[cstraub]

Ok, thanks.

Scott,
A split duration doesn't make more power than a single pattern and vice versa. The correct cam for the combination makes more power.

The given rule of thumb is when the I/E ratio is 75% the duration at .050" will be equal on both intake and exhaust. When you go below this number the duration on the exhaust is larger. When you go above this number the duration on the intake is larger.

Years ago I used the word "crutch" which now I look back and that is the wrong thing to say. I cam the intake track with a lobe that has the correct path for the valve. I cam the exhaust track with a lobe that has the correct path for the valve. Both are seperate paths but they are both ground on the same camshaft.

Duration is nothing more than time. If a port has a low flow number then it needs more time, duration, to exhaust the port. At 240# and 5'9" I need more time to run 100 meters then what Mr. Bolt would need.

 

[Gtogeoff]

So why did cams for V8s have extra exh duration? Was it because at the birth of the OHV V8 engine, factory cars had very restrictive exh manifolds, small pipes & restrictive mufflers, so the exh needed 'help' via more duration?? And bad habits die hard...Isky's Tech Tip 'Is extra exh duration really necessary' is a good read.

Watching the EMC over the years, I see new trends. Reverse pattern cams. [ LESS exh duration ], SP cams with higher ratios on the intake etc. At least one cam company, I think it is Erson or Howard, has a DEDICATED page of RP cams. You would not have seen that 20 yrs ago. So the thinking has changed it seems...
There is another benefit of RP cams I do not see mentioned. The exh component of overlap has the greatest influence on idle quality & vacuum. With a RP cam, you have less exh duration as part of overlap, so idle & vacuum improves. A logical extension of this suggests the idle benefit doesn't stop abruptly at idle but continues into the low range for a benefit. I always use RP cams now for street engines, & often use a little extra int duration, with no obvious downsides. A 468 cu in engine with 254/248 @ 050 cam 112 LSA, idles at 950 rpm in Drive. No way would it idle that low if the #s were 248/254 @ 050.
Interestingly, my Crower catalog in the SBC section says increasing rocker ratio on the INTAKE to 1.6 adds about 5* duration for more top end power. Notice it says to add rocker ratio to the intake for top end power, not the exh.

 

[Scottybigblock]

Scott,
A split duration doesn't make more power than a single pattern and vice versa. The correct cam for the combination makes more power.

The given rule of thumb is when the I/E ratio is 75% the duration at .050" will be equal on both intake and exhaust. When you go below this number the duration on the exhaust is larger. When you go above this number the duration on the intake is larger.

Years ago I used the word "crutch" which now I look back and that is the wrong thing to say. I cam the intake track with a lobe that has the correct path for the valve. I cam the exhaust track with a lobe that has the correct path for the valve. Both are seperate paths but they are both ground on the same camshaft.


Duration is nothing more than time. If a port has a low flow number then it needs more time, duration, to exhaust the port. At 240# and 5'9" I need more time to run 100 meters then what Mr. Bolt would need.

Thanks Chris, I'm just old school, been building engines for 40 years, but still learning.

 

[Arch Stanton]

So why did cams for V8s have extra exh duration? Was it because at the birth of the OHV V8 engine, factory cars had very restrictive exh manifolds, small pipes & restrictive mufflers, so the exh needed 'help' via more duration?? And bad habits die hard...Isky's Tech Tip 'Is extra exh duration really necessary' is a good read.

Watching the EMC over the years, I see new trends. Reverse pattern cams. [ LESS exh duration ], SP cams with higher ratios on the intake etc. At least one cam company, I think it is Erson or Howard, has a DEDICATED page of RP cams. You would not have seen that 20 yrs ago. So the thinking has changed it seems...
There is another benefit of RP cams I do not see mentioned. The exh component of overlap has the greatest influence on idle quality & vacuum. With a RP cam, you have less exh duration as part of overlap, so idle & vacuum improves. A logical extension of this suggests the idle benefit doesn't stop abruptly at idle but continues into the low range for a benefit. I always use RP cams now for street engines, & often use a little extra int duration, with no obvious downsides. A 468 cu in engine with 254/248 @ 050 cam 112 LSA, idles at 950 rpm in Drive. No way would it idle that low if the #s were 248/254 @ 050.
Interestingly, my Crower catalog in the SBC section says increasing rocker ratio on the INTAKE to 1.6 adds about 5* duration for more top end power. Notice it says to add rocker ratio to the intake for top end power, not the exh.

