[gtome]

There is def a difference with mine, at least for the first few seconds?? And I am getting 14 hg of vac at idle.

[guccieng]

Originally Posted by gtome

There is def a difference with mine, at least for the first few seconds?? And I am getting 14 hg of vac at idle.

if you notice a difference, you could have a bad vac can, a sticking vac can mechanism, or a restriction in the carb or hose that is making the vacuum come on really slow. the reaction of the vac can should be practically instant. i would remove your distributor cap and use a hand vacuum pump to see that you have free movement from the vac can. if you can pump it up quickly and the can slowly moves, then there's your problem.

[Keith Vrabec]

If only I could use heavy full throttle all the time in a street car....PSP (PA State police) wouldn't like it. SHould I take it to the track, I'd try it ported rather than full, and bump the static. And plunk on a set of E heads....

All depends what you're after and your comination. Theory is one thing, driving it daily is another. Gotta do what works for fuel, flexability and economy/effeceincy.

Sputtering up the mountailn until mechanical advance kicks in doesn't do it.

NOt starting on hot soak with too much timing sucks.

Flooring it with instant response in 4th and kicking a bimmer into the weeds and starting on hot soak and not downshifting for every little hill and running on 87 octane is fun. Compromise to shake a bit of performance to make it drivable? Of course. DOes it work? Come and drive it, tune it your way and make it better with all the constraints. It's not wrong, it's what works for the application.

Can always skin the cat tail to tip or tip to tail...Either way it's a skun cat. have to find what works by thinking about what you ask the engine to do and maximizing it for your application.

[kyle_blake]

i believe the comp cam card says to install this cam at 106 ICL (pretty sure its icl) and since lsa is 110 it is 4 degrees advanced. Thats what comp cam recommends to install it at... The term installed straight up from what I learned means dot to dot and should equal 4 degrees advanced in this situation. If all manufacturing is bang on and timing chain works out perfect. But the only way to truely know is to degree it.. !! As we've all learned!

However do all mechanics use the term in the same fashion, do they all degree? I think the term is relative because of how the cam is ground. To me installing it straight up dot to dot would mean neither advanced or retarded but simply installed on 110 ICL.

I believe most pontiac engine builders like to have an "early" intake event and thus prefer to advance it. It seems most cam manufactures agree too. At least with cams i've looked at, which is only really HFT. Plus with chain stretch etc one day you end up with the cam running probably right at 110ICL. I suppose that is much better than starting at 110 ICL and then being retarded after chain stretch and being 114 ICL. I'm using 4 degree chain stretch but who knows maybe it might be only 1,2 or 3 degrees...

Hopefully I'm using the right terms here, it's all based on what I've read in the forums. Anyway so his cam is advanced as per manufacture but it's not "more advanced" from the comp's "recommendation".

If I'm wrong anywhere in this reply, please correct me. For sure.
Glad all is well.

[tr709]

Well, I picked up the car today and it is pulling MUCH harder than before. Still some room for improvement on the low end. But We'll continue to work at it.

I'll keep updating as we make progress.

[screamingchief]

Originally Posted by kyle_blake

The term installed straight up from what I learned means dot to dot should equal 4 degrees advanced in this situation.

Not so fast,the term "straight-up" means the centerlines and the LSA are all the same,your later comments are more accurate in how that term is meant to be used.

Straight up on a 110° LSA cam means the ICL and the ECL would also be @ 110°.

This is true even in a cam that was ground with some advance in it,though in that specific instance it just means the cam needs to be retarded to get it "straight-up" if that's what one is after.

Another "old school" moniker for that terminology is split overlap.

Though for split overlap = straight up also requires that the cam in question be a single pattern cam (same intake & exhaust durations).

Also that term is'nt necessarily accurate anymore,especially with the advent of asymmetric lobe cams,as with those lobes,even if everything is the same (intake & exhaust duration ~ 110°/110°/110°),that does'nt necessarily mean the overlap is split 100% equally,despite what the numbers might suggest.

