Can't Time Engine Correctly

[Mark A.]

Re: Can't Time Engine Correctly

The valves were adjusted as per the guidelines for adjusting the 30-30 solid lifter cams. They are quiet, and I think they are okay. The balancer checked out good when I degreed the cam. It does not want to run well with 8 degrees. It seems better at 12. The engine is very lazy. When you rev it up, and snap the throttle closed, it returns to idle slowly. Should the secondary throttle stop screw be holding the plates open at all? What is the proper way to set this screw? Thanks, Mark

[Dennis O.]

Re: Can't Time Engine Correctly

Hate to be a pest, but is the harmonic balancer new or rebuilt? Both of mine failed after the first start of a fresh engine. Forty years of heat, weather, etc. can do a lot of damage. Try this : plug the vacuum advance; connect your vacuum gauge and advance the distributor to get the highest reading on the gauge; then retard it, just a tad. If it runs good, you have found your problem; if not, start looking at other things.

[Joe C.]

Re: Can't Time Engine Correctly

The valves were adjusted as per the guidelines for adjusting the 30-30 solid lifter cams. They are quiet, and I think they are okay. The balancer checked out good when I degreed the cam. It does not want to run well with 8 degrees. It seems better at 12. The engine is very lazy. When you rev it up, and snap the throttle closed, it returns to idle slowly. Should the secondary throttle stop screw be holding the plates open at all? What is the proper way to set this screw? Thanks, Mark

It's fine to use that "procedure", but you can't use the 30-30 specs if you installed a replacement LT1 cam. LT1 should be lashed @ .024/.030 according to the factory, and .017/.023 according to the "procedure".

When the idle speed is set properly, the secondary plates should be open very slightly less than the primary plates at curb idle with the engine hot and vac advance connected. It is not easy to see movement off of the "stop" with the carburetor installed, but practice makes it easier. The more foolproof way to check your settings, is that after you have solved your idle/vacuum issues (the secondary throttle stop position has NOTHING, at all, to do with your condition) and you have the idle speed and mixture screws set properly, is to remove the carburetor, drain it, and hold it up to a light source while looking through the throttle plates. You should see light through all 4 bores, but very slightly less through the secondaries than through the primaries. If this is not the case, then balance them as best you can while the carb is off the car. Reinstall carb, and make any further speed adjustments equally using both screws. Once the speed is set, put some LockTite on the secondary stop screw threads.............you should never have to touch that again................unless you change to a very different cam profile. A "check" on the accuracy of your speed balance setting, is to observe the amount of exposed idle transfer slots in the pri bores. The slots should appear as squares when viewed from below the throttle plates with the plates against the idle stop. The secondaries do not have idle transfer slots. If too much slot is exposed (the slot appears as a groove rather than a perfect square), then you will have off idle stumble unless you compensate with a modified pump shot. BIG cams with a lot of overlap force racers to drill holes in the primary plates which allow the proper slot exposure, yet bypassing enough air to allow a reasonable idle speed.

Did you use the speed pro version of the lt1? That is the most accurate one available. CS 1145R.

Symptoms you describe sounds like you have tight valves (if your cam and rockers are identical to factory units). You should go over them again and make sure that they are at .017/.023. If I were you, I'd add a couple thou to this. I like to compromise between "factory" and "tight" lash settings recommended by some. I set my 30-30 at .025/.025 rather than .023/.023 as "recommended". You would do well to set your LT1 at a fairly loose .019"/.025", cold. Check the indexing on your distributor cap. Make sure that it matches the way I described it in my previous post.

The vac advance canister should be connected to manifold vacuum rather than ported. It should be giving you an additional 17 degrees spark advance at idle (assuming it is idling at about 800-900 RPM with about 12 in-hg vacuum.)

[Mark A.]

Re: Can't Time Engine Correctly

Joe,
The cam is not identical. I believe the stock cam is a dual pattern, I put in a Comp Cams unit that is identical with the exception that the lift is the same on intake and exhaust. I will check the lash again. Thanks for the info. Mark

[Joe C.]

Re: Can't Time Engine Correctly

Mark,

The Comp Cam number that you gave me is the replacement for the GM 346 cam (30-30). This is quite different from the LT1 cam, as you should know. DO NOT expect idle vacuum at anything more than about 8-9 in-hg @ about 800-900 RPM. I hope that you realize that you installed a 30-30 cam in your LT1 engine. There is one further complication which you may have not realized, which I will explain in paragraph 3, below. The fact that you have a 346 cam installed, and, further, that it is timed at zero rather than 4 degrees advanced, will make your engine somewhat disappointing for you to drive at RPM's lower than about 5000. If you'd rather not open it up and re-time your cam, then you should know that the missing 4 degrees of cam advance makes your engine more detonation resistant than the factory timing. As such, you will be able to dial in much faster spark advance in order to compensate (somewhat) for the lower cylinder pressures developed at low/mid RPM ranges due to the retarded cam timing.

