brad8266

iTrader: (23)

Cool, and your explanation seems to be on point. You take a single blower that flows x CFM. It will flow that CFM regardless of whether that air charge makes it to the combustion chamber or not.

With the smaller cam the air charge will not make it to the chamber, which results in higher manifold pressure, while the larger cam will allow that same charge to make it into the combustion chamber due to holding the valves open longer, which results in lower manifold pressure but yields more power.

Thye general idea people have is that a bigger cam will always have a lot of overlap, which in general is not wanted in FI applications. This really isnt true. You can have a large cam ground that does not have much overlap, you just have to understand cam design a bit.

Too many people are always concerned about boost numbers, like thats what makes power. Its all about getting as much air into the combustion chamber as possible, not making boost.

obZidian

iTrader: (2)

exactly!!! now, this manifold pressure you mentioned, this is the same as the boost pressure seen on the gauge? Just want to connect the dots....

"my explanations seem on point" well, im no expert so i dont think im quite there yet... but thanks!! LOL

Got Me SOM

iTrader: (2)

i think actual testing needs to be done to verify these theories.

I'd gladly lend my car to a shop to have the testing done. (and i get to keep the cam that works the best )

WAHUSKER

iTrader: (4)

Mike made a good point about overlap with blown engines. Overlap will bleed off boost. I could make more power with a 117 lsa cam, but I like a little chop in my idle, so I made an informed decision & went with a 115.

NA cars run big heads & cams & spin high to make power....and they do this by moving more air. A boosted car doesn't "need" that much cam because we are forcing air in. That said more cam can make more power IF you spin it high enough. But it's all about matching the cam to your setup. Too big is as bad as too small.

sr71

iTrader: (17)

this is correct. when you want to make BIG power at higher rpms, you use a BIG cam.

my car has a big cam to shift my powerband up to match the higher boost threshold of my big turbos. it doesn't make sense to cam the car for low end when I don't see boost untill 3500. that would be a horrible mismatch. let's all remember that the entire combo is what counts...not just the cam.

I think you should cam the engine for the rpm range that you intend to use, THEN add fi. the only concession for fi that I would make is a wide lobe seperation.

and Vince, Phil has a larger cam for higher rpms and/or larger engines.

Got Me SOM

iTrader: (2)

i think everyone (including myself) is getting confused with duration and lobe separation angle when it comes to forced induction.

what do you mean when you say BIG cam?

some of the gn guys run 110 lsa's on their turbo engines so I dont necessarily buy the higher the lsa the better.

i think its all in where you want your the meat of your powerband to be, where you want your peak numbers to be, and maximizing the efficiency of your particular setup.

I definitely feel that you can't generalize and say to use brand x cam for turbo and brand y for superchargers. It has to be matched, to rear gear, torque converter, what heads your using, what your goal is with the car be it a street car or all out race car, etc etc. (I wonder if Comp Cams is a member of this site and can shed any info)

this is a good discussion, the only way to know for sure is to have two ls1 mules, one turbo and the other supercharged and try out and see which cams really work.

brad8266

iTrader: (23)

yeah manifold pressure = boost

brad8266

iTrader: (23)

You are following the misconception that low LSA numbers = more overlap, which is a false idea. A 110 LSA cam can be ground to have very little overlap. Overlap comes from a combination of durations and LSA, not just LSA alone. With that said, you can have a good FI low overlap cam on a 110 LSA.

Also when you run major power and big blowers/turbos you can afford to bleed off a little boost if it will help you in another area.

Read over the threads about cams that Patrick G starts, he is like the cam guru around here. he has a lot of great info on cams such as overlap, duration, and LSA.

brad8266

iTrader: (23)

"Big Cam" usually refers to the higher duration cams.

Got Me SOM

iTrader: (2)

right i agree with ya, you can come up with the same cam running a different duration and different lsa.

its confusing but makes sense. it would be cool to see some real testing though to prove the theories.

I spec'd my cam for my car 224/230 on a 115, at the time I thought I picked it right but now I'm not so sure.

brad8266

iTrader: (23)

Valve effects on boost are really not confusing.

Take a fan(blower) and connect it to a piece of pipe and put a cap(valve) at the end of the pipe to seal it. Now turn on the fan and cap the pipe closed for 1 second and you will have a slight amount of PSI built up in the pipe. Now turn the fan on and cap the pipe again and leave it capped for 1 minute and then turn the fan off. Now you will have a lot more pressure built up inside the pipe compared to when you left it capped for only 1 second. The amount of time that you left the pipe capped is the equivilent of a cams duartion. More duration = less boost which will yield more air in the combustion chamber to be burned.

Thats is the basics of valve events on boost.

brad8266

iTrader: (23)

Good discussion going on here!!

Got Me SOM

iTrader: (2)

absolutely this should be made a sticky

engineermike

Probably because most people can reach their goals with small cams and FI. Hey, if you can get in the 9's at 140+ mph with a smooth idle and you're not interested in going any faster, then why switch to a larger cam.

I had an interesting conversation with a local turbo guru tonight. He races a car that runs in the 8's with twin turbo's and a blow-through carb. At one point, he swapped from a 242/242-114 cam up to a 259/259-114 and, lo and behold, the car ran faster with the bigger cam at the same boost level.

Mike

andereck

Stop it Mike, you're just going to screw up what everybody knows as an absolute with that stuff. You can't run overlap with forced induction. Why you'll lose all the boost out the exhaust....lol.


and this particular discussion is not sticky worthy....least not yet.

brad8266

iTrader: (23)

Agreed

brad8266

iTrader: (23)

Did he have to adjust the wastegate to maintain that same boost level after the cam swap though? Or did it hold the same boost level after the swap?

Most of what I stated already was based on a supercharger since there are a lot of different variables that affect turbos such as exhaust velocity, which a cam can have affect on.

engineermike

He didn't say, but my guess is that he did have to adjust them. Every time I change any part in my turbo system or engine, I have to re-adjust the boost controller. If it were electronic, you wouldn't have to.

Mike

obZidian

iTrader: (2)

hmmm..

Got Me SOM

iTrader: (2)

you seem to know a lot about supercharging, especially since you work for ATI, how bout you share some of your knowledge and shed some light on the topic with us?

lets clear up the mystery.