OT why ar hemis powerful?

[mm]

OT An interest in hemi engines stirred me, and I found this site

which gave as the advantage of the hemi that 2 valves could be placed opposite each other, instead of next to each other, and thus could be made bigger, for more power.

I had assumed that the advantage was with fewer corners, etc. in the combustion chamber, there was less chance of power-losing knocking.

OTOH, now it occurs to me that if the combustion chamber is domed, it might be bigger than one with a flat top, leading to a lower compression ratio and less power.

So why are hemi engines powerful?

BTW The last time I posted off-topic, a couple nervous Nellys thought it was spam. It's not.

[Ed Pawlowski]

The hemi design was great in it's time pre-OHC. Other designs are as good or even better. What makes them so popular to day is their reputation and advertising hype. The pentroof allows four valves per cylinder. That have been many other types of head designs that just never had the advertising catch that the hemi achieved.

[Reed]

The pistons are domed to match the head

see

[Nate Nagel]

You're assuming that a typical hemi uses a flat top piston - they don't. Most have a domed shape piston and you are correct, the chamber shape allows a higher compression ratio on the same gas.

nate

[ds549]

todays hemi gets the air in and out of the engine more efficiently. todays gasoline hemi engines in production vehicles dont get horsepower by high compression ratios like in the 1960's , we just dont have the pump gas for it. todays engines get more power thru volumetric efficiency .getting the air in and out in a better way,many with hemi combustion chamber, and of course computer controlls on the engine. most of todays engines use exhuast manifolds that are made like the headers racers use, to increase volumetric efficiency.

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[mm]

I should have thought of that. I guess I'll never make it as an engine designer. That's a shame too, because I'm 62 now and looking for a new career.

Thanks. And to you too Nate.

[mm]

I guess this couldn't be true, becaus high mpg cars don't have especially high compression ratios, but I've always thought:::

Related to your last quoted line above: Unless this means a leaner mixture, and I don't think it does, doesn't this lower mileage, to burn more air and the accompanying gas?

Besides power, I've always thought that a high compression ratio means getting the most out of the gas. That every portion of its expansion applies power to the wheels of the car.

Doesn't that seem reasonable, and how come it must not be true?

[ds549]

GAS ENGINE HEMIS IN THE 60'S-70'S COULD HAVE 12:1 COMPRESSION RATIOS. TODAYS GAS CAR/ TRUCKS WITH HEMISPHERICAL COMBUSTION CHAMBERS ONLY RUN ABOUT 8:1 COMPRESSION RATION.. IT TAKES RACING FUEL TO BURN IN A 12:1 ENGINE OR THE ENGINE WONT HARDLEY RUN..

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[dpb]

mm wrote: ...

Efficiency and power are competing design features -- the hemi initially was designed for a higher power output per unit volume (remember when NASCAR really was "stock" car and the 60s muscle cars?). The higher efficiencies to produce better mileage are optimizing other parameters.

Not that all was totally bad w/ some consideration -- my '69 Charger w/ the 383 (not hemi) and the 4-bbl split-bowl Holley would average 18+ mpg at interstate+ speed. Around town where it was start 'n stop it only might do 10-12, though, but it surely got off the entrance ramp into traffic in a heartbeat! :) Dang, wish I had kept that puppy. :( Saw an auction on eBay the other day for about 20X what I paid new off the showroom floor...

--

[salty]

Not true. Any time you raise the horsepower generated per cubic inch, you increase fuel efficiency. You get more power from a given volume of fuel. Take a 2 lire engine, and add a turbocharger to it. Both available power AND fuel efficiency benefit. You are getting more power out of the same size engine from a given amount of fuel.

[Steve Daniels]

On Mon, 26 Jan 2009 08:32:46 -0500, against all advice, something compelled [e-mail], to say:

My Audi runs at 11.25:1 on 92 pump gas.

[mm]

Without any knocking ever? I too thought that would be hard. Is there a trick?

I agree with that. Just look at the girl who does 30,000 a month (or whateer the number was) and the one behind her who did 20,000. Teen-age girls. Who wants to be one of those. (Other than Norminn maybe, but we were talking aobut "real men".

[Jim Yanik]

[e-mail] wrote in news: [e-mail]:

not necessarily.

no,because you have to increase the fuel delivered when using a turbo or supercharger. They effectively increase -displacement-,meaning they compresss a larger charge of fuel/air mixture.(equalling a larger motor) The more air you cram in,the more fuel you have to add to keep the proper combustion ratio.

[salty]

Really? So the engine will only run at full throttle?

A turbo will ALLOW you to cram more fuel in, but that is not a requirement. At less than full throttle, you will be getting more power per volume of fuel used. A lower throttle setting will get you as much power as a higher throttle setting on a normally aspirated version of the same engine.

[Don Klipstein]

A turbocharger achieves more power per unit engine volume by cramming more air (to be used with more fuel) in the same volume.

Superchargers do the same thing but are powered in a different manner.

Both achieve an increase in power and decrease fuel economy slightly.

However, if 2 cars have equal power and one has a turbocharger and the other has a bigger engine, then the car with the turbo could possibly get better city fuel economy by weighing less.

- Don Klipstein ( [e-mail])

[Don Klipstein]

Due to cramming more air and fuel into the engine. If you only cram in more air and not more fuel, all you accomplish is a leaner mixture.

- Don Klipstein ( [e-mail])

[Don Klipstein]

Sounds fairly reasonable to me. My 1995 Oldsmobile Delta 88 LSS has the "3800 series II" engine. My owner's manual says the compresiion ratio is

9.4:1 and specifies 87 octane gasoline.

- Don Klipstein ( [e-mail])

[Ernie Willson]

Hemi engines can incorporate larger valves than other combustion chamber shapes. This means that they breathe better and have a higher volumetric efficiency (a measure of how much air (and hence fuel) the engine can swallow).

Because of the hemi shape the distance from the spark plug tip to the furthermost point in the combustion chamber is shorter in a hemi, than in any other chamber shape of the same volume. What this means practically is the hemi engine can carry more power without knocking than other combustion chamber shape.

HTH,

EJ > OT An interest in hemi engines stirred me, and I found this site >

[Ernie Willson]

On the contrary, Turbocharging increases fuel efficiency, whereas supercharging always decreases it.

EJ > >>

[Jim Yanik]

[e-mail] wrote in news: [e-mail]:

Yes,it is. ever hear of stoichiometric ratio? that's the proper mixture of air and fuel,for best combustion. you must maintain the proper air:fuel ratio. If you cram in more air,you have to inject more fuel,or go lean and burn pistons.

sorry,it doesn't work that way.

Still by using MORE fuel.