[ericnova72]

My compression calculation came out a lot different, I got 9.88-1. I used 57cc for the head(.006" cut would be about 1 cc 58-1=57), 6cc for valve reliefs, piston .025" in the hole(deck ht), .015" gasket. How did you get 10.66?? I get 8.6 for the old combo using head cc of 64.

I use this calculator www.wallaceracing.com/cr_test2.php

[v8fbrx7]

oh ok, this program doesn't really like valve relief settings they don't make sense so i went with flat tops, i know thats not right but it wont let me put the 6cc with the .025 deck height. thats probably why the compressions so high

[v8fbrx7]

ok fellas in need of your help again. ok so i have a holley 1850 carb, its got 3 vacuum ports, 1 big one for the brake booster and 1 full time vacuum and 1 ported vacuum. i was wanting to add a pcv valve cuz i'm oil filming my engine bay. now i have power brakes hooked up to the big one, and i have the vacuum advance hooked up to the full time vacuum(which apparently according to holleys website i should be using the ported source for that ) but i originally had it hooked up to the ported source and it ran too hot all the time and obviously felt like it had less power. now if i run a pcv valve i'm supposed to move the hoses around.

so i'm guessing my car is running full advance at all times which that doesn't sound like that could be a good thing. now i've been doing my research elsewhere other than this site too for advice and its all sounding about the same as here too.

now my dad on the other hand (whom i love to death and used to race his nova way back in the day, so he knows his stuff) is telling me that the 305 heads will bump the compression but it'll run like crap cuz they're still smog heads, and then theres no way i can set the timing at 12-14 degrees at idle with todays gasoline...now i know my dads pretty damn stubborn cuz thats where i get it from but is there any truth to that? i told him that theres no point to even having a vacuum advance if its running full advance all the time, unless i'm missing somethin here. please advise on this subject

the short version is

-adding pcv valve, need to move ports around according to holley
-when i used ported source for vacuum advance it ran hot and slow
-dad says i need to run full time vacuum on the distributor(set at 8 degrees now) and can't switch ports and run 12-14 degrees

[DoubleVision]

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.

[v8fbrx7]

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.

[cobalt327]

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.

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.

[v8fbrx7]

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

[F-BIRD'88]

Originally Posted by v8fbrx7

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

As long as your PB's function ok the T'd vaccuum set up is ok.
If the PB's don't work then you'll have to connect the PB's to the vaccum port above the #8 intake runner on the manifold.
I'll bet you have not recurved the distributor as I discribed to allow increased base idle (24ish deg at idle) timing without excessive max advance 34-36deg. (Limit the mechanical advance stop)
As far as porting your 305 heads, they need a full port effort. Not just a bit of bowl . These heads work very well when fullly ported. Air flow makes you go fast. You cannot do damage here short of carving right thru the port wall. This is THE CYLINDER HEAD to learn on. Hog the bowl out under the valve to 85-90% of the diameter of the new larger valve. (see the stock 1.84" valves seat, open the bowl up to just a hair smaller than the inner edge of the stock valves seat edge. Now its just right for the now larger diameter valves with a new alrger valve job.)
You want a slight venturri funnel contour under the new valve job.
Fully slim and contour--streamline the guide boss so the air can flow around it. Don't be shy here. Use a long shaft 3/8" cylinder shaped carbide cutter to reach deep in the intake port to do this. Raise the roof of the port all the way from the intake port flange to the bowl/guide boss zone by about .100" useing a Felpro #1205 gasket as a guide. Widen the port at the pushrod pinch as much as you dare short of busting thru the pushrod area. Do not make the port opening bigger than the #1205 gasket. DO not lower the floor of the port, just clean it up. radius the short side of the port in the bowl. You'll be removing about 10cc of metal total from each port.
Basically the same on the exhaust port. Do not make the opening of the exhaust port larger than the header tube opening. Do not use a header gasket as a guide. But the exhaust port needs to be open up and raised a good bit as well.
Only minor deshrouding around the new larger intake valve is nessessary. gauge the space around the old valve-stock chamber wall . the new 1.94" valve will be .050" closer to the chamber wall. All you need to do is re-establish that clearance around the new valve. Not that hard.

These heads have modest port air flow in stock form. Its up to you to bring out the power by increasing the port airflow. Its real hard to screw it up.
Don't be shy with the porting, but use common sense to not carve right thru.
Cut an old stock head apart to see the typical port wall thickness.

The critical thin spots are the push rod pinch area and the spot where the head bolt passes between the two intake port walls. You want to straighten these "common long walls" to the bowl but not bust thru into the bolt hole.
You want to open up the push rod pinch but not bust thru.

If you want to save money find some good used junkyard 350 motor 1.94" intake valves from a 350 sbc cylinder head and cut then down to 1.90" diameter in a lathe. Now have the machinist do a new valve job based on the 1.90" diameter valves.

Same on the exhaust side, use 1.60" good used 350 valves cut down to 1.55"
Do a 30 deg back cut on each valve too.
Now you can regrind them with plenty of margin thickness. Now you got a 1.90" x 1.55" custom big valve fully ported high compression head for your 307.
If you want to run a 1.6 ratio rocker arm for increased lift then lengthen the push rod slot in the head towards the rocker stud. I do it by hand with a cut off 5/16" rat tail file in a drill. Longer, not wider.

Again the 305 HO head is the cylinder head to learn on.
Use an old junk SBC head to gain skill on you want first. Then go nuts on your heads. They work very very well with the right effort.

If I can do it, so can you. The biggest skill you will need is the patience to do a full job.

[v8fbrx7]

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

[F-BIRD'88]

Originally Posted by v8fbrx7

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

The combustion chamber is one of the easiest to do. All you are doing is reestablishing the clearance around the valve edge that the stock 1.84" valve has in the stock chamber. The 1.94" valve has a .050" larger radius than the stock 1.84" valve. So you blend out the chamber by .050" around the new valve to get the same clearance relationship. The area that the chamber wall gets close to the valve is prestty small. Not rocket science and you neden''t get all carried away here. Do not try to match the chamber to the bore. Use an old junk stock valve in the seat to protect as you grind the chamber wall.

If you just got to stay with the stock 1.84" valve then fully port the heads with the stock valve. The only difference is the finished bowl diameter under the valve seat in the head. Do a 30 deg back cut on all the valves too. Use long shaft cutters to reach in the valve bowl to avoid hitting the valve seat with the die grinder. I buy the 6" shaft carbide porting cutters and cut 1" off the shaft.

You can get a good full port result with the stock valves but I prefer going bigger.

Experiment and find what base idle timing gives the best idle quality and low rpm throttle response (below 3000 rpm drive testing) Then recurve the distributor to allow this increased amount of base idle timing without excessive max advance.
You can use up to 24deg base inital at idle. But only want 34-36deg max advance. With the 416 heads you will need the Champion rv8c spark plugs and premimum gas. regular ain;t going to cut it.
I don't port cylinder heads for the public.

[cobalt327]

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.

[F-BIRD'88]

If you want to maximize the hood (scoop) clearance get a old 60's 70's era chev corvette 14" "L-88" "L-72" "LT-1" air cleaner and cut the choke horn off your holley carb.
This is the lowest hugging low profile 14x3" air cleaner and also has the best base shape contour. Works best with no choke horn in the way.

[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

[cobalt327]

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.

[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?