skinny z

I've been into this engine game for a while. While not as experienced as some, certainly more than most I'd would think. (I've beeen working on carbed engines for more than 35 years) I'm not really after any particular tuning advice but what I'd like to know is who has what regarding their idle vacuum.
Here's my example:
355 Gen 1 SBC.
750 Barry Grant vacuum secondary with 4 corner idle and the "idle eaze" feature.
Custom hydraulic roller built from Comp XFI lobes. 274/282 adv, 224/230 @ .050", 110 LSA, 106 ICL, 58° overlap at .006", 7° @ .050".
RPM Air Gap intake.
170cc iron heads. (RHS Vortec)
9.8:1 scr, 8:1 dcr.
Cranking compression 185 psi.
Timing currently set with 16° initial advance plus 14° via full manifold vacuum advance for a total of 30°.
Idle speed 850 in drive, 900 in park.
Depending on the engine temps and ambient air temps (I run a cowl hood) I have from 11-12" of idle vacuum.
Seems to me I should be able to find more.
The engine is relatively fresh in that it has about 20 000 miles on it so I would expect the ring seal and valve job are all sound.
I've researched hundreds of documents and papers and for example a guy like David Vizard often comments on he achieves " 12-14" of idle vacuum with a rock steady 600 rpm idle" with cams that I think are more aggresive than mine.
Seriously?
So...what have you got?
Even if you're fuel injected, I'd like to know.
Include as much info as you can.

Night rider327

iTrader: (13)

4-5" hg vac @ 1,000 rpm idle in park.
10.34:1 355 gen I sbc
.025" deck height
Wisco 2 VR flat tops 5cc
fel pro 1094 head gasket 4.100x.015"
Brodix IK 200cc heads, 64cc chambers
Howards Solid flat tappet #110812... 286/299* adv. 249/252@.050", 106 LSA
IIRC 79* overlap, IVO 41, IVC 65, EVO 78, EVC 38 advertised
@ .050" 38* overlap, IVO 18, IVC 51, EVO 52, EVC 20
Howards 1.6 ratio alum roller rockers
Edelbrock RPM intake, center divider wall milled down 1" deep x 2" long
1" open carb spacer
Holley 4150 HP 750 cfm mech. sec. carb with 4 corner idle.
18* initial timing, 35* total all in at 3200 rpm, 12* of vac advance on full manifold port

skinny z

Ran your cam specs through my cam manager program. Yeah, you're about 80° of overlap seat to seat .
It's seems obvious that the longer cam has killed some of your idle vacuum. Do you tune with a vacuum gauge? Our spark lead is the same. I've thought of dropping the full manifold vacuum for ported vacuum to see what might happen. It's been so long since I made the switch to full manifold vacuum that I don't recall the numbers.

Idle vacuum notwithstanding, I was originally running a .026" gasket with my .014" piston depth and (naturally) ended up with the same static compression ratio as you (similar pistons and head cc). Problem was, with the short cam, my cranking compression was over 205 psi with a DCR of 8.5:1. I couldn't keep it out of spark knock.

Thanks for the data.

Night rider327

iTrader: (13)

Your welcome. Yes I tune with a vac gauge and in fact drive with an autometer sport comp vac gauge mounted in the car.

I can't remember the numbers for ported vs manifold vac but I remember when I was dailing this one in, ported vac advanced justed on my engine. Response was lazy, a/r ratio was richer and fuel mileage was very poor. Poor now at 7 mpg but it was down to like 5 mpg on ported vac.

I had one engine combo before this one that was cranking at 218 psi but I tuned it to run knock free for the most part. I lost a ring land on a piston but that was after 4 years of daily driving and just about every weekend at the track. It wasn't easy to get out of detonation but by keeping coolant keeps under 190*, keeping inlet temp down with a true sealed to hood scoop cold air setup, 3 steps colder plugs, timing pulled back to 32* for street, 34* at track, 93 octane pump fuel, and having to jet carb pig rich. It seemed to work. The loose "2800" stall converter than acted more like a 3600 stall helped a lot I think, it kept the engine from being loaded as much around town.

skinny z

Your experiences are very similar to my own. Even down to trying to keep a high compression iron headed engine out of detonation. I only hope I haven't done any permanent damage doing what I've been doing. I'm not sure if my low vacuum (lower than it's been anyway) is due to the change in the weather around here, (not to mention the change in altitude because I've moved) or because I've broken something. Might be time for another compression test before I put the car away for the winter. I've also seen the same results with full vs ported manifold vacuum as you have.
I have a vacuum gauge and wide band O2 gauge in the dash and keep tabs on everything full time.
Getting the highest idle vacuum sees about a mid to high 13:1 AFR. That varies a lot with ambient temps though. I like to keep a lean cruise for the most part as I put on a lot a highway miles. So far it's been about 20 mpg highway which is saying something for mid 12 second car that weighs 3650 lbs.

