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Posted: Wed Oct 03, 07 9:21 am
by Dave-R
Depends on your heads and how much they flow. There is a point where opening the valve further does not allow more air/fuel to flow into the cylinder because of head restrictions.
Higher ratio rockers do open the valve at a faster rate as well as slightly more. Which is a good thing.
You do need more flow capability through the valve than the head actually flows though. So that the average flow through the valve (between open and closed) is as good as the max flow through the ports.
Posted: Wed Oct 03, 07 11:00 am
by Kev
Fantastic answer, Dave! Don't forget Pete is a brickie!! I'd say buy both sets and then plasticine the motor to see if the 1.6s fit. If they do, try them, then try the 1.5s and see which ones work best. (Just bung the extra dosh on your next quote!!
)
Posted: Wed Oct 03, 07 2:42 pm
by AllKiller
My Crower is a gross .585 lift with the old TRW high dome type pistons @ 12.5:1 with 1.5 rocker ratio on 906 heads, with 1.88 x 2.14 valves, no probs..
...not tried 1.6's as Dave beat me to a good set advertised for sale on here some time ago
Posted: Thu Oct 04, 07 2:55 pm
by Ian Z
Trying 1.6 roller tips meself currently, from stock rockers. So far seems to pull slightly stronger and defo cleaner at top end -(and hasnt hit pistons
)
Just need a long flat straight piece of blacktop and a stopwatch. Or, sell an organ and do SPR/Shakey/Yrk soon.
BTW, which organ is the heaviest / least required
Posted: Thu Oct 04, 07 3:01 pm
by Ian Z
is gross measured by advertised cam lift x lift ratio ( ie if 1.5 equals x1 and 1.6 equals 1.1 ?)
Posted: Thu Oct 04, 07 3:56 pm
by Dave-R
Valve lift = tappet lift x rocker ratio.
So if you know the valve lift of a cam with 1.5 ratio rockers you need to divide the figure by 1.5 to get the tappet lift.
(Valve lift/1.5 rocker ratio = tappet lift)
Then x 1.6 to get the new valve lift.
(tappet lift x 1.6 rocker ratio = new valve lift)
Posted: Thu Oct 04, 07 3:57 pm
by Dave-R
Many stock rockers are only give about 1.45 ratio anyway.
Posted: Thu Oct 04, 07 5:44 pm
by Ian Z
therefore I calculate 10% lift increase using 1.6 from 1.45. And 7% increase 1.5 to 1.6.
Posted: Thu Oct 04, 07 9:36 pm
by Dave-R
Faster rate of lift too.
Posted: Thu Oct 04, 07 10:14 pm
by Mick
.590 with 1.6 =623
Posted: Thu Oct 04, 07 11:13 pm
by Ian Z
So with Pete's setup, any issue with piston clearnces?
Posted: Fri Oct 05, 07 9:08 am
by Dave-R
Depends. He needs to check for valve spring bind as well as piston to valve clearence. Both can be put right one way or another.
Posted: Sat Oct 06, 07 3:40 am
by Anonymous
Dave wrote:Depends. He needs to check for valve spring bind
Was wondering when you were gonna mention that.