Just finished a mild restoration / upgrade / rebuild of my L48 1978 C3 and I am pleased with the results. I have learned a lot, especially the fact that reality has a way of tempering ambitions for projects like these. For example, I was going to stuff an all aluminium L33, a 2004r trans and a flaming river rack and pinion into it, among other things. Well the L33.. I sold, and the trans and rack in pinion are sitting unused in the shed. Nothing is as easy as it looks on youtube. These things considered, I have also learned that upgrading and rebuilding existing stock systems can be just as good and rewarding to drive. What I'd like is some opinions on the engine combo which is as follows:

Stock L48 block and rotating assembly Inspected and found to be in excellent condition
Summit 152123 iron heads These were already on the car when I purchased it; 165cc intake runner, 72cc combustion chamber, 2.02 / 1.6 valves
1.6 rockers
Sum K1103 cam 214 int./224 exh.,112 lobe separation; installed straight up, no advance or retard
Stock HEI initial timing of....9? Can't remember
Long tube Headers
2.5 dual exhaust
Stock '78 torque converter Stall speed of 2100 or so I think
Stock '78 posi 3.08 I think
Of course nice new timing chain, fuel pump, gaskets, pushrods (stock length), lifters, bits and bobs
Stock qjet - CFM ???

It starts, sounds great (don't think I wiped a lobe), maintains good and constant oil pressure, amperage, temperature and idle. It definitely has more go than before. Just wondering if the forum gurus can see something within the recipe that is clashing with itself, needs tweaking, or if something is missing. Thanks a ton, love my C3.

 

Looks like a solid basic L48 rebuild with close to the same perf potential.
The only mismatch i see is your SCR didn't go up much from the move from 76cc to 72cc heads (8.4 maybe), yet you used a cam suited for 9.5-9.6 SCRs and bumped duration UP a few more degrees with 1.6 rockers. this may also be why your initial timing is at 9

 

Ahh I see! Interesting. Apart from pulling apart the engine and slapping on different heads and cam, what could I do to dial this combo in so that it's better suited to itself? Would getting 1.5 rockers be an improvement? Maybe adjust the timing? I saw a hotrod article that said this cam performed best with around 12 degrees of initial timing. Thanks much.

 

I'd keep what you have and adjust timing to max the combo potential -- try bumping to 10/11 or even 12 and see if the motor runs better at those advances.

 

Your engine, or really any small block can benefit from a bump in timing from the stock spec. You want 36 degrees total with the vacuum advance unplugged, which will probably put you somewhere in the mid teens for an initial number. I would email Lars at V8fastcars@msn.com and ask for a copy of his timing papers.

If you haven’t touched the Quadrajet after all the other mods it’s likely running a bit on the lean side. You might want to look into increasing the primary jets a size or two. Depending on the individual carb number it might be 750 or 800 CFM.

 

Excellent idea, I'll hit it with the timing light this weekend and see about bumping up the timing. Maybe it'll help to dial it in. Thanks 👍

 

This is excellent advice. There is more to maximizing your ignition timing than setting the initial advance. Getting the timing curve correct will provide a significant improvement in performance.

Also, your diff is probably the original, untouched unit from 1978, with 3.08 gear ratio. When you decide it is time for a rebuild, you should consider changing to a 3.55 or 3.73 ratio. I changed mine from 3.08 to 3.73 and took 5 tenths off my quarter mile ET. You can feel that difference. On the highway, 65 mph is 3,000 rpm. So not too bad for a car that is not a daily driver.

 

Then you can use that 2004R to get the best of both worlds

 

I was thinking that about the transmission...until I realized that it's not worth it unless you have one of more performance oriented valvebodies and governors. Like out of a Monte Carlo SS, Olds 442 or Grand National. Guess what....the one I found isn't one of those ha! It's in great shape but the cost to upgrade it ....forget it

 

My $.02, I have a 200R4 in my 78 and you don't have to upgrade to handle XXX horsepower. You can keep it pretty stock with just a simple rebuild. The idea is to get the overdrive which is way better than the Turbo350 or whatever you have in the car now. I also have a flaming river rack. IF you have the complete set up with the cradle, it also is a piece of cake to install. And you will get rid of all those leaky hoses and valves underneath. I actaully think you did good on the engine work and the trans and steering upgrades will give you a really nice, fun and drivable car. Don't stop now!

 

Thanks much 👍. I'll mess with tweaking the timing and fueling and keep repairing / upgrading what's in the car for now. Maybe I'll save the other parts for a future build. I also have a 67 Riviera that needs my attention as well.

 

Lars is smart.