Anyhow,enough on that for the time being.

Dot to dot is just what it sounds like.

When a cam is installed "on the dots",then you can get what your describing.

That is a 110° LSA installed on the dots showing a 106° ICL and a 114° ECL per the cam card info,thus informing the cam installer that the cam was ground with 4° of advance in it.

Just because a cam is installed "dot to dot" does'nt mean it's installed "straight-up",that would only be correct if the cam was ground without any advance in it,and as a result,both the installed centerlines matched the LSA as described above.

Originally Posted by kyle_blake

However do all mechanics use the term in the same fashion,

Obviously not,but this is what comp has decided that term means to them.

This terminology often gets debated,so it helps to discuss this every so often to get everyone on the same page.

Originally Posted by kyle_blake

To me installing it straight up dot to dot would mean neither advanced or retarded but simply installed on 110 ICL.

That would be combining the two terms,and yes that can happen,but as stated above,that requires (barring any flukes in the chainset or such) the cam be ground with no advance,and that a "dot to dot" install on a 110° LSA cam puts the ICL @ 110° like you stated.

Most Comp custom ground cams will come without any advance ground into them,unless otherwise is stated @ the time of ordering.

And some "race" cams like the 290b6 solid are ground without any advance in them as well.

So yes,if everything is spot on,when a cam like that is installed "dot to dot" on the chainset,then yeah it should also be "straight-up" when checked during degreeing.

Originally Posted by kyle_blake

Hopefully I'm using the right terms here

You got the basic idea down,just a few rough edges to be cleaned up.

Originally Posted by kyle_blake

If I'm wrong anywhere in this reply, please correct me.

No problemo,got ya' covered see above ^^^^.

HTH.



Bret P.

[jakeshoe]

I was a bit disappointed today because it didn't seem to want to break the tires loose like it did on my last test drive. It does run much better IMO but it should be more responsive off idle IMO.

We learned it is very sensitive to timing and not what you typically expect. You could advance it some and it seemed to lose bottom end but still not have any detonation issues.

I'm really starting to think a converter swap might be the best thing to wake it up. A quality ~2500'ish converter.
It is just soft from a standing start. Once it pulls in the secondaries, it seems to run like it should. Fairly strong on the secondaries and I would guess it is shifting near or at 5000 rpm WOT.

I degreed the cam on the recommended 106 ICL. It has 4* adv ground into it, so the timing set is "straight up" or dot-dot but the cam is at the recommended ICL.

[Stuckinda60s]

Ported vacuum is NOT the same as manifold! To say it is ignores physics. Simply opening the throttle blades does not put full vacuum at the ports, it simply increases it. There is a always a pressure drop across the throttle blades, and it's greater at lower throttle openings. This means that the vacuum in the manifold is always greater than above the throttle, if it weren't, there would be no flow. It also explains why an engine with manifold will often pull hills better than with ported. It also explains why the timing just off idle isn't excessive after you increase idle fuel when you use ported but often is when you use manifold.

The purpose of vacuum advance is to allow the relatively small amounts of fuel, at idle, time to be consumed in the cylinder. The only way to have an engine which will idle properly without it, and without advancing initial timing to unmanageable levels, is to increase the fuel in the cylinder for the same amount of air. Because the fuel air charge isn't compressed as much, leaving more distance between fuel particles, this is particularly true for low compression engines. You do that with the idle circuit in the carb, and that increased fuel will be there for low speed driving. If you do that and run vacuum advance at idle then the engine won't idle as well as it will with less timing. Either the fuel is a band-aid to compensate for the lack of timing or the timing is a band-aid for the lack of fuel. It isn't necessary to have both and is actually counter-productive.

In other words, if you do add fuel than you are actually reducing the efficiency of the engine, and the engine may well not run as well at lower speeds when vacuum advance is present. Note that I said may not. It all depends on the fuel curve built into the carb, but one thing is certain, it will burn more fuel to decrease the time it takes for the flame front to travel across the cylinder.

