stevo1988

So my car has not been on the road for long, and I am driving it like an old lady. There has only been a few times when i needed to jump the gas pedal to get the hell out of a sticky situation and Im noticing spark knock occuring just after 2000 RPM. I am running a modified SBC 355 with a stock distributor with a good set of wires and brand new plugs. The motor runs at about 9.3:1 CR with a mild hyd flat tappet cam and 0.485" valve lift. the heads are cast iron with 2.02/1.60 valves. Additionally Im running a stock torque converter with a th350 trans going to a 7.5" GM 10-bolt rearend with ~2.5 gears (stock!) the motor breathes through a set of hooker long tube headers to a very UNrestricted dual exhaust with an X-pipe.

I tried anywhere between 8-deg to 14-deg of initial timing (no vac or mech advance) but unfortunately i dont have timing tape or a dial-back timing light so I am not sure what my total timing is, but i know its all in by 2800-3000RPM

I have no mechanical advance happening at idle so the springs must be a little stiff, but when i gradually rev past 2000RPM i get knock. If I punch the throttle it sweeps through the RPM quickly and no knocking occurs.

I am thinking about re-curving my distributor but Im trying to wrap my brain around the vacuum and mechanical side and how they work together at cruise but opposite at idle/WOT. Please help

ALSO: I just went for a spin and im only noticing the knocking after 2000RPM when i gradually accelerate in either 1st or 2nd gear. when i am in Drive (no kickdown) and i accelerate past 2000RPM gradually I do not hear knocking

stevo1988

Maybe a detailed list will be helpful

1985 Small Block Chevy 350, 4-bolt main, 2-pc rear main seal
Bored 0.30 over with Sealed Power DDP Series flat-top pistons (4-valve reliefs = 5cc i believe)
World Products S/R Torquer heads with 67cc chambers, 170cc intake/67cc exhaust with 2.02" intake and 1.60" exhaust valves
Manley Stock Replacement Stainless valves
Comp Cams Hydraulic Flat Tappet Hi-Energy 268 Cam (218*/218* duration @0.050" 0.302"/0.302" lobe lift, 110 LSA)
Comp Cams hyd f/t lifters
Comp Cams stock size 1-pc pushrods
JEGS 50-Series Gold full Roller Rockers w/1.6 Ratio (0.485"/0.485" valve lift)
Comp Cams Valve guides (machined rocker stud surface 0.125" and machined guide holes for clearance)
Comp Cams valve springs #981-16 Single spring w/dampers
Comp Cams retainers and locks
Milodin Double Roller timing chain set
JEGS 331 Champion Series High-Rise Dual-Plane Aluminum intake manifold
AC Delco Spark plugs
Delco HEI Distributor (Stock)
Edelbrock Performer Series #1406 600CFM Carb w/Vac-Secondaries, Elec-Choke
Stock Torque Converter
Turbo 350 Transmission w/Shift kit and no kick-down
Stock Driveshaft
Stock rearend (7.5" GM 10-Bolt with 2.5 gears)
The car is a 1985 Oldsmobile Cutlass Salon (not entirely sure on weight but estimating between 3600-3800 lbs)

v8sten

iTrader: (5)

first of all you need to get a dial analog timing light if you wat to fine tune your engine. Especially if you want to modify the timing curve of your hei. typically the centrifugal advance has around 20-25 deg of advance built in. so you are on the right track. Sounds like you are in the 11-1 or more compression ratio range with a flat top piston and 67cc combustion chambers. that being said you will not be able to run full timing on pump gas. with todays pump gas i would say you could safely go as high as 10.5-1 CR on pump gas and be able to run full timing. for ***** and giggles put a bottle of NOS Octane Booster in your tank and see how it runs. If the knock or pre-ignition goes away that proves my theory. more compression on an old school street motor does not mean more power.

stevo1988

I forgot to mention that I'm running the vacuum advance off the port above the throttle plates on my carb. I havent tried the other one yet, is it possible that maybe im advancing too fast in the mid-RPM level? If so will the other port bring the vac advance down?

When I calculated CR i was getting 9.35:1 when you include gasket thickness and the amount of CCs that the pistons add with their valve-reliefs. Plus at TDC i have about 0.020" before the top of each piston and the deck. I have been running 93 octane.

I want to mention that it only seems to happen when Im in either 1st or 2nd gear. It never happens in 3rd no matter how i try. I know i am getting to the limit of my initial timing because every so often if i start the car when its hot, the starter struggles a little bit to turn over. I have tried lowering the initial advance but i still get knocking. could i simply be advancing too much as I gradually climb up the RPM?

v8sten

iTrader: (5)

disconnect the vacuum advance. and it WILL run much better. seriously just disconnect it and plug the vacuum ports off. believe it or not with the centrifugal and vacuum advance both working it put the full timing around 50+ deg of advance. hence you get detonation or knock.

stevo1988

Thanks for the advice, I am going to try it tomorrow and see how that works!

Big Al

iTrader: (1)

The above has no relation to truth. Vacuum advance is exactly what it says. No vacuum, no extra advance. It's there to help with part throttle cruise. Don't disconnect it. Check your total advance and see if the vacuum advance is working correctly. It should not be "sticky." You can test it with a hand held vacuum pump like this:

http://www.amazon.com/Mityvac-MITMV8.../dp/B0002SQYUA

Or you can just put a timing light on it at idle and suck on the hose and see if it advances smoothly and returns the same way.

