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[BuickCamaro]

Degreed the cam and now the motor pops in the upper RPM's but will build 15 PSI of boost. What chart do I need to look at on the Fast system and is this a lean condition in the upper RPM's or just not enough timing. This happens in all gears . Or could I not have enough spring pressure on the valves and they are opening easier in the upper RPM's under that amount of boost. Car starts fine and doesn't pop on the low end at all!!!! Any help would be greatly appreciated!!!!!!!

 

[bobc455]

Did you record any of the runs that had popping? If so, does your dashboard include the Target A/F, Actual A/F and O2 Corr%? Those are the numbers that will tell you if you're lean.

If your valvesprings don't have enough pressure, then popping at high RPM's is a possibillity - do you know your valvespring pressures? What is "high" RPM for you? What size cam & rocker ratio, etc... some more info would help.

-Bob Cunningham
bobc@gnttype.org

p.s. including your name makes the conversation more friendly, and including your location will tell someone who might be near you so they can come over & help if they'd like...

 

[BuickCamaro]

I'll check on that I just got the car running again and now the starter went out so I don't have any recorded info yet on the laptop. As far as the popping at high RPM it is around 4,500 to 5,000 RPM, Cam size not sure off hand right now but will get the card out from the garage and give info on that tomorrow. And for the valve springs not sure on pressure but they are stock small block chevy valve springs in the heads.

The set up I have is the Fast system, 65#/hr injectors, stock heads but ported pretty well, 20 over JE pistons, Two center mains are steel, stock intake and plenum and throttle body, T-72 turbo with a 3" down pipe, and soon to be installed a 3,800 stall...... The car ran a 12.43@ 124 MPH (times are not close to MPH) with the cam being off 4 degrees now it's right but still pops..... Names Chris and from the Mishawaka, Indiana area.... Thanks Bob for some input on where to look as soon as the rain stops and I get the starter in it!!!!!!

 

[Craig Smith]

I'd bet you a buck you need more valve spring. Whatever your seat pressure is, you can subtract (boost x surface area of valve) from that number and that's how much seat pressure you really have. My guess is that this number will be 40-50 pounds at best if they are stock springs.

 

[bobc455]

Chris-

Doesn't sound like a sky-high RPM, but don't forget that when you have boost, it takes extra spring pressure to close the valves so upgraded springs are probably necessary. And from those times I'm guessing you have a bigger-lift cam than normal, too, compounding the problem. Since it started happening right after you changed cam timing, this is a possibility.

One other thing to check is to make sure that your spark plugs are a cold enough heat range- once they heat up they can ignite the mixture even while the intake valve is open (although that takes a bit of time for the plug to get that hot, it wouldn't be consistent in every gear like you say).

FWIW, running lean tends to create a pinging (i.e. detonation) sound more than a "popping". However running mega-rich can sometimes cause backfires through the intake manifold (popping) so checking your A/F ratio is the easiest first step.

Hope that helps...

-Bob Cunningham
bobc@gnttype.org

 

[BuickCamaro]

Thanks Craig I will use that to find out what I need, Bob The car has always popped through the exhaust since day one but we had to run so much timing that it wasn't funny because of the cam being off.... Here is the cam PTE cam new at the begining of last year.... .478 intake and .479 exhaust, 109 center line, seat duration at .006 is intake .262 and the exhaust is .268, duration at .050 is intake .218 exhaust .222, lobe lift intake .309 exhaust .310..... What springs should I use then????? I want to run 30 PSI of boost but if I don't figure this out then I will never get there... Built plenty of small block chevy's and raced NMCA but this buick thing has got me going, but I know I can make more power with less money with this set-up!!! Thanks for all your guy's help so far I really appreciate it!!!!!

 

[Craig Smith]

Call the Comp Cams tech line and tell them all this. They can pick the right spring for you. You will likely need to open the spring pockets up quite a bit because with 30 psi boost your valve springs will be huge. Count on close to 300 pounds of seat pressure to compensate for the boost.