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  #16 (permalink)  
Old 03-28-2012, 04:36 PM
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This is why I build my own custom mufflers and exhaust systems.

And build my own carbs and distributors.
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  #17 (permalink)  
Old 03-28-2012, 04:45 PM
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I'm looking forward to getting my exhausts back from the fabricator... proper merge collectors and a properly-sized straight through baffle should help with scavenging. I have a feeling the previous exhaust with its 2" baffle that had little half-moon shaped plates welded at regular intervals throughout its length and caused the exhaust gas to follow a snaking motion down each pipe may have been throwing the fuelling off and reducing burn time. The previous collectors weren't proper merge collectors, either. I'm pinning a lot on this new exhaust system I've designed (and it's costing a fair amount in stainless stock), so I hope it proves to be the magic bullet I'm hoping it will be.
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  #18 (permalink)  
Old 03-28-2012, 05:01 PM
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I'll bet it's in the module! Try a stock GM module.

In the EFI tuning it has been found that differant modules produce differant timing readings even though timing table has not been changed. It is called latency timing if your into tuning EFI and can be as much as 11 degrees off just between 2 differant number GM EST/ICM modules that are both for same car. Aftermarket modules were not tested.

http://www.thirdgen.org/techboard/di...ncy-table.html
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Old 03-28-2012, 07:05 PM
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Quote:
Originally Posted by v8hed
I'm looking forward to getting my exhausts back from the fabricator... proper merge collectors and a properly-sized straight through baffle should help with scavenging. I have a feeling the previous exhaust with its 2" baffle that had little half-moon shaped plates welded at regular intervals throughout its length and caused the exhaust gas to follow a snaking motion down each pipe may have been throwing the fuelling off and reducing burn time. The previous collectors weren't proper merge collectors, either. I'm pinning a lot on this new exhaust system I've designed (and it's costing a fair amount in stainless stock), so I hope it proves to be the magic bullet I'm hoping it will be.
From your description of what you had to work with there I think your new exhaust system is going to show significant gains.

Did you get to try it with open exhaust with just a extension pipe (collector extension) on the headers?
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  #20 (permalink)  
Old 03-29-2012, 06:01 AM
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Quote:
Originally Posted by F-BIRD'88
From your description of what you had to work with there I think your new exhaust system is going to show significant gains.

Did you get to try it with open exhaust with just a extension pipe (collector extension) on the headers?
No, I was initially planning on doing that, but spoke to a couple of people who confirmed these exhausts are very restrictive. The design of the original system speaks for itself and it's easy to see how restrictive it is. So, I'm confident the new system should be a big leap forwards. It's the only item in the drivetrain that's not easily measurable in terms of losses and it's the only thing that can account for the large discrepency between the figures the motor achieved on the engine dyno and what it achieved on the chassis dyno. What was interesting to me was the shape of the curves from the chassis dyno... the chassis used to have a totally different motor installed (but same drivetrain and exhaust), yet the shape of the curves are remarkably similar between the old motor and the new motor... dips and peaks in the same areas, etc. The old motor was also a 383, but had a Performer RPM FT cam, RHS 230cc iron heads, a Performer RPM intake and an Edelbrock carb (quite a mismatch... I didn't build it)... nothing like the new motor that's in there now, yet the shape of the curves are almost identical. If you weren't paying attention to the numbers, you'd almost think you were looking at the same motor just from how the curves look. This is another huge indicator to me that the exhaust is/was a playing a huge part in things.
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Old 03-29-2012, 08:17 AM
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Quote:
Originally Posted by F-BIRD'88
having the vacuum advance pegged all in at idle is not correct.

use ported vacuum.

if you use full manifold vacuum as a signal source, you should adjust the vacuum advance so little or no vacuum advance is actually added at idle.
and the vacuum advance does come in at hiway cruise speeds at part throttle light engine load Make sure it does not ever exceed 12-15deg at highest manifold vacuum (deceleration) by ensureing the vacuum advance travel is physically limited to 12-15deg.
if it pegs the vac advances at idle its not right, reguardless of what you have read.

Find the best rate of vacuum advance by adjusting the vac adv diaphram tension.
a tach and vacuum gauge helps get it dialed in right.
Manifold vac,,,just as a matter of debate for mild to mid duration cammed motors.

Every vacuum advance can I have ever tested and/or adjusted if that style of unit pegs out at 12 or 13 "hg vacuum. Most are all in by 11" hg. So if we use manifold vacuum on a lighter cammed engine making over 13" hg one will have it all in no matter how much one trys to adjust the setting. Limiting the total amount of vacuum advance is manditory in either case. What does this 383 pull for idle vacuum?

I disagree with not having it all in at idle in the first place, not only is it nearly impossible for most mild-mid builds due to adjustment restrictions I mentioned but in the case of high duration cam engines the vacuum level fluctuates too much at idle. The last thing you want is the vacuum advance canister actuator rod moving around when you try and tune the thing as it adds and subtracts advance at idle. Try it out, its no fun and when you drop it in gear things get worse. Best to have it set so that it is just all in with the tranny in gear. Under light load cruise the engine vacuum will be less and the vacuum advance will end up in the middle of the range which is what you want.

Basically engines making decent vacuum levels do not benefit from adjustable vacuum advance canisters because the can does not allow adjustment above the engine vacuum the motor is making anyhow.

If we tell someone not to use manifold vacuum and then say dont have it all in at idle it becomes important to know the engine vacuum levels at idle. Chances are that most engines talked about here for the average Joe build wont have a choice in the matter since there motors pull good vacuum and as soon as they hook up to manifold vac it is all in no matter what.
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  #22 (permalink)  
Old 03-29-2012, 08:23 AM
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Custom10... this motor makes 8-9" idle vacuum.
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  #23 (permalink)  
Old 04-04-2012, 10:41 AM
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Quote:
Originally Posted by v8hed
383 with GM HEI distributor (modded with aftermarket module, curve kit, coil and shimmed for end play). Before going to the chassis dyno, I'd set my timing up (with a good timing light) for 34* total. Checked it up to about 4,000rpm and I could see the advance curve adding more timing until around 3,000rpm then it remained static. As I said, I only went up to about 3,500-4,000rpm with the timing light... just enough to see that the advance curve had finished adding timing. On the chassis dyno, we were messing around with the timing (just adding/subtracting a couple of degrees to do some fine tuning). I asked the dyno operator to take it up to 6,000rpm and keep the timing light on the balancer. He said total timing went up to something like 50* by 5,500rpm! We checked with another light and same thing. Couldn't hear any pinging (although the car is v.loud). What I don't understand is how could this possibly be happening? I still don't believe everything is at it seems, since taking timing out resulted in less power. If is was really running up around 50* total timing, there's no way the motor would be making power. I've previously tried experimenting locking the mech advance out and didn't see over 36* (with about 14* initial). Since the holes in the base plate physically limit max advance, what's going on with this wacky timing reading? It had occurred to me that perhaps the ProStreet balancer I'm using could be slipping, but it's pretty new and looks perfectly fine to the naked eye.

Anyone got any other ideas or explanations? If not, my next step will be to try a brand new MSD HEI just to eliminate the distributor from the equation. What about spark scatter or some other kind of effect?

I have been reading up on ignition timing and found there is much confusion on vac advance.I havnt read all you discussion here so you may have covered it already. timing may go to 50 deg in certain instances as I understand it. Not being an xpert on the subject here are some discussions here on hotrodders about timing.

http://rockridgefarm.com/vettdoc/Timing_101.pdf
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Old 04-04-2012, 06:57 PM
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F-BIRD'88 is right on the money...

The HEI distributor does NOT limit maximum centrifugal advance like its non-HEI predecessor. Previously, the max advance was limited by a pin in a slot. The maximum could be limited by adding a bushing to the pin to shorten the travel, but this is for the pre-HEI distributors ('73 ('74?) and earlier). The HEI limits its advance by when the short arm on the flyweight reaches nearly to the center of the oval cam under the springs. HOWEVER, some aftermarket flyweights, have a much shorter arm and will have greater leverage to twist the distributor to a still greater advance after the flyweight appears to have reached maximum advance.

The stock flyweights have worked better for me with stiffer springs to tailor the curve. The stock weights have a longer cam arm and appear to limit more robustly than aftermarket "performance" curve kits.

In any event, F-BIRD'88 has the best solution to add a positive stop to the centrifugal advance with the HEI distributor.
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Old 04-05-2012, 04:38 PM
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Timing your engine

Many years ago I had the opportunity to get hooked up with an old NASCAR mechanic. He never used a timing light! He used a vacuum gauge to set timing. I know it sounds odd, but with his help I learned how to do this. Hook up a good vacuum gauge to the engine, a port near the carburator works best. Start the engine, bring the RPM's up to between 1700 and 2000. The vacuum gauge should be rock steady. If it wavers you need to adjust timing until it becomes steady. Of course if you want to run advanced or retarded timing use a very good timing light and set it to the degree's you want. But the vacuum gauge works and I have been using one ever since.
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  #26 (permalink)  
Old 04-05-2012, 05:00 PM
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Quote:
Originally Posted by dbrow6272
Many years ago I had the opportunity to get hooked up with an old NASCAR mechanic. He never used a timing light! He used a vacuum gauge to set timing. I know it sounds odd, but with his help I learned how to do this. Hook up a good vacuum gauge to the engine, a port near the carburator works best. Start the engine, bring the RPM's up to between 1700 and 2000. The vacuum gauge should be rock steady. If it wavers you need to adjust timing until it becomes steady. Of course if you want to run advanced or retarded timing use a very good timing light and set it to the degree's you want. But the vacuum gauge works and I have been using one ever since.
The problem w/this as I see it is there's no way to know what the total timing is, or at what rpm the mechanical or vacuum advance starts and is all in by, nor the amount of mechanical and vacuum advance.

But other than that...
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  #27 (permalink)  
Old 04-05-2012, 05:35 PM
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NASCAR engines of the 1980's and 1990's ran fixed, locked timing.

I dont understand setting timing for a racing engine at no load.
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Old 04-05-2012, 07:19 PM
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tIMING

So no load means they set the timing running around the trackk? No seriously, this can be done on a dyno and I'm thinking it probably is although I haven't been near a dyno in just about as many years. I stick by the vacuum method and have used it on many hot rodded engines. Then I have used a light with an advance dial on it to "fine tune" timing. I do not trust all balancers, some yes, but not all. Many times I have found the timing ring on a balancer slipped or plain just improperly installed. Use the vacuum gauge first, check the timing mark to see where the balancer is and go from there.
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Old 04-05-2012, 08:09 PM
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not the answer but feedback

i usually advance the cam then fine it with the distributor ,minor.it would make you lower the timing to hit 34 at 6000 rpm.that would hurt you low.but maybe youll have to retrace youre steps to the point of failure and adjust from there.i run a HEI with sbc vortec 350.dot to dot on the cam on comp cam mother thumper is fine 2-4 degree advance there or go alittle more.so i dont really want to have to do much more than an 1" to 1.5"advance manually on the dis.like the part bout mech advance only affecting low drive to an idle imma keep that in mind for when i next tune.love carbed out monsters and its true fine tuning and constant adjust comes with the territory.good luck with it brother.out
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  #30 (permalink)  
Old 04-05-2012, 08:49 PM
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Huh????? I read that twice and still have no idea on 90% of it.
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