Team Chevelle banner
1 - 7 of 7 Posts

·
Premium Member
Joined
·
8,120 Posts
Discussion Starter · #1 ·
Hey Guys .....is there a rule of thumb for Idle vacuum produced by cam spec ?? Example my old BBC UD solid roller 251/260 110 LSA made 9hg at 950 idle

New cam 252/268 108 LSA am I correct in assuming it wont make as much vacuum at idle and probably wont idle below 1100rpm 439ci BBC

Just got it running to-day and need to dial in the carb more but rough tune looks like 1100rpm idle with 6-7hg .......should expect I to get it to idle with more vacuum or is this pretty much the norm for a cam of this spec ... was thinking of increasing timing in the vacuum advance can ....to get better idle vacuum ??? currently at approx 1100rpm it has 30* of advance with vacuum advance disconnected and 40* with it hooked up ...... as I type this realize my dist curve set up to start advancing based on a 900rpm idle ....maybe need to recurve for this cam as again assume this one will never idle that low ..........Manual trans car
 

·
In Memory of Harold
Joined
·
1,796 Posts
I am not much on carb tuning, but your new cam, with a 108 LSA vs a 110 LSA, and 8* bigger on the exhaust at .050, will pull less vacuum than the UD 251/260.
Advancing your new cam may help increase the vacuum a little, so would a looser intake valve lash.
Sorry that I couldn't be of more help (Carbs just confuse me.......).

UDHarold
 

·
Premium Member
Joined
·
13,104 Posts
Hey Guys .....is there a rule of thumb for Idle vacuum produced by cam spec ?? Example my old BBC UD solid roller 251/260 110 LSA made 9hg at 950 idle

New cam 252/268 108 LSA am I correct in assuming it wont make as much vacuum at idle and probably wont idle below 1100rpm 439ci BBC

Just got it running to-day and need to dial in the carb more but rough tune looks like 1100rpm idle with 6-7hg .......should expect I to get it to idle with more vacuum or is this pretty much the norm for a cam of this spec ... was thinking of increasing timing in the vacuum advance can ....to get better idle vacuum ??? currently at approx 1100rpm it has 30* of advance with vacuum advance disconnected and 40* with it hooked up ...... as I type this realize my dist curve set up to start advancing based on a 900rpm idle ....maybe need to recurve for this cam as again assume this one will never idle that low ..........Manual trans car
=
Hi Bill
Not what you would call a cam guru but,,, what the hell here is my thoughts,, :D
Basically you are correct, the tighter LSA along with more timing is a lose-lose situation with regards to idle
I don't think it is going to get much better in a small engine with that timing
But the engine does not care what the .050 timing is, it cares how much total overlap you have from the time the intake starts to open to the time the exhaust closes.
.050 numbers are reasonable for comparing cams but they are just a rough guideline
The cylinder sees the timing from the time the valve opens to the time it closes
 

·
Premium Member
Joined
·
8,120 Posts
Discussion Starter · #4 ·
Thanks guys appreciate your info ..... just wanted to confirm that I should not expect this cam to make as much vacuum or idle as slow as the old one and spend a ton of time trying to get the carb adjusted to get the idle better I want to get carb close before I call Patrick to see what if any changes I need to make to carb based on cam change PV for sure .... and yes Harold i did in fact advance the cam 2* more then the card install spec ......
 

·
Registered
Joined
·
7,729 Posts
I would personally try more initial timing. The vacuum advance might be making the idle worse when the vacuum is so erratic. Just try experimenting by setting your base time to like 36* with no vacuum advance and see how it is. You might have to lock out the timing if that cures it.

Dave
 

·
Vendor
Joined
·
9,410 Posts
Hey Guys .....is there a rule of thumb for Idle vacuum produced by cam spec ?? Example my old BBC UD solid roller 251/260 110 LSA made 9hg at 950 idle

New cam 252/268 108 LSA am I correct in assuming it wont make as much vacuum at idle and probably wont idle below 1100rpm 439ci BBC

Just got it running to-day and need to dial in the carb more but rough tune looks like 1100rpm idle with 6-7hg .......should expect I to get it to idle with more vacuum or is this pretty much the norm for a cam of this spec ... was thinking of increasing timing in the vacuum advance can ....to get better idle vacuum ??? currently at approx 1100rpm it has 30* of advance with vacuum advance disconnected and 40* with it hooked up ...... as I type this realize my dist curve set up to start advancing based on a 900rpm idle ....maybe need to recurve for this cam as again assume this one will never idle that low ..........Manual trans car

Bill,
Looks like your questions have been answered.

Let me know how the throttle reponse is after you get some time on it...
 

·
Premium Member
Joined
·
8,120 Posts
Discussion Starter · #7 ·
Bill,
Looks like your questions have been answered.


Yes your right appreciate the responses ...... just wanted to confirm what I thought I knew , so I wont try and get more idle vacuum than about 7hg ...

Let me know how the throttle reponse is after you get some time on it...
Will do, might be a few weeks before I get it out on the road .............Damm it has a nasty idle but I will get it sorted out .....already talked to Patrick and he has given me some good advice
 
1 - 7 of 7 Posts
This is an older thread, you may not receive a response, and could be reviving an old thread. Please consider creating a new thread.
Top