Check total advance with the vacuum advance plugged. You need to rev the engine to about 4,000 rpm to see if it has really topped out. Then see what RPM the advance starts and ends at. For a street engine it should not top out at less than 3,000 RPM. I usually set total advance at around 34 degrees. You can get more power with more advance but you have to run decent gas. If it's knocking at 2,000 RPM that's where you need to check things.

It may also be carburetor related. Lean mixtures tent to knock.

Al 95 Z28

v8sten

iTrader: (5)

did unplugging the vacuum adv work, stevo?

bww3588

iTrader: (49)

I had Damn near the same setup you do. The 600 carb for me was not enough. It ran way too lean.

As stated, do not disconnect your vacuum advance. It's a great thing when it's working properly.

stevo1988

disconnecting vacuum advance did get rid of the knocking but it idled terribly and i get a constant smell of fresh gas when i drive. Throttle response suffered at low RPM but it really liked cruising and WOT above 2000 RPM. I guess this means that i have a lazy centrifugal springs and it advances too much causing the knocking.

If the 600cfm carb causes the motor to run lean at cruise then wouldnt it run good with the extra timing from the lazy springs?

Should i get stiffer springs or should i be running a bigger carb? (re-jet possibly?)

stevo1988

Update:

I used a vacuum gauge to monitor manifold vacuum so I could set the mixture screws. It idles alot better now (vac advance plugged) at about 700RPM pulling 14in of vacuum. The smell of gas has greatly reduced and pedal response is also alot better. The car seems to perform much better than it ever had and the only thing I am noticing is that my exhaust bellows a bit louder when cruising. I think Im going to keep it this way until i can get the Crane adjustable vacuum advance kit part # 99601-1

v8sten

iTrader: (5)

damn ......... and people doubted me.

bww3588

iTrader: (49)

Just because it's not working properly does not mean you were right. As stated, when vacuum advance is working, it's a great thing.

To the op, do you have your vacuum advance hooked up to ported or manifold vacuum? It needs to be hooked up to ported vacuum for it to work properly.

v8sten

iTrader: (5)

you can tune an hei to where it doesnt need the vac adv by simply changing the springs in the hei. and you can achieve full advance at around 32-36 deg just by the centrifugal advance. i have done this many times and it has worked every time on every hei dist i have ever worked on and fyi that is a whole lot of them. its very simple to do and performs and drives a whole lot better than trying to make the vacuum advance work.

Stevo originally what vacuum port was it hooked to? the one on the base plate or the one on the metering block? if you are to use the vac adv it should be hooked to the vac port on the metering block, just like bww said it should be hooked to a ported vacuum port. try that out and tell us how it reacts and how much full timing it get when it is hooked up, and if the knock comes back. my guess is it will run fine down low but the higher the rpm it will put too much timing into it and detonate (knock)

bww3588

iTrader: (49)

Agreed, you can make it work. Many people do just as you stated with vehicles that have donkey dick cams that don't pull hardly any vacuum. Of course these are generally race only cars.

I have spent probably just as much time as you have actually making the springs and weights do the work, making the vacuum advance work.

The thing that makes vacuum advance awesome in my opinion is the advance at less than half throttle that makes tooling along the interstate gravy train.

LilJayV10

iTrader: (39)

What's your intial and total timing?

Disregard. Just re read the first post.

My .02 is invest a timing light with a dial on the back of it. You can get one pretty cheap at Auto Zone.

stevo1988

I am going to need a dial back timing light to check total timing, My timing tab only shows up to 18 degrees advance and i did not use timing tape on the balancer (I should have!)
But without vaccum advance i am at 8 degrees initial. Anything more and it starts to knock very slightly around the same rpm as before.

As for the other questions, I got the best results using ported vacuum (metering block) than i did using the manifold vacuum on the port below the throttle blades.
Using manifold vacuum made the engine knock every time you gently press the gas at low rpm. Using ported made it only knock after climbing past 2000rpm which made it driveable as long as im not holding speed higher than 85kph

without the vac advance i had to play with the mixture screws and curb idle to get the same idle quality and actually it idles much smoother than it had before. It starts up without choke with the winter weather getting even colder. The motor seems to stutter as you ease on the gas at lower speeds but with a little footsies with the pedal i can get it to accelerate smoothly. It also accelerates much faster and the secondaries kick in really hard like a mechanical carb. I am not hearing any knocking and pinging but im not doing any other damage am i? I am monitoring my engine temp and it isnt higher than normal, i cruise at 180-185 F. I have also heard that GM used to sell crate engines with no vacuum advance on the dizzies

bww3588

iTrader: (49)

As far as I know, every engine up until the tbi years had vacuum advance, after that it was electronic. Every large cap hei distributor I have seen had vacuum advance.

bww3588

iTrader: (49)

If you're getting knock at 8 degrees initial, something else is wrong. I ran 14 initial and 32 total on the last 355 I built and it ran great.

What is your compression ratio? What octane gas? You might try stepping down to a colder plug to curb the detonation.

LilJayV10

iTrader: (39)

You should not have spark knock with only 8 degrees of initial timing. Are you using the correct balancer? If so is it possible for the outer ring has slipped? What timing tab are you using? I have seen aftermarket ones be way off.

If some of this has been answered i apologize its late and i'm half asleep.