I've read all this also, which makes cam stuff seem confusing at times.
I thought on a SBC changing to 1.6 rockers only gave you an extra 2 or 2.5 degrees duration. Is it really 5?
Same thing on exh duration, seen posts where all the exhaust pressure "pops" like a soda can as soon as the ex valve opens up and most of the exh is released during the beginning-mid cycle of exh. If so, then why hold it open even longer, to coax more scavenging through header pipe/exhaust system? Help draw intake charge a little during overlap?

 

[Gtogeoff]

I thought 5* extra duration was a lot also, but it states that on a number of pages of the Crower catalog.

 

[steelcomp]

I've read all this also, which makes cam stuff seem confusing at times.
I thought on a SBC changing to 1.6 rockers only gave you an extra 2 or 2.5 degrees duration. Is it really 5?
Same thing on exh duration, seen posts where all the exhaust pressure "pops" like a soda can as soon as the ex valve opens up and most of the exh is released during the beginning-mid cycle of exh. If so, then why hold it open even longer, to coax more scavenging through header pipe/exhaust system? Help draw intake charge a little during overlap?

The pressure is released almost immediately, but the ex gas still needs time to get out as the piston approaches TDC.

 

[BigRed-L72]

BigRed has a Single Pattern UDHarold UltraDyne Hyd roller in it.
233 @ .050 .625 Lift 109 Sep.
Same low compression, same heads and valves the car probably weighs a little more than an average Chevelle.

Time shows in Signature...I wonder what HP it makes !?

 

[steelcomp]

I thought 5* extra duration was a lot also, but it states that on a number of pages of the Crower catalog.

It depends on how much duration you're working with. Look at it as a percentage.

 

[69-CHVL]

Hi guys a bit info if u can help with cam selection on a bb 454 bored to 468 with 781 heads 2.19/1.88 valves (dont know if bowl work done ) I hope so
Perf. Rpm intake , srp forged piston 10.1 cr.
Holley 750 dp.

Looking into comp cam XR288HR hydro.roller cam 236/242 521/540 lift retrofit

Or XR294HR 242/248 540/560 lift

trying to hit 475 Hp margin or bit more

Should I buy a 2800 stall or go with a 3000 ? In a 700r4 auto tranny 3.73 gears

Any help appreciate it guys

Let me know if missing info to provide

The XR288 is about as big as I would go for a STREET 454, it will have some lope. The 294 will start to get rowdy and lose tq for no reason, and will not cruise as nice with the OD trans.

 

[cstraub]

So why did cams for V8s have extra exh duration? Was it because at the birth of the OHV V8 engine, factory cars had very restrictive exh manifolds, small pipes & restrictive mufflers, so the exh needed 'help' via more duration?? And bad habits die hard...Isky's Tech Tip 'Is extra exh duration really necessary' is a good read.

Watching the EMC over the years, I see new trends. Reverse pattern cams. [ LESS exh duration ], SP cams with higher ratios on the intake etc. At least one cam company, I think it is Erson or Howard, has a DEDICATED page of RP cams. You would not have seen that 20 yrs ago. So the thinking has changed it seems...
There is another benefit of RP cams I do not see mentioned. The exh component of overlap has the greatest influence on idle quality & vacuum. With a RP cam, you have less exh duration as part of overlap, so idle & vacuum improves. A logical extension of this suggests the idle benefit doesn't stop abruptly at idle but continues into the low range for a benefit. I always use RP cams now for street engines, & often use a little extra int duration, with no obvious downsides. A 468 cu in engine with 254/248 @ 050 cam 112 LSA, idles at 950 rpm in Drive. No way would it idle that low if the #s were 248/254 @ 050.
Interestingly, my Crower catalog in the SBC section says increasing rocker ratio on the INTAKE to 1.6 adds about 5* duration for more top end power. Notice it says to add rocker ratio to the intake for top end power, not the exh.

