Author Topic: Pent v Hemi.  (Read 10586 times)

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Offline stay`tee

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Pent v Hemi.
« on: November 22, 2014, 07:02:55 AM »
in percentage terms, how much more efficient is the Pent roof engine than Hemi's, ??,,
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Offline Rex Schimmer

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Re: Pent v Hemi.
« Reply #1 on: November 22, 2014, 03:07:12 PM »
Read Kevin Cameron's book "Classic Motorcycle Racing Engines" and you can see why all present day high performance engines are a pent roof design.

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Offline fordboy628

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Re: Pent v Hemi.
« Reply #2 on: November 22, 2014, 04:34:47 PM »
Read Kevin Cameron's book "Classic Motorcycle Racing Engines" and you can see why all present day high performance engines are a pent roof design.

Rex

x2   Read & reread the chapter on Cosworth DFV's . . . . .

in percentage terms, how much more efficient is the Pent roof engine than Hemi's, ??,,

Da**!!!!   Where do I start???

A)   More valve area per cyl bore area
2)   Less valve to valve clearance problems at high durations
d)   Less valve C/L to bore angle & less port to valve C/L angle
m)  Less chamber surface area for a given "volume"
z)   etc, etc, etc . . . .

For mech F/I, 2V semi-hemis like Lotus T/C etc, running 9500 MAX rpm:    130/135 bhp/litre

For mech F/I, 4V cossie types like BDG's, BH420R's etc, running 9500 MAX rpm:    145/155 bhp/litre

Original Cosworth DFV spec was 405 bhp at 9000 rpm MAX, for 135 bhp/litre
Final Cosworth DFY spec was 520 bhp at 11,000 rpm max, for 173 bhp/litre

Most pessimistic differential +7.4%
Most optimistic differential   +33%
Or do the math anyway you want.

Figure a conservative pickup of 5% even if your combo is not well sorted, due primarily to better low lift flow because of the larger valve area . . . . . .

 :cheers:
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Offline wobblywalrus

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Re: Pent v Hemi.
« Reply #3 on: November 23, 2014, 12:02:07 PM »
The old British bike engines were two valve hemi's and the new Triumph ones are Cosworth influenced pentroof four valvers.  It took a lot of time for me to realize they are different animals to tune.  Lots of unlearning had to happen.  It pays to do some research and talk with experts before tackling one.  They are better than hemi's in almost all respects.  The investment in time and effort to learn about them pays off.

Offline stay`tee

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Re: Pent v Hemi.
« Reply #4 on: November 24, 2014, 04:29:17 PM »
Thankyou for the heads up on the book Rex, looks to be an interesting read, have ordered a copy, :cheers:,,

The majority of performance information out there these days is based on the Hemi motors,, the challenge is to sift thru this information and translate it to the PentRoof designs,  :wink:,,,
First Australian to ride a motorcycle over 200mph at Bonneville,,,

Offline jl222

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Re: Pent v Hemi.
« Reply #5 on: November 24, 2014, 09:45:20 PM »
in percentage terms, how much more efficient is the Pent roof engine than Hemi's, ??,,

  What's the comparison if both only had 2 valves?

        jl222

Offline wobblywalrus

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Re: Pent v Hemi.
« Reply #6 on: November 24, 2014, 11:24:49 PM »
That is a good question.  Most of the pent roof designs have four valves and it is hard to tell whether it is the combustion chamber shape or the number of valves that make the difference.

Vizard, in his book "How to Build Horsepower" writes about some of the differences in tuning for the different configurations. It has been a big help to me.

Offline John Burk

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Re: Pent v Hemi.
« Reply #7 on: November 24, 2014, 11:32:23 PM »
Nail head Buicks must be the only 2 valve pent roof .

Offline Jack Gifford

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Re: Pent v Hemi.
« Reply #8 on: November 25, 2014, 01:33:31 AM »
This sketch is courtesy of Wikipedia.

A "penta" head need not be more than two-valve, although most are.
"Squish" can be used with either- I don't know if there's any reason for the sketch showing only the penta with any "squish".

