Quadrajet Problem Solving > Dialing in your rebuilt Quadrajet carburetor
1903 Won't Respond to Tip-In Procedure
Cliff Ruggles:
I actually like the early swirl port "Vortec" heads for a truck/towing engine. They bring the power in early and didn't fair two bad anyplace in your 355 by looking at the dyno sheet.....
bry593:
Yes, that is impressive for TBI swirl heads.
Here is a useful reference from the Crankshaft Coalition discussing Cam Duration vs Compression:
https://www.crankshaftcoalition.com/wiki/Cam_and_compression_ratio_compatibility
Cliff Ruggles:
Good info. I especially like this part:
"The chart will show why it is foolish to install a 230 degree cam in an 8.00:1 motor in an attempt to get a lope out of the motor. It might lope, but it will lack the power to pull the hat off your head. There seems to be an obsession with lope these days. Lope is simply the sound of the motor being inefficient at low rpm's because the ascending piston is pushing fuel/air mixture back up the intake tract through the still open intake valve and disrupting the metering abilities of the carburetor."
Although they didn't take CID into consideration the info is still valid and the chart reflects what you'd see from a 350cid. A 454 on the other hand will gobble up a 230 @ .050" cam and run very well with it, especially if it had moderate compression. Below is a dyno chart from a 455 Pontiac Super Duty build around 8.8 to 1 compression. The cam was a 230/236/112LSA HR with 281/287 @ .006" duration. It idled dead smooth making 16" vacuum at 650rpm's with only 10 degrees timing in it.
Put the same cam in a 350 engine with under 9 to 1 compression and I'll bet you'd be lucky to see 8-9" vacuum without running the initial timing clear off the scale at idle speed!......
tayto:
cliff, bry: i wish I would have known the dyno graph went lower than 3000. i suspect the peak torque is higher and a bit lower down in the rpm range. also, it made the most power at 26*. i had to persuade the owner/dyno operator to dial back the timing from 32*. he did so 2* at a time made about 10hp/15tq more peal power with 6* less timing!
EDIT: I currently have 3.42s with 30" tire, this thing needa more gear, I think i will goto 4.11s which will help a lot for towing IMO
Cliff Ruggles:
The better job that you do with the engine build the LESS timing and fuel is needed to make highest peak power. Those early swirl intake port heads must do a great job at cylinder filling early in the RPM range so it's now wanting a lot of timing and making a lot of power early.
Most of the engines I've done here in the last 25 years have made best power with around 28-32 degrees and another 10-14 from the vacuum advance.
My own engine runs equally as well at 28 vs 30 or 32 degrees at the track, so I run it with 28 degrees......Cliff
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