[JEC3039]

Starting this as a new thread versus my out-of-the-box thread.

Quick question about dual-pattern cams in E-heads. The traditional Pontiac iron heads benefited from 10-12° of extra exhaust duration due to the sharp short turn radius in the exhaust port. Does this continue to be true with the Edelbrock alum heads (round port specifically) or did Edelbrock make improvements in this area per chance that would allow the exhaust duration to be closed up tighter toward the intake duration. Just curious.

[steve25]

The need flow wise for a certain level of Exh to Intake ratio is very dependent on what the compression ratio is of the motor in question and if it will have Headers who's primary tubes are a bit bigger then the Exh flange opening in the head.

If you look at the flow numbers up to .500" lift for the out of the box 60579 head and total those numbers up we see 911 cfm of Intake flow and 641 cfm of Exh flow.

This makes for a 70% Exh to Intake ratio which is 5 % short of what's accepted to be good for a motor with about a 9 to 1 to 10.5 to 1 compression, but bolting on a set of Headers with primary tubes as stated above has been shown on my flow bench to pick up anywhere from 4 to 8 % more Exh flow at lifts above .400".

Edelbrock as others have, used the smaller 1.66" exh valve these heads have to crutch the the issue of low Intake port velocity at low rpm in 428 cid and smaller motors due to the overly large 215 CC Intake ports.

The smaller Exh valve helps to scavenge the Intake port at overlap and get the Intake flow moving on into the cylinder again at a faster rate, but not by much!

[slowbird]

Originally Posted by steve25

This makes for a 70% Exh to Intake ratio which is 5 % short of what's accepted to be good for a motor with about a 9 to 1 to 10.5 to 1 compression,

The smaller Exh valve helps to scavenge the Intake port at overlap and get the Intake flow moving on into the cylinder again at a faster rate, but not by much!

Where did you read this? Not one professional porter or engine builder that i know of cares about intake to exhaust ratio. The flow bench cant come close to duplicating whats going on in a running engine. Imo id throw that ratio info out the window.
How does a smaller exhaust valve help scavenge during overlap??

[steve25]

Why don't you post up this info that you speak of then?

Do you have your own flow bench to clearly see as I and others have what takes place like I have posted about and then corroleted this on running motors and there performance?

If I recall right it was Smoky Yunick who first , after tons of hours of flow testing finally settled on a needed ratio of 75%, so are you going to throw cold water on his findings now to?

[Steve C.]

Years ago when Chris Mays at Comp was helping me with a combo he suggested a single-pattern cam at the time with my higher exhaust/intake ratio on the ported Edelbrock heads which were about a 79 percent ratio. Admittedly the rpm operating range was a tad bit higher than many 5000 rpm truck motor combos out there and apparently that can influence the decision. But he also suggested a narrow 108 lobe separation. Both considered taboo in some circles here. The engine made peak power at 5800 rpm and it had a crappy 9.7 compression with the aluminium heads.

Not isolated though, I believe it was Bob Cook at Bullet that recommended a single pattern cam with a 108 lobe separation when Lee Attkinson upgraded from his "041" hyd cam to a 247/247-108 solid flat tappet cam. He picked up about 2 tenths and about 4 mph. That said, Lee has also posted since then with his thoughts and current design of cams with a very large amount of exhaust duration.

Quote of interest from Harold Brookshire, "Whenever the ratio gets around 85%, single pattern cams seem to work as good as anything." BUT this was not specific to Edelbrock heads. And back then in conversation the tech help at Crower they suggested I add an additional 6 degrees exhaust duration because it was a street car with full exhaust.

So opinions will vary. Problem is most of us don't have the resources or time to do cam swaps out of general interest.


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[Steve C.]

This topic is related....

https://forums.maxperformanceinc.com...aust-to-intake

And in my post #27 there is the full quote by Harold Brookshire made here at PY on the subject. Keeping in mind no specific type of cylinder head was noted.


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[slowbird]

Originally Posted by steve25

Why don't you post up this info that you speak of then?

Do you have your own flow bench to clearly see as I and others have what takes place like I have posted about and then corroleted this on running motors and there performance?

If I recall right it was Smoky Yunick who first , after tons of hours of flow testing finally settled on a needed ratio of 75%, so are you going to throw cold water on his findings now to?

Not sure why it matters but yes I do have a flow bench and i communicate often with others that have flow benchs
Example our 400 has 80% we run 10° more car runs 11.4s (not many iron headed dailey driven pump gas cars running that). Our iron headed tri-power pump gas 455 is over 80% it has 12° more it runs 11.8s weighs 4250lbs.
Fwiw our race engines are in the 60% range i sure as heck ain't giving up intake flow for exhaust. My opinion is based on my experience and talking to some really good ports and builders. I also don't see current OEM cars with that 75% ratio, if it was important i'd think they would try to come close. Most of the LS stuff stock is mid 60%, why wouldn't GM focus on improving that if it was so important?

Please answer my question about how the smaller 1.66 exhaust valve helps scavenging better vs a larger valve.

[Lee]

The Corvette LS7 (modern version) is one of the most efficient N/A pushrod motors ever. The racing version has a VERY efficient, low-restriction exhaust. Based on the old "theories" about camshaft design, you would suspect that the cam GM engineers designed for an all out racing (their GM Performance Parts Stage 3) would be a single-pattern, or even a reverse-split.

