Comparing single and dual pattern cam.

I'm not asking what is better, I am asking how one would compare the two. From what I have read, a dual pattern cam is preferred if you need to make up for poor flowing heads (right?). But, if you have heads that flow very well, would the longer exhaust duration work against you?

My 393w has 10:1 compression, Edelbrock air gap intake and a set of AFR-205 heads that have had a little work. The cam is a hydraulic roller 232/240 @ .050" .535/.545-112'. Now, if I were to pick a single pattern cam with similar characteristics, would I get a 232/232 @ .050"? Does the longer exhaust duration affect the power band or just help with scavenging? Thanks..

Mike

I think comparing the two is really quite difficult. The lift and duration numbers mean a lot, but they don't truly state the profiles of the cam.

As I understand it, and my personal experience is limited, a dual pattern cam is usually recommended to make up for poor exhaust flow rather than poor overall flow.

My thought is that eliminating the dual pattern in your specific instance will probably improve low and mid range performance, but may make the engine's octane requirement higher by increasing the dynamic compression.

So to answer your question, you would probably benefit from taking to the manufacturer of your current cam as they will have ALL of the data on your cam, not just the peaks.

I can't answer specifically from a Ford standpoint but because I've never built a 351 but a sbc similar to what you describe would be a fantastic street engine. Peak power would be 500 +/- at the flywheel and with a bit of a stall (or decent gears for a stick) would be very happy on the street. The longer exhaust duration is not to make up for poor flowing heads . It is done when the ratio of intake flow is higher than the exhaust flow by more than a percentage that I cant' remember at the moment. IIRC it's around 80%. It is very common for heads (old or new) that fit domestic V8s to respond well to a longer exhaust duration. When a supercharger is added the split can be by more than the common eight degrees for N/A engines.

This is a complex subject and I don't pretend to be an expert but as I understand it the intake duration sets the peak HP rpm. There is a certain amount of exhaust duration required to empty the cylinder. A poor flowing exhaust port requires the exhaust valve be open for a longer duration to achieve this than a good flowing one. In addition, it is desirable for the exhaust valve to remain open long enough so that cylinder filling can be enhanced by the pulling effect of the exhaust (scavenging) on the intake track during overlap.

In general, torque production goes down the longer the exhaust valve is open. This is compensated for in part by reducing the intake duration. Longer exhaust duration can also help hp to hang on longer at the power peak.

So if it is determined that an intake duration of 236 @ .050 is desirable for a given combo but that the exhaust port needs some help, one might use a split duration of 232/240 for the reasons stated above.

Just to throw this into the mix. I have seen 4 pattern cams being offered. The reasoning behind this is that the outer intake runners run leaner than the inner 2. So leaving the outer valves open a little more gets a better charge into them.

as mentioned, way too complicated but in short hesds that have more than around 25% difference in flow between the intake and exhaust will provide more plwer with a dual pattern cam because the longervduration allows the exjaust to get out and as the exhaust exits, it helps pull in the intake charge.

generalky, umless you have afr heads, s dual pattern cam is probably better . . that being saifd, even with dual pattern cam, the adr heads will make big hp thru the entire rpm range.

call bullet cams, they will spend some time on the phone with you.

if you have poor exhaust flow the comp xe series cams often work well . . the same person thsat designed the comp xe series, designed the lunati voodoo series.

Ok. Thanks everyone. I'm trying to come up with a plan for an engine and didn't know if (having good flowing heads) if the dual pattern cam could actually work against me.

The AFR heads shouldn't require a dual pattern cam. Some engines, like the 4V Clevelands can benefit from dual pattern cams at higher RPMs, due to the tight bend after the exhaust valve. This is one of the reasons that reduces the exhaust flow velocity.

Most engines have smaller exhaust valves and exhaust ports, than on the intake, but this is because the intake side is operating on atmospheric pressure (unless it has forced induction) and the exhaust side is operating on the combustion and moving piston pressures, forcing the exhaust gases out much faster than they entered on the intake side. The smaller valves on the exhaust is not normally a reason for a dual pattern cam, unless the intake to exhaust size difference is extreme.

Ok. Thanks everyone. I'm trying to come up with a plan for an engine and didn't know if (having good flowing heads) if the dual pattern cam could actually work against me.

Click to expand...

the best plan is too tell us what you want and we can confuse you with different suggestions but in answer to your question, no, a dual pattern cam will not defeat the purpose of the afr heads . . again, its complicated but that being said, i wouldn't get one with a big spread like 10 degrees or more . . also, in general, all other things being equal, a cam with 112 lsa will have slightly milder idle, and slightly wider but "softer" power band than one with a closer lsa like 112 or 108 etc . . 110 is the most common for street perf cars but is rarely used in fuel inj computer smog cars because it causes probs with the computer etc . . the 112 is second most common and is more commonly used in computer smog high perf engines . . i would stick with 112 or 110 lsa.

I am looking at another Dart block Windsor (427 or 438) with 10:1, AFR-205 or 220's with a Trick Flow EFI box style intake, TKO-600 5-speed and 3.50:1 gears with a TrueTrac. Since it will be a street car with power brakes, I'm thinking of a cam around 236 intake duration @ .050". I figured this should make more power than I need and still be plenty streetable..

