20mercury

Did some searches and would like your thoughts on what is needed to upgrade my 68 base 300hp to a L79 350hp.

Some results:

1. orange valve covers to chrome valve covers

2. 929 to 151 222/222 cam, where to buy an original? or are the aftermarket ones with this grind just as good?

3. 11/1 dome aluminum pistons

4. Hopefully rework heads from 391729 to 3917291, so can you rework the existing heads from 1.94 to 2.02/160??

5. carb 7028207 to 7028219

6. harmonic balancer 6" to 8"& timing cover, blk to chrome oil fill cap, decal, tach red line (later 2, probably not though)

Much thanks!

Shark Racer

It'd be doable. You'd be better off going to modern parts. Another 100hp for not much more cash.

Is your goal to create a clone or get more HP?

You probably wouldn't need to change the carb, just tune the existing one to work well with the new motor.

Easy Mike

Leave those off and you could have a 350hp sleeper.

20mercury

Thanks for the feedback, yep, likely a clone, I like originality but a reversible upgrade to a L79 is pretty appealing.

Anyway also like the sleeper thought, didn't Paul Newman put a Porsche engine in a Volkswagon, but this is a long way from that.

So what do you think are the must do items?
cam, heads and pistons? what is the need for a 8" balancer compared to a 6" ?

boat196

I would install the L46 camshaft, change your head gasket to obtain a true 10.3:1 compression. And port your heads, valve job and block the exhaust crossover. Have Lars rebuild your carb and set up your distributor.

cardo0

Ah the good old L48 to an L79 (L82) 350hp+ upgrade using GM parts. In my area i still find big vlv double hump heads w/samll 64cc chamber on craigslist - for cheap. They really should be pocket ported and need a good 3 angle vlv job. These will give the compression increase and save u from the bottom end rebuild w/new hi compression pistons. BTW the better chevy head that should just bolt on for more pwr would be a vortec but it needs a vortec intake also.

I dont have all the double hump head numbers but u want to look for tight 64cc chambers and large 2.02 vlvs. U may need the accessory holes in the ends also.

For the cam google Chevy #3863151 cam and look for Elgin, Melling for the best price in copies. Crane has 1 but a tad more $$$.

http://www.competitionproducts.com/E.../#.VTvNJvCTWhE

http://www.autozone.com/1/products/1...ing-22200.html

http://www.summitracing.com/parts/crn-967601

Good luck and if u have specific q's just post them.

20mercury

Much thanks!! Good to know others have thought and done the same thing, LOL's!!

Will check out as you suggest, thanks again!

So just to be sure I understand, I need to install an original 3863151 cam or one of the new aftermarket cams with the same grind which is just as good.

And I need to find and rebuild some old double hump heads. I thought double hump heads might be pre68??

And doing all of this will mostly get me a L79 clone?? And with this option, you do not need to change the pistons out? And the 6" to 8" balancer is not really needed? Or just retune the Q jet?

Not many accessory holes needed, the only luxury option on this 68 is power steering!

7t9l82

the 8" balancer was used for high R.P.M operation and depending what is actually in your engine may not be compatible. the 350 H.P engine had forged mini dome pistons the 300 H.P had neither . the 350 horse engine had rods that had a better heat treat. the 300 had smaller valves the L-79 had 2.02 valves. the 300 horse version was a passenger car engine and the L-79 was a high performance engine. i would be concerned the condition of your engine and if a compression increase and a higher R.P.M camshaft is a good idea. but that all said it could be done, i would use the better head with a good valve job a smaller chamber and some bowl work as already mentioned and an appropriate aftermarket cam with your distributor set up by someone competent like Lars and perhaps the carb too.

cardo0

Here are some heads near u on craigs list but are bare w/o the original small vlvs. Really nothing to install larger 2.02 vlvs other than the expense. But i dont know what heads u have now and maybe could use your current heads for vlv donors. http://batonrouge.craigslist.org/pts/4984814877.html
Or sometimes u can find a great price on large vlv smog heads and for <$200 can use those vlvs. Heck sometimes u buy the entire chevy small block for <$200 for donor parts.

Just need to keep shopping.

20mercury

Much thanks for all of the advice and info!!

The compression was good with a simple compression gauge, I thought the bottom end on the base and L79 was the same, no 4 bolt mains or other. But you make a good point, I understand others have upgraded the top end only to find the bottom needed an overhaul too.

