Wondering how much power I will get from switching from 1.5 rockers to 1.6 rockers. I'm thinking maybe 10-12 HP? They are worth about 30 thousandths more lift.

 

Depends on ALOT of things.

If the 1.5s are stock stamped sheet rubber and you put on something consistent and accurate, you'll gain ALOT, and more than mere HP, besides.

If the 1.5s are already good quality (NOT stamped sheet rubber), you'll gain ALOT less.

If the distance that the valve opens is The Bottleneck in your motor, you'll gain some.

If something else is The Bottleneck, you'll gain very little, maybe nothing detectable at all.

You may even break parts, depending on what cam, valve springs, etc. you have.

All in all, there's ALOT more to this than your 350 being bored .030" or what intake you bolted to it, which is all you've told us.

 

Sorry, Its a 355 with Trickflow 175cc heads that flow 242@.500 lift and 245@.600. Springs are good to .540 lift. Cam is Comp XE 268 flat tappet 224/230 477/488. 1 3/4 Stainless works long tubes into 3" magnaflow catback Edelbrock base with Superram intake and runners and Lingenfelter 58mm tb. Since the heads are capable of flowing more than the current cam lift (1.5 roller rockers) im hoping to gain some by going to 1.6 which would give me roughly 509/512 lift. Maybe a 10hp increase?

 

I would think 10 is reasonable on that combo. May not even need to change the exhaust and run just intakes. But both wouldnt hurt it

 

I doubt you'd even see 5 HP. Those heads are well and widely known NOT to respond particularly strongly to added lift, as illustrated by the flow numbers you posted.

Unless your 1.5s are defective in some way, the difference would be down in the noise and uncertainty that already exists from one dyno run to the next; statistically insignificant.

 

Doing back to back before and after dyno runs next week so we will see what they yield.

 

My guess is 6 HP gain. I would be real curious to see the results of your back to back dyno

 

Make sure you adjust the 1.5s first, so you're only measuring the results of ONE change.

No sense introducing "confirmation bias" http://en.wikipedia.org/wiki/Confirmation_bias where it's avoidable.

 

Will do.

 

Not sure how this comes into play? Its a fairly simple test. Run 1, swap and run 2. Preload done the same method.

Are you saying to experiment with preload?

 

I know dyno time is money, but IMO it would be best to try just 1.6:1 rockers on the intake first. Then come back and put them on the exhaust. Often you will find gains on the intake and occasionally loses on the exhaust from over scavenging/opening the exhaust earlier. Remember increasing rocker ratio is also going to increase overlap at .050".

 

I understand what you are saying but I'm pretty high in compression now 10.5-10.75 to 1 with this cam so I think it will help on the exhaust side too. But maybe I will try your suggestion just to see what the difference is.

 

I think that he's suggesting that if the engine is running well now, you might not think to set up the rockers properly. Who knows how long it's been since they've been adjusted. ....then of course a properly set up 1.6 would look like the hero, when in fact it was the proper adjustment all along, not the increase in lift. Hence the confirmation bias.

 

Exactly!


It's amazing how often this sort of thing happens... it's one of the classic hot-rodder mental mistakes.

 

If its running well now theres a good chance it was setup right. If you do the rocker swap the same way with the same preload turns on the rocker stud, it should be a fair comparison. I just was under the assumption that these steps would have been taken care of.

 

They were recently (within 1K miles) checked and adjusted so I think we should have a fairly clean dyno comparison next week.

 

If it was mentioned earlier, I apologize, but what KIND of rockers are on there now? ....I honestly hope it's worth it, and that you're happy with the results. That being said I tend to lean towards Sofa's opinion on rockers. There ARE legitimate gains to be had in certain situations, BUT....I think so many success stories are due to various other factors. I think it was Lingenfelter who said in one of his books that there was more power to be had on the average street driven 350 going from stamped 1.5 rockers to a good solid 1.5 RR, than there was from a quality 1.5 RR to a quality 1.6 RR. Said more eloquently of course. ....but it was a factor of flex in the poor quality (although super durable) stock rockers, AND the fact that proper geometry was set up from the get go. He went on to say that if you see a gain from the switch, then you probably weren't set up properly to begin with. ....but of course, whatever the reason, 10 hp is 10 hp, so who cares exactly WHY, right?!?!?! LOL

...and even if you DO have rock solid quality 1.5's now, and you see a significant gain, then awesome!!!! ...but it's worth having the discussion so when a search in the future is done, these thoughts, opinions, ideas, experiences are found, along with a simple dyno number result....legitimate as it may be.