If you look at the cams in the Japanese imports you will find almost all have larger duration cams on the intake then on the exhaust. Some of the OEM heads have 90% I/E ratio. The exhaust port flows so well it does not need much time (duration) to do its job.


If a engine does not have enough duration on the exhaust it will smoother at a given rpm range. In the car it will feel like it hits a wall.

 

[cstraub]

Thanks Chris, I'm just old school, been building engines for 40 years, but still learning.

I learn everyday sir and have changed my school of thoughts on a few things. The I/E ratio thing I learned over 20 years ago and have proven it time and time again. BB Fords are the worst of the worst when it comes to IE ratio. From the factory the heads can be in the low 60% range. I have a customer with a OEM headed 512CID Ford with a heavily ported set of heads with an IE ratio of around 56%. The split at .050" is 33 degrees favoring the exhaust. The engine makes over 900HP with a 4150 and 1006HP with Dom.

Now nobody in sound mind would send a customer a camshaft like that!!

Another example is Tracy Hicks of Wise Guy Rod and Custom. Tracy has a 66 Chevelle with an OEM headed 540 that I cammed many moons ago. The engine made 540HP on the dyno that we use today, Performance Automotive. Tracy and I have a mutual friend and friend had him call me. Heads had been ported so I told him I needed the numbers. After flowed it was found this set of 990's flowed very well on the intakes but he exhaust was horrible. The cam in it was 670/690 262/272 on a 112. The cam from memory was 680/650 258/282 on a 108. All that was changed was camshaft and engine picked up 146HP. This was a defining moment for me as this still today is the largest measured pickup I have seen based on my Theory of camming to I/E ratio.

 

[Gtogeoff]

Chris Straub,

Agree that many imports have cams with less exh duration [ Reverse Pattern, RP ]. RP cams are nothing new, but it seems to be new to Detroit....

I don't think you can unilaterally say a RP cam hits a wall with a RP cam. If the engine does hit a 'wall' & it is beyond the targeted rpm range, then that is a win-win because it will have a better idle, probably a bit more low end, all else being equal.

Jon Kaase broke all the 'rules' with his winning 2008 EMC engine.
The tight LSA [ 98* ] didn't 'go over a cliff' as the wide LSA proponents like to tell us; & the lesser exh duration didn't hit a wall. [ 246/238 @ 050 ]. This 403 ci engine made 663 hp @ 6000 & dropped a mere 11 hp to 652 hp @ 6500.

 

[cstraub]

Chris Straub,

Agree that many imports have cams with less exh duration [ Reverse Pattern, RP ]. RP cams are nothing new, but it seems to be new to Detroit....

I don't think you can unilaterally say a RP cam hits a wall with a RP cam. If the engine does hit a 'wall' & it is beyond the targeted rpm range, then that is a win-win because it will have a better idle, probably a bit more low end, all else being equal.

Jon Kaase broke all the 'rules' with his winning 2008 EMC engine.
The tight LSA [ 98* ] didn't 'go over a cliff' as the wide LSA proponents like to tell us; & the lesser exh duration didn't hit a wall. [ 246/238 @ 050 ]. This 403 ci engine made 663 hp @ 6000 & dropped a mere 11 hp to 652 hp @ 6500.

Geoff,
I'm saying that you need to cam for I/E ratio. If you don't then it will hit a wall. John Reed, one of my mentors, was doing RP cams in the 70's sir. It was John that figured out Restictor Plate racing cams first and they were reverse pattern.

Bottom line and I hate saying this cuzz all I can hear is my 2nd grade teacher Ms. Shootman saying...(Christopher you will use math all your life"). She right...it's all about the math.

 

[Gtogeoff]

Chris,
Agree with many of your comments! However, the Kaase example that I provided dispels a couple of myths associated with RP cams & LSAs. Raw numbers, not someone's 'theory'

D. Vizard uses mostly SP cams, & very often with higher ratio rockers on the intakes, effectively converting the cam to RP.

And then there is the maths.....
Why is 70% E/I flow the magic number? Who has PROVED by dyno testing various cams that this is THE number? Probably only DV & he does not seem to be a fan of extra exh duration, & in one of his books/articles, I remember reading he favours 4* LESS exh duration, a RP cam.