The ideal setup would be the minimum amount of fuel with the minimum amount of timing because the timing gives a negative torque against the piston as it's compressing a charge which is actually expanding at the same time.

For a street engine, which is generally using a carb which is basically off the shelf, It's been my experience that manifold consistently works better than ported, particularly when you consider how much fuel would have to be burned to compensate for the reduced timing.

As far as performance with ported vs manifold? As I stated, manifold is always higher than ported. You might not be able to see much difference at greater throttle openings, but it's there.

As Cliff said, when vacuum falls off to a certain point, depending on the can, there is no vacuum advance. However, if you are at idle or slightly off-idle, manifold vacuum will have the timing fully advanced. If you nail it, the timing is already advanced and takes time to return due to the lack of vacuum. In the meantime, the engine needs time for the fuel from the power circuit to be sucked into the cylinder. The time it takes for the advance to fall off helps keep the engine from bogging. It's not instantaneous. This is also true for a punch at lower engine speeds.

If you tune your package to eliminate vacuum advance. That's great, but most guys don't have optimized carbs and if they do, they don't have another to throw on if they have problems. A lot of guys are also looking to keep their fuel costs down. Don't you think manifold might be the best for them?

[tr709]

Just to add in, for the record we did try both vacuum sources yesterday and honestly there was no real difference.

Like I stated earlier immediate plan is to swap intakes and with the holidays coming I'll proably take some time to mull this over with Jake and take into account some of the great input received on this post. Although the converter change is tempting....

On a positive note this car is now a really nice cruiser! Nice power mid to upper, could have driven all day at 60-65 mph with the top down. Engine temps never strayed above 180*.

[Cliff R]

"Manifold vac gives better idle quality, but it looses the bad ass lumpy lump sound. I havent noticed any improvents anywhere else on it with FMV."

At the very most, this is what should happen switching back and forth between manifold and ported vacuum to the advance.

Just a note on ported advance. Many carburetors will have the ported source located too high above the throttle plates. This is most common with factory carburetors, as some of the ports were designed to operate the EGR valve, not the distributor.

A correctly ported source should show FULL MANIFOLD VACUUM with the slightest movement of the throttle plates right off idle.

When a correctly located ported source is used to employ the advance, the only difference in performance anyplace between MVA and PVA is at idle. The long and lengthy Googled-up crap we've been fed for years on this subject is highly flawed in many areas. Your engine does NOT have to have MVA to the distributor, it is a personal choice based on idle quality.

If your vehicle has to get thru emissions, you will probably find that running a LOT of initial timing will cause it to fail the testing at idle.

Those of you that like to tune, will also find that a lean idle mixture REQUIRES a lot of initial timing, and TONS of stinky exhaust fumes exit the tail pipes when this is attempted. This often makes the tuner think that the carb is too rich, when it is actually too lean. Keep in mind when reading this, if you don't put enough fuel in PPM in suspension for complete combustion at low engine speeds, a LOT of it will not get burned, and exit the tail pipes. It will drive you right out of the garage, and smell up your clothes, etc....sound familiar?

Bottom line, as it relates to this thread, Jake could just leave the vacuum advance unhooked for all of the heavy/full throttle testing, it is NOT a player here........Cliff

PS: when setting the secondary airflap spring tension, it should be relativley tight, not loose. A test drive should show smooth transition from idle to full throttle. This can be difficult to set on a street car, as on occassion the tires will be on good pavement and hook, and other times there is enough road debris/slippery pavement to allow them to spin. This creates two completely different situations for the carburetor, engine heavily loaded as it pulls thru transition, and engine lightly loaded as it moves almost instantly into the mid-range and top end power.

Put some DOT's on it, and you will quickly find the most ideal spring tension for the airflap........

[bnorris_74]

Although the converter change is tempting....

I hear lots of positive results from just that.