In reality, it is a BIG-split. Only 233 degrees @ 0.050" on the intake, but 276 on the exhaust. For reference: https://www.gmperformancemotor.com/parts/88958773.html

[PAUL K]

Originally Posted by Lee

The Corvette LS7 (modern version) is one of the most efficient N/A pushrod motors ever. The racing version has a VERY efficient, low-restriction exhaust. Based on the old "theories" about camshaft design, you would suspect that the cam GM engineers designed for an all out racing (their GM Performance Parts Stage 3) would be a single-pattern, or even a reverse-split.

In reality, it is a BIG-split. Only 233 degrees @ 0.050" on the intake, but 276 on the exhaust. For reference: https://www.gmperformancemotor.com/parts/88958773.html

That's an interesting grind.

[Skip Fix]

Just one reference point on lots of variables that can factor in when racing the old NMCA's Top Stock class we got factored HP depending on idle vacuum so lots of folks tried all different approaches. My RAIV 400 I had 5 different cams in it, different splits, LSs- a reverse split picked it up as did smaller headers. It had 85% E/I and on the flowbench the exhaust would flow 90% backwards(on intake setting) which is why I had reversion problems until we made those changes.
Probably about a 400hp motor. John Angeles in his RAIV SS car uses a single pattern cam.

[slowbird]

Originally Posted by Lee

In reality, it is a BIG-split. Only 233 degrees @ 0.050" on the intake, but 276 on the exhaust. For reference: https://www.gmperformancemotor.com/parts/88958773.html

Any chance that's a misprint?

[Steve C.]

JEGS states the same...

https://www.jegs.com/i/Chevrolet-Per...58773/10002/-1


From the LS forum

How does increasing exhaust duration and lift affect the torque and hp curves?

https://ls1tech.com/forums/generatio...hp-curves.html

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[Steve C.]

Opinions....

"When to use a longer exhaust duration is NOT about the head flow, it is so the engine can still produce very good HP ABOVE the true HP peak. To take advantage of this, the car will have to run a gear ratio that lets the engine rev up ten or 15 percent above the HP peak to cross the finish line. On a drag race engine , by going 10 to 15 degrees more on the exhaust duration, many times you will pickup 20 or more HP at the 500 to 800 RPM over speed. The peak might be better too, but the over rev HP is where the big gain will show up."

JOE SHERMAN RACING ENGINES

"If you throw out drag racing, where efficiency isn't much of a factor (just increase the stall converter, and turn it higher), you don't see the real long exhaust durations you would see 20 years ago.
It's all about the mass in the cylinder, the RPM range, and the exhaust port. On most 4 valve engines, you normally see a shorter exhaust then intake duration, and that's directly related to the exhaust ports that flow almost as much as the intake ports, and in somecases, outflow the intake ports at low lifts. If you look at circle track 2V engines that have to be efficient at lower RPM's, where the exhaust flows around 70-76% of the intake, you normally will see Exhaust durations that are even to 8 degrees longer then the intake."

Mike Jones

longer exhaust duration.... why and when??
https://www.speed-talk.com/forum/viewtopic.php?t=29382


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[Lee]

Originally Posted by slowbird

Any chance that's a misprint?

Nope. I've provided several similar grinds for a variety of LS engines, with excellent results.

The BETTER the exhaust, the BIGGER the split I can use. The peak HP doesn't really change. The biggest gains are in the midrange, and the powerband is extended (typically) as well.


For drag racing, with high stall speeds and rather narrow power bands being used, then single-pattern & mild-splits work well.

[steve25]

The smaller Exh on these heads attached to a flaring out lower section of the valve bowl( that's a bit more suited for actually a 1.71" valve ) produces a longer lived Venturi effect that keeps Exh gasses moving out of the cylinder even while the piston is the way down the Bore before the Exh stroke even begins, and as we all know the lesser the amount of hot positive pressure there is in the cylinder, the sonner the Intake flow will restart and thus air velocity stays more constant and in turn so does low speed Torque output.

[slowbird]

Originally Posted by Lee

Nope. I've provided several similar grinds for a variety of LS engines, with excellent results.

The BETTER the exhaust, the BIGGER the split I can use. The peak HP doesn't really change. The biggest gains are in the midrange, and the powerband is extended (typically) as well.


For drag racing, with high stall speeds and rather narrow power bands being used, then single-pattern & mild-splits work well.

Very interesting, i've done huge splits on our race engines to keep power going in high rpm and after peak hp. It's interesting that you're finding good gains in the mid range. Have you done any cams similar on Pontiacs?

[slowbird]

Originally Posted by steve25

The smaller Exh on these heads attached to a flaring out lower section of the valve bowl( that's a bit more suited for actually a 1.71" valve ) produces a longer lived Venturi effect that keeps Exh gasses moving out of the cylinder even while the piston is the way down the Bore before the Exh stroke even begins, and as we all know the lesser the amount of hot positive pressure there is in the cylinder, the sonner the Intake flow will restart and thus air velocity stays more constant and in turn so does low speed Torque output.

You know the only reason e-heads have 1.66 valves is so they could work with stock pistons. 1.77 will hit the valve reliefs in stock pistons (valve spacing is .050 moved from stock). If smaller is better then no one should be upgrading iron heads.