Hot Rod Forum : Hotrodders Bulletin Board - View Single Post - 350 SBC / Vortec Heads: Cam Choice?
View Single Post
  #6 (permalink)  
Old 12-10-2012, 06:20 PM
oldbogie oldbogie is online now
Registered User
 
Last photo:
Join Date: Apr 2007
Location: Seattle, Wa
Posts: 6,686
Wiki Edits: 0

Thanks: 3
Thanked 403 Times in 348 Posts
Quote:
Originally Posted by andrewakessler View Post
I love this message board. I have no clue how I'd complete this project without it.

While doing some research last night, I came across this "low cost" 350 SBC build: Small-Block Chevy Build - 391HP Chevy 350 Engine For $2,613 - Hot Rod Magazine

What I liked about the article was the fact they included a full build list in Excel that I could follow. The issue I had with it though was the inclusion of the "oval track" cam.

Everything else seems to line up with the info given here though, so I'm feeling pretty good about it.
I had not seen that issue, made an interesting read. I agree with you about the cam BIG TIME. Here again is cam and piston choice raising its head to show you how subtle differences in parts selection make big differences in power. I'd never use one of those circle track cams for a street build; these are often lift mandated cams for which more duration is substituted in an attempt to drive out more power at higher RPMs.

This leads you down several paths not optimum for a street engine

- A rougher, lower vacuum and higher RPM idle. This leads off to several things like limited vacuum for power brakes, a higher stall speed converter for example.

- Lack of compression with this set up, the later closing intake at 71 degrees say compared to the XE268 at 61 degrees requires more compression to get the power up.

- The low lift against the longer duration doesn't take advantage of the Vortec's better porting. Whenever you can use parts that take advantage of more lift with less duration you're better off to go that way at least on the street.

- Excessive cooling, if you read Smokey very carefully, you'll see he leads you to maximum power is achieved with a cold mixture being stuffed into a hot engine. Not a cold mixture into a cold engine. In his waning days on this earth he was working on a super hot engine as an approach to way more power. He's not the first to go down this path, but for half a century it's been in view theoretically but the materials engines are built from and perhaps physics interactions we don't yet appreciate have stymied this effort. Still running cold tap water through the cooling system did nothing to help.

It once again shows that what works for a rules limited race engine and what can be optimal on the street for a performance engine are not the same things in all cases. While close to the 400 horse barrier this engine just isn't going to get there unless you dive into the pistons and replace the cam with less duration having more lift. I mean you can get there with tuning but itíll be too edgy to stay there long; a different parts selection is needed to get up on that plateau and not have to constantly fiddle the thing to stay there. The condition HR found itself in with 1.6 rockers and 1-3/4 headers is that both of these, if not actively working against the combination, certainly didn't add anything. The 1.6 rocker opens the valves faster as well as higher. While higher could work to their advantage on the Vortec head, faster just made the overly rampy cam act like it had more ramp. That worked against the compression ratio and burn characteristics of the combustion chamber with the circular dish pistons. The 1-3/4 tube headers reduced the overlap signal that could have been used to help with initiating intake flow in what David Vizard calls the 5th cycle.

I'm sure with tuning this engine could crack the 400 barrier but it would not be economical to operate with the late closing intake and lacking compression. So while cheap to build it would be more costly than necessary to operate.

Bogie
Reply With Quote