And I am pretty sure I have the original 391729 heads, will check. Great tip on Craigslist and cheaper than what I checked on ebay. And these Craigslist heads are 3917291, same vintage as my 415000 last day of March 68. Pretty cool find, much thanks! So I could get these Craigslist heads and install the 2.02/160 valves plus the cam and I would be a long way towards a L79. What is guess on how likely a magnaflux check would show bad? I guess you are looking for cracks, so if it checked bad after you bought them, well you might have some expensive boat anchors.

Thanks again for all of the advice and help!!

cardo0

Your decision to just top end the motor with used chevy parts is a good one. Too many remove the engine in their vett trying to overhaul and the car never runs again. The top end is good place to learn on. Why spend thousands when your just learning and dont know exactly what u want or what the correct match of part is. Why spend thousands and a huge amount of your time when u can spend just hundreds and and much less time and still have a good result.

Your missing 1 digit on your head casting number. Only 3917291 could have had large vlvs but they would have had the smaller 64cc chambers too. U need to pop a vlv cover and look at the casting number on your head. More likely u have 3917290, 3917292 or 3917293 heads with larger 76cc chambers and smaller vlvs.

Any used part could be cracked. Thats whats nice with buying local/craigslist is u can look them over - bring a 4x magnifier. Or make the deal on tested good after sale or return ok. I think pay-pal supports returns on ebay deals for this if agreed to.

As for larger vlvs u may want to shop local cylinder head shops for used - many will have stock piles or bins full. As long as the guide clearance is correct nothing wrong with reusing them. Cutting larger seats for the larger vlvs should not be an issue and u would want a good 3 angle vlv job and to price out pocket porting charges if the head shop does it. Not runner porting but pocket porting - usually a shop will have a cutting tool for this and it takes very little blending work. My heads were PP for only $100 back when i got them. PP gives u the most bang for your $$$.

20mercury

Much thanks again for all of the help! Will print all of this out and start the hunt (also convinced the wife-better half, that this is a worthy endeavor indeed!!)

7T1vette

Your problem will be running 11:1 compression with cast iron heads on 93 octane fuels. You can do it, but might have to de-tune the ignition timing to eliminate spark knock. That build would be better with aluminum heads. (Modern alum heads would flow better, also.) You could still paint the aluminum heads, if looking 'stock' is important to you.

cardo0

Good catch 71. Normally 11:1 would be to high for a short cam and i could not recommend the tighter 64cc chambers. But the 3863151 cam has a late closing for the intake vlv (290 duration, 114lsa) which reduces the dynamic compression and u can read it was used in the higher compression L79 (11:1). That and most actual chamber volumes are conservative and measure larger than published figures. I would measure the vol above the piston on the short block (yes while in the car w/heads off) and then the heads chamber vol. If <11:1 i would run them with the 3863151 cam. And u can always run the numbers for a dynamic c.r. with that 3863151 cam for what static c.r. u expect to run now.

20mercury

Thanks for the tip on 11/1 compression, I have got an airport a couple of miles down the road, could mix in some aviation gas I suppose or retard timing a little.

You are correct, should be 3917291, I got the 391729 out of an old 68/69 NCRS manual which evidently had a typo.

Anyway, removed the 2 valve covers and evidently my 68 has a checkered history , driver head is 3917291, B248 (Feb 24, 1968)
which is supposed to be for a 1968 327ci 300hp or 350hp

And, the passenger head is a 3890462 D206 (Apr 20, 1966) which is supposed to be for a 1967 327ci 300hp or 350hp

So, I suppose an almost 50yr Corvette could have lots of interesting parts?

Anyway, I am thinking these are 64cc heads, and if I understand correctly, I would be able to just pull these heads off and have them reworked for 2.02/1.60 valves??

cardo0

Yea, u already have the 64cc chamber heads. 3917291 and 3890462 are both 64cc chamber heads. So my guess is more than likely they are large 2.02 vlvs also - but just my guess. So whats your block numbers - casting and VIN? Maybe u have the L79 motor? What do u use for octane gas now?

Well since u already have the hi compression heads i think u need to change your strategy. Do u want to continue to use your heads? Are u willing to remove a head (or both heads) and measure compression ratio volumes (and check vlv size to)? I guess what im saying is u need to find out what u have before buying anything and then consider the options.

Maybe a more modern aftermarket cam might help get u more power. If u just want to change cams now i would measure cranking pressure (compression test) then pull the cam to see what it is. Since at least one head was replaced but looks like both maybe PO swapped cams also. Block? Lets look at the block numbers first.

20mercury

Much thanks for the response!