It'll be interesting to see what others have to say.

InfernalVortex

iTrader: (4)

I have an xe262 (flat tappet, 218/224 @.050, 262/270 total)

355, around 9.5-9.8:1....

my biggest regret in this build is not measuring piston-deck clearance and quench. So I have no idea what my real world SCR or quench is... but according to theoretical specs going by my piston compression height, I should be ~.065 in the hole, which by my calcs ended up with my at around 9.7:1 But if Speed Pro was lying about my pistons having a 1.560 compression height... then who knows. Gotta live with it now...

Anyway, holley 650 DP, Edelbrock Performer RPM intake, 355, etc blah blah.

Anyway, with around 15-18 degrees idle timing (it settles different every time I bring it back to idle... whatever, max timing is always the same so... )

To the point... 700 RPM idle - 14" vacuum.

I feel like I should have WAY more than that... but it is what it is.

Night rider327

iTrader: (13)

Wow. 20 mpg on a full weight 12 second car is pretty darn good. What trans, stall (if auto), and rear gear are you running?

My old combo was a th-400 with the loose 2800 stall, 3.42 gears.
New combo is same th-400, 9.5" tight 3500 stall, 3.89 gears in a ford 9"

I know the 3 speed trans is a big part of my bad fuel mileage but major diff in my 7 mpg and your 20 mpg.

Night rider327

iTrader: (13)

Yours is inline with what I see as normal for a build like that.

I have built many milder combos like yours and end up in the 14-16" range at idle.

The 355 I have in my S10 is close to yours. A little more compression. 10.25:1, 216/228@.050" 112 LSA cam, vortec heads, 15" vac at 650 rpm idle

One of my dad's engines 9.2:1, 355, brodix IK 180 heads, 204/214@.050" 112 lsa cam. 16" vac at 650 rpm idle

skinny z

You can get an excellent idea of what your compression ratio is through a compression test. A while back I started building a data base of engine combinations and test results and correlated that to altitude and temperature. The results were, for the most part, quite accurate. My 205+ psi corresponded to the 10.4:1 of my initial build and my current 185 psi relates to the 9.8:1 I have now. Naturally it's related to running or dynamic compression rather than the static ratio.
I'm finding that ignition timing plays a large part in idle vacuum which should be obvious. It just seems that so many are pegged to limiting their initial timing to the standard 12 degrees or so. There's much to be had with more. Just watch the total of course.
Your 15-18 is better than a lot of what I've seen. As for your varying values when it returns to idle, I've found a couple of things that can cause this. One is too loose a spring on the advance weights and there insufficient tension to pull the weights back in. Proper lube on the advance mechanism is important too. The other thing I've seen, and this is case with my own distributor is that it's a little on the worn side. I get a big jump in advance amounts in the first 100-200 rpm of the advance curve and then it settles to a more linear curve as you would expect. Something I'll have to address in the off season. One other thing to look out for is if you're using full manifold vacuum to pull in the advance. Unless set carefully, the higher vacuum of the higher idle speed keeps the vacuum advance engaged and raises the rpm. Then it's a circular thing where the advance keeps the rpm up and round and round you go.

skinny z

I'm running a 700R4 with a 10" TCI converter. The best I can foot brake to is about 2200 rpm before I start to creep. It does flash in excess of 3500 on the chassis dyno though. 3.73 rear gear. Cruise rpm at 70 mph is about 2400 with the lockup engaged. I've tuned the AFRs to an ultra lean 15.5-16:1 at cruise (depending on ambient temps). My timing at that rpm is 40 degrees. I could use more too but it's a struggle with my dynamic compression ratio. When I accelerate moderately from cruise (so as not to kick out out the lockup clutch or down shift) I can hear the engine rattle. Something I hope hasn't busted a ring or piston.
Maybe you should consider a gear ($) vendors overdrive. We had one in a Chevelle I helped tune and it made a huge difference. We tried to set it up to split shifts too but the electronics weren't available at the time so it was strictly an overdrive unit. That Chevelle, with a 355, a Comp XR288HR and 3.73 rear gear knocked down in excess of 20 mpg too. A lot of tuning went into that one. Ran mid 12s at 3600 lbs or so.

skinny z

That's -2 degrees of overlap @ .050".

That's -15 degrees of overlap @ .050".

skinny z

Perhaps I should include in the data, the elevation.
I've since moved from 740' above sea level to 2450'.
I'm not sure how much that affects the vacuum overall.

