Intake/exhaust valve diameter ratio on older OHV8s

[PackardV8]

Let's take as a given the old SBC 2.02/1.6 I/E valve diameter ratio of .792 is the norm/golden mean. It's a given a larger intake valve usually produces more high RPM power than a larger exhaust valve.

I work primarily with Studebaker and Packard V8 engines. The Studebaker V8 used a .92453 I/E ratio and the Packard V8 used a .84375 ratio.

Is it possible that in the bad old days, a larger exhaust valve was seen as a way to reduce pumping losses at relatively low RPMs and thus increase efficiency and fuel economy.

If not, what other reason might Studebaker engineers have had for such a large exhaust valve?

jack vines

 

[patprimmer]

Studebaker offered their engine in supercharged form for quite a long time and supplied many cars with supercharged engines. That may have influenced their thinking.

Hmmm does increased compression which of course leaves a smaller chamber and less exhaust to scavenge impact on this and the trend to smaller exhaust vs intake valves. Certainly the after market modern SBC heads further increase intake to 2.08" or even bigger while retaining the same 1.6" exhaust valve

Heck, I'm currently working on a SBF Yates head that has I think 2.25" inlet and 1.6 exhaust

Regards
Pat
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[MikeHalloran]

I think Pat got it about right; with a mechanically driven supercharger, the intake valve can be smaller, hence the exhaust valve can be bigger, than an engine expected to perform its best in unsupercharged form.


Mike Halloran
Pembroke Pines, FL, USA

 

[franzh]

Studebaker only supercharged the 289 engines (and a few limited production 304's in the later Avanti) starting around 1957. Studebaker introduced the 232 V8 around 1951 and due to engineering cutbacks, did relatively little to their engine basic design. Smaller intake valves lead to increased intake flow velocity which in turn helped with low end torque. I doubt that they cared much about pumping losses, as they all touted using a vacuum gauge to establish best fuel economy (drive with the highest vacuum possible for the maximum economy!).

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[PackardV8]

Just trying to understand how engineering thinking changed over the years. Maybe the hi-po SBC heads weren't the best example. The later production SBC heads had 1.94"/1.5" valves, for an even greater .7732 ratio.

That would indicate the Chevrolet engineers had decided that a larger intake was more beneficial at higher RPMs without killing low RPM velocity.

jack vines

 

[franzh]

A lot of early (1963 and up) SBC heads had the 1.94/1.6 valves, on 327 and really stout 283 engines. Some of the built 327 engines in 67 went to the 2.02 heads (they were called the 327 Fuelie heads). They really didnt work well on the street as the intake air velocity would drop the gas droplets out and they would hesitate and stumble. Later Gen II SBC heads with port fuel injection solved a lot of the large intake valve problems. I would put the advantage on port fuel injection rather than the intake to exhaust valve ratio.

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[140Airpower]

PackardV8, the exhaust/intake size ratio of the old SBC was among the smallest back in the day. It apparently had to do with the short, high, excellent exhaust port compared to the usual contorted, downward-aimed port found on most engines in the '50s and early '60s. Chevy engineers stated that they found the SBC exhaust port with the 1.60 valve matched well with an experimental 2 intake valve head they designed for LeMans in 1963. This very head or a similar head design was later used by Micky Thompson at Indy.
Chevy did not take the GS Corvette with the 3-valve head engine to LaMans apparently due to the infamous GM "NO RACING" edict.

The SBC exhaust port was very short, making the engine more compact. The stock header rerouted the exhaust to a downward trajectory. When used in racing, the stock header was irrelevant leaving the short, high exhaust port and free-flowing headers to reach their full potential. Compromising, even siamesing, the exhaust port was common on other engines in those days.

 

[Lou Scannon]

The Mopar B engines had very similar exhaust port configuration to the SBC, and the same spark plug access issues. Some of the hi-po engines featured pretty wild, upward looping exhaust manifolds that refrained from undoing the good work of the free flowing exhaust ports. Yet they also had very generous 1.75" exhaust valves, combined with 2.08" intakes. Since they were fairly large bore engines, there probably was no constraint to make the exhaust valves any smaller.

