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350 with 305 heads?

  
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350 with 305 heads?

 
robbyxr200 robbyxr200
New User | Posts: 13 | Joined: 07/07
Posted: 07/16/07
08:20 PM

Im putting some new heads on my 350 and I have a set of old 305 heads and I also have some 350 heads from a old truck and i was woundering what heads I should use? the motor is going to be going in to my 86 chevy 4x4 I was thinking about puting a cam in it also. any sugestions?  

GibTG GibTG
Guru | Posts: 1985 | Joined: 10/03
Posted: 07/16/07
08:42 PM

I say neither. Vortec heads are far superior to either one, by the tune of 30-40 horsepower. Sure 305 heads will pump up the compression, possible even to levels that may cause detonation on pump fuel, but the small valves and small ports will not be liked by added displacement of the 350, that those heads weren't meant to support. If you use the 350 heads you have a slightly larger port and valve but then you're stuck with a large and in-efficient 76cc chamber that hamper the engine's power production anyway.  
~Gibs

robbyxr200 robbyxr200
New User | Posts: 13 | Joined: 07/07
Posted: 07/17/07
04:25 PM

also how do I know what cc my heads are? im always getting cylinder heads i dont know what ones are good  

CLYDE1LS7 CLYDE1LS7
New User | Posts: 43 | Joined: 07/07
Posted: 07/18/07
10:08 AM

I could spend an hour talkin bout pro's and con's either way.Big valves dont do crap 4 u till high rpm.So,if ur gonna wind the thing out youll need big valves and big runners.Otherwise 1.82 or 1.94 valves are fine.Back aruond 91'a shop here did a flowbench on those 2 heads cause of the hype bout 305 vs 350 valve size.they used a 2.02 chev truck,and a 305 car head to compare.there was no diference in flow rate for a 350cid till above 6,500 rpm,therefor no loss of hp to worry about on a street engine running less than 6,000rpm,less of a blower or the such.Mill the 350's .040 gets about 8.75,run the 305's youll get 9.0 to 9.5.Little gain 4 the money unless u wanta wind it.  

robbyxr200 robbyxr200
New User | Posts: 13 | Joined: 07/07
Posted: 07/18/07
04:32 PM

right now the truck smokes the valve seals are junk and I have to keep changeing the plugs o anything is better then that I want to make it sound good and have as much horsepower for cheep I have a edelbrock intake mainafold and a holley 600 carb some headers a supercoil just little stuff I dunno what im gunna do yeat but the 305 heads are sounding good I wont be winding it up much  Grin  

GibTG GibTG
Guru | Posts: 1985 | Joined: 10/03
Posted: 07/20/07
11:39 PM

I would love to see that flowbench that measured flow rate of the heads on an engine in relation to rpm's! It's awfully new to me...

"The cc's" of the heads usually refers to the volume of the combustion chambers or intake runners. The casting number (should be a six or seven digit code) may shed some light on this.

I guess if you have low expectations it doesn't really matter what heads you use but just make sure you don't get into a situation where you have too much compression. Most 305 heads have 58cc chambers but some are even smaller, down to about 53 cc's minimum. With flat top pistons and even a loose piston to head clearance in a 355 10:1 is no problem with this small of a chamber and 10:1+ with iron heads is probably going to be too much for pump gas with a stock or near-stock camshaft. You don't want to make a highly inefficient engine that needs to have timing removed to prevent detonation even on pump premium. That will lead to a very expensive and troublesome vehicle...

Here are a couple examples of static compression ratio calculators to find out (in the ballpark) where you're at...

http://kb-silvolite.com/calc.php?action=comp
http://www.mindspring.com/~steveflyer/compression.htm  
~Gibs

CLYDE1LS7 CLYDE1LS7
New User | Posts: 43 | Joined: 07/07
Posted: 07/23/07
02:12 PM

Perhaps I wasnt accurate with my statement,Ill try to correct the terminology.Any engine produced by a manufacturer has a standard flow rate for its stock intened performance level.Back in the late eighty's,when the HYPE was big on 305 heads going on a 350,valve size restricting flow was the concern.So this shop did a flow test to see where the cc restriction started to b effected.The needed cc's for a stock configured 350 sbc where very well maintained by a 305 head.Jim said if you where to rev the engine to or past 6,500 rpm,you would b likely to see gaining loss's on a dyno,the further it reved.These where his figures from the numbers produced by the flowbench,not a direct result of the flowbench.My opologies for the misconsept.As for comp.,not all 350 pistons are flattop's.My 84'had 7.8 stock dishtops.305 heads wont bump that far with them.If you look,there are some articles in yrs past in CC or this mag. that say the norm is estimated around 9.5 or so.And lets not forget he can run a thicker headgasket at lowered cost.Summit Engine Shop used to build 355sthat way,and advertised them at 9.5 comp. drivable on PUMP gas.  

GibTG GibTG
Guru | Posts: 1985 | Joined: 10/03
Posted: 07/23/07
03:12 PM

Would you like to tell me the volume required by a 350 to make power at such and such RPM? I really was always under the idea that minimum cross-sectional area and induction length decided where an engine would make its peak power, not just volume all by itself.

