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  #16 (permalink)  
Old 07-28-2008, 09:48 AM
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biscuit,
here's the knowledge base spacer article...

http://www.nhra.com/dragster/1999/is...echnology.html

Answer A:
for heat soak/percolation/vapor lock/etc... (use any oem) approx 1/4-3/8" thick 4 hole carb vapor lock insulator gasket from your local store for $10?
it works fine and very likely the 31 squirters will again give excellent crisp motor response....
the Edelbrock 4 hole insulator gasket is $14?

Answer B:
your specific combo, of different brands, pretty darn well matched parts may well like having a 1/2" of the divider wall missing alot at WOT on your total car combo.....
(EX:it can work: the "stock" Edelbrock Airgap RPM dual plane does have about a 1/2" of the center divider ground away so it does behave a bit like a open plenum)....

Answer C:
my best guess for why your motor does like the open spacer alot and is a total bear to tune out the bog is partly/largely(?) due to the 1.6 rockers.....
the 1.6 rockers do totally change the operating characteristics of the cam and A/F flow dynamics....
those rockers are the one item that's "different" on your motor and weren't used to engineer/design your motor's performance parts....
your 227-234 cam/180cc heads/intake do make a excellent mid range rpms carb signal....so just minor carb adjust for the open spacer is the typical solution (with 1.5 rockers)....
the "fly in the oitment" is the 1.6 rockers????

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Last edited by red65mustang; 07-28-2008 at 09:55 AM.
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Old 07-28-2008, 09:24 PM
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Thanks guys. I enjoyed both articles and passed them on to a friend. Looks like I have some reading to with respect to understanding oxygen sensors.

I am going to fiddle with the jets and the timimg for another few evenings before I throw in the towel and throw in the four-hole spacer. Biggest reason being I am enjoying the learning process associated with getting it wrong and then making stepwise improvements.

The 1.5 rockers is a compelling idea if cost were not part of the equation. Desktop Dyno shows more-or-less no output difference between the two rocker ratios. One more learning expereince...
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Old 07-28-2008, 10:32 PM
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Quote:
Originally Posted by biscuitville
After installing a 1/2" phenolic carb spacer to avoid percolation, my throttle response has been altered much more than I expected. The car nows bogs heavily upon full acceleration. I am running a 9.9 static/8.5 dynamic CR gen I 355, Brodix IK 180 heads, Perfromer RPM dual-plane intake, Holley 650 cfm double pumper (with choke tower cut and smoothed out), and Hedman 1.625" long tube headers which expel through 2.5" Dynomax mufflers. The cam is a Lunati Voodoo 227/233 and the carb had a set of 69s up front, 77s in the rear, and a 6.5 power valve. The squirters were 0.031"; switching to 0.035" and then 0.037" incrementally helped lessen the bog time. It seems a little too obvious to think that I should move up to 0.040" or larger, especially since the car accelerated so crisply without the carb spacer and using the 31 squirters. [The plugs look a little black and wet to me, before and after any changes.] My question is (finally) "am I tuning the carb incorrectly"?
Thanks in advance
You're probably trying to tune out a problem that has nothing to do with mixture ratio.

- Lets get to vacuum first. Vacuum is about 85 to 90 percent dependent upon the timing characteristics of the cam, barring the presence of a vacuum leak. Everything else, ignition timing, compression to a small extent the exhaust, and certainly the size of the carb are things that are playing in the remaining 10 to 15 percent of the variables.

- A spacer doesn't always make things better, though I realize you put this on for heat management issues, it none the less has changed the characteristics of the intake system by making the plenum larger. That alone will tend to move the power band up the RPM range. It will also contribute toward damping the signal on the carb which will tend to change fuel circuit response. With greater lag at low speeds for which you will try to cover with fuel jetting and the power valve, only to discover the things too rich on the top end. This almost always gets you into air correction jet changes in the carb and stuffing boxes in the manifold, fuel dams, vortex generators and the like. It can get to be a real PIA that can drive changes in the type of booster venturis the carb uses.

- Then the problem gets worse if you used an open spacer. This not only makes the plenum bigger by the size of the spacer, whoa, it now lets any and all cylinders to see the total CFM capability of the carb and they see the total size of the plenum. The power band really moves upstairs because the low to mid range RPMs can't develop enough volumetric need to signal the carb in the rather tight band of information to fuel flow it wants to see. Like above, your tendency to cover the bogs and flat spots is to throw more fuel. That as usual doesn't work for the same reasons already stated above.

