So i have been using the factory gauge in the car since I have owned it and I didn't realize it was so far off. I thought I had a screwed up T-56 when I was turning 1900rpms in 6th gear at 60mph when the T-56 in my Vette turns like 1200rpms in 6th at 60mph. So after learning the OEM tachs in these cars are regularly wrong I Installed an aftermarket tachometer and my RPM range came back to what it should be. Now I have realized that I'm shifting a lot earlier than I thought I was. My OEM gauge says I shift at around 6300 rpms. Now I realize I'm barely hitting 5,000rpms before power drops off. I tried to push her past 5k but the power is long gone. The cam range is supposed to be 1800-6000rpms. Are the factory Vortec heads choking the engine off at 5k? She really stops pulling... Mods........

357, stock crank, forged rods using ARP bolts, KB hypereutectic pistons, XE274 cam, Summit stage 2 intake, Holley 600cfm carb, Long tube headers, 3" cat back, no cats, flowtech afterburner muffler, accel HEI dizzy.

Anyone know why the power would just drop off so rapidly at 5k?

 

Hook up a vacuum gauge to manifold vacuum, tape it to the glass & drive the car hard. If the carb is too small the gauge wont get to zero at WOT under load.

 

I've had a vacuum gauge hooked up for a while now. Gauge has always shown that the carb is a little too small. But according to calculators estimating the air needed I should be fine for 5,000rpms and starting to run out closer to 6k not 5k.

 

Those calculators are based on theoretical numbers and are correct, but do a little research into how carburetor CFM's are determined. It's a black magic art with several different methodologies, so there are different points of view about how to pick the right carb.

That said, I would trust the vacuum gauge over anything else. If it's indicating your carb is too small, get a bigger one.

http://www.carcraft.com/techarticles...arb/index.html

Note the formulas are useless section. And the section afterwards.

 

When the car is floored the vacuum gauge reads like .3 vacuum. You can't tell me a 600cfm carb is killing power that badly. I don't believe that.... Plus with how many cars that are overcarbed on this site I don't believe the carb is THAT small. Yes the engine could prob use a 650 but I don't think 600 is way too small where power would just dump off at 5k... Could it be timing related?

 

Here's another interesting article on it. I'll let you make up your own mind.

http://www.candsspecialties.com/ratings.html

BTW I run an xe262, much smaller than your 274, and it pulls to around 5500. After that it's done. I dont think it's the heads, it's the cam. What is your compression ratio? And what valve springs are you using?

 

Back when my car was still FI I used to run the XE262 actually. haha. It's been awhile since I figured the compression ratio so I don't remember exactly. I would assume it's at it's lowest 9.5:1 and 10:1 on the high side. I think it's closer to like 9.7:1 though. How can the cam drop off at 5k when it's rated to 6k? Should pull until at least 5800rpms. I used the recomended valve springs Comp cams Part# 981-16. I also am using 1.6rr's also.

 

You may be going into valve float, especially with 1.6 rockers.

http://www.ehow.com/list_6889982_val...-symptoms.html

You dont mention misfires... but Im suspicious since it seems like your motor just hits a wall at 5k. That sounds like some descriptions I've heard of valvetrain issues. The 981 is barely enough to handle the 274, with 1.6 rockers I'm curious if it's just too much. Could be seat bounce too. Take the valvecovers off and look VERY closely at every spring for damage.

What is the installed height on those springs?

Does the vacuum gauge get noticably more jittery as you get to 5000 RPM's and beyond?

Have you ensured your mechanical advance in your distributor is working properly? Have someone rev it to 5k and make sure the spark advance is where it needs to be. I've had my mechanical advance mechanism seize up before, had to pull the cap and get it moving again. Replaced the bushings and weights and springs after that and hasnt happened since, but all the power came back. Could be that your motor is just outrunning your spark. So be sure to rule that out.

 

The vacuum gauge stays very smooth as I approach 5k. I'm not sure if the vacuum advance is working correctly. I'll look into it. I have never got it set right i don't believe.... I use the light spring and my base timing is set are like 12 degrees advanced i believe. It never ran right having less though... Seems to run better the more timing it give her... I have never been good at doing timing or tuning the carb even. The timing issue seems more likely considering I'm using the lightest springs available for the dizzy. You have to forgive me it's been around 2 years since I have done anything to the ignition or tuned the carb... I don't really remember that much... I'll check it out and get back to you.

 

Vacuum advance doesn't come into play at wot, its there for part throttle fuel economy.
You need to add you initial timing (6-12*) & mech advance (20-30*) to get around 34-38* total. Vacuum advance adds another 12-20* on top of that. But dont worry about vacuum advance now. Get the total in the 34-38* range by using a timing tape or a dial back timing light, you might have to either slot or weld up the holes for the mech. advance.
Lets say your car likes 10* at idle, then w/ mech. advance that adds another 30*, that might be too much, so you'll have to keep the mech advance from going that far by limiting how far they swing out, so it'll only add 26* instead of 30*.
Vortec heads are really modern, they like less timing then the old heads.

 

I have always read the Vortec heads like around 32 Degrees total timing. Is that true?