 

Apart from timing and fuel tweaks, here's a thought process I had:
1. I've read the springs on the heads (sum 152123) that were already on this engine have a max lift of .490 in.
2. With the 1.6 rockers the exhaust lift is .49099777 or something close.
3. I've also been seeing threads where there is discussion regarding valvetrain geometry and binding issues when using 1.6 rockers.
4. Saw other threads promoting 1.6 on intake and 1.5 on exhaust.
5. I have 1.5 sitting around somewhere.

Would 1.6 on the intake and 1.5 on exhaust help my compression ratio issue while also providing a valvetrain environment less conducive to binding or other geometry issues? Thanks!

 

USE the 200R4, stock it'll take 330 ft/lbs of TQ. Your engine build is probably very near that

 

GroovyVette,

Rocker arm ratios having nothing to do with compression ratios.
C.R. is the volume in the cylinder squeezed from the bottom to top. Don't even have to have rockers on the head. Simple math.

But wait, there's more. During this compression stroke one of the valves is still open. How can that be?
I won't get into all the stats on Dynamic Compression Ratios, but you get the idea.

I am not a big fan of 1.6 rockers. If an engine came with those, fine, obviously keep them.
When you start messing around with valvetrain you can get into trouble real quick. Engineers wanted 1.5 on GEN 1 for a reason.

The 1.6 ratios can bind pushrods in the cyl heads.
Can make the rockers strike the retainers.
Can make the retainer strike the valve guide.
And can put additional (unneeded)) stress on the guides.

How can one avoid all these poor events from happening?
Buy the right cam!
Want more lift? Buy more lift.
Want more duration? Buy more duration.

It's too late for you, your short-block is all buttoned up and done. But I do not recommend 1.6 rockers.
Comp Cams has a 1.52. Much safer, less stress on the valvetrain. And worth a couple horse.

(Rumor is, stock rockers are closer to 1.4 something)

 

Ok
I don't know the Summit heads, but whoever built that did an OK job.
See this thread for more specifics:
https://www.corvetteforum.com/forums...e-numbers.html

As stated, your CR may be low 8s now.
It could be much better with 64cc or even 57cc heads. That is what I would have used.
The cam is pretty decent selection tho.
This will probably run on 87 regular gas.
But it has almost no quench, so maybe not.

New 57cc TFS heads could be worth another 30HP.

1.6 rockers are a waste on this cam, and may cause lift issues.

The best thing you can do is to perfect the ignition timing curve.
Lars papers describe the whole procedure.

Here are the basics:
36* total advance, at ~3200 rpm
Perhaps as much as 15* at idle.
It may involve modifying the distributor to get both. the 36* is way more important that the 15*
Only 10-12* from the vacuum can,. It must have a limiter, and it runs on manifold vacuum, not ported.

If you have a stock curve, and change it to the above, it will feel fantastic, like free 30HP!

Carb & intake are fine for this engine. QJets are the best! For stock L48s
The 200R4 is a sweet move!

 

Timing is definitely on the docket this weekend, thanks all for the great tips. I'll take a look at those 1.52 rockers. I'm more concerned with trouble free and fun rather than all our power.

If anyone knows....would a 200r4 with a KC valvebody and governor work (shift point wise) with my engine? I've checked out the turbo Buick forum and everyone is going on about how not all 200r4s are equal and the ones in the standard car (mine came out a 1987 Olds cutlass with a 307) will not perform well behind more performance specific applications. Again I'm not looking for an all out rocket, just quick fun and capable.

 

In the older TH400s and TH350s, there was a mechanical governor that controlled the shift point rpm but at WOT only. The vacuum controls controlled it elsewhere.
If that governor did not shift at the rpm you wanted, it was relatively easy to raise the shift rpm. Change the springs or grind some material off the spinning weights. They work just like distributor weights.
Pedestrian engines, like a 307 olds, did have lower rpm shift settings. The special performance engines did get a special higher rpm governor. But in the speed shop we just modded the stock ones.

A LT-1 or L88 automatic would shift exactly at it's 6500 RPM redline at WOT from the factory. Everything else was more like ~5300 rpm.

Hopefully a 200R is similar? I thought it was like a TH400 but with an extra gear?

 

Hey all,

Update:
Swapped to 1.5(2) rockers, found a few ill fitting spark plug boots, completely rebuilt the rear suspension (shocks, fiberglass spring, new half shafts, all new trailing arms with bearings, spindles, etc.), and basically tidied up - WOW what a difference, the thing rips from down low all the way up. Found a few suspicious looking witness marks on some of the pushrods and noticed that many were sitting low in the pushrod slots - as in touching the lower quadrant of the slot. Not the case with the new rockers, feel a lot better about the valve train geometry. On to lights and windows - speaking of which - any tips for aligning the windows because they are not able to roll all the way up without contacting the window- stripping.

 

1.5 rockers, rock!
(pun intended)

Good for you.

Pushrods WERE rubbing the slot in the cyl head or guide plate?