"Schiefgehen will, was schiefgehen kann" - das Murphygesetz

 

[140Airpower]

Hemi, the Mopar big wedge and other engines, Mopar small block, Ford small block, later AMC, BBC... It was a good design for an exhaust port. The ratio of valve sizes were more even in those engines. The original SBC heads had 1.50 exhausts and 1.72 intakes, more in line with the other engines. I think the SBC with the early 1.50/1.94 and 1.60/2.02 heads were only Corvette "fuel injection" versions and were the most racy of the group. Those original "fuelie" castings were sought after up until the late '70s even though late '60s heads with the same valve sizes were more available. I had sets of both styles, but though I drove "aggressively" I didn't race so can't confirm what was better.

 

[patprimmer]

The so called fulie heads where first introduced on the FI 327 Corvette and soon offered on a few other HP engines. What really distinguished them was a smaller chamber for high compression. The 1.94/1.5" valve size configuration was std for Fulies as opposed to the earlier FI 283 heads Power Pack heads which had even smaller valves but still the smaller chambers. Later casting Fulies.

Almost all 350s had the 1.94.1.5 valve sizes mostly in lower compression heads.

The 2.02/1.6 was mainly only supplied as a heavy duty 'homologated" part for racing. Although they gave a rough idle and poor low rpm power in the day, I currently run aftermarket heads with 2.08/1.6 on a social ski boat and it is very calm and tractable and has impressive fuel consumption in this application which cruises mostly at 2500 to 3500rpm, but must idle at about 600rpm so the dog clutch operates smoothly.

Regards
Pat
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[140Airpower]

Pat, That's basically right. Before they put the BBC in the Corvette in 1965 1/2, the hottest engine was the 327 with FI and carburetted with 1.94/1,50 valves on "double hump" or "camel hump" heads rated (gross) at 360 and 340hp respectively in 1963,4 going up to 2.02/1.60 valves and 375 and 365hp in 1965. Those were solid lifter engines that sounded wonderful, like heavy duty sewing machines. They also introduced a hydraulic cammed version of the 365hp carburetted engine (2.02/1.60) that had 350hp for the Chevelle and which could be dealer optioned on the Chevy II (this allowed Bill Jenkins to run with the Hemi Dodges and Plymouths). But, after the BBC arrived they dropped the racy 327s except that they kept the 350hp hydraulic engine in the Corvette for a couple of years. The 350 cu in version of the 1.94/1.50 valved heads was a new casting that was also used on the '67-'69 302 but with the 2.02/1.60 valves. That engine was created for the TransAm series.

The valve size ratios on the classic hipo SBC was out of the normal range. However, all along they also had engines with the 1.72/1.50 valve heads with lower compression than the old Powerpak heads for more pedestrian applications.

 

[dgallup]

I got a sixty-nine Chevy with a 396
Fuelie heads and a Hurst on the floor
She's waiting tonight down in the parking lot
Outside the Seven-Eleven store
Me and my partner Sonny built her straight out of scratch
And he rides with me from town to town
We only run for the money got no strings attached
We shut `em up and then we shut `em down

Tonight, tonight the strip's just right
I wanna blow `em off in my first heat
Summer's here and the time is right
For goin' racin' in the street

B. Springsteen

----------------------------------------

The Help for this program was created in Windows Help format, which depends on a feature that isn't included in this version of Windows.

 

[PackardV8]

"sixty-nine Chevy with a 396; Fuelie heads" over the years Bruce has made millions writing songs about things of which he knows little first hand.

About like the Beach Boys Little Deuce Coupe lyrics: "Just a little deuce coupe with a flat head mill" and the next verse, "She's got a competition clutch with the four on the floor" I was there in the day and I never saw a '32 coupe rod with a flathead and a four speed. They just didn't fit together.

jack vines

 

[140Airpower]

Poetic license guys. Poetic license. Give them some slack.

Springsteen may not have hot rodded 396s, but the Beach Boys were real hot rodders during the golden era.

