Guys I was doing some desktop dyno stuff last night and it seems that 1.6/1.5 combo is only worth about 1hp and 1ft torque on paper... My questions is it worth a little more in real life I know paper is not always right. Was thinking of comp pro mag rockers for 1.52 or a lunati set of 1.6/1.5 or just all 1.6 what do you recommend.. I wanna get these bought this weekend as I'm near start up again...and If i go 1.6/1.5 will I need 8 push rods 1 size and 8 of anotrher???

Brian

64,
As you'll find from various posts in the past, it's generally best to put 1.6's on the intake not the exhaust. Vizard has done dyno tests demonstating the benefits of this and the loss of performance by having them on the exhaust. But it all depends on your motor. Gains are usually not that great as you say, but sometimes a little extra lift/duration helps for smaller cams.

You shouldn't have to worry about the pushrods, as long as the ones for the 1.5's were correct.
Good luck!

Can anybody explain to me why an engine might not produce as much power with 1.6's on the exhaust? I have heard this before and it doesn't make sense to me. :confused:

Both David Vizard and UD Harold recommend using them only on the intakes. I don't know Harold's reasons but Vizard has run extensive dyno and track test that, in most cases, show a power loss with 1.6's on the exhaust. Ocassionally an engine will make a bit more. He mentions that the exhaust side responds more to extended duration than to increased lift. He never states why this might be so.

To my reasoning, and there is nothing definitive about this, it has to do with maintaining velocity in the exhaust port. By opening the exhaust more with a high ratio rocker, velocity drops and any scavaging effect is diminished. Extending the duration, particulary on the opening side, allows the chamber to be cleared effectivly and still have velocity near closing. Increasing the lift causes the gases to exit the chamber at a slower velocity near and during overlap having spent its velocity earlier in the cycle. I really don't know that this is the case, but that's my reasoning.

All engines are different and the only way to know for sure is to test. However, the evidence I've see suggest that 1.6's on the exhaust usually cost power. I was told early on by a number of drag racers to use higher ratio rockers only on the intakes. I expect they learned this from experence.

Brian, you'll need to check PR lengths. I have HS 1.5's on the exhaust and 1.6 Comp Pro Mag's on the intake. I needed .100 longer than stock with the Pro Mag's and .100 shorter than stock for the HS's. I don't know why this was so, but that's what it took to get the geometry correct.

That sort of goes to show how the Chevy ex ports are relitively efficient vs the intake.Notice how many SBC's run great with either single or dual pattern cams... Both stock and aftermarket heads have keep increasing the intake valve size bigger and bigger with no where near as much increase in exh valve size.

A Buick for example a 1970 455 stock and optional cams had huge differrences in lobes.

Intake : .389 lift 290 dur 17-93
Exhaust: .460 lift 322 dur 93-49

Optional Cam:Timing figures only

Intake : 316 dur 28-108
Exhaust: 340 dur 98-62

The Buick engineers must have felt the Exhaust needed all the help it could get with it's poor exh port design.

You should look at the cam timing that the old hy-po- Buick Nailhead valve 401 type motors had to use to get good power due to the "torturous/crooked" route the exhaust ports had to take since they were on the intake side of the "half a pent-roof" chamber design the old Nailhead motors have.

It's a real shame GM didn't let Buick make a true pent-roof head for the old motor!!!

pdq67

Glad this post came up. I was going to put 1.6's on my vortecs exaust's as the flow numbers continue to rise as lift goes up, you would think that would help power...

bored&stroked brings up a good point there, if I remember right max flow on the exhaust side of AFR 195's is over .600 lift, so wouldn't there be a gain, up to .600 lift but then if you exceed that number then you would begin to see a decrease in performance. The way I see it is, you can cram all that air/fuel in there you want, but if you can't get it out, what's the point. I hate it when topics make you go cross eyed!