i picked up a 350 4 bolt main the other day for $700 its getting swapped is like the next week or so. for right now its going to stay the stock bore but what i wanted to know is should i used the bone stock 350 heads (casting number unknown right now) or should i use the 305 heads that have the port and polish? now i think the 305 heads are 58cc (the 305 heads are off a L69 305 HO) but what i don't know is what the differents in the valve size and all that. what i'm doing just for right now is makeing the 350 run the best that i can with what i have. if the cam in the 305 is still useable then we are goign to use it... if not then its goign to stay stock. so far the mods are going to be a MSD 6A box, a Rev kit, the PRM intake, the headers, jetting the carb, during the winter i'm going to take it all out and bore it .030 over put a forced crank and all that BS. i'm just worried that if i go from a modded 305 to a stock 350 that my 1/4 mile time isn't going to do what the 305 did. if the 350 is slower then that i'm going to be pissed. so do you think it would be wise to use the 305 or 350 heads? give me all your idea's thanks :-D

PS the 305 ran a 15.1 it would run it all day long.

 

the higher compression won't come anywhere near making up for the lousy flowing heads. Higher compression isn't worth the big differences in horsepower that people think it is, but good heads ARE.

 

well this is why i'm asking cuz the 305 heads have been ported and polished.

 

ever hear the phrase "You can't polish a turd"?

Port and polish it all day and you're still not going to flow as much air as your typical junk L98 iron heads.

 

Yes you can Jim...

Evry been to a carshow and looked at Chiny Chit?

 

LoL well the 350 i have doesn't have the L98 heads like i said in the first post i said the casting number is unknown the 350 is comeing out of a el camino from around the 70's

 

It all really depends upon which 350 heads you've got, and which 305 heads were ported & polished.

If the 305 heads are 416 castings, then they will out-flow typical 70's 350 smog heads. Add decent porting to the 416's, and you'll get very good results on a 350 - especially if the 350 has dished pistons.

The only really bad 3rd gen 305 heads were the LO3 (TBI) swirl ports. The LG4, L69, and LB9 heads are actually pretty decent.

 

see thats what i have.. i have the sotck heads off a 1986 Monte Carlo SS with the L69 HO motor so i have the L69 HO head.... i thinki need to find out the casting numbers on the 350 first and see whats up with them. guz i'm going to be pissed if i'm slower then a 15.1

PS anyone know the casting numbers for the L69 HO heads?

 

Not true....

If you had access to a flow bench you'd know the myth about 305 heads is false.


305 HO heads (416,081 castings) only flow slightly less than cast iron l98 heads.
in stock form. Mostly because of the smaller intake valve.
The ports are almost identical and of the same volume.

I have ported and flowed many sets of 305 heads and with larger 350 sized valves i get way more flow and power then what you'd get on a stock l98 head. (L-98 heads can be improved too)
I also have a set on my 350 powered car now.
A significant power improvemant when replaceing low compression 76cc smog heads like 882's 993's etc.

Right now it's down with a blown headgasket but the performance is real good. As fast as my vortecs were on the same motor .
Unfortunatly no reliable timeslips yet.

Did get a 13.45 et on a bad run.

 

Last 3 digits are "416".

 

Sorry Jimbo I have to disagree. It doesn't matter how many times Fbird'88 post the flow numbers, there are still people who won't believe it . 350 guys have been putting 305 heads on cars for years. There are several guys in the 12s or faster with them. Pull the valve covers off some of the oval track cars running in stock classes and yep you'll see 305 heads. I am porting a set of 601s right now. They'll do until I can afford something bigger. Yeah we like those cheap turds.

 

so you guys just put bigger vavles on them and port them? what will it do tothe compressions on the motor?

 

If your raplaceing large chamber heads then it will raise iit.
How much depends on the actual volumes and clearances of
your motor. Most stock motors have quite a bit of deck clearance so the actual compression ratio doesn't usually end up exessive.
When you install the larger valves you have to sweep out the chamber walls a bit so your finished chamber volume is usually a little bigger than 58cc's. 60 to 62 is typical.
A little flat milling the head can get 58cc's if that what you desire.