I guess I don't know your definition of "support" but I doubt that 305 heads can support anything above 5500 rpm in a 350. If you can even MAKE the engine peak after 5500 you know that there are cylinder heads that would show HUGE horsepower gains at that rpm because of how in-efficient some 305 castings from 1980 are...

Before guessing at the compression ratio go to an actual calculator and figure it out. I will tell you all the specs I'm using for my numbers...

4.03" bore
3.48" stroke
.040" gasket (8.2cc)
.030" piston to deck clearance
58cc com. chamber
8cc dish

= 10.01:1 static C/R

- change that to a 53cc head (which some 305 heads are) and you're above 10.5:1. If this is a decked and rebuilt engine the piston to deck clearance could be A LOT tighter. Any of these numbers are too high for pump fuel with a near-stock cam and iron heads. Overall, I don't think 305 heads are a good idea. Spend an extra couple hundred dollars and do it right and buy vortec heads. There are less guesses involved, less trust in a machine shop, and more power.  
~Gibs

CLYDE1LS7 CLYDE1LS7
New User | Posts: 43 | Joined: 07/07
Posted: 07/23/07
09:07 PM

Ok,I didnt come on this forum to argue with people!...robbyxr200,? for you friend I have question's for this app.
What is ur deck height off crank center? Are your heads to b milled and how far? What kind of pistons will b run? What rpm range do you wish to b at? Will this truck b in deep mud running low end for torq? And finally how much CASH can you dump in this thing to reach these height's.Without cash,your either gonna mill stock350's to their limits to get comp. or run 305's with .060 Fel-Pro Gold gaskets to drop under 9.8 and run a 170.thermostat,to avoid detonation.By the way New GM heads require a ton of new parts to keep the warranty.I run only well seasoned castings.IF THEY WERE GOING TO CRACK,THEY PROBABLY WOULD HAVE BY 100,000 MILES!  

GibTG GibTG
Guru | Posts: 1985 | Joined: 10/03
Posted: 07/24/07
12:15 PM

There is a difference between argument and debate. I believe that debate will bring out the best course of action, I wasn't being forceful I thought was just stating facts. Hopefully at the end of this the original poster can see the best way out...

I'm glad you're changing your tune a little but still there are some ideas that are less than optimal. Milling costs money, in the case of angle milling it can cause numerous aligment issues. Pushrod length, head bolt location, the header face, the intake manifold face, etcetera. These need to be corrected and all of this costs money. So you could spend a couple hundred dollars and still be down 30+ horsepower under vortec heads. If you decide to go with the 305 heads you would need a .060-.070" quench clearance just to hope that it would run on 91 octane. That loose quench would cause a 15-25 horsepower loss in its self. Bring this back to the small valves and ports and you could be huge numbers under vortec heads at 5500-6000 rpm. This loose quench would also make this engine more prone to pinging, in other words you waste a lot of time and are on the brink of detonation and loose a lot of power. A vortec head has a much more efficient chamber, it would allow a regular composition gasket and may even run 9.5:1 compression on cheaper 89 octane. I personally don't see a better way...  
~Gibs

CLYDE1LS7 CLYDE1LS7
New User | Posts: 43 | Joined: 07/07
Posted: 07/24/07
05:25 PM

I never said there not better,simply more costly.Ive not looked up price for V's in a few yrs but can bet by the time he gets all the crap to bolt them on,new bolts, rocker arms,pushrods,all the goodies the General requires to keep them warrantied till the casting is seasoned,at a guiss close to a grand,if the budget is tight and the desire to learn exists aguy can make the cheaper heads work nicely.I mean think of it,if he has time,detonation and comp. both can b reduced by polishing the chambers.With a little time thats basic stuff a guy can teach himself using the right tools at minimal cost.Milling,yes can get costly,but the shop I use up here is still under 200.00 for a set of heads with the valves done.A long way from a 500.00 bill.And IF I was to recommend a set of heads,go to World for sportsmans,for 850.00 and put some real #'s in that badboy.Ive always told people,do it cheap,or go to the other end of the spectrum.The middleman always costs more than the outcome is worth.And if a guy gets educated on the physics of fuel mixture,flow,volumetric efficiancy,and flashpoint he can do a lot with cheap heads for little money.Just a little booktime goes a long way.Afterall,isnt that why we are here,to help the beginers not to get fed full of .... by some sales guy trying to make money.Wish I had this forum when I was 16-20 yrs old.Woulda saved a ton of cash.  

robbyxr200 robbyxr200
New User | Posts: 13 | Joined: 07/07
Posted: 07/24/07
05:29 PM

it has stock pistons as far as I know I have no idea if there flat top yeat ill know in a few weeks when its out of the truck im going to use it as just a truck to drive on the road  and i want to make this truck fast and it has a 4 speed trans with the granny gear so i dunno what to do i cant get vortec heads right now maybe someday i dont havethe cash right now for now im just gunna get the best of the few sets of heads i have iv got 2 sets of 350 heads and one 305 set i was thinking about getting a high performance cam  

CLYDE1LS7 CLYDE1LS7
New User | Posts: 43 | Joined: 07/07
Posted: 07/24/07
08:20 PM