I have a new GM25 converter that I'm going to get installed when I have my TH350 rebuilt. Mine is a little lazy from the get go but really comes alive at higher RPMS. I think a higher launch RPM would really make a difference.
I'm also thinking seriously about a Continental if my money situation ever improves.

I've been lurking and reading this thread with a lot of interest. Great posts and reading.

[Cliff R]

The Continental 13" unit would be perfect for this vehicle. It acts and drives like a stock converter, yet will flash high enough on full throttle starts to get the engine quickly into the real power range. They are coupled near solid above the stall speed, and couple equally as well as stock full size converters for "normal" driving. Also less expensive than the popular 10" units......Cliff

[PontGuy]

A bit surprised that the cam timing made that much of a difference, but good work!

If the engine is making good power once out of the hole it seems to me that you may be at the point where compatibility of the parts may be the main issue with the low end. The cam does have some duration to it and the 2.93 gear, Performer manifold, log exhausts and relatively low stall converter are all working against the car being really strong off the line.

I agree with Cliff on the vacuum source. Once you dump the primaries the vacuum advance is not really relevant.

Still watching to see what happens next!

[grandville455]

Glad its working out!... I don't really see that manifold being a problem on a little 400. I ran that same manifold on my 455 and ran mid 13's with it! If u haven't opened up the ports yet that can help if your head ports are opened up any? otherwise i doubt u will see any difference between it and a stock one. I would save your money there and just put it into some better gears or the 13'' cont like cliff said. gears alone would make that thing a tire smoker big time! even some 3.23's would be enough

[guccieng]

both my chebby and poncho give exactly the same amount of vacuum from ported and full manifold. that's with an edelbrock afb clone, a holley street avenger, holley hp street, and a cliff's q-jet. absolutely no difference in vacuum after the throttle is touched.

[GAGuy]

I applaud Jake for taking on this car with so many people watching/waiting and giving their 2-cents worth. I would imagine there is an extra amount of pressure with this project.

It sounds like Jake has done a great job trying to diagnose, fix and optimize a combination that somebody else put together. There is certainly more than one way to put a muscle car combination together and starting with a messed up combination that someone else created can never be easy. If money were not an issue, it would be alot easier to just throw a bunch of parts at it to make it work right.

I think all would agree that as far as this car's problem (a lack of power and an inablility to smoke the tires) vacuum advance is a moot point. This car's problem is the cam is not a good match for the gears/converter in the car or depending how you look at it the gears/converter don't match the cam. (its too bad the cam installed is such a controversial piece). Which you change to get the combination right just depends on your personal preferences.

[chiphead]

I agree, vacuum advance is a non-issue when it comes to these performance complaints.

Under hard accel, manifold vacuum goes away and so does vacuum advance.

Now that the cam is installed correctly, I'd focus on carb tuning, exhaust and drivetrain mods. A perfectly-tuned engine will only give so much power. To optimize the use, you've got to optimize converter and gearing selection. A 2500 stall and 3.23-3.45 gears would be great.

I don't think this combo will see big gains going to a '67-71 intake. Performer is pretty close to a factory intake. I'd upgrade the exhaust, then gears, then converter, then RPM intake. And then it will need a fuel system upgrade, as the ability to revv higher more easily will require more fuel.

[tr709]

BTW I am very pleased with my dealings with Jake and I am comfortable recommending his services

Good input on the intake, important point I failed mention before is that the intake was port matched to the heads and the heat crossover was blocked.

[intense]

There is some great information on this thread ! But I am getting concerned about my combo .. I have a xe 284 in a 72 400 with 355 gears ,16heads, 1.65s, a 2800 stall converter and 3inch exhaust .. It has never been driven out of the driveway , but sounds great .. next year after winter after its finished i guess im hoping its ok.

[gtome]

I agree that the vac is a moot point in this case. Although it does seem with mine I have a little better throttle response with manifold vac.
I may hook the vac gauge up to the ported sorce today and see how much throttle it takes to get the full vac numbers..... just for my own curiousity.