I bought my ’71 last year and have driven it ~1,800 miles since, including the 1,000+ mile trip home. #s matching, stock aside from what I’ll detail below, and won its class at the Portland Roadster Show after a two-year restoration. It’s pretty gorgeous.

Over the past year, we worked on getting to drive like it looks. Insulated the entire interior, and addressed a number of small issues. The next bullet item is to get it to run better. M20 trans, 3.36 rear, base engine.

When the resto was done in 1999-2000 (it only had 4,500 miles on the build when I bought it 22 years later), the owner at the time, for some unknown reason, spec’d this cam shown in pic #1.

Specs on the engine build are in pic #2.

The Clevite pistons spec’d are no longer manufactured, but I think these Silv-o-lite in pic #3 are the same.

A look down one of the cylinders in pic #4.

What one of the plugs looks like, pic #5.

I reached out to the builder, who’s still in business. He reviewed the build, and said the only things he could add were:

• The porting on the heads was just minor clean up
• Decking on block and milling on heads and intake manifold were minimal
• Roller-tip rockers are 1.52 ratio
• Overbore was .030”
• The cam was not his choice! It was installed straight up.

The timing is curved per Lars’ specs, with the 36 degrees in by 2,800 rpm. I replaced the carb float, and set level per Lars’ spec. I replaced the stock vacuum advance with a B28, since there’s only just over 5” of vacuum at idle. Vacuum advance is connected directly to manifold vacuum; no solenoid.

I have the idle set for 1,000, but if it idles for any length of time, it’ll creep lower, and I’ll blip the throttle when it gets to 800. It’s weak on the bottom end, but it can’t breathe at the high end (where the cam might actually work). Accordingly, drivability is mediocre.

I’m not sure what to do at this point. I’m guessing a cam, and maybe also a head, change is in order; maybe more? I’m not looking to make this thing a rocket, I just want it to run well, and be fun to run up through the gears. Your suggestions and insights are very much appreciated!

 

What carburetor is installed on this engine?
Is the balancer new and hasn't slipped?

 

Stock Q-jet. Balancer was replaced; doesn't seem like it's a timing issue.

 

Perhaps something restricting the exhaust system? My highly cammed car had inserts in the tail pipes when I got it (likely to reduce the noise with the turbo mufflers). Once I took them out the car goes like a "bat out of hell" at the higher rpms.

 

did anybody change the primary power piston spring to match the low camshaft vacuum?

 

Exhaust was all new, stock Corvette Central replacement at time of resto. And it sure doesn't sound like it's restricted ; )

My guess would be "no". I thought that was a Holley thing?

 

The base L48 is a dog by modern standards, your average v6 min van has more power. Go time a 0-60 run or a 1/4 mile time and compare it to that era before getting too wound up. Even with a good tune it’s not going to be fast.

With 10:1 compression, a decent set of heads, and the right cam 400 hp isn’t that hard in that 350. IMO 400 hp is about where these cars get fun.

 

Why are you using manifold vacuum advance? You should use ported vacuum advance and limited the centrifugal advance on the retard side. I haven't finalize my timing table yet (electronic spark timing controlled, but timing is timing) but for example my timing is locked at 25 BTDC crankshaft degrees from idle to 1200 RPM's, 32 BTDC crankshaft degrees at 3200 RPM's. Manifold vacuum or kpa doesn't come in at idle. And my camshaft is a lot smaller (206/212@.050, 112 LSA). With your camshaft you might be better with a locked 36 BTDC or whatever it wants.

 

Same with Quadrajet, the spring has to be selected for low vacuum. Holleys the power valve has to be selected.

Spring kit

 

I know it's not going to be a bullet. But it can still be fun, which it really isn't right now. I had my first SBC 48 years ago, and have had a number of them since. I've got a pretty good idea of how it should run, but not exactly sure how to get there. I'm thinking aluminum heads with 64cc chambers, and a roller cam of as-yet undetermined spec, but I wanted to see what others had to say before I tear into it. There are many on this forum who know more about these engines than I do.

On another note, my daily is a '15 SS sedan... LS3 and Tremec 6060. My wife's daily is a '17 F-150 RCSB with a Coyote. Neither one is slow ; )

I set my timing up per Lars's suggestions. I was running ported vacuum, but switched to manifold per his (and many others') advice. With that cam, it definitely runs better with more lead at idle, but if I ran that much static, I'd have to limit the advance curve. This way is easy.

Well, I just learned something new. I would bet the spring hasn't been changed, but for that price, I'm on it tomorrow. FWIW, I don't think it's a bog; it's more like inadequate cylinder pressure at low rpm.

 

1. You are correct to run manifold vacuum
2. The combination you choose will determine what cam you can run. For example. If you just replace the heads, but dont take compression into consideration bigger cams will lower your idle vacuum. It can be made up with a lot of timing but it isnt worth the effort.
3. You look like you have a flat top piston so you need to get a compression calculator and plug in cylinder volume and valve reliefs to get yourself up to 10.5:1. You can go higher if you have 93 octane with aluminum heads without detonation but 10.5 is pretty good all around. 62cc or 58 cc may be a choice
4. You want a head that has a 185 to 195 runner volume with a cam like that. You can go bigger but a cam like that should get you around 400 hp. A good flowing head at that volume will liven up your motor and will be able to go up to 500 hp if that happens in the future. Getting those components you can talk to a Cam company about your goals and they can spec a cam
5. Spend the money for a good set of lifters like Cranes. If you can afford it go with bushed rollers versus needle bearings
6. You will want headers, even a block hugger shorty is better than rams horns
7. Finish off the exhaust with 2.5 pipes all the way back. 2.75 would be better. C2 style round 2.5 side pipes with C3 covers is a good option
8. A low deck aluminum intake will fit under the hood. The Aluminum will save weight and a newer style will flow better tgan old cast iron. Dual plane intakes promote low end torque which will give you off the line grunt.

