Hot Rod Forum : Hotrodders Bulletin Board - View Single Post - Ported -vs- Manifold - Vacuum Advance Scenario
View Single Post
  #5 (permalink)  
Old 01-12-2009, 02:13 AM
curtis73's Avatar
curtis73 curtis73 is offline
Hates: Liver. Loves: Diesel
Last wiki edit: How to find cheap parts
Last journal entry: Found an LQ9 today
Last photo:
Join Date: Dec 2005
Location: Carlisle, PA
Age: 43
Posts: 5,472
Wiki Edits: 16

Thanks: 31
Thanked 52 Times in 44 Posts
Originally Posted by techron
it sounds to me from your post like you are trying to use the vacuum advance for a substitute for centrifigal advance and locking the dist. at 36*. how does that engine crank with 36* advance without kicking back??? how is it at 1000/1500/2000 RPMs at full throttle with 36* advance???
Not at all. 8.5:1, plus inefficient 049 stocker heads, plus a 218/224 cam meant needing a ton of initial advance. Having an automatic tranny (even with a stock stall) means you will never have 1000-2000 RPMs at WOT. My converter flashed to 1800, so a split second and you were past 2000.

The more important measurement (when using ported vac) was how it did just off idle at 2000 rpms with the vac can at max advance on a hot so-cal, sea-level day... and it was fine. I measured as high as 52* total advance at simulated cruise on the dyno with no detonation. If I tried really hard I could get a tiny touch of audible detonation on the hottest days, but a swap from 50/50 coolant mix to 70/30 water took care of that. Hot cranking never gave kickback, but I could tell it was working a little harder when hot.

SO, i stand by my earlier post:

ALWAYS use manifold vacuum...
And I'll stand by mine: Using absolutes to apply one "best" solution to the billions of possible combinations that exist in the new-school as well as the old-school is a little narrow-minded.

PS: i still work on early vettes, i'm doing a body off resto-mod on my 64 coupe. i built a 383 stroker with 10.7-1CR (but i know how to make it live on pump gas) i'm also helping a friend/noob 65 vette owner build a late engine 383" stroker because he wants to be faster at the strip...
Sounds sweet in a light Y-body like that. I'm currently building a 383 LT1, 11.03:1 that should make somewhere north of 445 hp, pass CA OBD2 smog, and it will actually burn 87 octane with the obvious use of the factory knock sensors and some very advanced chamber designs. I'll tune the PCM to run on 91 where its designed to run, but so far I've done a few that passed OBD2 with flying colors at 425-435 hp and 11:1, including a couple 396s. I'm working on some new head port and chamber designs with AI to see what else we can squeeze out of the old LT1.

to the OP----> go with MANIFOLD VACUUM-->ALWAYS...
And I'll stick by mine... Proper vacuum source --> always. There are no absolutes. techron, I totally respect your experience, but there are a lot of things that worked well with leaded gas in the 60's that just don't fly today, especially when considering the modern combustion chambers, quench theory, more modern valve event controls, etc... you have to admit that comparing a Vortec head and modern gas to a camel hump with leaded gas are two vastly different things... as evidenced by your 38* total timing recommendation. There is just so much that has changed that its not really logical to make absolute statements like that. I agree that for the most part your statement would be true up until about 1972, but in the 37 years since then, the widely varying range of engine combinations renders that kind of absolute advice obsolete.

Again, I'm coming from a totally respectful perspective. I don't mean to neglect your experience in the industry, but as a former engineer/consultant for Rochester Delphi, I was constantly required to know the latest and most advanced combustion physics on the planet. Of course, my greatest contribution was whittled down into the TBI/swirl port engines, so maybe I should keep my mouth shut
Reply With Quote