[Firebob]

Finally decided to join the 20th century and go to manifold vacuum instead of ported. Now I figure I should revisit the tune on my carb because I'm now running about 20 degrees more advance almost all the time. That's going to have to change tings a bit.
Any thoughts on what I'm in for on the Qjet?

Robert

[TedRamAirII]

I'm experimenting with that now. I think we will need a vac advance can that has less advance than a normal one, now that we will have a lot more advance at idle. I have a old Sun Distributor Tester. Its confusing with Dist Degrees and RPM, that needs to be doubled.Plus I think it needs to be all in at idle speed. So your idle vacuum needs to be enough to pull all it in. I bought a Vac Advance from Orielly 2-5310 it should have 15 degrees, but I am going to maybe limit that. I have just begun. I have not gotten anywhere but I need to start somewhere. 93 octane with 72CC chambers is not getting it done. I would for sure get the timing like yoiu want, before the carb. http://www.digitalcorvettes.com/foru...ad.php?t=75830

[Tom Vaught]

Lot of discussion on both sides of the bench on what is better for a Pontiac engine with 72 cc heads as far as Ported Vs Manifold Vacuum.

There was a post in the street section a bit ago where a guy was using a vac distributor can from a later GM engine that seemed to work well for him.

I have used the old 427-435 HP and the 454-469 hp type Corvette Vac advance cans as those engines had low vacuum due to the camshaft used.
More engine vacuum and they seem to get a lot of advance at idle. Then you can install a "Advance Limiting Bushing" on the vac can stem and at
max vacuum you only have a given amount added to your engine.

Tom V.

[Navy Horn 16]

Originally Posted by TedRamAirII

93 octane with 72CC chambers is not getting it done.

Go to your local airport and get some 100LL. A few gallons per tank and your problem is solved.

[RocktimusPryme]

Interesting article from the corvette forum. Though his advice is slighly contrary to some I've seen on here. Specifically I've seen people say below 12 initial timing with the vacuum disconnected is fine.

[Blued and Painted]

A mentioned in other threads, ported and manifold vacuum is the same everywhere except at idle. So main metering and power enrichment should work the same as before.
The idle speed/throttle angle may need to be reduced. Then re-visit the idle mixture screws. Adjusting to highest vacuum/rpm at a given throttle angle.
After adjustments, make sure the car will still idle if distributor vacuum is disconnected and plugged. You dont want it to stall if there is blip in intake vacuum signal.

[steve25]

Let me ask a loaded question here!
If a given motor is making enough cylinder pressure ( by means of Head , Cam or both) to call for a need to delay the rpm point where full mechanical timing can be put in to above 2800 rpm , then what would be the need to have the Vacuum can hooked up to full Manifold Vacuum.

[Cliff R]

With some engines there is simply no need to run a lot of timing at idle.

This topic gets significant coverage on the Forums, but much of the information is "regurgitated" from the Lars article, and most folks doing so really don't know what's going on with these things.

Anyhow, we tune for a living here, and have all sorts of vehicles brought in here that have troubles in that area. Some actually come from great distances. We've had them trailered here to Ohio from as far away as Georgia after every possible resource available to the owner was utilized and still poor end results.

Recently had a PRESTINE 1968 455 powered Hurst Olds Cutlass brought in here from a little South of here. It was a very rare care, and had spent much of it's life as a full race car, then meticulously restored back to "stock" and sold that way. The proud new owner couldn't get it to run well despite having every "guru" in his area take a stab at it. It showed up here with the timing over 30 degrees at idle and it ran very poorly. It turned out that part of the restoration included a W-30 camshaft in the big 455, the one that is close to the Pontiac RAIV cam, so it was a bit lacking in idle fuel and someone had "hacked" up the carb and hand drilled the main jets out to .080" attempting to get more fuel to it.

The carb was returned to it's stock calibration with a slight modification for more fuel at idle and some bypass air. The super-light spring kit was removed from the distributor and it was returned to stock with a new vacuum advance to add less timing.

We set the timing to 12 degrees, 22 from the mechanical advance and 15 more from the VA via ported vacuum. With the added idle fuel it idled perfect in and out of gear. The owner drove the car and returned with a big smile on his face telling us it had never ran anywhere near this good since purchasing it. Another success story and we didn't have to run a ton of timing at idle speed to get it done.

I could easily fill up many pages here with specific examples and how we cured them with carb and distributor tuning, but time is limited. I will say this, each engine combination should be set up for what it wants and needs for idle fuel and timing at idle. Doing one without the other typically yields less than satisfactory results.

WAY before any of this happens folks should do enough research to make good choices for compression ratio and camshaft used. If this done correctly you'll very seldom, if ever find your engine wanting or needing a lot of timing at idle speed.

As for specific tuning, each engine combination will be it's own deal, and always give it want it wants for idle fuel and timing for best results, and NEVER simply add a ton of timing to one to cure insufficient fuel delivery at idle speed from the carburetor, that's just a "crutch" fix.....IMHO......Cliff

[PontGuy]

Originally Posted by Blued and Painted

A mentioned in other threads, ported and manifold vacuum is the same everywhere except at idle. So main metering and power enrichment should work the same as before.

Exactly right but often not understood by newbies to the old tech. The manifold vs ported decision only impacts idle quality, and maybe throttle response off idle.

Carrying this just a bit further, whether the connection is ported or on the manifold, vacuum advance drops out and has no impact when you step on the gas hard and the engine is under load. Vacuum advance only adds timing when the engine is loafing along and during light acceleration. So you can't compensate for a poor mechanical advance curve by messing with the vacuum advance, or visa-versa. Another point that is not obvious to newcomers to the old tech.

