SBC freshen up
12-13-2013, 10:06 AM
#22
Launching!
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A few updates. I've been daily driving the truck and spending a great deal of time tuning and dialing things in.
Hooked up the wideband gauge & sensor and have been dialing in idle,cruise & power AFR's
The Mighty Demon's throttle shafts were wore out and creating un-predictable idle speed from deceleration and it was tough to dial in AFR's; replaced the base with a generic holley style base and modified the idle fuel ports to work with the demon main body.
A few things I've noticed about my engine that may or may not be due to the combustion chamber mods.
I had a carb base gasket vacuum leak, combined with the wore out throttle shafts, occasionally at idle the AFR would peg max lean at 22:1. The engine never stalled, it kept idling. It idled rough at those AFR's, but never stalled.
While dialing in a leaner cruise I noticed under a load at highway speeds around 2.5k-3k rpm the engine can get into high 17's low 18's before the engine would start to stutter. It's now dialed back to high 14's low 15's.
I wanted to see what kind of ignition timing the engine wanted at idle, so I hooked up the vacuum gauge. Turned the dizzy, re-adjusted the carb for 900rpm and low 14's AFR. Did this in steps and when the engine made max vacuum at 900rpm and same AFR's I checked the timing with the light and it landed at 35 degrees.
I pulled the dizzy out, locked the mechanical advance. Maintained the vacuum advance for cruise. Indexed the magnetic pickup for rotor phasing. At the same time I pulled and rebuilt the Demon. Set the dizzy back and eyeballed it for enough advance to fire up. Set the carb up with bench settings of .020" transfer slot exposure and 1 turn out from the fuel screws.
It fired up in half a crank and idled at 400rpm, stone cold in winter, with a 224/230-110 cam. I couldn't believe it.
Adjusted the timing for 36 degrees and re-adjusted the carb.
https://www.youtube.com/watch?v=xXs5dUx76Pg
It's not perfect by any means, the more fuel and timing I throw at it the better SOTP performance it has. Which tells me the chambers aren't as efficient as I would have hoped for. However, Dialing in AFR's for a low 14's idle, low 15's cruise and low 12's power. With a mix of highway and town driving this old SBC with a stall converter and tall rear gears gets better MPG then my '09 GMC 5.3L with a PCM tune and is alot more fun to drive.
This week I lowered the *** end down a bit. The old stepside bed is significantly lighter then the original, combined with the staggered tire diameters. My once level 2"/4" drop had developed quite the rake:
After:
The bolt in frame notch will arrive this afternoon, then to get to cutting:
I also threw on a set of cheapie universal slapper bars. But as it is right now the rubber stop contacts the leaf spring and not the eyelet. I've got some more work to do on those.
Hooked up the wideband gauge & sensor and have been dialing in idle,cruise & power AFR's
The Mighty Demon's throttle shafts were wore out and creating un-predictable idle speed from deceleration and it was tough to dial in AFR's; replaced the base with a generic holley style base and modified the idle fuel ports to work with the demon main body.
A few things I've noticed about my engine that may or may not be due to the combustion chamber mods.
I had a carb base gasket vacuum leak, combined with the wore out throttle shafts, occasionally at idle the AFR would peg max lean at 22:1. The engine never stalled, it kept idling. It idled rough at those AFR's, but never stalled.
While dialing in a leaner cruise I noticed under a load at highway speeds around 2.5k-3k rpm the engine can get into high 17's low 18's before the engine would start to stutter. It's now dialed back to high 14's low 15's.
I wanted to see what kind of ignition timing the engine wanted at idle, so I hooked up the vacuum gauge. Turned the dizzy, re-adjusted the carb for 900rpm and low 14's AFR. Did this in steps and when the engine made max vacuum at 900rpm and same AFR's I checked the timing with the light and it landed at 35 degrees.
I pulled the dizzy out, locked the mechanical advance. Maintained the vacuum advance for cruise. Indexed the magnetic pickup for rotor phasing. At the same time I pulled and rebuilt the Demon. Set the dizzy back and eyeballed it for enough advance to fire up. Set the carb up with bench settings of .020" transfer slot exposure and 1 turn out from the fuel screws.
It fired up in half a crank and idled at 400rpm, stone cold in winter, with a 224/230-110 cam. I couldn't believe it.