 

My suspicion is that your dynamic compression ratio is too low. Hard to fix without changing the heads or pistons.
however, if you don’t want to change any of that, a small sbc supercharger, about 5psi will raise your DCR and probably work well with your cam.
im not sure what heads you are running, but if the heads don’t flow well then you wouldn’t be getting the most out of the boost

 

You need a much smaller cam.......and if you are not running headers and a very good exhaust, this engine will never get close to running right.....
First problem is the Q-Jet.....I love Q-Jets, but only for engines that have more than 13 inches of vacuum......they simply cannot meter fuel on a big cammed engine properly without a shitload of mods that I have no info on.......
You have posted zero info on what heads they are, what intake you have...no pics of the engine, no idea what the distributor is....etc.......
With huge cams like this, having headers on the street is very important......the scavenging of the exhaust pulses at the collector is crucial for getting the charge out of the cylinder due to the huge overlap......you choke this up with manifolds and/or **** exhaust and it will run like a lazy turd.......if the heads are stock...this engine has about 9.5 to 1 compression...which is not enough on a 350 for this duration to run at low RPM with any hope of crispness.....
Two ways to fix this engine......
1) Bolt a pair of Dart 180 heads on it with 64cc chambers.......but you will need to measure for pushrods.......the 64cc chamber will get you 10.5 to 1.....the cam will like that......you will also need at least a 2101 Edelbrock Performer or 2701 Performer EPS.....and a Holley Classic 650 Double Pumper.....or 650HP/Street HP.......and long tube headers........it will make well over 400hp but will be somewhat soft off the bottom.......it will pull like a train though once it overcomes that gear, and gets on the cam......
2) Stick a mild cam in it like the Summit 1103 cam.......I just put this cam in my 82' 305 Z/28 with 9.5 to1, headers, 2101, and a Q-Jet...it loves it......your 350 will love it too.....you can keep the Q-Jet but I would still run the 2101 and headers to get the most out of it......this will be a fun engine that you can drive anywhere.......13-14 inches of vacuum......mild rumble......and if the heads are mildly ported, you can expect 370hp or more........

Personally.....if it were mine, I would go with option 2 if it is in good health besides the cam......much cheaper and a LOT less work......and still a solid low 14 second or high 13 second car......

Jebby

 

Wrong.....especially with this much overlap...he needs ALL of the advance he can get at idle.......even if he could get 20 degrees initial, you would still need 10-12 degrees vacuum advance, maybe more......at idle. In fact...this cam would probably work locked out at 36 degrees.........
Much easier to dial in what you need electronically and play around......

Jebby

 

Jebby, I respectfully disagree and here's why. Let's say that your engine idles best at 25 BTDC and 32 BTDC gives the best WOT performance. Your distributor centrifugal advanced range is 22 degrees and your vacuum advance range is 15 degrees. So you set the base timing to 10 degrees BTDC at idle with the vacuum disconnected. You connect the vacuum advance and life is good. But wait, you start to accelerate and all of a sudden the car feels a bit sluggish or hesitating because you just lost 15 degrees of timing. What's even worse you have a automatic, put it in drive and drop some RPM's, which results in less vacuum, which results in less RPM's, which results in less vacuum and so on and so forth.

When I first setup my Megasquirt timing table it was mirror image of a HEI distributor and what I described in the last paragraph is what I experienced. But we both agree that the OP would be better off with a locked 36 degrees BTDC. Yep it's certainly easier to play with electronic spark timing and you know what's even better, if my idle RPM drops, my timing advances.​​​​​

 

Have you ever read Lars Grimruds paper on setting up a distributor curve and which vacuum can to use?

 

Nope.

 

maybe you should, your theories on timing curves and vacuum advance need some updates.

 

Easier yet, Just install the summit 1102 cam or one similar in specs. It's much better suited for low CR engines. The assumed 9.0 -9.5 CR for your engine is based on the heads actually being 76 cc's (if that's what they are). GM was very optimistic in the chamber measurements. On my '77 the 76 cc chambers measured out around 80 cc's. A further reduction in your CR.
Since we know nothing in regards to chamber size, head gasket thickness, compression height for pistons, or where the pistons reside in the cylinder at TDC, making any cam recommendation based on assumed CR is going to be flawed.

OP, You'll note on the pistons you linked as the same as the ones installed that the CH (Comp HT) is 1.540", this is .020" lower than stock. Which means unless the block was decked .020" then your pistons are that much lower in the cylinder at TDC. Stock CH is 1.560" putting the pistons .025" down the bore at TDC. With 1.540" CH pistons this would put them .045" down the bore unless material was removed from the block to compensate. What this all means is less CR, less cylinder pressure making for a lazier engine, particularly with that cam.

 

Trust me I played with the timing table every which way to Sunday. It idles smoothly, nice crisp drivability, and goes 0 to 60 in 5.47 seconds. I think I'm pretty good.