 

[Lou Scannon]

Then there's Joanie Mitchell with her "57 Biscayne"... but we digress

"Schiefgehen will, was schiefgehen kann" - das Murphygesetz

 

[140Airpower]

'57 Biscayne? Oh I had one of those. Or was it a '57 Impala?

Sometimes song writers deliberately say something impossible or unlikely the way Chuck Berry was "...campaign shouting like a southern diplomat..." when you expected him to say "...Southern Democrat...". Poetic license.

 

[Marquis]

When looking at valve sizes but perhaps more appropriately, port flows, viewing it in context with the valve lift profiles often sheds light on scenarios.

The Mopar B and RB engines as standard had much long exhaust durations (relative to intake) from the factory and yet didn't flow significantly worse than their competitors from the other two- about mid way

http://www.moparmusclemagazine.com/techarticles/5115_cylinder_heads/photo_06.html

From my work on the Jaguar Supercharged engines I found that a longer duration exhaust was indeed very beneficial to torque pretty much all the way through the rev range. These were fixed cam timing engines so the quote isn't distorted by optimums for VVT.

http://www.auto-scape.com

 

[140Airpower]

Marquis, greater exhaust valve sizing and cam duration appears to be appropriate for supercharging. From your experience with supercharging what is your take on the compression vs expansion ratios for supercharging vs turbocharging where the turbo can capture additional expansion? Does it pan out?

 

[Marquis]

Earlier exhaust valve opening will certainly benefit the Turbo charger as more enthalpy is availiable to the turbine...IN THEORY- however with fixed cam timing this engine would potentially suffer at lower loads and lower engine speeds in terms of brake thermal efficiency- as it wouldn't be getting the most work out of the cycle.What about using variable cam phasing on the exhaust? Good idea. The other thing to bear in mind is that we're getting to stages where at WOT higher engine speeds the engines are savagely over fueled to keep the turbines cool. This indicates that there is ample energy in the exhaust stream that can't be fully utilised already and on top of that we're wastefully overfueling. For this reason earlier exhaust valve opening (lower expansion ratio) to help the turbo at higher loads and speeds may not really seems appropriate...
However with the impending implementation of COOLED EGR on GDI engines to keep exhaust temps low and mitigate knock there may be an opportunity.
If there is- it would need to be done thoroughly with extensive DOE techniques as there are SO many variables (as already touched upon) that interact with one another!

In terms of boost pressure and what the cylinder sees- supercharging and turbo charging have very similar requirements:

Design a combustion system that mittigates the onset of knock as much as possible
With the higher levels of boost on down sized boosted engines (turbo charged)- attention needs to be paid to oil control and in cylinder residual levels- to ensure 'super knock' or "megaklopf" isn't initiated.

Before such high levels of boost I was about to say that on a supercharged engine overlap is nearly always minimised to avoid short ciruiting of fresh charge.

This isn't the case on some older OEM production Turbo charged engines- such as the Porsce 993 which has quite alot of overlap- I would cite that perhaps because the back pressure is potentially higher in a Turbo charged engine this isn't such an issue. Also it's not a 4 valve engine so perhaps just looking at degrees overlap is misleading...

GDi engines can again use overlap to effect even greater scavenging taking advantage of alot superior charge cooling and the freedom of injecting whenever they want to avoid short circuiting (this is what BMW has been doing to great effect)

Again- as boost pressure start to climb further- I think we will probably see overlap minimised in certain regions of the rev range (variable cam phasing) to control in cylinder residuals and exhaust durations may start to get limited for the same reason.

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Sideways To Victory!

 

[patprimmer]

I think one big complication is that typically with a mechanical driven supercharger with even a moderately OK exhaust, it can be assumed the inlet manifold pressure will be well above the exhaust manifold pressure. This greatly assists scavenging and is easily overdone thus wasting inlet charge straight out the exhaust if overlap is not restricted.

On a turbo, the pressure in both inlet and exhaust is increased a lot. Who knows which side is highest at any one time. You might have scavenging during overlap or you might actually get exhaust flow back into the inlet port, reducing the potency of the next charge. To really know, we need data for real pressure measurements on both sides

Regards
Pat
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