Then there is the flow figures that the E/I ratios are calculated from. How representative is the actual exh flow when measured with negative pressure [ depression, 28"of h20 being the most common ], when the exh flow is under positive pressure, expelled out of the port by the high pressure of the burned combustion charge??
I see no correlation to 28" of depression being relative to several hundred psi of pressure.

DV, in his latest porting book also mounts a convincing argument that measuring intake flow at the same depression for all lifts does not provide a representative picture of the actual port flow operating in the engine.

Not saying that there is never a place for extra exh duration. Some Ferd heads as you say have very poor exh flow & might need help.
However, I think for most street/strip engines that won't see part 6500 rpm, running a little less exh duration & little more intake duration will see an overall performance improvement.

Look at turbo cams, most have less exh duration. Crower specs a BBC turbo cam with just 212* @ 050, with a 6250rpm redline! So, even with extra exh gas to be expelled because of the extra HP generated from the turbo, they think that 212* is still enough duration to get the job done...all the way to 6000+ rpm.

 

[cstraub]

Chris,
Agree with many of your comments! However, the Kaase example that I provided dispels a couple of myths associated with RP cams & LSAs. Raw numbers, not someone's 'theory'

D. Vizard uses mostly SP cams, & very often with higher ratio rockers on the intakes, effectively converting the cam to RP.

And then there is the maths.....
Why is 70% E/I flow the magic number? Who has PROVED by dyno testing various cams that this is THE number? Probably only DV & he does not seem to be a fan of extra exh duration, & in one of his books/articles, I remember reading he favours 4* LESS exh duration, a RP cam.

Then there is the flow figures that the E/I ratios are calculated from. How representative is the actual exh flow when measured with negative pressure [ depression, 28"of h20 being the most common ], when the exh flow is under positive pressure, expelled out of the port by the high pressure of the burned combustion charge??
I see no correlation to 28" of depression being relative to several hundred psi of pressure.

DV, in his latest porting book also mounts a convincing argument that measuring intake flow at the same depression for all lifts does not provide a representative picture of the actual port flow operating in the engine.

Not saying that there is never a place for extra exh duration. Some Ferd heads as you say have very poor exh flow & might need help.
However, I think for most street/strip engines that won't see part 6500 rpm, running a little less exh duration & little more intake duration will see an overall performance improvement.

Look at turbo cams, most have less exh duration. Crower specs a BBC turbo cam with just 212* @ 050, with a 6250rpm redline! So, even with extra exh gas to be expelled because of the extra HP generated from the turbo, they think that 212* is still enough duration to get the job done...all the way to 6000+ rpm.

You mean this Jon Kaase? Ed Morels 482CID FE engine I designed the combination. Jon built and dyno'd it. Ed's heads have a 58% I/E ratio and the duration split at .050" is around 24 degrees. For an old Y block 850+ HP's not to bad and Jon was very impressed.

 

[cstraub]

D. Vizard uses mostly SP cams, & very often with higher ratio rockers on the intakes, effectively converting the cam to RP.

I just did the cam for a SB Ford build that Vizard is doing and will be in the new book. Great guy.

 

[cstraub]

And then there is the maths.....
Why is 70% E/I flow the magic number? Who has PROVED by dyno testing various cams that this is THE number? Probably only DV & he does not seem to be a fan of extra exh duration, & in one of his books/articles, I remember reading he favours 4* LESS exh duration, a RP cam.

The number is 75% and that was taught to me by John Reed and confirmed by Joe Petelle. This was all during the 80's when Cup racing was getting the sponsorship dollars and the R&D was ramping up. John was doing cams then and Petelle was porting heads.

I'm blessed to have had the mentors that I have had over the years.

 

[Gtogeoff]

Chris,
75% confirmed. Who confirmed it was 75% & how was it confirmed? Dyno testing a variety of cams with heads having different flow ratios? Where can I see the results? The number is really immaterial. I see many #s bandied about. It seems to be a pet theory, with no actual comparison testing to prove it.

And it seems to me to be one of these myths that grows over the years & becomes fact.

