[77 TRASHCAN]

Cliff, got an interesting Saturday project going on today???

[STEELCITYFIREBIRD]

The car idles well now?
Little more back ground on your deal with this engine>??

Fresh rings, bore, rering or valve job, etc.? Have you driven it under load to seat the rings yet?

[footjoy]

Originally Posted by STEELCITYFIREBIRD

The car idles well now?
Little more back ground on your deal with this engine>??

Fresh rings, bore, rering or valve job, etc.? Have you driven it under load to seat the rings yet?

Sent carburetor to have it reworked it not running

Greg

[Cliff R]

"Cliff, got an interesting Saturday project going on today???"

I had a local customer bring in a carb for his 350 engine. It's in a 68 Camaro and racing it in the Stock Eliminator Class.

It was an expensive carb purchased from one of the "big names" who build them for Stock and Super Stock Racers.

I found some issues with it including damage to the sealing area under the fuel inlet seat and it was leaking past the gasket. Not sure why they used a Carter casting but I see a LOT of those type issues with them. I finished it up by mid-day and worked on a 455 I purchased recently........

............John Deere 455 that is!........

I also shot a couple of ground hogs and made some more carpenter bee traps.

Elena also found a new friend.....

[SD455DJ]

Originally Posted by 389

Dennis I would try removing the vacuum advance and bringing up the initial to 18 or 20..... You will need a 160 thermostat to do this... I would try 89 gas too if your using 87..

The 2801 is an excellent cam for a powerful street Pontiac motor..

389 - Not sure I'd want all that initial timing...starting would be a kinda hard with the almost 10 to 1 compression and mild cam timing. It dyno'd at 380+ hp/450+ lbft with the 12 initial and 36 total on 93 octane pump gas, and that's where we left it. Right now we haven't driven it with the vacuum advance operational and it was super torquey without it.

Curious why you would suggest so much timing???

Dennis

[Will]

Originally Posted by SD455DJ

389 - Not sure I'd want all that initial timing...starting would be a kinda hard with the almost 10 to 1 compression and mild cam timing. It dyno'd at 380+ hp/450+ lbft with the 12 initial and 36 total on 93 octane pump gas, and that's where we left it. Right now we haven't driven it with the vacuum advance operational and it was super torquey without it.

Curious why you would suggest so much timing???

Dennis

Getting the initial up to 14-16 *might* make the engine feel more "snappy" down low but unless you shorten the curve you'll have too much total and will pound out the rod bearings in a hurry. I can't imagine running 18-20. I've run higher initial advance on lower compression engines but you probably don't need it at 10:1 and you're flirting with detonation at that CR with that cam so I'd probably just leave it alone if it all runs good. I hammered out a set of rod bearings running too much timing because the engine just felt so good and responsive with 16 initial but I had a 22 degree curve and that gave me 38 total which I knew I didn't need as the car ran best at the track with 32-34.

[SD455DJ]

I agree with Will's assessment and will add that we have found generally that engines with 9.5 and higher compression need less initial timing (8-12 degrees) and total between 32-36 degrees depending on the actual compression, cam, and heads. Lower compression engines require another 4-8 degrees to both initial and total. Every engine is unique and we work to find out what ignition timing and fuel curve suits each the best, most the time on the dyno. That gets us in the ball park and we tweak again once the engines are back under the hoods realizing that the real world changes things a bit.

We do like to run vacuum advance on the street for better drivability and fuel mileage, but try to limit the cannisters to the 10-12 degree units (when we can find them, or use aftermarket adjustable) and 14-16 degree units for the lower compression engines. We find the final settings are always a little different between two identical engines...defies logic, but there are enough tiny differences that add up to require different optimizations.

Obviously, when you step up to a RAIV cam in a 10:1 400, it's a new ball game and more initial timing and different jetting is needed.

Sorry for the thread hi-jack Greg!

Dennis

[Cliff R]

There are very few good vacuum advance canisters left that provide ideal timing plus correct cut-in and all-in points.

There used to be some aftermarket units available that controlled the amount of advance with a small allen wrench thru the vacuum fitting, but most being sold these days only increase or decrease the spring pressure.

They are however very easy to modify for how much timing they add.

Well thought out engine combos well seldom if ever need much more than about 12-14 degrees initial timing and 10-15 from the vacuum advance. I set up most of the distributors built here with 10-11 degrees mechanical (20-22 at the crank) shooting for 10-12 degrees initial once the engine is placed in service. It is rare to find an engine that wants, needs, likes or responds well to much more timing than that at idle speed. In almost all cases when a troubled engine combo shows up here for custom tuning where the owner/tuner is having to run the timing clear off the scale just to get it to even try to stay running below 900rpm's or so, they simply have made a very poor cam choice or the carburetors idle system doesn't have enough fuel at the mixture screws, or a little of both.........Cliff

[footjoy]

Originally Posted by SD455DJ

I agree with Will's assessment and will add that we have found generally that engines with 9.5 and higher compression need less initial timing (8-12 degrees) and total between 32-36 degrees depending on the actual compression, cam, and heads. Lower compression engines require another 4-8 degrees to both initial and total. Every engine is unique and we work to find out what ignition timing and fuel curve suits each the best, most the time on the dyno. That gets us in the ball park and we tweak again once the engines are back under the hoods realizing that the real world changes things a bit.