These problems can be worked out but without a good background in the theory of whats going on and a closet full of carb parts, it can get really dicey trying to find a solution.

Check your air cleaner for being wet with fuel, that will tell you if reversion is also a problem. Reversion can make the engine seem lean, while it's swimming in fuel. Or it can form standing waves just under the boosters cutting off signal to the main metering. The different shape boosters you see for carbs are in one sense an attempt to get around this by moving the booster in relation to known standing waves in the air flow.

Knowing that you had a good combo before, I'd start by trying to get back as close to that as I could. Maybe a stack of 2 or 3 gaskets between the manifold and the carb would be a solution that minimizes gains in plenum size, while reducing the heat flow into the carb to an acceptable level.

Check this out, http://store.summitracing.com/partde...5&autoview=sku

While this is an open plenum, it does cut the size down to a quarter inch which will make life a little tougher for transfer flow from one side to the other. But better yet, using this idea, some hand tools, and piece of hardware store sheet metal with a couple carb to manifold gaskets you can carve one out in an hour that keeps the throttle bores isolated if that was your original configuration. The metal doesn't need to be a quarter inch thick, simple thin gauges around a sixteenth inch would be just fine, all you want is a place to catch and dissipate heat before it gets to the float bowls.

Bogie
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Old 07-29-2008, 10:08 AM
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biscuit,
your welcome...
for the future, a more effective way for shifting the power band up or down (and keeping decent tuning control parameters) is by advancing (more low end) or retarding (more top end) the camshaft which changes the relationship of where is the piston in the bore (further up or down) when the valve does begin to open....
CAUTION: retarding a cam for more top end power does reduce the valve to piston clearance!!! (your 1.6 rockers may not allow retarding depending on your pistons and those Brodix heads clearance)

more info at the page bottem:

http://www.lunaticams.com/Tech/Cams/CamSpecTerms.aspx

last thought:
the 1.6 rockers do increase the cam duration...
more cam duration means the motor wants/needs more base timing to make more low end TQ...
(EX:your 19* base is 2-3* more than typically needed by that cam)
it's normal/typical for a really big cam (290+?) to use 20-22 base and only 16-14 cent....
very very often just a 2* base timing change does make a world of difference for performance...
it's possible (?????) that the combo of 1.6 rockers and adding the spacer is adding more duration/less signal....so more base is needed?

was 36* all in at 2100rpms a typo????
(36* all in at 2800+ would be more typical for no WOT detonation at your CR unless the car weighs next nothing and you have very deep gears for no load on the motor while winding up)
there is a very very slim chance (?????) that changing the dist springs may help (a tiny bit?)....
(to much timing to soon can hurt just as much as not enough soon enough depending on the car combo)....

a favorite Max Keith (a member) saying: :ALOT more "gooder" is not necessarily "more" good!!!

edit add:
here's a link to my favorite "basics" ign tuning article.....
another saying that is so true so often is "80% of all carb problems are in fact ign/timing problems"

http://www.gnetworks.com/v4files/bar...withimages.pdf

Last edited by red65mustang; 07-29-2008 at 12:23 PM.
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Old 07-29-2008, 02:41 PM
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Quote:
Originally Posted by oldbogie
You're probably trying to tune out a problem that has nothing to do with mixture ratio.

- Lets get to vacuum first. Vacuum is about 85 to 90 percent dependent upon the timing characteristics of the cam, barring the presence of a vacuum leak. Everything else, ignition timing, compression to a small extent the exhaust, and certainly the size of the carb are things that are playing in the remaining 10 to 15 percent of the variables.

- A spacer doesn't always make things better, though I realize you put this on for heat management issues, it none the less has changed the characteristics of the intake system by making the plenum larger. That alone will tend to move the power band up the RPM range. It will also contribute toward damping the signal on the carb which will tend to change fuel circuit response. With greater lag at low speeds for which you will try to cover with fuel jetting and the power valve, only to discover the things too rich on the top end. This almost always gets you into air correction jet changes in the carb and stuffing boxes in the manifold, fuel dams, vortex generators and the like. It can get to be a real PIA that can drive changes in the type of booster venturis the carb uses.

- Then the problem gets worse if you used an open spacer. This not only makes the plenum bigger by the size of the spacer, whoa, it now lets any and all cylinders to see the total CFM capability of the carb and they see the total size of the plenum. The power band really moves upstairs because the low to mid range RPMs can't develop enough volumetric need to signal the carb in the rather tight band of information to fuel flow it wants to see. Like above, your tendency to cover the bogs and flat spots is to throw more fuel. That as usual doesn't work for the same reasons already stated above.

These problems can be worked out but without a good background in the theory of whats going on and a closet full of carb parts, it can get really dicey trying to find a solution.

Check your air cleaner for being wet with fuel, that will tell you if reversion is also a problem. Reversion can make the engine seem lean, while it's swimming in fuel. Or it can form standing waves just under the boosters cutting off signal to the main metering. The different shape boosters you see for carbs are in one sense an attempt to get around this by moving the booster in relation to known standing waves in the air flow.

Knowing that you had a good combo before, I'd start by trying to get back as close to that as I could. Maybe a stack of 2 or 3 gaskets between the manifold and the carb would be a solution that minimizes gains in plenum size, while reducing the heat flow into the carb to an acceptable level.

Check this out, http://store.summitracing.com/partde...5&autoview=sku

While this is an open plenum, it does cut the size down to a quarter inch which will make life a little tougher for transfer flow from one side to the other. But better yet, using this idea, some hand tools, and piece of hardware store sheet metal with a couple carb to manifold gaskets you can carve one out in an hour that keeps the throttle bores isolated if that was your original configuration. The metal doesn't need to be a quarter inch thick, simple thin gauges around a sixteenth inch would be just fine, all you want is a place to catch and dissipate heat before it gets to the float bowls.

Bogie
Excellent - for my 420hp 351W flat tappet cam'd motor - I have found that using two phenolic carb spacers help me with heat and top-end power. One 1" four-hole spacer on top of a 1" open spacer - that might be too much for your set-up - don't know? Good luck.

Last edited by Sixtyninemercury; 07-30-2008 at 08:53 AM.
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Old 08-03-2008, 01:08 PM
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Thanks to everyone for the tuning help.

Gentlemen,
So here is the current solution which appears to be working the best. The front jets were set at 73, the rear at 79 and the 6.5 power valve was left in. Idle vacuum in neutral (975 rpm) was 12.5" and 10" in gear (700 rpm).

I checked with the guys at Brodix and they told me it would be fine to run the LV 60103 cam with 1.6 rockers.

The hesitation at WOT is now gone. I checked the air filter for fuel reversion and I am not seeing any.

Through all of my changes, my engine temp has been a consistent 205 F.

I rechecked the timing and here are the numbers:
Degrees (+) Engine RPM
16 975
20 1150
24 1300
28 1800
32 2350
34 2800

The vacuum advance is still not hooked up. I attempted to do via manifold vacuum and the engine wanted to cut out. Since several folks have mentioned the importance of using the vacuum advance, I would still like to hook it up but thought I would ask the following first. Would I need to make changes to the idle screws or the intial timing to get a smoother idle?
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Old 08-04-2008, 12:39 PM
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Quote:
Originally Posted by biscuitville
Gentlemen,
So here is the current solution which appears to be working the best. The front jets were set at 73, the rear at 79 and the 6.5 power valve was left in. Idle vacuum in neutral (975 rpm) was 12.5" and 10" in gear (700 rpm).

I checked with the guys at Brodix and they told me it would be fine to run the LV 60103 cam with 1.6 rockers.

The hesitation at WOT is now gone. I checked the air filter for fuel reversion and I am not seeing any.

Through all of my changes, my engine temp has been a consistent 205 F.

I rechecked the timing and here are the numbers:
Degrees (+) Engine RPM
16 975
20 1150
24 1300
28 1800
32 2350
34 2800

The vacuum advance is still not hooked up. I attempted to do via manifold vacuum and the engine wanted to cut out. Since several folks have mentioned the importance of using the vacuum advance, I would still like to hook it up but thought I would ask the following first. Would I need to make changes to the idle screws or the initial timing to get a smoother idle?
Your mechanical advance comes up pretty fast, that combined with the vacuum is probably too much advance, which is why you're getting the reaction from the engine you see.

Vacuum advance is beneficial for an engine that sees a lot of low to moderate (idle to 2500 RPM operating where the throttle is not open a lot. This makes for a low density fill in the cylinder which burns slowly, so more spark advance is employed to give the mixture more time to completely burn. This is a benefit to fuel economy and emissions by not wasting fuel as unburnt energy going out the exhaust.

However, vacuum advance can be a problem as an engine is set up more like a competition engine. As cam timing gets longer with large overlap periods and late closing intake, the manifold vacuum decreases because greater throttle opening is required to keep the enigne idleing. The low speed mixture is still low on density, but now for a different reason, the mixture is being reversed pumped back thru the late closing intake.

There are a couple solutions hot rodders typically take with ignition timing:

The first, more oriented toward hotter cammed and faster turning engines based upon overall gear ratio and typical operating speeds, is to use some combination usually of more static advance and a faster but shorter mechanical curve. A variation on this theme can be less base advance with a faster and longer mechanical. But usually these engines idel better with a lot of initial advance.

The second, more orientated toward a milder, but still could be a performance level cam, with none to small amounts of overlap and a reasonably early closing intake valve where manifold vacuum is fairly high and the operating RPMs fairly low. This requires a lot of low speed speed advance that is sensitive to engine loading as defined by manifold pressure (vacuum). These usualy employ low to moderate base advance with a long and slow to come in mechanical, with a large amount of vacuum advance to fill the void between the low base and late to come in mechanical. By "long" I mean that there are a lot advance degrees in the timing plate within the distributor, this would look like long slots in the plate. "Short" would fewer advance degrees in the advance plate. Since the total advance engines like is pretty much fixed around 30 to 40 degrees a lot of this depends on the combustion chamber shape, number and location of sparkplugs. The effect is how to get the best performance from the enigne between idle and about 2500 to 3000 RPM which is where mechanisms for advance come into play. Against a given counterweight mass in the centrifugal, the return springs control how fast the advance comes in. Heavy springs slow the rate of advance against the rate of building RPM. Light springs allow the advance to come in faster.

Vacuum advance can be used with a hotter cam but this is usually an adjustable can type that allows the mechanism to be adjusted to the cam and how the engine is operated.

From all these combinations that I listed, you have to find out what works best for your situation, this is the tuning part of building the beast and it can be difficult to do. But to say there is one perfect solution for everybody, is to under estimate the complexity of getting the engine tuned to how its used and how you want it. Yes, in the latter statement there is an element of being personally satisfied with how it runs against how you want it to run. I guess that's artistry in tuning. So in the end, if you don't have vacuum advance, things can still be OK with the proper accommodations in the base and mechanical.

Bogie
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Old 08-04-2008, 04:56 PM
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different vac cans have different amounts of degrees per inch of Hg curve calibrations and different ranges for total degrees possibly added and how much Hg before any timing is added.....
(there are 100's of different ones)

the particular vac can you plugged in may well have way too many degrees per inch of vacuum or if it is a adjustable unit it may be adjusted full on and adding way to much to soon....

here's a pic' graph example of a (Acell) adjustable unit (which reads in dist *'s, multiple x 2 for crank *'s added).....

because your combo only has 12.5Hg at idle, to find out how many vac advance *'s are being added at high Hg with your particular unit wind the motor up to 3000+ with the vac adv plugged in and read your timing....EX: 52* plugged in minus 34*'s of base and cent=vac can is adding upto 18*'s depending on how high the Hg is (regardless of the rpms)

the main reason I would want "some" manifold vac advance on your ride is the only 10Hg at a stop light....
carbs just don't make a decent "atomized" A/F mix with much less than about 12Hg.....
using this chart example, at 4 turns and 10Hg it will add a minimum of 10*'s to the 16* base at a red light and that will raise the carb Hg a bunch (very likely the Hg will be well above the 12Hg needed on the 4 turns line so that the vac unit will add 14*'s to the 16* base for clean plugs in traffic)....

if you can't get the vac adv to work, and you do alot of intown driving, I would bump the base up to 18* and that should make 12Hg+ in drive at 750 idle....
(if your heads best timing is 32-34*max, adjust the base back to 16* at the strip)

edit add: that chart 'could' represent 8 different non adjustable can models (model 1 turn, model 2 turn, etc) your vac can may well be a "model 8 turn".....adding 28* on top of the cent and base....
typically on a performance motor adding 10*-14* is plenty and not run into part throttle detonation

I hope this post helps, it's a bear for me to explain vac advance.....
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Last edited by red65mustang; 08-05-2008 at 06:59 AM.
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Old 08-04-2008, 08:49 PM
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Red and Bogie

Thanks guys for the thorough explanations and the graph. Responses such as yours are much more useful than the one-liner dogma often put out. Will attack the timing with a new approach tomorrow evening.
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