305 - Engine Modifications/Rebuild: Need Some Advice


PeteA216
05-14-2006, 02:09 AM
Hey guys.... I'd like to get a few things straight that I keep getting different views on... (First off the engine is a 305 CID from a 1977 Monte Carlo)... what is this that I keep hearing about a non-roller cam, a roller cam, and a semi-roller cam? Which cam's have to be broken in upon initial startup and which ones don't? How much bigger than stock can a cam be before serious engine work is needed to accomodate the larger size? Also, which roller rocker arms can and can't be used on a chevy 305... I know roller tips can, but what about the full roller rocker arm setup? Heads.... how much (or how far) can they be milled to increase compression before it gets to be too far that it will risk hurting the engine?

Thanks in advance for any information at all!! I Appreciate it!

silicon212
05-14-2006, 04:14 AM
Rocker arms - you do not want aluminum rockers on a street car. Period. They do what they do well, but don't have the longevity for the street. Factor in about 3-6K miles on a GOOD set, so stick with the chromemoly roller tips. In this regard, the Magnum rockers from Competition Cams / COMP are very good and are recommended. They are what I use on my car - and the set currently installed has over 250k miles on them - that's a quarter million - still going strong.

Heads - the only heads you can safely mill are the pre 1978 ones (if your heads are stock, you're golden). These can be differentiated from the earlier ones by looking at the area under the spark plug - the good (pre-78) heads have the edges generally even across while the later heads have "notches" between the 2-3, 4-5, 6-7 bolts along the bottom. Also, on the underside (the deck surface), the water jackets on the good heads are drilled in, while the later heads have cast-in ports along the intake side of the chambers (the quench area) that are trapezoidal in shape. The later heads are called "lightweight" heads because the deck surface is thin and cannot be milled very much - these heads are also quite prone to cracking. I cannot remember what the casting number is for a set of these older 305 heads as it has been about 18 years since I've worked a pair, but something tells me "334" are the relevant numbers to look for.

Roller lifter cams do not require initial break-in in the way that flat tappet lifter cams do - so a roller cammed engine can be started and run at idle during initial start up. As for maximum lift - if you're going with stock rockers the max lift is in the .450 range at the lobe. Any more than that will require rockers with longer slots in them - if you go with the COMP roller tip lifters, you can use a larger lift cam. The biggest thing to look out for on any engine as far as lift is concerned is piston to valve interference. This is best checked with an amount of modelling clay added to the piston crown, the heads installed along with the intended gaskets, spinning the engine BY HAND for two revolutions, breaking down the heads and measuring the thickness of the clay. It's a little time consuming, but this will give you an idea of what you can run. Any cam with large lift is going to require stiff valvesprings - what is required depends on the cam maker - you want stiff springs to prevent piston/valve interference due to valve float at high RPM.

One more thing about roller cams vs. flat tappet cams is that you will need aftermarket rollers in an engine without such a provision (most blocks 1984 and older) or factory rollers along with the retaining bracket (1985 and newer blocks *generally* have the provisions for this, even if the engine is factory equipped with flat tappets). On factory rollers, there is a retainer that bolts to the front of the block (behind the timing sprocket) that retains the cam preventing "cam walk" which can occur with roller lifters. If your block doesn't have this, you will need a special "button" and timing sprocket. The button attaches to the inside of the timing cover, on the cam centerline, to prevent the cam from "walking" forward. Also, aftermarket roller cams, typically billet steel, require a soft, bronze distributor gear in place of the factory gear.

PeteA216
05-14-2006, 11:34 PM
Wow, you are like a god of knowledge to me right now. Thanks so much!!! This has helped me tons! One more thing....

Quote: Heads - the only heads you can safely mill are the pre 1978 ones

The engine is from a 1977 Monte Carlo, so odds are these are the "good" heads. Being the correct heads, about how far can they be milled before it'd cause problems such as interfereing with the valves or bringing the valves too close to the pisions? Is it possible to raise the compression too high just by milling the heads?

Thanks again!! I just printed off this thread.

silicon212
05-14-2006, 11:54 PM
Wow, you are like a god of knowledge to me right now. Thanks so much!!! This has helped me tons! One more thing....

Quote: Heads - the only heads you can safely mill are the pre 1978 ones

The engine is from a 1977 Monte Carlo, so odds are these are the "good" heads. Being the correct heads, about how far can they be milled before it'd cause problems such as interfereing with the valves or bringing the valves too close to the pisions? Is it possible to raise the compression too high just by milling the heads?

Thanks again!! I just printed off this thread.

If your aim is to increase compression, you'd fare much better by either performing a rebuild with flat top pistons, or using a newer set of heads with 58cc chambers. Along with possible valve clearance issues, removing too much off of the deck surface will necessitate shorter pushrods to keep the valvetrain geometry intact.

Your engine, if it's original, will have the millable heads on it. As for "good", that's open for debate. While the older heads can be safely milled, the newer heads flow better and are available with the aforementioned 58cc chambers. There's a set that Blue Bowtie recommends for 305s, I can't remember what the casting number is, but these can be modified to run 1.94 intake/1.5 exhaust valves which will open that 305 up and allow it to breathe. This is the key for getting power out of these motors. To answer your question, I believe the heads can be safely milled .10", but don't quote me on that. That will raise your compression up HIGH. It will also throw your intake manifold out of alignment with the heads and require the shorter pushrods as above.