Thats a heavy machine,going fast is not easy with that kinda weight,but youve heard that Ill bet.As I have stated,on that cash limit,fast wont happen in 1/4 mile terms.But with a 4spd youve got cam options an auto takes away.I like Crane and Lunati myself.I dont rec. comp cams.Seen a guy get 3 bad ones back to back few yrs ago.That convinced me.As for selection,give Crane Cams a buzz,they will be glad to help you at the tech line.Just went thru that on a FORD 300 inline for a buddy of mine.I didnt know a thing about ford inlines so to do the install I called to b sure.VERY helpfull staff,cant say enough good about em.They have HUNDREDS of cams available 4 Chevs as does Lunati.You dont have a lot of cash,so get booksmart to the beast.Dont b afraid to do a little headwork @ home either.It is simple as basic math.Dont grind into your ports,youll lose low end,but intake gasket matching to the heads and chamber polishing helps low and mids.With the right carb and a serious dual exhaust you can make nice performance.Hope these sug. help.  

GibTG GibTG
Guru | Posts: 1985 | Joined: 10/03
Posted: 07/24/07
09:39 PM

Clyde, You do make a good point. If there isn't room in the budget for horsepower then that's fine but I guess my advice is don't make this small budget force your engine into a poor condition, such as detonation on pump-fuel.

Gasket-matching probably couldn't hurt but depending on the size of the runner at the manifold face there may not need to be any grinding. A slight mis-match here in either direction can be beneficial to a mild engine so I'm not usually too critical about a perfect match...

Polishing the chambers on the other hand is a bad idea. If you are trying to make carbon NOT adhere to the combustion chamber surface you're hurting power. Depending on the mixture and oil control of the engine it will probably carbon (or soot) up after some time anyway but putting a mirror polish on a chamber is a lot of work for no gain...

I highly recommend Comp Cams. Your buddy that had a few "bad" cams probably had a bugaboo in the system elsewhere. And even in the rare occurance that I've had a Comp cam go out they sent another without hassle. And even in that case it was user error. The equipment these companies have today rarely has an issue, what's more likely to cause a mistake is the user breaking-in the cam. There are a lot of variables in break-in. Even if you get past the oils and break-in procedure if you or your friend had three cams go out of the same block you could debate that block had alignment issues at the lifter and lobe interface.

Where porting actually shows benefits is at the bowl, more specifically the seats and slightly below the seats. As-cast heads usually have a "line" or a very pronounced ridge below the seat, especially after a machinist has "knifed-in" the seats and DEFINITELY when there are larger valves being installed. These ridges need to be blended into and contoured into the seat. I guess first off though you need to find a high-quality machine shop to do the valve job. I second the advice to pick up a book. David Vizard's book specifically for modifying cylinder heads for a small-block Chevy is great.

I think there is some information that is vital that isn't here. What are the casting numbers of the three sets of heads you have to work with? What are your goals for this build? "Going fast" just doesn't define your goals very well. What exactly is your budget with this project? If you don't know exactly at least talk about some of the parts you intend on buying, what you hope to reuse, etcetera. Specifics just help decide what's best for your build.  
~Gibs

CLYDE1LS7 CLYDE1LS7
New User | Posts: 43 | Joined: 07/07
Posted: 07/25/07
12:19 PM

Well Ive gotta tell ya in the neer twenty years Ive been buildin you are the first person Ive ever heard say polishing the chambers isnt all the way around benificial.Matter of fact,my old boss spent 17yrs doing nothing but porting heads between the two race shops he worked at,and that is the first detination controll either used.And some of the BIG money heads hes worked with,I hold his word in a VERY high standing.I could not tell you the name of the place in St.Louis,MO he worked at but Ive heard guys at the track refer to the place being top notch.And I never said for the flow,just det. controll.As for the comp cam issue,if I build an engine for big hp,it gets all the good stuff done.We spared no expence on that engine,and the shop it went to was reputable to say the least.All three went out  on  break-in within 20min or so.Everthing else was right,triplechecked,and after we even sent the *** spec sheet to them.We got no explanation.just a lot of overhead.YES they replaced the cam&lifters.What bout the other stuff gaskets,oil,etc.and the time we had in the thing.Ive built prob. 150 engines with no issues like those.But that was my only exp. with comp.,GRATEFULLY!Lots of guys have told me as you have.To each there own friend.This young guy here has a lot of desire to learn from what I gather,but not a ton of cash.So he,mills the 350's .030 to get sum comp.Then the head deck is so thin if he heats it up at all the heads prone to warp and crack.How is that good advise.A thicker fire ring will help to prevent those issues when the block & heads start shifting around under stress and high temps.So if he starts with a stock deckhight,305 heads stock or.010 under,.060 gasket,polishes the chambers to avoid det.,and runs a moderate cam @ the right degree of timing,the only condition I see being an issue is high rpm flow.If to go with big rpms,as I said befor,use the 350 heads at a greater risk if their milled to raise comp. ratio.And I cant tell you how many 350 heads Ive replaced cause being milled to get comp,then they crack under heat & stress. DOSENS if to even speek of it.  

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