springs and things...
Le Mans Master
Joined: Aug 2000
Posts: 8,838
Likes: 31
From: Australia
"I thought the cam was .415 (as you say). And, you can not change it's "gross lift". I would think the cam lift multiplied by the rocker ratio would equal the valve travel (in inches)."
Right formula; wrong starting number.
.415 is the lift (at the valve) with stock 1.5 rocker
Cam (lobe ) lift is as stated .280 ish.
Right formula; wrong starting number.
.415 is the lift (at the valve) with stock 1.5 rocker
Cam (lobe ) lift is as stated .280 ish.
Burning Brakes
Joined: Dec 2006
Posts: 936
Likes: 2
From: Tyrone PA
I thought the cam was .415 (as you say). And, you can not change it's "gross lift". I would think the cam lift multiplied by the rocker ratio would equal the valve travel (in inches).
So, .415 x 1.5 = .6225" of valve travel for a stock '89 intake valve
And .415 x 1.6 = .644" of valve travel for a stock '89 with 1.6 rockers.
Effectively, this would be the same valve travel as if you'd left the stock rockers on the car and changed to a cam with .443 (intake) and .459 (exhaust) lobes. And, since head flows are based on cam lift (right?), then you need these numbers to predict flow.
We are saying pretty much the same thing -- except for the gross lift part.
gp
So, .415 x 1.5 = .6225" of valve travel for a stock '89 intake valve
And .415 x 1.6 = .644" of valve travel for a stock '89 with 1.6 rockers.
Effectively, this would be the same valve travel as if you'd left the stock rockers on the car and changed to a cam with .443 (intake) and .459 (exhaust) lobes. And, since head flows are based on cam lift (right?), then you need these numbers to predict flow.
We are saying pretty much the same thing -- except for the gross lift part.
gp
Race Director
Joined: Nov 2003
Posts: 12,221
Likes: 446
From: Overland Park Kansas
2020 Corvette of the Year Finalist (appearance mods)
C4 of Year Winner (appearance mods) 2019
Will kiss my Grits!
Thanks for the info.
Thats not how I thought it worked. With so many head options for a given motor, this seems wrong...
While we're on the subject....
Talk about 1.7 vs 1.6 rr. Are there problems with changing to the higher value? Cost? Clearance issues? etc.....
Thats not how I thought it worked. With so many head options for a given motor, this seems wrong...
While we're on the subject....
Talk about 1.7 vs 1.6 rr. Are there problems with changing to the higher value? Cost? Clearance issues? etc.....
Last edited by GREGGPENN; Jan 10, 2008 at 04:41 PM. Reason: typo -- hard to read....
Burning Brakes
Joined: Dec 2006
Posts: 936
Likes: 2
From: Tyrone PA
It really is strange, you would think all cams would just advertise the lobe lift, but it isn't so. Just one more thing that could be simpler than it is. I know that with 1.7 rockers you can run into retainer and spring clearance problems, you would need to be extra careful on spring and retainer choice. With a stock L98 or LT1 you can run stock springs with 1.6 rockers, but I don't think you can get away with 1.7 rockers. As always, as lift increases there is more potential for piston to valve clearance issues, but if you are talking stock L98 cam, heads and pistons then there would be no problem. The other issue is that you need narrow body rockers, and self aligning if you aren't using guideplates. I don't know of anyone who makes narrow body 1.7 sbc rockers. You would also need to check the pushrod length. I think till you go through all that trouble you would be better off to swap out the cam and run 1.5 or 1.6 rockers to get the desired lift.
Race Director
Joined: Nov 2003
Posts: 12,221
Likes: 446
From: Overland Park Kansas
2020 Corvette of the Year Finalist (appearance mods)
C4 of Year Winner (appearance mods) 2019
One reason I say this is because on stock '89 heads, the difference in flow from .4 lift to .5 lift increases from 185cfm to only 195 (per TPiS). 1.6 rockers would get you 5cfm.
I'm thinking woopdeedoo.
Last edited by GREGGPENN; Jan 15, 2008 at 04:36 PM.