Furthermore.............if you adjusted this per LT1 cam specs, and even worse, if you followed the "tight" lash "procedure" to the letter then your intake valves are way too tight, causing your extremely low idle vacuum. Please take my advice and, if you like, then use the complicated procedure outlined in the "paper" to get your cam on the base circle, but lash them all at .024/.024, cold. You may lash them all at .023/.023 if you wish, but the .024/.024 setting will make that 30-30 cam idle a little bit more like an LT1.

Your Comp Cam is an exact dupe of the factory cam, EXCEPT that it is ground with zero advance/retard, so the GM 114 LCL with "straight up" settings as you indicated, has the intake lobe center at 114 ATDC and the exhaust lobe center at 114 ABDC (inlet and exhaust centers are coincident with lobe centerlines). The factory 346 cam used identical lobes to your reproduction cam, with the same 114 degree LSA, however; the factory cam was ground with 4 degrees advance. This means that if the factory cam was installed with the timing marks on the sprockets aligned, then the factory cam would have been running with 4 degrees advance, putting the events as follows 110/114/118 (inlet lobe centerline, lobe centerline, exhaust valve centerline). You indicated that you installed and degreed your cam with zero advance, which means that it is running at 4 degrees retard with respect to the stock 346 cam timing. Your engine, as now configured, will deliver reduced torque at low/mid RPM range as compared to a stock L76/L84 of 1964-65 vintage, with boosted horsepower at RPM's above (about) 5000 RPM.

[Mark A.]

Re: Can't Time Engine Correctly

Joe,
Thank you for all the insight. My idle vacuum is about 9-10, as you stated it should be. I didn't know about the exact replacement cam being available. I'll have to live with this one now. I haven't road tested it yet, as I live in northern Wi. I'll see what it feels like under load. Thanks, Mark

[Joe C.]

Re: Can't Time Engine Correctly

Joe,
Thank you for all the insight. My idle vacuum is about 9-10, as you stated it should be. I didn't know about the exact replacement cam being available. I'll have to live with this one now. I haven't road tested it yet, as I live in northern Wi. I'll see what it feels like under load. Thanks, Mark

Don't fret Mark. The 30-30 cam is the best kick in the ass you can get in a vintage SBC. Ask any owner of 1964-65 L76/L84 Corvettes, and 1967-1969 Z28 Camaros!

You did what I would have done (except for the cam retard), only you did it unintentionally.

You only need to be aware of the fact that your cam timing is not optimal for street use, but it is optimal for drag racing with a close ratio gearbox with minimum 4.11 gears. It will make more horsepower the way you have it timed, but at the expense of low/midrange torque. If you get discouraged by some lack of torque, just remember that you have an additional 23 cubes over a L76 and 48 cubes over a Z28, which will help make up for the 4 degrees of cam retard. You also have the option of retiming the cam to have it run 4 degrees advance, rather than zero as it is now.

Another option would be to loosen the lash by a few more thousandths (to .027/.027) above what I consider optimal at .025/.025 in order to recover some bottom end.........albeit at the expense of horsepower.

You need to discuss your induction and exhaust systems in some depth, as your cam likes less restriction, especially in the exhaust, than the milder LT1.

[Mark A.]

Re: Can't Time Engine Correctly

Joe,
Fortunately I have 4:11 gears, and an M21 in the LT-1. That will help somewhat. I don't like the idea of bleeding off compression with the retarded timing. Mine CC'd to 9.15 CR. But I guess I'll have to live with that. It always was a rather peaky engine anyway, even with the stock cam. Then the crank sawed itself in half, and the fun was over for awhile! I'm looking forward to getting it on the road, and trying it out. The induction is stock, with the Holley, and the aluminum manifold. The exhaust is stock, coming off at 2", and opening up to 2.5". The mufflers are stock replacements. I appreciate all your time, helping to sort this out. Thanks, Mark

[John H.]

Re: Can't Time Engine Correctly

Mark -

1. Is your vacuum advance connected to the (stock) "ported" source, or to a full manifold vacuum source? You want it connected to full manifold vacuum so you get vacuum advance at idle.

2. Which vacuum advance unit do you have? Your "30-30" cam will only pull 9"-10" Hg. vacuum at a 900 rpm idle, and if you have the stock vacuum advance unit, it won't be fully-deployed at that vacuum level, resulting in an unstable "dithering" idle.

[Joe C.]