We do like to run vacuum advance on the street for better drivability and fuel mileage, but try to limit the cannisters to the 10-12 degree units (when we can find them, or use aftermarket adjustable) and 14-16 degree units for the lower compression engines. We find the final settings are always a little different between two identical engines...defies logic, but there are enough tiny differences that add up to require different optimizations.

Obviously, when you step up to a RAIV cam in a 10:1 400, it's a new ball game and more initial timing and different jetting is needed.

Sorry for the thread hi-jack Greg!

Dennis

Its all educational for me. I just have to find someone who can build a distributor for me to get more advance at part throttle in overdrive at 2300 rpm. If thats possible.

Thanks

Greg

[Formulajones]

This should go without saying but appears that it needs to be mentioned.

Anytime you feel you need to run more initial timing, you can't just turn the distributor and call it done. You MUST go back into the distributor for breaker plate modifications and limit the centrifugal advance the same amount to keep your total timing in spec. Other wise turning the distributor advances the entire curve.

That's pretty self explanatory but sounds like guys are just turning the distributor for more initial and then having detonation issues.

On your timing question Footjoy, with overdrive and low rpm cruising, anything below where your total mechanical advance stops, the vacuum advance is going to be a player in those rpm ranges at light throttle cruising.

[SD455DJ]

Originally Posted by Formulajones

This should go without saying but appears that it needs to be mentioned.

Anytime you feel you need to run more initial timing, you can't just turn the distributor and call it done. You MUST go back into the distributor for breaker plate modifications and limit the centrifugal advance the same amount to keep your total timing in spec. Other wise turning the distributor advances the entire curve.

That's pretty self explanatory but sounds like guys are just turning the distributor for more initial and then having detonation issues.

On your timing question Footjoy, with overdrive and low rpm cruising, anything below where your total mechanical advance stops, the vacuum advance is going to be a player in those rpm ranges at light throttle cruising.

X2! We will reduce the built-in advance (10 degrees) if we know we are going to have a high initial setting (15 degrees) in a low compression engine such as a 455HO. Like Cliff said, keep the total mechanical advance between 32 - 36 degrees. I may have to drive my Tempest without the vacuum advance just to stay out of detonation since I have 9,94 compression, mild cam timing (2801) and using 93 octane pump gas.

Dennis

[Formulajones]

Originally Posted by SD455DJ

X2! We will reduce the built-in advance (10 degrees) if we know we are going to have a high initial setting (15 degrees) in a low compression engine such as a 455HO. Like Cliff said, keep the total mechanical advance between 32 - 36 degrees. I may have to drive my Tempest without the vacuum advance just to stay out of detonation since I have 9,94 compression, mild cam timing (2801) and using 93 octane pump gas.

Dennis

I'd be curious to know how it works out. Looking at the 2801 it's close to the 068 as far as duration and LSA.
I use the Melling 068 in my 400 with 10.13:1 (about exactly where you are for the most part) and all we have is 91 octane here. I'm actually running 14 initial in it, only because I set it up with 20 centrifugal, thinking it might only run at 30 or maybe 32 total, putting me at 10-12 initial, but when I found it runs best times at 34 degrees total timing, of course that put me at 14 initial. I just left it that way, wasn't ambitious enough to dig back into it. The vacuum advance has always been right at 10 degrees additional at light throttle cruising. Been a few years now, I drive it daily and have raced the snot out of it always looking for more. It's been absolutely detonation free, very good mpg at 16-17 highway, and never complains when cranking after a good heat soak. It seems to be a very happy engine.

[SD455DJ]

The initial 10 mile drive without the vacuum advance connected went without incident and I was 'breaking it in' with full throttle blasts in second gear up 5000 rpm (or more) and let it engine brake to continue seating the rings. There wasn't a bit of detonation or pinging...but the chambers and pistons are pretty clean yet. The engine was very peppy without the vacuum advance, but I will try it eventually to see how it runs. I may end up turning back the initial to 8-10 degrees (32-34 total) if there is any rattling going on. We'll keep the tune sharp, as we do with all our cars. It does start well after a good heat soak too...I'm always grateful for that!

Dennis

[Cliff R]

There would be no negatives running vacuum advance during "break-in". It only applies at light engine load. If it's "peppy" without it adding another 10 degrees of timing or so at light load will make things even better......IMHO.

When we read about folks experiencing pinging when using vacuum advance they are either adding too much, or their engine combination isn't quite ideal and making too much cylinder pressure early in the RPM range.

Even my 455 with over 11 to 1 compression gets another 10 degrees of timing via ported vacuum. It runs fine without it hooked up and barely noticed as I've forgot to hook it up a few times, but the added timing helps burn a lean mixture and minimizes carbon build up, etc.....

[footjoy]

OK I got the 2bbl back from Dick B and it started up and carries 15" vacuum. 15 will be fine I don't have PB.

Thanks there is some good info in this Thread.

Greg

[STEELCITYFIREBIRD]

Good base line, note the ignition advance /rpm @ idle and compare after seating rings.
Good you made progress!