Adjusted the timing for 36 degrees and re-adjusted the carb.
https://www.youtube.com/watch?v=xXs5dUx76Pg
It's not perfect by any means, the more fuel and timing I throw at it the better SOTP performance it has. Which tells me the chambers aren't as efficient as I would have hoped for. However, Dialing in AFR's for a low 14's idle, low 15's cruise and low 12's power. With a mix of highway and town driving this old SBC with a stall converter and tall rear gears gets better MPG then my '09 GMC 5.3L with a PCM tune and is alot more fun to drive.
This week I lowered the *** end down a bit. The old stepside bed is significantly lighter then the original, combined with the staggered tire diameters. My once level 2"/4" drop had developed quite the rake:
After:
The bolt in frame notch will arrive this afternoon, then to get to cutting:
I also threw on a set of cheapie universal slapper bars. But as it is right now the rubber stop contacts the leaf spring and not the eyelet. I've got some more work to do on those.
Last edited by 04SilveradoMykk; 12-13-2013 at 10:15 AM.
12-14-2013, 12:58 PM
#24
Launching!
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A $150 for the Innovate MTX-L, I see no reason not to have one.
I've tuned two carb set ups using the MTX, first was a dual weber DGAS's on an opel V6, and now this Mighty Demon SBC. I can't imagine tuning a carb or EFI without one.
My demon was *** backwards with "as shipped" settings. As expected with carb too large for the app. Really rich cruise, and scary lean high throttle/high RPM.
I've tuned two carb set ups using the MTX, first was a dual weber DGAS's on an opel V6, and now this Mighty Demon SBC. I can't imagine tuning a carb or EFI without one.
My demon was *** backwards with "as shipped" settings. As expected with carb too large for the app. Really rich cruise, and scary lean high throttle/high RPM.
Last edited by 04SilveradoMykk; 12-14-2013 at 01:04 PM.
08-11-2014, 10:05 AM
#26
Launching!
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A little update, I decided to retire the Comp XR276HR-10 cam after suspecting a few issues with it. I wanted to take this opportunity to try a cam with a tighter lobe separation. I found a Howards single pattern cam that caught my attention. 282/282-108 (229/229 @ .050") .480"/.480" valve lift with 1.5 rockers, with my 1.7's it will be .544"/.544"
I opt'd to install the cam -4 crank degrees retard, which in actuality just removed the cam's ground in +4 degrees. Giving it a 108 lobe sep on a 108 intake centerline. I did this in hopes to tip the dynamic compression in favor of pump gas.
With the engine back together, fired up and timed. It ran great, It felt like it lost a little bit of low end but opened up in the higher RPM. Which I anticipated and was looking to shift the power up higher in the curve.
I wanted to see how the engine re-acted to a bit more plenum volume so I dug out my 2" open spacer and gave it a shot. It brought back the low end torque and opened up a bit more as it reved.
The carb we see here is a Summit brand (holley 4010) 750cfm vacuum secondary. It's a good carb, and a bit oversized for the engine as evident by how lean it goes when throttle is opened up and the engine is screaming. Despite going up as much as 10 jet sizes and air bleed tuning.
I've got another Demon to try, a 650cfm Annular booster. We'll see how it does with the new carb soon.
I also want to see how it acts to stacking my 1" tapered spacer on the 2" open.
I opt'd to install the cam -4 crank degrees retard, which in actuality just removed the cam's ground in +4 degrees. Giving it a 108 lobe sep on a 108 intake centerline. I did this in hopes to tip the dynamic compression in favor of pump gas.
With the engine back together, fired up and timed. It ran great, It felt like it lost a little bit of low end but opened up in the higher RPM. Which I anticipated and was looking to shift the power up higher in the curve.
I wanted to see how the engine re-acted to a bit more plenum volume so I dug out my 2" open spacer and gave it a shot. It brought back the low end torque and opened up a bit more as it reved.
The carb we see here is a Summit brand (holley 4010) 750cfm vacuum secondary. It's a good carb, and a bit oversized for the engine as evident by how lean it goes when throttle is opened up and the engine is screaming. Despite going up as much as 10 jet sizes and air bleed tuning.
I've got another Demon to try, a 650cfm Annular booster. We'll see how it does with the new carb soon.
I also want to see how it acts to stacking my 1" tapered spacer on the 2" open.
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