There are lots and lots of better (And more expensive) heads but if you don't mind working them a bit and are on a budget, they work well.

 

well short blockis stock so with the stock heads its like 9.5:1 i believe so would it go up to 10:1?

 

If the motor is a stock motor from a el camino from the 70's it is likely to have dished pistons and 76cc smog heads.
actual measured compression ratio will be right around 8.0:1.
No more than 8.5:1 bolting on 305 heads will raise it about 1 compression ratio.
These motors typically have at least .025" compresion deck clearance. Even if it was rebuilt, most replacement flat tops have .045"+ compression deck height. You won't know for sure untill you get the heads off it and look and measure. But I wouldn't sweat it.
Like i said you'll be opening up the chambers a bit for the larger valves so you'll be ok.
Don't bother bolting on stock 305 heads without the porting and larger valves. thou. You'll be disappointed.
They'll likely be ready for new valves any ways.

 

the heads only have 2,000 miles on them from the last rebuild btu i don't know what size vavles are on there. iwill ask the guy that did the work and see

 

Measure them yourself. A simple tape measure or ruler will do.
You want 1.94" intakes and 1.60 exhaust valves for a 350.
stock 305 HO heads have 1.84" x 1.50 valves.

A simple valves job and new budget valves won't cost that much.
you can sell of your used stock small valves too.
Don't forget the porting.

 

well is the porting your talkgin about different formthe port and polish that the heads already have?

 

Can't tell I can't see them.

The polish is mostly eye wash. If the intake ports are polished
you'll get fuel wash problems. should be somewhat rough.
Only the exhaust ports and chambers should be polished.

If only the bowls have been worked on, do the rest.

 

well what i mean is do i have to post out the holes where the valve's go? the heads have a 3 angle porting and polishing job on it in both the exhust and intake . so all i have to do is put in the bigger valve's?

 

does anyone know?

 

To install larger valves you'll need a new valve job.
The valve seat diameter in the head will need to be cut larger to match the new larger valves.
*machining required* as the saying goes.

To maximize the flow increase of the new larger valves
some more "posting" (porting) may be nessessary.
Typically you want the bowl diameter below the valve seat to be
about 85/88% of the diameter of the new valve .
All sharp machined edges on the lower angles should be be smoothed after machining. especially on the Short side radius where the floor of the port curves to meet the valve seat.
Porting heads is all about gettin' in there and getting dirty.

So, what size valves are in the heads now?

 

stock 305 size how much we talkign about to get the new ones put in?

 

If the heads need nothing more than a simple hot tank cleaning
and 3 angle valve job I'd say $125/150 plus the valves.
About $120 to 160 for decent budget/performance valves.
If the machinist has to do additional work, could be more.
Best to take a head in a discuss it with a local automotive machinist in your area.

can U take some pictures of your heads with 2 valves removed
to show how much porting has been done?

U can get an idea of the cost of new valves here
www.competitionproducts.com

What is the casting number on the 350 heads?

 

well the 305 heads are still on the car. and i don't know the casting on the 350 heads yet. i need to get tjhat info ASAP (meaning the casting #'s)

 

My86Firebird,

Did you get those casting numbers yet? I'd like to see this thread continued.

Later,
-Ben

 

nope i didn't ... i don't have time to take the 305 heads to to taken in and have then fitted for the 350 valves so i'm just goign to stick with the 350 heads will i can get some dart or world heads.

 

Just take off the valve cover from one of the 350 heads and write down the 8 digit number. It is right on top of the head, between the studs. Post it here and we'll help you out

If money is tight, you can't beat a home port and polish job.

 

its kinda hard to so that when the new motor isn't at my house LoL

 

]

You got a point there

 

hell i haven't even see it yet... LOL i paid for 1.6 RR MSD 6AL the holley jeting kit and tons of other ****... i haven't seen any of it yet LoL