Night rider327

iTrader: (13)

I'm not sure how much it effects it but I know elevation, temp, humidity all effects cranking compression. So if it can change cranking compression it has to have some effect on vacuum

skinny z

I have a math formula around here someplace that ties it all in together.
On one of my tours, one car in the group, that had a typical idle vacuum of about 7" at or near sea level could barely manage 3-4" once we made our way to the Grand Canyon.

Orr89RocZ

iTrader: (20)

9.0:1 comp turbo motor
246/252 at .050 hyd roller. 113.5 lsa
245cc heads, single plane efi

About 11-12" at 1000 rpm idle

Thirdgen89GTA

iTrader: (5)

stock displacement LT1.
11.1:1 Static Compression
231/239 duration @ .050
282/290 duration @.006, 110LSA. 66° overlap.
Ported stock castings, ported intake, LT's wide open, no exhaust. First start on the new combo. Still on initial tune. About 16-17" vacuum at 900rpm according to the PCM. Mech gauge showed 13", but I had the PCV valve unplugged too (oopsie)

skinny z

22 degrees of overlap at .050" compared to my 7.
My understanding is that turbos have little if any effect on idle. What do you figure your idle timing is?

skinny z

The LT design certainly allows for a healthy compression ratio. Nice idle vacuum too.
For what it's worth (as you probably know), some collector extensions on those LT headers will make a world of difference. Low speed torque goes for a dump with the exhaust pulse created due to the short collector length.

IROCThe5.7L

427 SBC
10:5:1 compression
AFR 220 Heads (65cc)
Hydraulic roller cam: Duration @ .050" Lift: 242/248. Lift: .576/.600. 112LSA
HSR intake
Hooker 2210's. 3" dual exhaust.
12-13" at 900rpm idle

skinny z

Another fine example. How about your idle timing?

Orr89RocZ

iTrader: (20)

Turbo is like a restrictive muffler lol. Think my idle timing is up around mid high 20's. i used to keep it at 30

91 camaro racer

iTrader: (2)

350
9.5 compression
tf 195 heads
hyd. flat tappet cam, .458 lift, 218 duration at .050, 110 LSA
rpm performer intake
14.5" at 850 idle

InfernalVortex

iTrader: (4)

All of these apply, although when I am checking total timing and base idle timing, I disconnect my vacuum advance, so it isnt affecting the numbers for vacuum or anything else.

But I definitely thing the weight posts or something in there are a little worn. They always end up a little sticky, and whenever Im at the autocross I always give it a quick rev before my run to make sure I get the weights kicked out and moving. I wasnt really sure about what was a safe "lubricant" in there, considering there's high voltage spark flying around inside the distributor I didnt really know if I wanted to put a flammable oil under it all.

And I also have very light springs on it.

I just figure it doesnt really matter all that much so I haven't tried to "fix" any of it yet, but if there's a good reason to I will probably just throw a heavier spring on one of the weights.

IROCThe5.7L

Running 26 degrees of timing at idle.

skinny z

For those that have taken the time to tune their engines, it seems that this is rule rather than the exception. I'm running 30 degrees at idle with half of that supplemented by the vacuum advance. That in itself can present a problem or two with a higher compression engine. Still it hepls to support excellent idle quality.

TinnMann2

I know this post is a little aged.

85' 5.0L LG4, q-jet, vac advance, stock cam and pretty much everything else is stock except for 2300 stall and rebuilt 700R4 (prepping for vortec swap). I was recently tuning with a vacuum gauge to get my timing right, it idled like garbage and had brutal cold starts. I started at a weak 8" then started advancing and looking at the vac gauge, not the timing tab. now im at 17" vac at 3400' elevation, it runs so much better when i timed it with the vac gauge, but my vac advance canister hit the intake manifold and i think it could run even better if i could advance it a but more. Do you think my distributor is out one tooth??

skinny z

I would doubt that the distributor is "out" a tooth however advancing the timing as you have, has pushed the stock placement to the limit it would seem. I get the same thing with my engine although it's far from stock, especially with the taller Air Gap intake.
As a remedy, when I found I couldn't get any further rotation out of the distributor in order to increase the initial advance setting, I pulled the distributor out, rotated the oil pump drive counter-clockwise so that I could drop the unit back in one tooth from it's original position. I then move the plug wires around the distributor accordingly. This allowed me to rotate the distributor away from the manifold interference and set the timing where I wanted. To nail it down more precisely, I also have a degreed harmonic balancer and rotated the engine to the amount of spark timing I wanted. If was easier then to determine where the number one position ended up on the distributor cap.
That little set of instructions may be difficult to follow but that's what needs to be done if your situation is like mine.