My reading on the subject suggest dual pattern cams with more exhaust were originally intended to overcome inefficency in stock exhaust systems.This theory seems to have carried over to performance cams where it may not be so relavent.
As the more performance minded of us are running headers , upgraded exhaust and / or better heads what is the advantage of a dual pattern cam over single pattern other than to bleed off some bottom end torque?
I realise most of the popular L98 / LTX cams have more exh duration but some such as my ZZ4 cam have much more exh lift (.470 / .510) where as the Hot cam ( a proven runner) is .528 on both.
With a upgraded exhaust system it would appear to me that one would be running the risk of over scavenging the engine in extreme cases.

Question.
What are going to be the different characteristics of the same engine with a relatively "balanced" cam such as the hot cam or with one with more exh event (lift x duration)?

 

Asking about cams is like asking men who the most beautiful woman in the world is. This one has great legs, but that one has better cleavage. You really need to decide what you want, legs or cleavage. Now that I've been a smart ***, cam selection really needs to take in many considerations. More exhaust duration may still be better with an improved exhaust. Duration is just one piece of the puzzle. Are you looking for bottom end power or top end. Is this to be a daily driver, hot street, or track. Manual tranny or auto. Rear gears come into play. How much are you willing to do with computer turning. I'm no expert at all. I know what cams I've put in hot street cars running carbs in the past with no computer. When I build an improved motor for my 85, I'll have to ask an expert.

 

I understand cam selection.
Was just seeking opinions on engine behaviour with the two types.
Will one give say, more torque over the other at a certain rev range?
Will one give more Hp ?
Etc

 

Yea, it isn't so much in the exhaust system but in the head itself. Figure if a head has around a 80% I-E flow you can use a single, but under 80%, some say 75%, use a split. Comprende?

 

Yup. This is the basics. The SBC exhaust port on the 23 degree head sux. The large overlap is to make up for that. The more you stretch out your exhaust, the rowdier your idle will be. I would go with a good split unless you are going SCed and then I would go with a HUGE split.

 

Camshaft theories, especially for small block chevys, are all over the map. I will try to keep this relavent to our purposes and not ramble on. SBC, and many other engine families, generally have cylinder heads that will experience intake flow peak BEFORE the exhaust flow peak. This is why you will see greater lift and duration numbers on the exhaust side of a cam spec'd out for stock or mild engine combos. This is an attempt to increase efficiency and utilize every last bit of available flow on an otherwise less than optimal engine combo. When a cam company designs a dual pattern cam, they are usually assuming it will be going into a car that will be somewhat limited by its exhaust system because of physical factors and compromises caused by the chassis (NOT a racecar). Even with headers, street cars still have long exhaust systems with bends, cats, mufflers, relatively small pipes for fitment, etc. We actually have greater flexibility with our intake/induction systems simply based on their physical location and ease of access & modification. The dual pattern design seems to favor street engines that need idle stability, operate at lower RPMs, and have smaller displacement. When you have the ability to reduce the amount of restriction or compromises in your exhaust (i.e. no cats, bigger pipes, fewer bends, hiflow mufflers, etc.) you can begin to bring the exhaust characteristics of the cam closer to those of the intake side. That is, if your desire is to further increase power, you can increase the intake's lift and/or duration to match the exhaust's without adversly affecting efficiency or robbing power from exhaust 'restriction'. BUT, this can still be a 'grey area' since things like compression, cubic inches, RPM range, and streetability are still important considerations to a cams overall specs. Also, since we are always talking about ported or better aftermarket heads, we need to know head flow numbers to ensure we don't 'leave any power on the table' by going with a too small of a cam. Again, with a typical PORTED 23 degree SBC head, you still can have exhaust flow peaks at higher lift numbers than the intake flow peaks. Whether or not you can utilize this extra exhaust lift will depend on a combination of the factors I spoke of earlier. The following are GENERAL statements, so keep in mind emissions, programability, fuel consumption/quality, and vehicle use, as additional considerations and apply them as needed. We will assume we are dealing with better than stock heads with typical flows peaking around .550" to .600".
Higher compression with free flowing exhaust probably likes a single pattern. Larger displacement with larger exhaust may like a single pattern. High RPM, with appropriate gearing, may favor the single pattern. The more 'racecar like' you make it, the more you need to consider the single pattern cam.
Remember that LSA selection, installed positioning, lifter/camshaft type, and staggered rocker ratios are all useful techniques for optimizing a bigger cam and extracting a little extra power, in some cases, without sacrificing street manners or driveability.

