IROCaholic

Alright guys I am running a comp XE256 cam with mildly worked over 416 heads on my 350. The rest of my combo is in the sig. What I am wondering is what would be the effect of adding just 1.6 rr on the exhaust valves in my engine. I know that these heads like dual pattern cams and I am thinking that this will boost my power enough to knock a couple tenths off of my times for when I go to the track this weekend. Basically what I am wondering is where it will move my power band. Right now its 1000-5200 rpms what do you think it will be after the rocker swap or is there a formula to figure it out. I am wondering because I still have the stock stall in my trans and dont want the car to be sluggish of of the line. I basically just lookin for a way to get a little more power out of my combo before I swap the heads and cam again this summer. I appreciatte any input.

blacksheep-1

I'd try it, what the heck. You never know. Keep in mind that you'll need to check the clearance with the guide slot. They will usually hit on the bottom of the slot nearest the stud when fully open. So be careful. What I did was put a light down in the lifter galley and looked for light around the pushrod at full open. It would be nice if you could run this back to back on a dyno.

paulmoore

Instead of using 1.6 roller rockers on just the exhaust, try using the 1.6 rolloer rockers on both the intake AND exhaust for cylinders 1,2,7 and 8. The reason for this is because most of the air/fuel distribution comes from the center of the manifold. Cylinders 1,2,7 and 8 are the furthest away, and the added lift and duration under the curve will allow for a little bit more cylinder filling. It will help with cylinder power balance, and may even help with improving fuel mixture metering. This is a trick that they use on carb'd motors, but I would think that it might still have some impact on an EFI motor, especially TPI.

kevinc

It's interesting that the guy who can't get his engine to run, barfing gas out the carb and spooging oil at the pan gasket, can summon up the confidence to give out advice. Especially about fuel distribution.

Take a look at his other recent posts and then decide whether you want to follow his advice.

The bigger rockers will only help if your heads can flow at the new higher valve lift.

If you have flowbench data on your heads post-modification, easy decision. Some ports "stall" at a given lift, and opening the valve up more doesn't add any extra CFM of flow.

I'd experiment, but I'd keep things symmetrical on the cylinders. No mixing ratios on exhaust ports from cyl to cyl.

paulmoore

Kevinc: Just because I am having a hard time getting my engine to run doesn't mean that I am dumb. I wasn't aware that this board was made to bash other people because of their problems. I thought that we are all here to help, but I guess I could be wrong. The reason why I have so many oil leaks is because the car/motor took me 4 years to build. The engine sat for such a long period of time that all the seals dried up. And as far as getting the engine running right, I think that my valve lash is too tight, so I need to adjust it. The only reason why I posted that little tidbit of info is because I have an old Chevrolet Power book, and it described in detail why some of the guys who race in the trans am series use offset rocker arm ratios. So when I get to work tonight, I will bring the book home and scan those pages so that IROCaholic can see what I am talking about.

kevinc

Actually, the other guy who posted help thinks your valve lash is too tight.

I'm not bashing you for your problems, and if you look you'll see I offered some advice also.

I'm bashing you for trying to give out "expert" advice when you can't even get your own engine to run.


As you continue reading, you'll learn that wet-tract carbed intake tricks don't generally have the same effect on a multiport fuel injected system. That trick addresses fuel distribution problems which aren't present when there's an injector aimed at each intake valve.

RB83L69

One other thing they failed to mention in that old Power Manual (I believe you're looking at the one with the 4.3 6-cyl on the cover?) is that the tricks in question are specific to the intake used in that series back then. Things like that don't generalize well.

In a street motor, there is no benefit to leaving the intake at lower lift. That is, the engine will make incrementally more power if you raise the exhaust lift only; and it will also make more power if you increase the intake lift.

Typical street motors with stock heads have FAR POORER exhaust flow than intake flow; which is why they make cams like the XEs with the exh lobe bigger than the int lobe. Race heads, including ones such as those used back when that catalog was still called "Power Manual", don't need that; and often benefit from the intake lobe being bigger than the exhaust. That was a "spec" head, IIRC; and things that work with those heads, like the intake they were using, also don't generalize well to other motors.

The XE256 already has a significantly bigger exhaust lobe than intake. In other words, the mfr already too into account the asymmetrical properties of the heads they were designed to work with (stock ones for the most part). Therefore there will be limited benefit, if any, to increasing the exahust lobe and leaving the intake one at a lower ratio, in a motor using "mildly worked" 305 heads.

pennylessz28

1.5 exhuast 1.6 intake.

Best Bet

RB83L69

No benefit to that either. Not with those heads and that cam especially. Same reason. The correct E/I ratio is already ground into the cam.

vernw

iTrader: (2)

What's your opinion if you're using a SuperRam on a 383 with a 218/224 @ 050, 112 LSA, 497/502 lifts hydraulic roller in some mildly ported 200cc iron Sportman II heads at 10.5 compression? I've got flow numbers on these heads if that would help you form an opinion. Still don't like the 1.6 intake and 1.5 exhaust RRs? Think 1.6 all around would be any different?

Just curious.....

RB83L69

If the head flow E/I ratio is like stock, at around .7 - .75 @ mid-lift (.300-.400") which IIRC is the case with S2 heads, then the cam is already set up correctly for them; and doesn't need to be adjusted with the rockers.

Think of it this way: If you had the same motor, with 1.6s on both E & I, would the car go faster if you downgraded one or the other?

Then where would the "advantage" be in only upgrading one side, compared to upgrading both sides?

vernw

iTrader: (2)

That's why I've been looking into going full 1.6 rockers instead of my current 1.6 intake and 1.5 exhaust setup. You're pretty close on the flow ratios, the flow numbers are 78% at .300 and 74% at .400, do you think full 1.6 RRs will be noticeable increase, or would it be a waste of money in your opinion? Appreciate your input!!!

IROCaholic

I am glad I asked this question because it has brought out alot of diverse opinions. I myself am a little skeptical on 1.5 and 1.6 thing, but Ive seen more than a few articles in performence magazines showing the effects of everything mentioned and alot of them showed more power with 1.5 Int. and 1.6 exh. The only one I can think of off of the top of my head is the 350 they built for the my generation camaro. They had a cam with simular specs to mine and some aluminum heads and they test each different combo of rockers and cam out significantly ahead with the 1.5 and 1.6 swap. I will probably try both and post some results. I am really just trying to get the most out of this combo before I start saveing for my 383/built T-5 that I plan on swappin in the future.

paulmoore

Ok, here is a scanned picture of the article that I was telling you about. After reading it, I'm not sure that it would be worth the minimal gains you would see for the money you'd have to spend to get different ratios of rocker arms. And like kevinc said, this is mainly for an engine with a single 4 barrel carb. Anyway, here is the pic.....