Hot Rod Forum : Hotrodders Bulletin Board - View Single Post - chevy milage best option, transmission, heads or rear end
View Single Post
  #8 (permalink)  
Old 06-04-2010, 05:39 AM
cobalt327's Avatar
cobalt327 cobalt327 is offline
WFO
 
Last wiki edit: Intake manifold
Last journal entry: 1980 Malibu Wagon
Last photo:
Join Date: Mar 2009
Location: Atlanta
Age: 59
Posts: 5,037
Wiki Edits: 1616

Thanks: 128
Thanked 597 Times in 546 Posts
Quote:
Originally Posted by 350pano
When you say accelerator pump, is this the powervalve in holly type carbs, or the cam on the throttle plates?
What I mean is the plastic cam on the throttle shaft that actuates the pump.

As delivered, the white cam has the least amount of volume but a cam can be modeled to provide less than this amount. If you have a selection of pump cams, I'd suggest you use the white cam as a template to scribe an outline on a "larger" cam (a cam that you presumably wouldn't be using) to use as a starting point for making your own custom cam.

This might take some trial and error but might result in some savings.





Quote:
Haven't considered shooter size yet, will do some reaserch on that and get back to you.
The smallest Holley shooter is 0.025". You can go smaller by soldering the holes closed on a tube discharge type shooter, shown on the right, below:



The whole thing w/using a bare minimum accelerator pump volume and shooter size is that the engine will need to be up to operating temp to run correctly (w/o any flat spot off idle). This may or may not be a concern, depending. You wouldn't want to just sit there while the engine was completely warmed up before pulling away, but if you didn't have a lot of stop and go as soon as you pull out of the driveway, it might not matter nearly as much.

That said, I'm willing to bet that the engine will run fine w/less than the white pump and possibly w/less than a 0.025" shooter.

Quote:
With regard to better flowing heads do you mean have mine ported etc, or get a more modern design with smaller chambers? If I get smaller chambers/higher compression, would I need to use higher octane fuel?
A more modern design head w/smaller chambers like the Vortec head would be my choice. If the quench is kept relatively tight (on the order of 0.040"), because the heads have a good amount of quench/turbulence "built in" so to speak (due to the shape of the combustion chamber), they will support more CR on the same fuel than the smogger 882 heads.

IMO, there's little to be gained by fully porting the 882's- but if that's going to be what you run, pocket porting and a 3-angle valve job will help flow. A 30 backcut on the valves will improve flow as well.

Quote:
Finally, your information on octance numbers etc are interesting. I have noted on this site some people run their chev on 87 octane. Our lowest is 91, which is roughly 4 higher than 87, so it seems I am running on the lowest (cheapest) type possible.
I would agree- 'your' 91 octane is 'our' 87, so to speak.

Quote:
I am getting a little ping justoff idle both excellerating andf decelerating. I believe this is caused by the vac advance coming in to quick or too much to early. I am trying to taylor it, but will probably just retard timing by a few degrees, as our 91 gas is 5c cheaper than the 95 octane.
I'd be interested to see if a higher CR from the better Vortec heads and timing optimized for performance rather than ping resistance would more than offset the additional cost of a higher octane fuel, if you follow me.

In any event, don't take any advance away from the engine if you can tailor the vacuum advance better to avoid the transient ping you described.

Here are some parts that might be of interest to you:

The ACCEL #31035 is an adjustable vacuum advance can for GM HEI that allows infinite adjustment to BOTH the amount and rate of advance. Comes w/instructions and tool.

To limit the amount of advance w/an OEM or some adjustable (for tip-in only- NOT total advance applied) cans like the Crane can, you will need to physically limit the vac can's travel w/a VACUUM ADVANCE LIMITER PLATE- Crane #99619-1, #99619-1 INSTRUCTIONS.

ADJUSTABLE VACUUM ADVANCE CAN KIT- Crane #99600-1, #99600-1 INSTRUCTIONS.
Reply With Quote