as to be shot at without result.  It's not enough that we do our best; sometimes we have to do what's required

Offline M Spencer

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Re: Hemi heads
« Reply #2 on: June 20, 2018, 08:08:00 PM »
You can fit Larger Valves in the head .  >:D
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Offline katana

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Re: Hemi heads
« Reply #3 on: June 24, 2018, 05:08:37 AM »
Quote from: Matt Spencer on June 20, 2018, 08:08:00 PM
You can fit Larger Valves in the head .  >:D

Maybe but at a cost of lower compression ratio and wider included valve angle giving worse flow figures. Ok in a blown motor but superceded by shallow angle 4V patterns!
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Offline M Spencer

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Re: Hemi heads
« Reply #4 on: June 24, 2018, 11:11:50 PM »
The So called ' Black Art ' 9 in the 1960s ) of COMBUSTION .

Raymond Mays , Jaguar H.E. ( V-12 ) Lean Burn - And SPUTHE H.D. Had Similar Combustion Chamber Tecnology , around 1980 development ( obviously late 70s , produced early eighties . )



Stepped Closed valve face high / relationship to plug . / Bathtub ( shaped ) Chamber , Squish Bandminterface .

Intake Charge ACROSS Exhaust Valve lowered the Ex Valve Temperature . So it didnt melt .

The Sudden , at T D C ,  increase in charge density , incresead the Strato Chemic ? ( I think it was ) Density ,
as in it was suddenly Fuel Richer , the Fuel not compressing as did the atmosphere .
Thus the Charge Density was o.k. not to pre ignite , which wouldve otherwise occured at that fuel / air ratio
( Lean , for the 21 mpg 5.3 litre Jag. ) 14:1 .

The Sputhe was more about controling tempretures , and was as capeable as the latest ' High tech ' GS 1000 S of Wes Cooley .
Which had all ' the latest blather ' , 4 valve , narrow angle , Multi Cylinders . BOTH were AIR COOLED .
Though I think the Suzuki pretended ' Oil cooling ' ( Said it was the FIRST , typical Jap Tripe ) As Was the COMMANDO ,
Oil Jets to Underside of Piston .
A bit like our S P I , Bypass channel ( hole ) to Big End / underside of piston crown . Prduceing Lower Piston Crown Operating Temperature .
« Last Edit: June 24, 2018, 11:46:11 PM by Matt Spencer »
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Offline M Spencer

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Re: Hemi heads
« Reply #5 on: June 24, 2018, 11:34:04 PM »
Heres something on Hemi Heads ,


shown here on those tarted up Volkswagons , . . . er , Porsches .  S?P



THE ' Hemi ' Head . ( Chrysler ) Chamber .
 

Quote
“Compared to a big-block wedge, the thing that surprises me most about a Hemi is the torque curve. If you build a Hemi and a wedge that both have a similar cam and compression ratio, the torque curve jumps up extremely early in the rpm range with a Hemi,” Corey reports. “It might not jump up as high, but Hemis produce a much flatter torque curve that extends much higher in the rpm range without dropping off.

Should be equitable , even tho were ' 2 stroke ' , not unlike a ' 3 cycle ' Rotary .  S?P ACTUALLY its the Combustion Chamber ' Cycles ' , ' Relevant .

http://www.hotrod.com/articles/an-801hp-gen-ii-hemi-for-the-street/

TWO stuffed with Aftemarket Commando Heads ( Herb Becker on Left )



STOCK 750 Commando Chambers , Eccentric Squish Bands ,


ACtually the result of ' engine development work ' .  LL~

They shifted the Crank Back & the Rods Out , to fit the bigger bits in the old 88 / 99 Dominator . ( Early Hogslayer was Two 810 Dunstall 99 Dominator )

Riley & Triumph ( Holder of world speed record , 56 to 74 ) and previously OPeugeot ( which Aston Puloined ) were Hemi ' Tecnology ' .
Initially to get lighter parts & Valves that didnt melt as quickley . ASTONs S V was of comparable performance in the 7.000 rpm Era .

Which may be a clue to the ' Appropriate ness ' of the Hemi Configurations usefullness Re F2B .

Another relevancy was the avoidance of irregularities , ( that created ' Hot Spots ' ) which pre ignited the High Octane  :P 70 rated fuel . Pre War .
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Offline M Spencer

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Re: Hemi heads
« Reply #6 on: June 24, 2018, 11:44:30 PM »
The E R A , Riley Derived , as was said to be Edward Turner TRIUMPH  ( Pre War / 1930s remember . The Peugeot was 1908 / 1912 )

Quote
The aluminium head is an entirely new production, The valves are set at 90 degrees to one another in a hemispherical head, and are operated by Riley-type rockers and Short push-rods from the cam-shaft on each side of the engine.



https://www.motorsportmagazine.com/archive/article/june-1934/43/first-era-racing-car-completed

The Biography ( Raymond Mays ? ) was fascinateing , shoestring budget Grand Prix Cars . Think they near all still survive . seen BIRAs at Whenuapai .
Were Two there . in the early 1980s . Built largely with disposal store fasteners etc , and remachined salvged W D suplous materials were possable .
Highest Specificaton Materials . !
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Offline Chris Wilson

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Re: Hemi heads
« Reply #7 on: June 26, 2018, 08:48:51 PM »
G'day Matt,  if you consider that the radius of the bore simply must match the radius of the head,  then the compression ratio will be massively low - hence the domed pistons that raise it back to a usable level.

Now, a normal modern two stroke model engine has  never had, to my knowledge, anything like what you are describing above  so is all this waaaaaay off topic?

In other words I have never seen a true hemispheric head on any model two stroke - have you?
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MAAA AUS 73427

You have enemies? Good. That means you've stood up for something, sometime in your life.
 Nothing in life is so exhilarating as to be shot at without result.  It's not enough that we do our best; sometimes we have to do what's required

Offline M Spencer

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Re: Hemi heads
« Reply #8 on: June 26, 2018, 09:35:41 PM »
In other words I have never seen a true hemispheric head on any model two stroke - have you?

BSA Bantam . !  S?P ( SORRY , the Jap ones are the TOY ones .  LL~ )



SAAB .



Villiers .



 ;D n1
 D>K D>K H^^
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Offline M Spencer

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Re: Hemi heads
« Reply #9 on: June 26, 2018, 09:45:46 PM »
NOW We're Getting Awkward !



Im Thinking you mean a ' Full Hemi '  , as a squish band Hemi , As Per G 15 , is still a ' True ' Hemi .

OS 35s & a myrid of others have a Hemishperical chamber , albeit bisected by a blaffle reciptical slot .

Though ONE might Argue that they're a Shallow Hemishere , rather than a full half geometrical hemishpere , if being annal .
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