What I do know as fact is that we are now seeing more & more engines built with SP & RP cams, higher ratio rockers on the intakes. As I already mentioned, at least one cam manufacturer has a dedicated page of RP cams. None of the above would have been seen 20 years ago, so there has been a shift in thinking....just like the spark plug was once placed closer to the intake valve & now it has done a 180, with modern heads having the plug closer to the exh valve.
As knowledge is gained, ideas change.

The Jon Kaase engine I mentioned in the earlier post WON 2008 EMC using a RP cam, 246/238 @ 050.
In the same contest, a 406 FE Ford used a 240/243 @ 050 cam with 1.9/1.76 rockers; another engine,a 429 Ford which generally have a very low E/I ratio, used a SP cam.

More examples.

EMC entrants;
[1] As reported in PHR, Feb 2010, EMC engines. A Jon Kaase Boss 429! 273/265 @ 050 RP cam. Another Boss 429, RP cam 254/246 @ 050; & another 429 266/262 @ 050; Ford 4.6 L mod engine 244/240 @ 050.

[2] Crower Cams entries in the EMC. Dan Crower quote: "Believe it or not, we ran 1.3 rocker ratio on the exh side! It didn't hurt power at all...we were surprised to see it. ...Our research has shown the exh presure escapes quickly when the valve cracks open, so the piston movement is simply pushing out the residual.' A Crower entry from another year, 470 BBC used a RP cam, 256/250 @ 050.

[3] Joe Sherman's winning engine, as reported in PHR April 2003. A SP cam was used 234 @ 050, 1.6 int rockers, 1.5 exh.

 

[USMC_Spike]

Bottom line and I hate saying this cuzz all I can hear is my 2nd grade teacher Ms. Shootman saying...(Christopher you will use math all your life"). She right...it's all about the math.

Chris,

As former teacher from a large urban school district
what you just mentioned warmed my heart.

That is the same thing I used to tell my problem kids.
To learn the math, you will use it all your life. And
once you know it, people won't be able to rip you
off because you'll be able to do the numbers and
check it. Along with math is power.

Then came Fraction Walk Multiplication Talk,
followed by The Multiplication Race: Drill
and Thrill.

As Tweety Bird used to say:

Mister, it's twoo, it's twoo!

Cheers,

USMC_Spike

 

[steelcomp]

Chris,
75% confirmed. Who confirmed it was 75% & how was it confirmed? Dyno testing a variety of cams with heads having different flow ratios? Where can I see the results? The number is really immaterial. I see many #s bandied about. It seems to be a pet theory, with no actual comparison testing to prove it.

And it seems to me to be one of these myths that grows over the years & becomes fact.

What I do know as fact is that we are now seeing more & more engines built with SP & RP cams, higher ratio rockers on the intakes. As I already mentioned, at least one cam manufacturer has a dedicated page of RP cams. None of the above would have been seen 20 years ago, so there has been a shift in thinking....just like the spark plug was once placed closer to the intake valve & now it has done a 180, with modern heads having the plug closer to the exh valve.
As knowledge is gained, ideas change.

The Jon Kaase engine I mentioned in the earlier post WON 2008 EMC using a RP cam, 246/238 @ 050.
In the same contest, a 406 FE Ford used a 240/243 @ 050 cam with 1.9/1.76 rockers; another engine,a 429 Ford which generally have a very low E/I ratio, used a SP cam.

More examples.

EMC entrants;
[1] As reported in PHR, Feb 2010, EMC engines. A Jon Kaase Boss 429! 273/265 @ 050 RP cam. Another Boss 429, RP cam 254/246 @ 050; & another 429 266/262 @ 050; Ford 4.6 L mod engine 244/240 @ 050.

[2] Crower Cams entries in the EMC. Dan Crower quote: "Believe it or not, we ran 1.3 rocker ratio on the exh side! It didn't hurt power at all...we were surprised to see it. ...Our research has shown the exh presure escapes quickly when the valve cracks open, so the piston movement is simply pushing out the residual.' A Crower entry from another year, 470 BBC used a RP cam, 256/250 @ 050.

[3] Joe Sherman's winning engine, as reported in PHR April 2003. A SP cam was used 234 @ 050, 1.6 int rockers, 1.5 exh.

I think you should start designing cams. :thumbsup: