327???

What advantages does one hold over the other?

Currently the cam in the car is a Comp Magnum 270HR, 215/215 w/ .532/.532 110lsa. This cam seems to act larger than it really is, it has a fairly rough idle and doesn't come seem to come alive until 2500-3000rpms and pulls to 6000.

Not really looking for a cam recommendation, just thoughts on single pattern grinds vs dual pattern grinds.

Sonix

iTrader: (1)

From what i've heard;

Stock style cylinder heads that are notoriously weak on the exhaust side (vortecs come to mind, but almost all stock castings) benefit from added exhaust flow, to help balance it out. This can also benefit a car with inferior exhaust flow (not heads, but poor headers, bad y-pipe, or gasp-manifolds!)

A single pattern seems to do better when you've got a good flowing exhaust, aftermarket heads with decent exhaust port flow, etc. F-bird'88 tells me that a single pattern will improve mid-range torque over a dual pattern.

I have a dual pattern xe268h, and notice it seems to be sluggish below 2500RPM or so, and on paper comp says 1600rpm it should be waking up.
I'm going to a single pattern on F-bird's suggestion actually, albeit a significantly larger one.

I would think your cam should be quite tame. Do you have very large ports? (that can increase the apparent size of the cam, right?)

discostu

[quote=Sonix;3508674]Stock style cylinder heads that are notoriously weak on the exhaust side (vortecs come to mind, but almost all stock castings) benefit from added exhaust flow, to help balance it out.[quote]

That's the same thing I've read...
The dual pattern was created to "crutch" up the weak exhaust port flow on most vehicles.
If you exhaust flow is more then ~80-85% of your intake flow, you should switch to a single pattern cam.
As a reference, stock Vortecs flow about 220/160cfm, which is slightly over 70% exhaust to intake ratio. This would necessitate a dual pattern cam. The worse your ratio is, the more exhaust duration you need.

327???

I have Trickflow 195's, at .500 lift they flow 253/177, giving them a 70% I/E relationship. If you look at .600 lift it's a bit better at about 75%. In the lower lifts it's about 70% the entire way up.

My exhaust flows well, longtube headers, 2.5" duals, straight thru mufflers.

With this cam/engine combo and the stock converter it would barely spin the tires.

Damon

Doesn't surprise me too much about the behavior. It's got a relatively tight 110* LSA, for one. Plus, roller cams almost always have less .050 duration than a flat tappet for any given advertised duration. Nothing wrong with that- rollers are more sluggish than flat tappets under .050 but after that they blow away flat tappets for valve acceleration and lift.

The direct face-off for your roller cam in the Magnum line would be the Comp 270H. It's got 224*/224* .050 duration for the same advertised duration as your roller cam (270). The fact that you've got almost 10* less .050 duration doesn't tell the whole story, though- the roller cam will still have considerably more "area under the lift curve" than the flat tappet cam becuase it moves the valve like a bat outta he11 above the .050 lift point.

However, advertised duration still has a significant effect on idle quality, vacuum, etc. So it's still going to idle and pull vacuum similar to the flat tappet 270 Magnum cam, which I have some personal experience with. And yes, the idle with the 270 Magnum is definitely a bit "choppy" and vacuum levels are nothing like a stock cam. Not the kind of cam you want to run against a stock stall converter. A 2500 stall converter would be the minimum I'd recommend, with 3000 preferred.

discostu

That's one of the biggest mismatches I've ever heard of.
That stock converter is killing your performance. You said it yourself, the engine comes alive at 2500-6000, but your stall is sitting at 1200-1500rpms I'm guessing. That's over 1000rpm where your engine is lugging hard against the converter, no wonder you can't spin your tires.
How about your computer tune? That would be my next guess.

Orr89RocZ

iTrader: (20)

stock headed LS1 guys are seeing good results with reverse pattern cams...with more intake duration than exhaust..i wonder if that would work for us, even tho the exhaust port is weak. but then again ls1 heads onthe exhaust are still fairly weak compared to the intake.

i've heard larger duration cams keep cam overlap higher which means rougher idles and more revs/high end power related

F-BIRD'88

The engine does not know if you bought a dual pattern cam or single pattern cam or what ever. it only sees the valve timing. It wants what it wnts for best overall power. Not what looks good or popular on paper.
Most of the catalog cams listed on the cam companies catalogs are "'dual pattern" with more exhaust duration than intake duration cause that is what the most customers think they need. UUUUUh I got a cam that is "not too big", yet "its a little bigger/better" cause its got extended exhaust duration.
The added exhaust duration may extend the usable power band a bit and looks good on paper, but at the cost of some midrange torque usually. Works good on very small mild cams on near stock motors with manifolds and restricted exhaust where there is little or no real scavaging effect from tuned headers to pull the exhaust out.
Marketing has a lot lot lot to do with what is promoted in print by the camshaft companies. Not every motor is cammed right with a "dual pattern" cam ground on 110 LSA. The intake lobe sets the basic rpm range. but the exhaust lobe, its timing opening and closing is critical to the overall effect of the headers on the motors torque curve. Sometimes a motor with a free flowing exhaust system with specific header tuning actually wants less exhaust duration than intake duration to avoid overscavaging.
sometimes, less is... more.

A popular cam like the Comp XE 268H10 (224-230@.050) is good example. it is promoted as being the "largest cam for a stock converter". In reality the torque is soft @ and below 2500 rpm. it really gets busy at 2800rpm Just like the single pattern 280H (230-230@.050) magnum. In reality both need a 2800 stall in a 350ci application. So why not just get the 280H magnum single pattern cam and get the needed 2800 stall and really go fast.
Mean while if you really wanted a cam that really does work with a "stock stall" the 268H magnum/hi enery grind (218-218@.050) really does have the torque to work with a stock stall especially when ground on 108 LSA and advanced a bit (103-104intake c/L) now you really have a cam that will pull at the critical 2200 up rpm point.

But most people buy a cam based on what looks right for them on paper. Comp cams is in the business of selling lots of off the shelf camshafts to pay for advertizing so what ever does the best marketing job, gets promoted as the latest best grind of the day. Not what the motor actually wants for best overall power. The people that build and test custom built motors on a dyno and in the car, know better. they want and need a custom ground camshaft. Their business volume is a lot lower than the masses of people who pick a cam based on magazine articles, so you the average buyer actually subsidize them allowing the cam companies to offer real custom ground cams that make more dyno tested power at a reasonable cost.
So keep on buying those HE XE VUDUU truk pawmax dual pattern 110 LSA cams. the custom engine builders/drag/dirt racers are depending on ya.

327???

About the converter, I know it was a bit of a mis match, but the new one made a big difference, it flashes to about 4000. It might actually be a bit large. It knocked .3 off my ET and the car still has a bunch in it.

Bet run was 12.6 at 108 with the car weighing 3320lbs going down the track.

Edit; The car has a perf rpm air gap and a holley 600 and 750 both vs(i have both carbs but am having tuning issues with the 750 so i put the 600 back on to finish the year out)

327???

I guess the next question would be; how "good" does an exhaust have to be in order to take advantage of a single pattern cam? ie. my exhaust? (yup, this is where I start looking for cam recommendations)

I've been looking around at various custom cam grinders, does anyone have any opinions on various places?

Anyone know how much cammotion gets for one of their custom grinds? I don't think it said on their site.

Just for the heck of it I went through cammotions lobe catalog and spec'd my own custom cam. 225/228 at .05 .553/.563 w/1.6's 108lsa 105icl retarded 4deg. I plugged it into dd2k for laughs and it made 30 more hp than an xr276 with the same tq and 80 more hp than my current cam.

F-BIRD'88

desk top dyno will not model your motor with you exhaust system close enough to pick the right LSA or exhaust duration that is best for your car. It only models a fixed unspecified (optimum exhaust design) that has little to do with your car.

Most engine builders that dyno their stuff will tell you that a typical hot rod SBC with open long tube headers will want a cam with near equal or near equal exhaust and intake duration and a tight 106 to 108LSA and a some dialed in advance. This makes the best fat torque curve with good top end power in the usable rpm range. used on most auto bracket racer and dirt cars for racing.

When you add a muffler to the mix you change things. Much of the scavaging effect of the header tubes that helps power is now dampened by the mufflers restriction. The motor wants less overlap now. the natural pressure difference that causes air to move thru the motor at overlap is upset.
So ya got to change the exhaust timing to make th best of it. (reduce overlap and increase the exhaust opening point to take advantage of the exhaust blow down when pressure is high in the exhaust and cylinder.
(Wider LSA and longer exhaust duration)
Airflow is always controlled by relative pressure. Air always move from a high(er) pressure to a low(er) pressure.

The exact exhaust timing events that are best for your motor with a closed exhaust is very hard to nail down. But as a rule a single pattern cam of the right duration usually results in the best torque curve (not peak hp) even with a typical but decent closed exhaust system with headers.
You might want to open the LSA a few deg to 109-112.
Wider LSA has the benefit of idleing nicer and cruising nicer and giving better fuel economy at part throttle tan say a torque making 106 or 108LSA.
it is always a trade off when picking a cam that is best on the street.

F-BIRD'88

will you ever be running with open exhaust headers?

327???

I always look forward to reading a post by you, give me a little while to read and reread what you've written and I'll be back with some more questions.

BTW, I believe dd2k about as much as my 8yo cousins cam advice.

Edit: no, i never really plan on it. but I MIGHT take the exhaust off once in a while

F-BIRD'88

ddyno is much more accurate if you use the true seat to seat timing events of a cam. that unfortunatly is neither found by the advertized duration or can be accuratly calculated from the .050" dur numbers.

You want actual net "seat to seat at the lash point +.004" A cam doctor camshaft measuring tool can get you these numbers.

Its also much more real world if you approximate the actual cfm airflow that the motor uses. Not the the carb CFM rating. A typical performance 350ci SBC that makes +/- 400Bhp and favours a 750cfm carb,,, uses around 550-570cfm of air. try it. Once you enter the numbers like this the accuracy is very good . eg: even if you swap ona tunnel ram and two 660cfm carbs the motor doesn't use 1300cfm of air. it might pick up 30cfm. when you add 30cfm airlflow to 550cfm when jumping up to a tunnel ram and twin 660cfm carbs from a single carb and dual plane manifold , you get the real world expected power increase that such change actually creates.

How do you determine how much air your motor actually uses at WOT? you can use a vacuum guage at WOT max rpm and by math and knowing the rated cfm of your carb @1.5" vacuum and knowing the actually manifold vacuum on a guage at WOT and 6000rpm, (it will be between .25" and 1.5" on the guage) a .75 to 1.00" reading is good for best performance. calculate the actual airflow of your motor. now punch that number into DDyno and play with cams etc etc.

But treat it as a learning tool to show trends when stuff is changed around . it's still not exact.

F-BIRD'88

Whoops wrong thread