I am looking at another Dart block Windsor (427 or 438) with 10:1, AFR-205 or 220's with a Trick Flow EFI box style intake, TKO-600 5-speed and 3.50:1 gears with a TrueTrac. Since it will be a street car with power brakes, I'm thinking of a cam around 236 intake duration @ .050". I figured this should make more power than I need and still be plenty streetable..

Click to expand...

comp cam xfi236hr hydraulic roller cam . . if you want a fairly nasty idle, i would get it ground on a 113 lsa for efi or a 112.5 lsa for a carb . . the shorter lsa makes it harder to tune the idle if you have an efi . . either way, it will get you around 500 crank hp on most dynos with decent bottom end . . this is a fairly unknown cam . . i would also run this cam with 1.65 rockers instead of 1.6"s for your app.

with your trans and gears it will very strong, however, this being said, if you want it stupid fast, use 3.73 or 3.91 gears . . it will run in the 10's and the 5th gear will still be fine on the freeway . . the 600 trans also have 2 options for 5th gear ratio . . unless you drive a lot at 75 mph or more, i really wouldn't run 3.50 gears with that set up . . imo, opinion they are the numerically lowest gear that should be used in your app.

another option is a custom cam from someone good like cam research.

you also need the R box intake then with a 90 mm throttle body . . probably need a big air flow sensor and around 42 lb injectors too but an efi guy can help you better with that stuff . . i would run the efi slightly rich like around 12.5.

run as much compression as you can . . it will likely end up around 11.0:1

you may still need an auxiliary vacuum can with this particular cam.

the 205 heads are plenty big for your app and this cam etc . . the 220's are for massive hp and will cost you some bottom end power . . although bigger is often better, it isn't always better.

.

.

running a stroker mellows out larger cams. You can use more lift and more duration with the set up you describe and I think 3.50's will be fine. Your car limits your rear tire size and height and at that power level, hooking up is going to be something that requires big sticky tires. With 1st gear ratios combined with a 3.89 (3.91 gears are nearly impossible to find) and a tire limited to 28 inches in height and about 11 inches in width, any more gear and you'll never hook up.

Look at my profile and see what I've built and done and what I am running-I am not just guessing

the bigger the engine, the more it will "melow" out a cam, its not specific to strokers.

that particular series of cams is a bit odd, and on the surface their specs might belie how wicked they really are.

if the tires spin you simply buy high traction tires and install traction master or cal trac traction bars and toss a few sand bags in the trunk etc . . even put the battery in there if you want.

i see little point in building a high hp engine if one does not utilize the power by using gears that are best suited to the app. . . that cam and combo wants 3.73 gears, its not a guess, although it will "work" decently with 3.50's . . that being said, i personally would not use anything numerically higher than the 3.70's because of how i drive now and if you use 3.89 or numerically higher, first gear will be a bit short imo but i have seen guys run a t5 with 4.11 and say that 1st is not too low but they have a totally different view point than i do, lol.

gears are no prob, my friend sells them.

3.70

http://www.currieenterprises.com/cestore/Product596.aspx?id=1867

3.89 plenty close enough to 3.91

http://www.currieenterprises.com/cestore/Product596.aspx?id=1843

I'm running caltracs and Nitto 315 extreme drag tires and at 19 PSI, I still have traction issues, though they are manageable. I'm running a T-45 conversion. Anything bigger requires tubs and that means some serious and expensive modifications to the car that are very hard to reverse. 3.70 might be a good choice for his build, but usage is paramount to the decision. If the car is to be driven primarily on the street, limiting the power to match the chassis' capabilities is pretty important. Without extensive mods, our cars will not put the power to the road at levels of 450+ RWHP. So you end up with an expensive tire burning machine.

I talk about strokers mellowing cams as the cams are often advertised for stock displacement.

Of course I do not follow my own advice and choose a cam that is one step hotter than I can ever imagine needing

i agree 100 percent with your traction comment and your low first gear comment and i have doen similar things as you by putting in a bigger cam than i knew i needed lol . . i know that some people prefer to be able to roast their tires just because its fun and they arent concerned about shaving 2 tents off of their quarter mile time . . i fall more into this category nowadays especially knowing that i might be more inclined to get into trouble if i had a high hp car with high traction so this is mainly why i mentioned the 391s.

i have also actually used sand bags in the trunk in the old days when the best tires we could buy were bf goodrich ta radials, lol.

Thanks everyone.. Even though I was looking to gain power all the way around, this is a 100% street car. I have the 232/240 cam in my 393w and I wish it had more in the bottom end. I have the TKO-600 and I chose the 3.50 gears for the freeway. I drive my cars on the freeway regularly. I figured a 427w would get me all the bottom end I could need. Maybe I'll look at the 427w with the that I have. Anyways, thanks again..

Simple

dual = 4V heads. Ford designed the 4V to have wide power band they are somewhat unique standard hop up techniques and logic do not apply. The exhaust valve port is only part of the reasoning there is much more.

Single = 2V heads.

? ok.. I appreciate the info but the more I try to understand it, the more confusing it is..

According to Dan Jones and a few other's... 4V heads have very large ports. The intake. The 4V exhaust port has a hump then a sharp drop with an exaggerated area change. The port exit area is very large but much of it is wasted and the flow velocity low. The resulting poor intake to exhaust flow ratio requires a cam with additional duration (typically 10 degrees or more) and lift on the exhaust side for best results. Reason 4V heads work best with a dual pattern cam. " Pontiac Motors need a dual pattern cam due to the same reason.