About a month ago I posted results for my H/C upgrade. In those results I posted this dyno sheet.
As you can see, the car takes a dramatic nose-dive at roughly 6400 rpm. Early speculation was that it was an exhaust choke or valve-float. We considered valve-float unlikely because the heads are running MTI double springs. I am running Yella Terra rockers.
I was contacted by Mikey (Rapid Motorsports) the other day and he said that he had another customer come to him with the same drop-off problem...he said the dynos looked identical. The customer was running 5.3 heads, TR230, Comp 987 springs, and YT rockers. Anyway, they swapped out the YT rockers for the stock ones and the drop-off disappeared. My questions are this, has anyone else had a similar problem? Fixes? Thoughts? If you want more background on my H/C setup, here is a link to the original thread.

Thanks!

 

Wow, that's weird. Any speculation from Mikey as to the cause of the problem with the Yella Terra's? I hadn't heard of this being a problem until now. Maybe the Yella Terra's need to be set up differently? TTT for you. I'm curious about this as well.

 

...since that is close to the red line I'm not too concerned....

...it would be interesting to know the cause, however...

...can you verify that the valve train geometry was correct including the pushrod lenght and preload?

 

I havent dyno'd mine with the YT's but have "felt" no drop off above that RPM. I cant see a these rockers causing a drop off. The only change would be flex at high RPM's and the YT's are WAY stronger than the stockers. Very strange.

 

stiffer rockers transmit more energy to the valvespring, and therefor if you are close to float from insufficient springs, you may be taken over the line.

The valvetrain is a system, and needs systematic design and troubleshooting.

chris

 

When the problem with my setup was noticed, Mikey and I never even considered the YTs to be the problem and since the heads are MTI Stage II with double springs we didn't think that valve float was an issue either. I'm running .600 lift...Jayson at MTI said these springs were good to .615 lift. We didn't think YTs until another one of his customers came to him to diagnose the same sorta thing and the fix was the change to stock rockers...Anyone have any experience with YT's tech dept.?

As for the valvetrain setup question...the pushrod length was recommended by MTI (7.4) and the setup was done by Mikey...he knows his sh*t, I'm sure it's correct.

 

There have been several people that had the exact same issues with the 987 springs not being setup properly for enough pressure. The graph would fall off at high rpm.

I had the same issue...TR re-setup my ARE heads and now all is well.

 

Great Numbers and Mikey's tuning is #1! I am also running the Yella Terras and also dynoed at Performance Specialties and I am not having this problem. I'm curious as to what could be the actual problem? What did Mikey have to say?

 

Just have Mikey put the stockers back on and then dyno & see if the problem persists. What ratio are they.. 1.7 or 1.8? Are the lobes X-ER's?

 

The rockers are 1.7 ratio, the lobes are XE-R...

 

Here's a graphic that SLP presented in another thread:



I notice a drop off at upper RPM's as well.

 

I think I have seen this on two cars now, and it has been mysteriously after a YT install from stock rockers. We suspected valve spring float. Hmm, gotta start doing some more research now.

 

TTT...anyone else have problems...any thoughts...

 

987's and an XE-R cam is a big no no... There isn't enough spring pressure. I had the same issue with stock rockers. I switched to Comp 977's w/ Comp R lifters and YT Rockers and all is well now...

 

Mikey's other customer had been using the Comp 987s, my heads are set up with MTI's double springs, I think they're manufactured by Isky, but not sure...anyways, Jayson @ MTI said they're good to .615 lift...

 

What would a valve spring have to do with the problem going away with the changing of the rockers... There was 2 cars, both with different springs, but same problems and same rockers. Changed rockers and the problem goes away..

 

Maybe the change in mass has caused a harmonic in the spring somehow. The YT's are lighter.

 

It's a rocker arm. Why would it cause the graph to fall off? Looks to me like improper valvetrain geometry some how. I also lean more to the spring not being suited for the cam. If someone tells you that a spring is good for .615 lift, your just taking there word. ALWAYS get a open/closed seat pressure on the springs. Then find out from the seller of the cam what pressures are needed to keep the cam in check. I myself like a little more pressure than what is needed for comfort. It sounds like your MTI springs are at the max and giving out on the big end. If your going to run XE-R lobes, you need a stiffer spring to keep things right. I reccomend swapping to the new Crane duals.

 

I have heard of this same problem so i'm taking off my Yella Terra's also. Several Top Tuners filled me in that the stock rockers are the way to go.

 

If someone would just make one valid point as to why a rocker arm could or would hurt top end performance, I'd be very interested in hearing it. There isn't much diffrence over the stocker other than stregnth. Beast96Z====>