DenisM
05-15-2006, 12:12 AM
Roller lifter cams do not require initial break-in in the way that flat tappet lifter cams do.

how do you break in a flat tappet cam?

PeteA216
05-15-2006, 11:24 PM
So if I were to go to a less of an extreme and say about .08" instead of .10" would that be sufficient enough to not throw the intake as much out of alignment? Couldn't I just get thicker intake gaskets to compensate for the lost material? If not what would you recommend I'd go down to? Also, when I got shorter pushrods, should I get them to be the same amount shorter as I get the heads milled? (Ex: Mill heads at .08" & get pushrods .08" shorter than stock) I have the resources to get the heads milled for a very low price and thats one of the reasons I don't want to simply get a new set of heads. Also, at a .08 milling or whatever you'd recommend, what would be your rough estimate of the compression ratio? I know stock with no mods is somewhere around 7ish:1. <- I Think. As for flow, could I just port and maybe polish the intake and exhaust ports to match the size of the gaskets that will be used? I did it once just for shits when I was younger to a lawnmower engine... didn't do much of anything, but then again, lawn mower says it all. Anyway, thanks again for the help!!! Its VERY much appreciated!

P.S. Silicon212 If you have a paypal account, give me you're email address and I'll send yah a few bucks for all the help you've been!

Oh and one more cam question.... when a roller cam would want to walk in an engine not roller cam ready, what keeps a flat tapped cam from walking? Thanks again!

silicon212
05-16-2006, 12:38 AM
DenisM - when you first fire up an engine with a new flat tappet cam, you need to run it up to 2,000RPM and keep it there for 20-30 minutes. The cam gets its lubrication via oil slung from the crank. A new flat tappet cam is going to wear FAST, until the parts wear together (cam to lifters etc). During this critical stage, the cam MUST be kept in an abundant supply of oil and at RPM lower than 2,000 - there's just not enough oil slung off the crank to keep the cam lubricated during this critical rapid wear time. After about 30 minutes, the cam is broken in and the engine can be treated normally. If you don't follow this critical break-in routine, expect your cam lobes to go flat before the cam gets ten thousand miles on it. See my post pertaining "cam walk" below to see why this is so critical.

Pete - interesting that you bring up lawnmower engines - this is where I cut my teeth. I built a running lawnmower engine at age 9 - this is what happens when you have a bored kid in the summer with nothing to do, but play with a broken lawnmower. The first one was a Briggs and Stratton 3.5HP engine with the "PulsaJet" carb (a whopping 9 cubic inches). That was 28 years ago, and I've been building engines since. To get back to the topic, milling heads is something that race engine builders do to get the perfect compression ratio - along with cleaning up the chambers to equalize the size among all 8 chambers. If you don't mind molesting, and possibly sacrificing your heads, you can try it if you want. Remember, what you mill off the heads, you will also need to mill off the end-seal areas of the intake, and you will also have to mill half that much off of each cylinder head mating surface on the intake. Milling to .08, you can expect to remove about ~8 cc.

Stock compression ratio on the 305 is approx. 8.5:1 - this mod will take it to about 9.5:1 on dish pistons, so 92 octane expensive gas, here you come!

Port the heads but don't polish them. Actually, port-match the heads and perform a little bowl-blending, but keep it conservative. Polishing the heads will kill your low-end power (reducing peak torque and moving the power band up the RPM scale). It will also cause your car to flunk emission tests if you have those.

I took a different approach to raising compression on my 350 when I built it in '93. I started with a set of "492" casting double-hump heads with 64cc closed chambers. I also decided to use Kieth Black 22cc reverse-dome pistons which brought the compression ratio to the ~8.8:1 range after performing a "cleaning" mill on the heads (to remove any warpage - they are after all old heads) of approx. .020". The combination of these heads with these particular pistons (which have a D-shaped dish with the flat portion in the quench area of the chamber) enhance combustion as well as increase compression resulting in a broader power band. The heads were conservatively port-matched and blended, and the intake valves were lightly unshrouded. The result was and is an engine which is about 275HP on single exhaust (2.5").

AND - the reason why rollers walk and flat tappets don't has to do with the design of the cam lobe. Flat tappets (lifters) really aren't flat, but are slightly convex. The cam lobe , looking at it sideways, is slightly angled along the high-point rather than being completely flat. The high-point of the angle is on the rear-facing edge of the lobe. This imparts a spinning motion to the lifter to minimize wear, and it also forces the cam back into the block.

Roller cam lobes are flat and level along the top of the lobe to maximize contact with the roller lifter, which is also completely flat across for the same reason. Due to this, there is no acting force to move the cam backwards into the block, so the cam can "walk" forward. Of course, this can have disastrous consequences if this happens, so a button or plate is used to retain the cam in the block.

wilfie27
05-21-2006, 09:46 AM
I believe the casting # is 434 on your heads and another thing you might want to watch for is the water jacket above the spark plug getting too close to the firing ring before milling. Certian SB Chevy heads are bad about this.

1986Z28
05-22-2006, 02:56 PM
i think "292" is another good head

onlineclick
05-30-2010, 06:49 PM
Hi everyone

Just introduce myself: I am a man (says my wife), I am 50 years old (hmm, that looks bad isn't it?) and I am a terrible bad programmer (I say myself).
My hobbies: computer (of course), my 17 years old son and of course my wife. I like to play billiard, I do a very little bit and very simple programming in VB and I try to make a site for my billiard-club in the near future.

Look forward to 'meeting' you all.

Add your comment to this topic!