I'm curious about the 'penta' name. The guy credited with inventing the penta head (at Peuguot in 1911) wasn't named Penta. Perhaps it meant that the valve included angle was 72 degrees, making the chamber shape a portion (two sides) of a pentagon? (The sketch shows very close to 72 degrees).
« Last Edit: November 25, 2014, 01:52:57 AM by Jack Gifford »
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Offline Milwaukee Midget

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Re: Pent v Hemi.
« Reply #9 on: November 25, 2014, 01:35:48 AM »
I don't know that a non cross-flow design like the Nailhead would be considered a "penta", at least with respect to a more modern engine, but I like conjecture.

I'm on a steep learning curve myself on this topic, but it's more complex than simply a blanket statement or percentage estimate regarding one design over the other.

Valve shape, valve boss size, bowl shape, port size, cylinder filling - keeping it simple only complicates things.  :wink:



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Well, I guess we're making a LOT of progress . . .  :roll:

Offline John Burk

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Re: Pent v Hemi.
« Reply #10 on: November 25, 2014, 01:15:31 PM »
Saw a dragster at Englishtown where the guy made 4 valve heads for his Buick engine . Busy racing and didn't find what the exhaust valve train was like . He crashed the car . Never heard anymore about it .

Offline Rex Schimmer

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Re: Pent v Hemi.
« Reply #11 on: November 25, 2014, 02:02:15 PM »
I think the pent roof design, which has been around for as long or longer than the hemi, was really originated because of manufacturing ability. A lot easier to machine than a true hemisphere. The pent roof design only starts getting better than the hemi only when the angle is reduced to less than 20-30 degrees, the Cosworth DFV, which was the pioneer in efficient pent roof design, had an included angle of 32 degrees, 16 degrees per side. When the angle is reduced to this area then the maximum valve size in a two valve design becomes restricted so most if not all of the latest engines are four valve design. To get compression in a high angle pent roof or hemi engine you need to run a piston with a big lump that goes up into the combustion chamber, this makes the piston heavy, increases piston surface area, and disrupts the incoming charge, all of which reduce combustion efficiency. One of the indicators of combustion efficiency is the amount of ignition lead required for max horse power, the DFV's spec timing was 35 degrees where as many hemis can require as much as 50 degrees and some of the latest pent roof, 4 valve motors are below 30 degrees. Cosworth found out that efficient combustion was even more important that the ports air flow ability to produce good horsepower. All of this is from Kevin Cameron's book, which is another reason, if you happen to enjoy well written technical information on engine design, to buy the book.

Rex
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Offline manta22

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Re: Pent v Hemi.
« Reply #12 on: November 25, 2014, 05:41:49 PM »
The Cosworth DFV also had a flat- plane 180 degree crank that caused such severe vibration that it broke things on the cars. The first practice run in a DFV equipped GT40 shook the needles off the gauges. See: "Racing In The Rain" by John Horsman.

Regards, Neil  Tucson, AZ
Regards, Neil  Tucson, AZ

Offline Freud

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Re: Pent v Hemi.
« Reply #13 on: November 25, 2014, 06:49:50 PM »
I do not rely on technical info to judge engines. I use the sound of the exhaust as they go past me.

Down wind side, preferably on the return leg of a record run and early morning.

So far Chrysler hemies, huge Whipple blowers and nitro are winners.

There may be more efficient power plants but none are louder.

Rock musis has convinced me that loudest is the most attended.

Must be the best?  Don't call me to argue. I can't hear the telephone.

FREUD
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Offline Milwaukee Midget

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Re: Pent v Hemi.
« Reply #14 on: November 25, 2014, 10:51:52 PM »
I do not rely on technical info to judge engines. I use the sound of the exhaust as they go past me.

Down wind side, preferably on the return leg of a record run and early morning.

So far Chrysler hemies, huge Whipple blowers and nitro are winners.

There may be more efficient power plants but none are louder.

Rock musis has convinced me that loudest is the most attended.

Must be the best?  Don't call me to argue. I can't hear the telephone.

FREUD


Few prettier sounds than the Mopar Hydrocarbon Octet.
"Problems are almost always a sign of progress."  Harold Bettes
Well, I guess we're making a LOT of progress . . .  :roll: