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  #76 (permalink)  
Old 03-16-2010, 07:37 PM
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Lets put it like this. Due to low octane unleaded fuels without any anti detonation agents like the leaded high octane gas of old, the compression ratio can`t get too high without detonation problems. On a SBC you can usually scrape by at 10:1 on 93 octane. Since your using 305 heads it will give it great low to mid range power and the power will start to fall off around 4500 rpm, no big deal for a street machine. Next is vacuum advance. If you read the catalyst of factory chevy vehicles, you`ll notice where it says on setting the spark timing to set it with the vacuum advance disconnected and plugged, this proves it`s supposed to be connected to manifold or full time vacuum, if it were connected to a ported source there would no need for it to be disconnected as ported only pulls vacuum off idle. If you run it on ported vacuum with a low timing setting the result is very poor and sluggish low end performance and horrible gas mileage. It can also cause it to run hotter temps than normal. The reason why vacuum advance on a manifold source is needed on street machines is because they use lean mixtures, lean mixtures burn slower than do rich mixtures, since the burn is much slower it needs the extra advance to ignite the mixture sooner which the vacuum advance provides. Since your running 305 heads and that jacked the compression ratio up, you`ll need to get either a adjustable vacuum advance or set the timing 2 degrees lower at a time until it no longer detonates. If it ran hot with the vac advance on a ported source this could mean the mixtures are too lean, did you rejet after making the changes? Usually I would recommend setting the base timing to 12 degrees before top dead center with vacuum advance disconnected and plugged, but since your running a higher compression ratio, start with it at 8 degrees before top dead center then reconnect the vacuum advance to a manifold full time vacuum source and test drive. You may also want to pull a few spark plugs, if they are white it`s too lean.

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  #77 (permalink)  
Old 03-17-2010, 01:06 AM
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ok well that helped clear up some things, and i don't exactly have a chevy catalyst to be looking at on my rx7 hood lol so i wouldn't know. As for it being too lean, i hardly think thats the case cuz this car has the richest most potent exhaust fumes i've ever experienced lol. now keep in mind i HAVE NOT done my cam/heads/HEI swap yet, hopefully this weekend now that its warm and i'm done with school finally, but just throwing that in there.
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  #78 (permalink)  
Old 03-17-2010, 06:40 AM
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You want the PCV to feed the bottom (base) of the carb so the fumes are evenly distributed.

Take the PB vacuum from a large port located on the intake manifold if you have one.

The other ports are for distributor vacuum advance. If you've decided on manifold vac. for the advance, block of the other ported port w/a cap.

Quote:
i'm guessing my car is running full advance at all times which that doesn't sound like that could be a good thing.
This is not how it works.

W/o going into a whole 'nother detailed explanation (it has been covered a million times- try using the search), when the vacuum falls off, like while the engine is under a load, the vac. adv. also decreases w/the amount of available vacuum- it's not on full advance all the time, because the vacuum isn't high all the time.
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Old 03-17-2010, 06:28 PM
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ok i T-ed the 3/8 line to the brake booster into my pcv valve and into the big port on the back of the carb, does that work ok? i dont have a port on my intake manifold at all. i noticed my idle raised after i did this do i have to readjust the timing for this amount of vacuum now? and yeah cobalt327 i understand the vacuum advance i guess i was in the heat of the moment lol
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Old 03-17-2010, 07:42 PM
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no fbird88 i have not recurved the distributor, forgive me for this is my first SBC build so i'm kind of a newb when it comes to that, ESPECIALLY the porting lol, they still have the 1.84 valve in them but they're freshened up with new everything. i wouldn't mind porting the intake and exhaust ports but when it comes to the combustion chamber stuff i don't have A CLUE what i'm doing, i wouldn't mind sending you these heads if you wanna hook em up lol cuz i'd rather not mess something up
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Old 03-17-2010, 08:12 PM
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Quote:
Originally Posted by v8fbrx7
no fbird88 i have not recurved the distributor
I would strongly suggest you do the distributor advance recurve before anything else, and tune the engine to its optimum so you:
1. Can enjoy what you have, and
2. Have a baseline to know what any additional changes actually do in the way of increased/decreased performance.

You might want to get a decent book on porting Chevy SBC heads- there are some areas that are more critical than others and actually, the combustion chamber is probably the MOST forgiving, AFA a small amount of metal removal wreaking havoc on the flow/velocity- unlike the short turn, where you can REALLY mess things up if you get the least bit 'heavy-handed'.

There are articles like THIS all over the net that will give you an idea of what's involved.
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Old 03-26-2010, 06:04 PM
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ok so as far as the recurving the distributor goes, i've never done this before, and i see either vacuum canister or the mechanical weights/springs advance kit, which am i supposed to modify and are their certain kits in particular you'd recommend? thanks again
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  #83 (permalink)  
Old 03-26-2010, 07:00 PM
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Quote:
Originally Posted by v8fbrx7
ok so as far as the recurving the distributor goes, i've never done this before, and i see either vacuum canister or the mechanical weights/springs advance kit, which am i supposed to modify and are their certain kits in particular you'd recommend? thanks again
Most parts stors have a "high performance" section to them. There you will find advance weights and springs for either points or HEI ignitions- use the one that matches what you have (HEI, I'd guess).

There you may also find an adjustable vacuum advance can- you might have to ask them to order one- for the HEI OR points dist.

Before getting the vac. can, see if yours is still good (holds a vacuum, and works as it should). If it's good, hold off till later to see if you actually need an adjustable vac. can or not.

You should start out w/good wires and a cap and rotor in good shape. It would be a good idea to strip your dist. down and thoroughly clean and lube it. HERE is a tute on how.

HERE is one of hundreds of tutes on working w/the HEI advance.

Crane had good kits, Mr. Gasket are what you will likely find- they're alright too. Use the original weights and center weight cam to begin with- usually all you need from the kit are the springs.

Good luck.
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Old 03-26-2010, 10:52 PM
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ok well after reading that tutorial its saying to use one medium and one heavy spring for the mechanical advance, BUT when i do that it gives too much advance with the stock vacuum advance canister so i need an adjustable one right?
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  #85 (permalink)  
Old 03-27-2010, 12:15 AM
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Quote:
Originally Posted by v8fbrx7
ok well after reading that tutorial its saying to use one medium and one heavy spring for the mechanical advance, BUT when i do that it gives too much advance with the stock vacuum advance canister so i need an adjustable one right?
IF the overadvance is occurring when the vac advance is "active", then yes. You can make a blocking piece for the vac advance that limits the travel, as well.

This is from F-BIRD'88:


What you're looking at is the purple-lined drawing of the part that is atop the vac advance arm. Ignore the locked out advance- that (locked out adv.) is not what you want.

You might want to try both heavy aftermarket springs to see if they help enough to delay the advance- but not if they cause the advance to come in too late. It's trial and error, but you're on the right track!
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Old 04-06-2010, 09:55 PM
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so will this be what i need? http://www.summitracing.com/parts/SUM-G5212/
i have a skip white HEI

they didn't have the crane kit in stock and my points dizzy vacuum advance canister took a crap on me today so tomorrow night starts the overhauling, got the 1094 gaskets today.

thanks again!
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Old 04-09-2010, 08:25 PM
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ok i think i'm starting to understand that now lol. well as an update i'm ALMOST done putting my engine back together after doing my cam/heads swap after many headaches/hangups so i'll post what i think about the outcome which after all this mess it better be nothing short of amazing lol
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  #88 (permalink)  
Old 04-09-2010, 08:39 PM
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Quote:
Originally Posted by v8fbrx7
ok i think i'm starting to understand that now lol. well as an update i'm ALMOST done putting my engine back together after doing my cam/heads swap after many headaches/hangups so i'll post what i think about the outcome which after all this mess it better be nothing short of amazing lol
Can't wait to hear about how it turns out. That's a pretty cool project you have going on!
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Old 04-09-2010, 10:56 PM
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on a different note, my exhaust...i want to run my headers but i've been contemplating redoing my entire exhaust while i'm at it because its SUPER loud in the car. plus i'm not sure if a 2.5 y-ed to a 3 inch exhaust is killing my low end torque as well, any insight on this? theres NO way i can run duals on my car.
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Old 04-10-2010, 06:53 PM
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ok well everything is installed...and i cant get the damn thing to start, it took forever to even act like it was going to start and it did for a short time almost fire up but nothing since...what the hell? whats the deal?
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