i have a comp xfi 292 that i will be running in my 383. i was considering advancing my cam 2 or 3 degrees for a little more bottom end. will this help shift my powerband down a little bit? i dont really want to spin my motor to 7k if i can help it.......and i am looking for as much bottom end as possible. what do you guys think?

i am NOT interested in buying a different cam. i just have a question about fine tuning this one. thanks.
_Nick

 

it can shift the curve some...it can also increase the chance of kissing a Piston with an intake valve too. IIRC, that is a pretty big cam, so that means you really need to be careful.

 

The only way to know for sure if you will have interference problems or not is to install the cam advanced and then measure the PTVC

 

If you want more bottom end, go with a smaller cam.

Most of Comp Cams grinds are ground with advance already into the lobes, by the way. Call them and ask.

 

probably won't run into PTV with a 113 lsa dude

 

On a cam this big...I wouldn't trust that.

 

its not an ls1... a stock lt1 doesn't have PTV issues until the duration gets well above 250. much less on a 383 with an aftermarket piston with bigger valve reliefs.

 

Do you speak from experience?

 

/thread

 

It's not a stock LT1 though, you said that yourself.. what if he has the block decked and gets the piston up out of the bore...

IMO the only way to know for sure is to check, and why the hell not if the engine is setting there on the stand. 20 mins to know what your PTVC is, is worth it to me.

 

i have a set of 7/16 stud 1.5 ratio rocker arms i have considered using instead of my 1.6's. i will be clay checking the ptv clearance when i get that far anyhow. my main concern is that i have the 918 beehives and i am worried about floating them. will the slightly shorter valve lift with the 1.5's help with how hard this cam likes to rev? or is that in the duration of the cam? if im thinking correctly, the shorter rockers will only affect valve lift and not duration. that is why im asking.

if it matters, the heads are fully ported lt1's with 2.02/1.6 stainless valves, milled .03. the intake is also a ported stocker. the block is zero decked with flat top mahle's.

 

Few years back I remember one of the Impala guys having to fly cut pistons in a "stock shortblock" LE2 setup, I suspect that "stock shortblock" had been decked.
Just a few months ago a guy had a set of LE heads and cam and did not check PTV and it took out the shortblock in 20 miles.

These were NOT 250degree setups, both stock shortblocks.

Point being, if there is any doubt, measure.


To the original poster, maybe you can find someone doing a stroker build from a heads/cam setup that would swap you? Their old 350 appropriate cam for what you have.

 

i have a 3.75'' crank and a 4.030'' bore.

 

I looked at your sig but missed what you clearly posted in the body of the first post.

Even more reason to need to check PTV especially since the rotating assembly and block have been messed with.

 

If you don't know, then you don't know.

So I agree that it's best to check. I mean, "why not?" since it's so simple to do. BTW, I prefer the clay method, too. i remember I use to "borrow" some of my kids' 'Play Dough' years ago, LOL.

Comp, Lunati and probably others tend to agree that changing one full rocker arm ratio effects duration by about 3 degrees, give or take a degree. This comes up pretty often during interviews with their engineers.

I use to play around with advancing and retarding cams years ago on different cams in different engines, both street and racing. I found a movement of at least 4 degrees was needed for me to see any real difference.

Here's an example, which you can skip reading if you don't like reading stuff:

Many years ago I built a 8 second quarter mile 468 BB for a team using what was then a new lobe profile from Crane - An inverted Flank Roller.

Cam made the engine sing, but after a couple of passes the driver, Skip, came to be complaining that the engine went flat at the first MPH light. He'd prefer to be able to drive out the back door in case he had to get back.

So even though I had degreed-in the cam according to the cam card, I retarded the cam 4 degrees; Jesel belt drive. After a couple of more passes he came back reporting that the car was now softer out the gate but continued to pull through the ET lights, but he still wanted more in order to pull past the last stripe.

After some back and forth, I finally convinced him to leave higher to help the launch but moving the cam phasing any more than I already had wasn't a good idea. I was concerned because when I referenced my book on his engine to see what PTV it had, I was concerned about not having enough clearance with more retard dialed in. So we left it like that.

I realize this example had me moving the cam in the opposite direction from what you're considering and that I went 4 and not 2. It's just an example of how re-phasing effected how that car ran.

Hope this helps.

Jake

 

You're not serious are you? I wouldn't even considered running those springs on my little xfi cam because of the aggressive lobes.

Have fun grenading your top end

 

That is a VERY good point. Those are the springs Comp recommends along with their WAY too low 6200rpm max for that cam.

Going to need a good bit more spring.