Will weak valve springs look like a high rpm miss on a dyno graph. How else can I check? I am running ex-612 springs from Combo motorsports and I was recently told to go with 977's from Comp. Do the heads have to be machined for 977's? Thanks

 

Anyone?

 

Weak springs can show up as a misfire caused by the valve float. And yes, the 977's will require a larger seat.

 

Thanks

 

Is it possible to have a high rpm miss without having a low or mid rpm one? I truns great down low. I can usually tell if I have a miss cause in 6th gear at 1500 rpms is shakes violently. But now its just as smooth as ever until I hit about 5000-5500 rpms.

 

Yes.

 

Hood up a vacuum gauge and partial throttle it up the rpm you think its floating at. If you partial throttle it you will have about 20 in of vac, once your valves start to float you will see the gauge bounce around.
If you need help you can give me a call me

Ravi
Speed Freaks Inc - Performance Dyno Shop
209.609.0355

 

Where exactly would you tap in that vacum gauage??

 

i hope the 612's aren't the problem, thats what i have still to go in mine,

 

white night you can tap the line into any vacuum port on the intake manifold, I would unplug the vacuum line going to the optispark. That way you don't set any EGR or EVAP ses lights. Just don't unplug your fuel regulator vacuum line.

Ravi
Speed Freaks Inc - Performance Dyno Shop
209.609.0355

 

Valve float shouldnt look like a simple misfire or spike on the dyno. The curve should suddenly fall off is all. If you think about it.. theres no way to float a valve for just an instant. The problem will start at some rpm and get worse as you keep going so the graph will die off very suddenly and never pick back up again as you find with a misfire. This is all assuming you are floating every single valve using a relatively weak valve spring for the cam. I suppose you could have 1 that is bad causing that 1 valve to float in which case you could have a high RPM misfire with that 1 cylinder.

 

I also I have the EX612 valve springs with my 1.7 roller rockers and I am having some issues as well. You can look at the graphs on the bottom of the link in my sig. The power drops off like a rock up top. 40-50 hp instantly. I have heard these springs are not so good, so I ordered up the crane 10308 spring kit that I used with my 306 in my last car. I was thinking about ditching the 1.7rr's as well, but I think I will try out the bigger springs first.