LOnSLO

iTrader: (15)

First, this is not a "how much power" question. Let's start with a head that flows say 250cfm on the exhaust port and 300 on the intake. Then, throw in a cam with say a 230 degree intake and exhaust duration. Assume this is on a bone stock LS1 short block. Is 230 degrees exhaust enough to both purge all exhaust gas, AND bring in a 100% clean charge of intake? How can we figure exactly what is enough exhaust duration for a given combo. IMO, a slight amount of unburnt fuel and air should be exiting the head at WOT. This means the intake charge will be 100% fresh air. I'd just like some experienced opinions on this. If this is not enough info to make a suggestion, then I will throw in some more specs. FWIW, this is NOT my combo. I just want to hear some discussion on theory.

703crewchief

LO,
I have a cam with 230 degrees duration on the exhaust side. The head's exhaust port flows at about 220 cfm. Also we are using Dynatech step tubes. The engine makes no torque, I think that the exhaust is actually pulling too much of the charge out of the cylinder! This setup was put together for nitrous, and runs like crap straight motor.

LOnSLO

iTrader: (15)

Interesting. These kinds of issues are what I want to focus on.

Cal

iTrader: (1)

You can't just make a statement on how many degrees are needed; it depends on what rpm you are running at, what the intake duration and resulting overlap is, and whether you have headers. It also depends on whether you are going to run NA or FI.

blazinredls1

Im pretty sure lonslo is talking about running NA and im sure if he was talking about that big of a exhaust dur. on a cam, hes gonna have headers, but im just suspecting here.. GOOD post!!

LOnSLO

iTrader: (15)

I realize there are many variables here. I'm not looking for an exact number answer, but rather some opinions on how much duration is useful, and how much is too much. Forgot to mention overlap in above post. Let's assume it's on a 110 lobe seperation. Just use the engine model above to create opinions.

h53speed

Is this for an all out race application? If so there is a place featured in a magazine I have that takes into acount head flow #'s, displacement, compression, air/fuel and some other factors and can determine the best cam for the engine. It can also factor in nitrous bias. I guess that this computer program is about as accurate as 2500 bucks R & D or so the magazine states and it only costs $40. That would probably give the best answer.

-Wes

Cal

iTrader: (1)

When I mentioned rpm, I meant you have to look at the rpm range you need to run at. I used to run a cam with 270 degrees on both exhaust and intake. It did not like to run below 2500rpm and had to idle no lower than 1200rpm, but it pulled like crazy beyond the safe rpm of the engine I had it in. I have a engine in my RC helicopter that idles at 2500 rpm, and makes tons of power right up to it's 20,000 rpm red line. See what I mean? You can use a lot of duration but it changes the useable rpm range.

chino_man279

iTrader: (7)

What affect does increasing the exhaust duration have if keeping other things (intake duration, lift, LSA, etc) the same? In terms of idle, power band, peak power, flatness of power curve....

LOnSLO

iTrader: (15)

OK Cal, let's give it a usable rpm range of 5000 to 7000.

Vents

iTrader: (9)

Lonslo,

spending 30 mins or so with the guys at comp will solve your inquiry completely. first of all, the problem with most of us is that we tend to apply our "SBC" inclined experience with the LS1 motor. this is bad. in stating that i mean, a conventional split on these motors is not a good idea specifically because the head flows totally diffrent than the conventional SBC head that most of us are used to working around. this is not a bad idea but it IS not the best idea for an optimum result. sure the car will work, but will it work at it's maximum potential all motor? no. as you mentioned, a standard split is one that will begin the scavenging effect sooner than a single pattern. well thats fantastic, but the exhaust ports on the LS1 flow so well, that when your scavenging begins, inbetween the overlap, your actually pulling out un-used air and fuel! that's why alot of the motors that have fully ported exhaust runners make less tq percentages that many stock headed motors. it's crazy but look at #'s nitrous motors w/ stock heads vs ported headed cars are making. sure many other variables come into play here also, but think about it thats the basis of the problem! thats why you see so many companies going to single patterns and even the reverse splits. these cars dont need alot of cam, thats why you dont see alot over the mystical "230" @.050 number. my personal choice was a standard split because i planned on running on the bottle, and i also compensated in my head work. i hope that helps at least some.

LOnSLO

iTrader: (15)

That does help Vince. I have talked to several well known head porters who tend to agree that the reverse splits tend to work better as these heads have excellent exhaust flow. During the past few weeks, I have been comparing two different cams on identical cars. First car has headers only and a 230/236 .592/578 112 cam. Second car has headers, a TB, and a 230/226 .591/.569 112 cam. Both cars went to the dyno untuned. First car put down 392rwhp. Second car put down 394rwhp. Not any difference at all, power wise. The 236 exhaust cam lopes a lot harder at idle, and is harder to keep idling. The 226 exhaust cam has a lot less lope, and is easier to start and keep running. Remember, they are untuned as of now. I will be comparing numbers and times on both cars in the future to try and get a solid answer on which works better.

h53speed

The highest hp and tq 346 LS1's that I have seen in the last month are all standard split cams. Look at what cartek and LG are putting out these days. Those are standard split cams and they are putting down some serious numbers. More than any reverse splits I have seen.