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Why only two valves per cylinder??

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Old 02-05-2003, 09:31 AM
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johnfm
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Post Why only two valves per cylinder??

Ok, we've got two plugs per cylinder to improve burn efficiency but why only 2 valves per cylinder??

We know that more smaller diameter valves flow more air than fewr large diameter valves, and I expect Porsche was making engines for their racing cars with more valves per cylinder, so why just 2 in the 964?

I'm not complaining, but 69 HP/litre in a 1250-1350 kg car is not incredible for a 1990's car made by a company like Porsche.
Old 02-05-2003, 09:59 AM
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Christer
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</font><blockquote><font size="1" face="Verdana,Tahoma,Arial,Helvetica,Geneva">quote:</font><hr /><font size="2" face="Verdana,Tahoma,Arial,Helvetica,Geneva">Originally posted by johnfm:
<strong>I'm not complaining, but 69 HP/litre in a 1250-1350 kg car is not incredible for a 1990's car made by a company like Porsche.</strong></font><hr /></blockquote><font size="2" face="Verdana,Tahoma,Arial,Helvetica,Geneva">Not (delibarately) being pedantic but it must be more like 1250-1500kg car...my car weighed in at 1352kgs/2988lbs (from memory) with half a tank of fuel and it has no aircon, no airbags and is not a Tip. A C4 with aircon has got to be looking at 1500kgs or as near as dammit (what does the manual say - 1460kgs?).
Old 02-05-2003, 10:19 AM
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johnfm
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You are being pedantic - the gist of the question is 2 valves per cylinder....

..please chime in on that too if you like!!
Old 02-05-2003, 10:23 AM
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Irishdriver
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Officially my car - a '92 C4 - is 1450 kgs empty.

Add a slightly overweight driver a, few CDs and the necessary fluids and we're talking a normal running weight of probably the best part of 1600 KGs.

As for valves, more valves tend to leave you with less low down torque and more top end power. I like the compromise as it is. Porsche made the right decision.
Old 02-05-2003, 10:38 AM
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Wil R. Ferch
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Hi:
Wil Ferch here ( from 3.2 list)...
Officially, Porsche didn't offer 4 valves on the air-cooled versions because there would be too little material left in the head for adequate cooling. All of Porsche's racing cars of that and prior eras ( as well as 944/968/928 series) could use 4 valve heads because they were water cooled...at leas the heads were water cooled on the air cooled racing versions. Notive that the later "996" 911's ...water cooled...are 4 valve head designs.
--Wil Ferch
Old 02-05-2003, 10:53 AM
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Dave R.
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I thought the main reason Porsche ran 2 valves instead of 4 valves was heat related; with an aircooled head it is too hard to keep 4 valves cool enough. Valves dump a lot of heat through the valve seat to the head, and with 4 valves instead of 2 there is less head material to draw away the heat.

If I recall correctly from Bruce Anderson's "Performance Handbook", Porsche has raced with 4-valve heads but the heads were liquid-cooled, not aircooled.

[Edit: Wil posted while I was typing, looks like we agree!]
Old 02-05-2003, 11:20 AM
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johnfm
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OK, I'll stop whingeing about only two valves then!!
Old 02-05-2003, 11:41 AM
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Christer
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And I'll stop being pedantic... <img border="0" title="" alt="[Wink]" src="wink.gif" />
Old 02-05-2003, 11:47 AM
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J-McDonald
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Everything that I've read is that Porsche stayed with 2 valve heads on the 3.0 SC, 3.2 Carrera and 3.6 964/993 motors because 4 valve heads would generate excessive heat, which could not be dissipated by oil/air cooling alone.

For this reason, the 935's flat 6, 24 valve motor has watercooled heads, but is otherwise oil/air-cooled.
Old 02-05-2003, 01:38 PM
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Ruairidh
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</font><blockquote><font size="1" face="Verdana,Tahoma,Arial,Helvetica,Geneva">quote:</font><hr /><font size="2" face="Verdana,Tahoma,Arial,Helvetica,Geneva">Originally posted by johnfm:
<strong>I'm not complaining, but 69 HP/litre in a 1250-1350 kg car is not incredible for a 1990's car made by a company like Porsche.</strong></font><hr /></blockquote><font size="2" face="Verdana,Tahoma,Arial,Helvetica,Geneva">I still haven't figured out how that 250bhp lump behind me propels my car so quickly, so responsively to such speeds. I've driven many cars with more cc; valves; bhp and cylinders and still don't understand how come 250 Porsche Horses feel like 350+ of other manufacturers?

What I'm trying to say is that the power-weight ratio doesn't tell the whole story here - although I don't know why not?
Old 02-05-2003, 02:58 PM
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RSAErick
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I agree, Ruairidh! I think it's called torque.

Does a 4-valve design affect where (at what RPM) that the engine delivers the power?

It seems like the 4-valve cars I've driven are very "revy", but deliver their power at higher RPM's with less torque. At the same time, many of the 2-valve cars I've driven (American muscle cars) are not "revy", and deliver their power at lower RPM's with more torque.

Is there a correlation?
Old 02-05-2003, 03:19 PM
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Jim Michaels
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Good torque, and the motor sits right over the drive wheels.
Old 02-05-2003, 03:42 PM
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johnfm
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Erick

the correlation you seek comes from cubic inches:

torque is effectively the amount of work you can get out of an engine per revolution - this is why you can increase peak power by increasing engine RPM, as BHP is a function of HP and RPM.

So, though little 1500 cc engines can produce impressive PEAK power, this is often at very high RPM, as they burn less fuel per cycle.

A small flat 6 with 2 valves per cylinder wil produce less torque than a 3.6 litre flat 6.
Old 02-05-2003, 04:22 PM
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RSAErick
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Thanks John, I understand what you are saying. But let me try another analogy:

I've driven a 944 and a 944S. The main difference is 2 valves/cylinder for the 944 and 4 valves/cylinder for the 944S.

944 is 158hp and 155 lb/ft of torque.
944S is 188hp and 170 lb/ft of torque.

While the 944S feels more powerful when you get the revs up, the 944 actually FEELS peppier at lower RPM's. Some even claim that the 944 is a faster autocrosser than a 944S!!! This leads me to surmise that the 944 torque curve peaks at lower RPM's, whereas the "pull" comes in later (albeit higher) for the 944S.

My question:
Is the difference in the shape of the torque curves a result of 2 or 4 valves - or is this simplifying things WAY too much?

Thanks.
Old 02-05-2003, 06:19 PM
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Herbie
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The behavior of torque curve between 2 valves and 4 valves engines is due exactly to this different distribution.

At higher rpms, the target is to achieve the more air/fuel mixture flow into the cylinder, so a 4valve engine is better.

At lower rpms, air/fuel flow is low, and only 2 valve is not an obstacle; but, with only 1 intake valve, the mixture increases speed obtaining a better filling of the combustion chamber.

To collect both advantages, modern 4valve engines use a variable phase distribution, that at low rpms reduces opening time to emulate a 2 valve engine.


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