[Cliff R]

Correct, vacuum advance is a load sensing device, and only applied at light engine load/high vacuum situations. With any heavy or full throttle vacuum quickly falls off to about nothing so no additional timing will be added by the VA. It will also be in at idle IF you have it hooked to manifold vacuum, AND have a vacuum can that is fully applied at the vacuum the engine makes at idle speed. Many vacuum cans are NOT well suited to using manifold vacuum advance at idle speed, hence why they make adjustable units.......Cliff

[TedRamAirII]

No arguing with Cliff. The RAIV Tune Up Spec used 15 BTDC @ idle. With a distributor degrees of 10-12 degrees. So that makes total timing of 35-37 degrees, if I am reading it correctly? The Vac Advance was 10 distributor degrees, all in at 15" (ported). Of course that was in the good old days with high octane leaded gas at the pump. My plan was to used Manifold Vacuum, and limit the total so I can use 93 octane. I believe my Comp Ratio is close to 10.7

[Cliff R]

Real RAIV engines actually do well on pump fuel even at pretty high compression ratios without a lot of special tuning from the distributor. This simply happens because of the long duration of the RAIV camshaft and how much overlap it has.

A very good customer of ours has a RAIV powered GTO and drives it a lot on a steady diet of 92-93 octane pump gas w/o any issues anyplace. A few years back when it was time to "freshen" up the engine he was talked into lowering the compression down to 9.5 to 1 and installing an XE274 camshaft. The builder told him it would actually make more power AND be fine on pump gas as well.

He was wrong on both accounts, and he ended up pulling the engine back out, putting flat top pistons back in it to get it back to the stock compression, and swapped out the cam for a real RAIV grind. Power was restored and he's back to enjoying the vehicle again......Cliff

[TedRamAirII]

I'm sure my tune is way off, so we will see what happens. I'm planning a call for QJet parts when I figure out what I want/need. Thanks Cliff.

[Firebob]

How much advance did an original vac can add on a 70 455 4sp GTO? How many total degrees should you run just cruising down the road? And how much is too much? 45?50?

[Cliff R]

The 1970 engine has 87cc heads so pretty high compression, and small cam. It's not going to want or like a lot of timing anyplace tuning it for pump gas. I'd start out with a pretty short mechanical curve and not a lot of initial timing, and about 8-10 degrees from the vacuum advance.

You have to let the engine tell you want it wants, and it will vary some depending on quench distance, position of the camshaft (ICL), fuel quality/octane, engine running temperature(s), altitude, air quality, auto or stick shift, etc.

That engine is a perfect candidate for the RAIV camshaft, which will make it a lot more user friendly on pump gas for sure, plus it will make a LOT more power as well.......Cliff

[steve25]

For part throttle / best gas mileage highway cruising you want as much vac advance as you can get away with per rpm in worst case scenario's, as in high engine temp, high ambient air temps and lower then normal octane fuels.

Many factory Dizz's came set for 52 degrees or more total at part throttle.

The lower a motors compression , the slower the peak cylinder pressure builds and the more need for greater levels of added part throttle vac advance.

[Firebob]

Ok so maybe that wasn't the closest example motor to what I have. So if running higher compression and a bigger than " normal" cam, then you probably don't need as much total timing?

[Formulajones]

That's going to be determined on a car by car basis.

If you mean total mechanical advance under full throttle, a dyno or some track time will find the spot where best power is made.

If you mean total with vacuum advance added for part throttle and light cruise mode, that's going to be a trial and error experiment. But for the most part, adding another 12-15 degrees on top of your mechanical total advance would get you right in the ball park.

It's not unusual to have anywhere from 45 to 50 degrees of total advance in light throttle cruise applications (vacuum advance working) but to find what works best will take some drive time because there are too many variables that affect what a particular car will like. Not just engine combination, but the weight of the vehicle and the rear gear ratio will play a roll.

[Cliff R]

One needs to consider here that the more things you do to increase combustion efficiency, such as high compression, tighter quench, well chosen camshaft, etc, the LESS timing is needed at every rpm to best power and least fuel consumption.

I've seen very, very few of these engines that like over 50 degrees at light throttle cruise. Most of the ones that we do here end up around 42-48 degrees total when you add up initial, mechanical and what is added by the vacuum unit. Engine that need more timing than that will typically be relatively "low" compression and lots of quench in them, and/or having cams that are basically too big for what they are doing.

For most very well thought out engines base timing will end up around 8-12 degrees, about 10-12 degrees from the distributor (20-24) at the crank, and another 10-14 or so degrees from the vacuum advance. There is also no need or benefit to have a super-quick mechanical curve with those engines either, all in by 2800-3200rpms or so is fine.

We ALWAYS recommend using a vacuum advance on anything street driven, even pretty high HP stuff still benefits from a few additional degrees at light throttle for less throttle angle and increased fuel economy.

I still see PLENTY of folks buying "race" type distributors w/o vacuum advance, then running a super-quick timing curve, all in by 1500-2000rpm's or so......and they still believe they are accomplishing the same thing as running vacuum advance. Those set-ups are "old school" thinking, left over from the days when you absolutely had to have a double roller timing chain, high volume oil pump, "camel hump" "202" heads, 3/4 cam, some sort of "high rise" intake, gas guzzling aftermarket carb, and dual point distributor w/o vacuum advance.....Cliff

[RocktimusPryme]

Cliff I'm probably misinterpreting something your saying there. But I don't understand your math. 8-12 base I understand. But your saying the distributor for your ideal setups is only adding 10-12 to the base? So at WOT with minimal vacuum signal your making leak power at 20-24* total timing? And you said most of your engines with vacuum at part throttle end up 42-48.

But even using the largest numbers your giving there it only adds up to 38*.