oh also old vac distributors are a great reference point but keep in mind factory carbs don't ever run at stoich, they run on the rich side. so guess why they need all that extra advance?
.... to burn all that extra fuel off!
so just 'cause a carb n points motor runs 50 degrees doesn't mean you should
Originally Posted by steveo
Just FYI, in my experience non feedback carbs actually run LEANER than stoich at cruising speeds. My factory Q-Jet on the 1983 G20 Van would cruise at 16.5-17:1 at light throttle down the highway and only richen up with moderate throttle.
I had one of those blinky LED afr gauges back in the day and swapped TBI to Carb to TBI numerous times on the same engine. Even a "performance jetted" Edelbrock 1405 would peg the meter lean and cause the LED to disappear lean until the throttle was opened.
Last edited by Fast355; 08-27-2014 at 11:58 PM.
you might be right; i've only had a couple carbed smallblocks and they were both factory rich (probably 12.5:1 judging by the plugs)
but lean cruise AFRs need more timing too.... a perfect afr doesnt need as much advance to burn completely
Thanks for the advice steveo, Got another question though. It seems like my engine is a little high in the kpa values. Like you just mentioned 35kpa @ 2000 rpm. My motor is more like 60kpa @ 2000rpm. Do you believe that is normal i.e. my cam or something contributing to this higher kpa?
Richard
1969 C-10 SWB Step
355 Small Block
Stock Vortec heads
Howard's Cam (110951-08)
Stealth Ram, Holley HP EFI
700R4 Trans, 2200 stall & 3.73 gears
He just tunes 2 door V8 powered sports cars. Those of us tuning box like trucks/vans/suvs will see different loading than a light, aerodynamic car. I run about 50-70 KPA at 2,200 rpm @ 70 mph and that drops to 40-60 KPA at 2,600 RPM @ 85 mph.
yep, cams that make more high end power produce poor vacuum.
i just looked at your specs. it's a fairly large duration but low lift cam. i wouldn't expect a ton of cruising vacuum.
you shouldn't even be driving it at 2000 rpm. with that cam you should cruise at least 2500rpm. notice you also have a very narrow timing map, half your timing map is just decel range now.
since you're running vortec iron heads (designed for low end power only) with high rpm operating cam (an odd choice, since now you have weak bottom end and poor flow on the top end), you'll probably find throwing timing at your fairly narrow operating range doesn't help a lot.
no offense but welcome to the 'i put too much cam in it' club
I hear you definitely got what I was asking for there, I was looking for a low lift cam (bc vortec head) but wanted a good sound. (Close overlap). Guess I sacrificed to much in the other departments. I still can do one hell of a burnout though...
Could you try to explain to me a little more about the narrow timing map comment? You were able to determine that from the cam?
just like i said, since the timing map is rpm vs vacuum, and your cam makes way less vacuum than a normal one and operates at higher rpm, you only get like 1/3rd of the timing map in realistic operating range. its good enough, just not as much fine tuning as you'd probably expect looking at the graph.
Posting Permissions
Reply With Quote
Bookmarks