KJL

I read in articles where this can improve low end performance over running 1.6 on both.

VETDRMS

I've heard of running the 1.6 on the exhaust, especially with nitrous, as the more exhaust you can scavenge, the more power your N20 will make.
:cheers:

no_radio

I've seen this done on magazine cars, and some crate motors offer that setup as well, but I don't know what the advantage is...given that the exhaust port is smaller I would think you would want MORE lift for maximum scavenging. I guess it's a low end thing, like you said...
Jeff

MotorHead

Similar to making a dual pattern cam out of a straight up cam, you could keep the low end you have( torque and vacuum) and have minimal increase in performance, probably not really worth it unless you have a restrictive exhaust :D


[Modified by MotorHead, 3:36 PM 2/13/2003]

Chris O'

"you could keep the low end you have( torque and vacuum) and have minimal increase in performance"

Is that why in DD2000 mine made stuff all difference when I input the increased lift from 1.6 rockers?

Schmucker

I'm doing it on my engine, but only because then we only need one set of 1.6 rockers for two engines. There is less going out of the cylinder than going in, so it isn't much of an issue. Some intelligent porting will give you more gain on the exhaust than the 1.6 rocker would, such as matching the cylinder bore to the head so it doesn't hit that little ridge there.

fauxrs

If you have data on the flow of the heads you can tell if there is any benefit to these rockers if - for example your cam lifts .498 with 1.51:1 rockers then the lift would be .52 with the 1.6:1 rockers.

Then check the flow tables for the head for example a gm 882 head flows 204 cfm @ .40 lift and 205 cfm @ .50 lift - so you can see there is little to be gained with the additional lift. The valve lift curve hits the mid-lift numbers such as 0.300- to 0.400-inch twice in each valve lift curve while peak lift is only achieved once per cycle. Judging a cylinder head by its mid-lift flow potential can be beneficial in choosing the right cylinder head for a street engine.

A dart iron eagle head actually drops flow between .40 and .50 lift on the intake side - so it would be a losing proposition on that head. Most aftermarket heads continue to improve in flow to at least .50 and often beyond.

Also I've read that an optimum ratio is for the exhaust to flow 70% of the intake. which is why heads with a weak exhaust port can benefit by increased rocker ratio on the exhaust side (i.e. 1.6 E --- 1.5 I)

cardo0

Depends in your camshaft duration KJL. David Vizard (the Wizard) writes in his book his dyno tests with cam duration over 270 degrees 1.6 rockers on the exh. showed reduced power. Something about more duration on the exh. with dual pattern cams is the right direction (as is currently popular) - more time for cyl. blow down. Only "short" cams (<270) showed power gains with 1.6 rockers on both int. & exh..
I think Crane will sell mixed sets of 1.6 and 1.5 rockers saving a few $$ over two sets.

OatBoy

if i remember correctly, its just like motorhead said, it increases vacuum at idle while still giving u performance boosts at higher rpm. just like a dual pattern cam.
this of course would only work for naturally aspirated motor non-nitrous motor.

BillsVette

I run 1.6 on the intake and 1.5 on exhaust to "even out" the lift difference of the Comp dual pattern solid cam (242/250, .508/.534") I'm running.
With AFR 195 heads on a 383, I feel it was to my advantage to increase intake lift (net .541 w/1.6).