dmoor88

who's running this combo? anyone using 1.7 rockers on the exhaust side for scavenging?

lt4 fd

EXACTLY what I was going to post before I scrolled down a tad... really like some info on this. Any head or cam gurus think that the 1.6/1.7 I/E would make more power and or torque than strait 1.7's?

dmoor88

I know that the exhaust ports on LT1's leave something to be desired and thought that this would help with it. I know there are a few guys doing this, where are they?

TransAm2k4

How can you run 1.7 on LT1. Is there any problems beside having to swap springs for more lift. What about piston clearance with stock pistons, rods, crank. What is the max lift that the stock pistons ... will permit?

dmoor88

depends on what cam you use with it. may be a good idea to go with composite LT4 valve covers to clearance the 1.6 and 1.7 NSA rockers.

depending on what cam you use, you may have to get shorter pushrods.

TransAm2k4

I'm running a cc306 and tall valve covers that i even had to grind the alternator to fit.i was doing the math and with the 1.5 my cam lift is .510 and .540 (what i'm running due to stock springs) 1.6 .544/I and .576/E with the 1.7 everywhere it would be .578/I and .612/E. If i would run 1.6/1.7 it would be .544/I and .612/E In my case would it help? Or is my cam already grind to compensate?

dmoor88

some people say the cams we're speaking of are already ground for extra exhaust scavenging, but the question is, are they ground enough or could they use some extra help?

what about 1.65 rockers?

Jon 97TA

from what I've read, increasing exhaust lift with a larger rocker ratio, doesn't add very much power, like increasing the intake lift with a bigger rocker does, and can even decrease power especially with a cam that has so much exhaust. If you already have 1.6/1.6, I would say it wouldn't be worth it to switch to 1.6/1.7, but you never know.

dmoor88

I know there are many who say just go with a larger lift cam with 1.5 rockers, which makes a lot of sense, but Im still real curious...

Jon 97TA

I meant it would probably not be worth the money to go to 1.6/1.7, if you have 1.6/1.6 now, and the 1.6/1.7 might make less power than 1.6/1.6 anyway. I read this in David Vizard's cam and valvetrain book, if you wanna check it out. It's a pretty good book.

dmoor88

I know what you meant. I just know the LT1 exhaust ports are 'underpowered' and could use some assistance in scavenging exhaust.

I heard where the intake is open a certain amount and if you change the exhaust to open farther than the intake, than you'd lose power, but is this proven somewhere?

Jon 97TA

I probably couldn't explain it, but you get a bigger effect by increasing the exhaust duration rather than lift, which is why the exhaust duration is so high on the 306, as it is probably for stock heads, or heads with a similar I/E ratio. It also depends on the setup, with yours you might gain 10 hp, but my understanding is the majority of the time you'd lose power, but there's only 1 way to know for sure 100%.

The closest thing to proving this would be: http://www.amazon.com/exec/obidos/AS...469763-8731856 look in the chapter about rockers.

dmoor88

how about this. if the cams mentioned already have extra 'built in' exhaust scavenging, what about putting the 1.7 rockers on the intake side and 1.6 rockers on the exhaust?

Jon 97TA

That might be worth a shot. Get 8 1.7 RR's used from ebay or somewhere, test em back to back on the dyno and sell whichever rockers make less power and check for P-V issues just in case.

dmoor88

not a bad idea. changing rockers doesnt take that long, it would almost be worth dynoing with 1.7 on the exhaust and then 1.7 on the intake, leaving 1.6 on each opposing side.

hmm...