Ok to answer your questions:

Block is 18S415000 VO319HE
(last day of March 68 production, & matches the vin tag, & March 19 1968 engine assembly, HE is base engine 300hp

block casting number is 3914678 with a C138 casting date (March 13, 1968)

Yep, with the 66 head, I am beginning to think somebody beat me to it. But talking to the 2 previous owners which goes back a little over 10years, they both swear it is all original, but obviously while the rest of most everything is matching (4sp) or date matching (except q jet and alternator and 4.11 rear end) somebody has swapped one of the heads. No big deal to me. I agree, if somebody has swapped one head, they very likely had both off and so again I agree, I bet they put in the 2.02 valves and maybe hardened valve seats too.

I have not driven it very much, had to redo the front end and a laundry list of stuff, it has a 4.11 rear (AP2365, not original either, ha!), and feels like it is a good runner to me. Not like my 08 LS3 mind you, but pretty good. I have been running 91/93 no alcohol and have not heard any pinging.

Yes I think all you say is good and makes sense. I think I will get the front end back together (already, changed clutch, flywheel, cleaned/painted frame, repaired windshield frame/glass, all interior/dash out and in, ect, ect... and drive it for a little while.

I will then check the compression again and then pull the heads off and then measure a couple of valves as you say. Also, pull the timing cover and see if I can figure out what cam I got, I thought cams are stamped on the end?? & What is the best way to measure compression ratio volumes?

Agree, need to first find out what I have now.

Anyway, this might have a funny ending and somebody might have already spent the coins to upgrade this one which is ok by me!

Thanks again for the advice and help!

cardo0

Ok if u still have body work to do then u dont want to get ahead of yourself taking the engine apart. Your idea to drive it for a while is a great idea and learn how to tune it for performance. I bet with 4.11 gears that car will have plenty of wheel spin and those tighter chamber heads on your HE 300 hp motor will add torque also.

With those heads it puts your engine hp somewhere between the HE L46 300hp and the HP L79 350hp motor.

To determine what cam u have really requires u to remove it as the GM cams had the symbols behind the first bearing journal - u might be able to remove the lifters and see this with a light but i doubt it. Aftermarket cams usually have their cam marks on the front face of the cam and u could see this with the cam timing gear off.

To measure compression ratios accurately u need to take a head off. Then u need to get the cyl u test to TDC (accurately with a degree wheel) and use a little grease to seal the grove above the top ring land on the piston. Cut a piece of plexiglass to cover the cyl bore and drill a fill hole near the top side of cyl bore. We found on this forum (REEL8VTR) red axle grease will hold the plexiglass onto the block (i tried petro jelly and it wouldnt hold well enough to hold the p-glass to fill the vol above the piston). Then fill the cyl with oil or something easy to use with a graduated cylinder, 1ml = 1cc. This gives your volume above the piston. Next measure the head chamber the same way but u can do this with the head on the bench and level. Those 2 volumes plus the compressed gasket volume gives u the static vol to accurately calculate the c.r.. When your ready for this let us know and we will help u work out the entire formula - easy to do.

But u can measure your cranking pressure now. Just pull the plugs and block open the carb throttle blades. Engine should be warm as possible but dont burn yourself getting the plugs out. Cranking pressure will tell u how high your compression is with the cam u have. Most stock chevy 350's are like 150psi. A hot street cam and engine is more like 200psi. Gives u and idea of where your engine is now performance wise.

good luck

7t9l82

64 cc heads were not uncommon back then, the 11-1 motors would have had mini dome pistons the 10-1 motors flat tops. the small chamber heads could easily have the small valves.

LS4 PILOT

I did this to an L -48 in the late 70 's .

I used a set of old fuelie heads from a dragsters 327 . They had been worked pretty good ..going by the wall to wall trophies he had.. Got em for 75 bucks ....a deal for a teen.

I added a 350-350Hp cam. I don't trust the metal in cams today if they are flat tappet .....too many failures IMO.....I d bite the bullet for an Isky or find a NOS from GM .

A local dirt track engine builder , did the work , set up the carb, recurve dizzy , tapped holes to mount my AC , even made a bracket . Big exhaust with cherry bombs. Alum. Valve covers, open air cleaner.

It looked stock and was night and day performance . Stock the L48 was pressed to break the tires loose. After the hop up ..no problem . Engine had no hesitation or flat spots, it would pull like crazy far as I dared rev the engine.

With 3.08 gear the builder claimed he ran 140mph plus late one night on the interstate bypass area outside of town.

I know it was a lot of fun ......the young sailor I sold that corvette to , could not wipe the grin off his face after the test drive.....he was writing a check as fast as he could ...thinking I might back out.of selling . Naw I had my next corvette already in the bag