 

Seems pretty long to me.

 

Is there a difference between 'rambling' and 'blubbering'?

 

Very small difference. Blubbering is more like crying though. Like when you forgot Valentines day or something pathetic like that.

 

No, 'pathetic' IS staying up until midnight rambling on the computer!

 

I just got home from shooting pool so I don't feel too pathetic.

Now I'm just trying to spread some "joy" around.

 

Question.
What are going to be the different characteristics of the same engine with a relatively "balanced" cam such as the hot cam or with one with more exh event (lift x duration)?[/QUOTE]

I consider the Hot Cam to be dual pattern because of its 218/228 duration @ .050". The fact that is has the same I and E lifts reflects on its intended design as a 'match' for the LT4 head, which does have a higher exhaust flow peak, but has almost a 80% E to I flow ratio. Any less intake lift would be underutilizing the LT4 head, and any more exhaust lift would require more valvespring and/or reduce reliability without enough increase in power to warrant this 'trouble'. I don't believe you would see much more power if you raised the Hot Cam's exhaust lift UNLESS you were to increase compression, or increase cubes, WITHOUT further improvements to your exhaust system.

 

great info thanks

 

Thank you 500hp , just the sort of informed comment I was after.

 

rodj,

I went through the same type of analysis. When I first put on my TFS 23d heads, I used a split pattern, 264/270 & 112LCA Comp Roller on my Xfire. I read an article by Vizard on cam selection and weas intrigued. So I sent my head flow specs to Motors and Machine to see if I was leaving any HP on the table. I had measured static compression and it was over 200psi/cylinder. This had me suspecting that I was losing something due to pumping losses and needed some more overlap. The spec came back suggesting that I could use a single pattern cam. So I installed one with 10d more overlap. Its a 270/270 with a 108 LCA. My 1/4 mile dropped by about .4ths. The M&M analysis was worth every penny of the $40 I spent.

 

Interesting info ,Dominic.
My original interest in the subject can about from whether I could squeeze any more power from my 95K L98 , basically a ZZ4 spec engine with siamised base , SLP runners and LTs .Has run 13.103 , looking for a 12.99.Am not looking to spend any money on this setup as building 383 race engine for drag only C4.
However on my parts shelf I have a hot cam and the 1.6 rockers ; others have run the hot cam with stock 113 heads with success so was looking to see if I would see any improvement with a rocker change on the intake or the complete setup.
Engine pulls strong (1.84 / 60ft ) but still suffers lack of breath over 5000 , even with a 58mm.
Cam , manifold and headers have taken it from 13.9 to 13.1 but mph has only gone from 99 to 102 suggesting a lack of top end power.
ZZ4 cam is rated 355Hpp @5200 with a carb so with TPI am prob not far off cam peak.
Any thoughts?

 

Port your heads and your base,. sounds like you cant get any air through them at RPM.

 

rodj,

My Xfire ran a 13.3 wih a 2.1 60' last October with the new cam and with 'some" tuning using a WB. The cam/head/headers change took my Xfire from 15.1 to 13.3. One thing I found was that the motor was running lean at top end so I went from 10psi to 20 psi FP. Remember I am talking about a Xfire with 80# injectors. That was also on street tires at 35psi. The low end torque makes it impossible to do anything but slip clutch at the line. A good portion of the issue building top end power has to do with the manifold you're using. Clearly I have made the decision of working with the limitations of the Xfire manifold. Knowing that, I looked to optimize the power I could generate in the sweet spot the manifold gives me. So I would never expect to have a power peak at 6500rpm using the m,anifold I am using. The TPI manifold isn't really much different from the Xfire in terms of the torque and HP peaks due to the runner lengths.
As for the heads, if you read Vizard's articles, the primary measurement for heads is the low lift flow numbers, not the high lift numbers. That has to do with overlap. The next key is the efficiency of the combustion chamber design.
Finally, tuning is an absolute to maximixe the changes. I'm hoping to see high 12's next time out.

 

Dom - Do you have additional info on the M&M analysys? eg How to go about getting one. I have not heard about before. Seems like a good investment.

 

I am more than happy with the performance to date , just looking to see if there was anything left.Thanks for info.
I shall now concentrate on cam choice my race engine
4 bolt 383 , forged Scat internals ,10.7:1 ,LT1 manifold,TFS 195 heads. Street manners won't be a problem.