Landracing Forum
Tech Information => Technical Discussion => Topic started by: stay`tee on November 22, 2014, 07:02:55 AM
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in percentage terms, how much more efficient is the Pent roof engine than Hemi's, ??,,
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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
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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:
Fordboy
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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.
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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:,,,
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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
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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.
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Nail head Buicks must be the only 2 valve pent roof .
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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).
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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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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 .
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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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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
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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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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.
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... Few prettier sounds than the Mopar Hydrocarbon Octet...
So... you can tell by its sound if a particular hemi V8 is a Mopar? :roll: I can't. But Mickey's Pontiac hemi sounded quite "pretty" to me when I watched it take Top Eliminator at the '62 drag nationals- which included beating Garlits' Mopar hemi. [And then there's the sound of my M/T hemi at 9,000 RPM, haulin' a 14,000 lb. transfer sled... :-D]
Moral of story: please don't use terms 'hemi' and 'Mopar' interchangeably...
Back to topic: Regarding Rex's ignition timing comment- my hemi never liked more than 30.5 degrees advance (blown alky).
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Jack - has anybody (you for instance) ever written up the full story of the Pontiac M/T Hemi? I've got a couple of period Hot Rod magazines with some detail but the full story must be worth telling. Apologies if you already have and I've missed it.
Take your point about Mopars and Hemis but I reckon its now a generic term much like Hoover for vacuum cleaner. I too have a Hemi that is not a Mopar :wink: It's 1.8 litre twin cam cross flow Lea-Francis that will go in my early 50's sports racer. Over here in Europe you can pick up LeaF engines for very little money, while the vintage racing people pay silly money Riley stuff without ever realising that Hugh Rose designed both engines. They are very similar. He worked as chief designer for Louis Coatalen at Sunbeam on - amongst other things - the Silver Bullet so even has LSR form of a sort.
Here's a pic of the LeaF head
Robin
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Take your point about Mopars and Hemis but I reckon its now a generic term much like Hoover for vacuum cleaner. I too have a Hemi that is not a Mopar :wink: It's 1.8 litre twin cam cross flow Lea-Francis that will go in my early 50's sports racer. Over here in Europe you can pick up LeaF engines for very little money, while the vintage racing people pay silly money Riley stuff without ever realising that Hugh Rose designed both engines. They are very similar. He worked as chief designer for Louis Coatalen at Sunbeam on - amongst other things - the Silver Bullet so even has LSR form of a sort.
Here's a pic of the LeaF head
Robin
Sort of off topic but very interesting is the fact that there is pretty much a DIRECT connection from all of the high performance British engine manufacturers/constructors and their engineering staffs, all the way from the Brooklands racing of the '30's to Ilmor Eng Ltd. of today, and no doubt beyond. Many of the most influential individuals had proteges who spun off their own companies, or did design work on the side. Also of interest are the connections of these folks and their small companies to LARGE manufacturers with huge resources and support possibilities. It would be very interesting to build a "connection tree" of all these folks. I suspect it would get out of hand, quickly.
And I'm sure there are connections I am NOT aware of . . . . . .
Sorry for the thread hi-jack.
:cheers:
Fordboy
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As a side note to the side note......
Mopar/Chrysler owns the name "Hemi". Ask Nick Arias, he sells a hemi head conversion for big block Chevys, LS-Chevys, & now the Ford Windsor engine not to mention his own 8.3 & 10 liter hemi engines. Anyway he now calls them "Hemispherical" cylinder head conversions in print as he has received several cease and dissect letters from the parent company.
The last picture is the 10 liter engine use in the Arias lakester before it was sold.
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My good friend, Steve Nelson, is replacing the V4, DOHC Ford B block in his lakester with one of the Arias four cylinder setups with his new design hemi head. He is looking for the F fuel lakester record.
Jack, regarding your blown alky motor, one of the possible reasons that it liked 30 degrees could be that you probably didn't have a big pop up on the piston to make compression. The pop up gets in the way of the incoming charge and typically requires more ignition lead. Just thought.
Rex
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Another thing I've noticed on the dyno regarding engine timing is the combustion chamber type, even for hemi engines. The "shallow" early 392 type chamber required less timing than the later "deep" chambers on the 426 type engines. The deep chambers seem to work well with blown fuel but the shallow chambers better blown or unblown on gas. Just saying I noticed the big chambers didn't respond as well on gasoline as the small. On fuel in a blown application all bets are off!
And Rex, I know about Steve's engine. Going to be an interesting season in 2015.
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Flame travel is slow till the mixture is compressed . A fellow I know put the mag in a couple of teeth off in his top fueler . 150* advance and it didn't hurt anything .
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The fuel/air mixture was set rich, between 12.5 to 13.5 to 1 when I started to tune the pentroof Triumph. The old hemispherical chamber Triumphs liked that rich mixture. The new engine ran bad. The mixture was leaned out to stoichometric ratio and it produced the best power at this setting. An old Triumph would burn up at stoich ratio. The timing advance is less than with the old style engines, too. The best way to tune these engines is to put them on a dyno and to try different fuel/air mixtures and advance curve settings. The motor will tell you what it likes best.
One amazing thing is, when this pentroof engine is tuned very well, it has relatively low brake specific fuel consumption. The old hemi style ones tended to produce power in proportion to fuel use. This modern Triumph engine does not do that. It is a strange beast.
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Nay Wobbly, the old one's the strange beast! :-D :-D :-D :evil:
Pete
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Robin- Pontiac Enthusiast Magazine did a good feature on my M/T hemi (Mar/Apr '99) in which I tried to share most of what I've learned about it (article titled "Great Mickey's Ghost!", by John McCarthy). There's a copy of the article on the wallaceracing.com website (go to bottom of home page, click 'search wallace racing site', search for "Gifford").
The Lea-Francis head is interesting- never saw one before.
Rex- My hemi V8 pistons have the traditional shape of any early Chrysler hemi race piston. The dome is somewhat truncated, to get the 10:1 compression ratio I wanted.
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Peter, are you calling Bo a strange beast? :evil:
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Peter, are you calling Bo a strange beast? :evil:
There are a lot of us fall into that category. Old English motorcycles fit the description even better. :-D :-D :-D
Pete
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I apologize in advance for slightly "detouring" this thread:
In Honda ads from about 1980, I see a trademark symbol (TM) by the word "pentroof"- and I assume Honda was claiming ownership of the trademark. How could that be? I thought the word went all the way back to the nineteen-twenties when some Indy roadster engines used four-valve pentroof chambers?
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Interesting. In the early 2000's Shelby TM'd 427, FE, Shelby, Shelby American, any many more. Never could figure out how 427 could be trade marked? So I'm not surprised Honda got pent roof.