Re: Can't Time Engine Correctly

Joe,
Fortunately I have 4:11 gears, and an M21 in the LT-1. That will help somewhat. I don't like the idea of bleeding off compression with the retarded timing. Mine CC'd to 9.15 CR. But I guess I'll have to live with that. It always was a rather peaky engine anyway, even with the stock cam. Then the crank sawed itself in half, and the fun was over for awhile! I'm looking forward to getting it on the road, and trying it out. The induction is stock, with the Holley, and the aluminum manifold. The exhaust is stock, coming off at 2", and opening up to 2.5". The mufflers are stock replacements. I appreciate all your time, helping to sort this out. Thanks, Mark

Good luck. The 1971 LT1 has closed chamber heads, which means that your chambers are about 64 cc's. If your nominal deck height is still 9.025" and you used a shim steel head gasket of about .021" compressed thickness, then you must be using flat top pistons. The compression you quote, 9.15:1 is static compression ratio, correct? Unfortunately, that's another very big deficit, as the 30-30 cam, set at 4 degrees advance (110/114/118), as factory, can easily tolerate static compression ratios of 11.25:1 using a very aggressive timing advance scheme. With the cam set at zero advance, like yours is (114/114/114), the 346 cam will easily tolerate static compression ratios of 11.50:1 using the same aggressive timing scheme. Of course, these SCR's are assuming you are running good Wisconsin high-test of 93 pump octane, with no additives required, at all. You should design a street driven SBC with minimum 8.0:1 DCR (dynamic compression ratio) and maximum 8.5:1 DCR unless you intend to use race gas.

If the SCR is indeed 9.15:1, then this reality, coupled with the retarded cam timing and likely restricted exhaust makes for a very unsatisfying experience in the rev range below 5000 RPM with your 30-30 cam. I wish that you had asked some questions before embarking on your project. I can only offer you a "band-aid" fix for your mismatched compression ratio, and that is to remove the timing cover and re time the cam so that it is running 6 degrees advanced, which is 2 degrees more than GM stock spec, and 6 degrees more than you are presently running. In addition, dial in as much spark advance as possible. I can confidently assure you that your engine will tolerate 20 degrees initial advance, with 16 degrees centrifugal, all in by 2200 RPM. You do not want anything more than 36-38 degrees total advance initial plus centrifugal) @ WOT. Of course, you will maintain your 17 degrees vacuum advance, which you will remove from ported vacuum and transfer to full manifold vacuum. Finally, you will widen your valve lash to .027/.027 (despite Comp's recommendation of .024/.024), which will have the effect of making the cam have less duration, and thus will bleed off less cylinder pressure at low RPM's.

Again, understand that these measures are mere "band-aids", designed to compensate (somewhat) for your engine's insufficient static compression ratio. The proper fix would be to rebuild your short block with the original type domed pistons, while at the same time setting the cam at 4 degrees advanced, as original. At that point, you would still run a very aggressive advance scheme, with pump premium fuel, with tappets adjusted at .024/.024 per Comp's spec.

Even with these corrective measures, you are still leaving a lot of horsepower untapped unless and until you install a very low restriction exhaust system, preferably fed by tube headers. The 30-30 cam loves your intake manifold, as well as your carburetor, although a 750 dual feed, mechanical secondary unit would make an explosive difference in power, given the fact that you are running 4.11 gears. If you port your heads well, you stand to gain another 40-50 net flywheel horsepower.

At this point I would not be concerned with vacuum advance at all......................disconnect and plug it. Buy yourself an adjustable vacuum canister from Accel. You can set it to operate at the vacuum level your engine develops at idle. Ideally, your vacuum advance should be fully deployed at 2 in-hg lower than your idle vacuum. In your case, with the 30-30 (depending on how you lash your valves) that means 6 or 7 in-hg shall provide full vacuum advance. An added benefit of an adjustable canister, is that you are able to set the deployment limit as well as the deployment rate. In its present state of tune, you can rest assured that the engine will not detonate at part throttle cruise with maximum vacuum advance, which is generally about 16-17 degrees for a SBC. Total advance will then be 20 initial + 16 vacuum + 16 centrifugal = 52 degrees cruise total.

[Mark A.]

Re: Can't Time Engine Correctly

John,
My vacuum advance is connected to the CEC solenoid, and is using the stock distributor, and vacuum advance canister. Thanks, Mark

[Mark A.]

Re: Can't Time Engine Correctly

Joe,
The heads on my LT-1 are #3973487. They are 75cc, according to the book.
My intention when rebuilding this engine, was to keep it as close to original as possible. The corvette shop manual only lists the lobe lift of the 71 LT-1 cam. It states ex .323, and in .305. The Comp Cams unit is ex .323, and in .323. As no duration figures were given, I called Comp and asked there techs if this was the replacement cam for the 71 LT-1, which they said it was. I didn't see the extra lift on intake as a problem, but the timing of the cam is obviously different. You state the exhaust as restrictive, yet it's larger than the factory sidepipes on my 69 390 hp. Anyway, when I get it up to 6000 rpm, it should still scare the neighbors! Mark

[Joe C.]

Re: Can't Time Engine Correctly

Joe,
The heads on my LT-1 are #3973487. They are 75cc, according to the book.
My intention when rebuilding this engine, was to keep it as close to original as possible. The corvette shop manual only lists the lobe lift of the 71 LT-1 cam. It states ex .323, and in .305. The Comp Cams unit is ex .323, and in .323. As no duration figures were given, I called Comp and asked there techs if this was the replacement cam for the 71 LT-1, which they said it was. I didn't see the extra lift on intake as a problem, but the timing of the cam is obviously different. You state the exhaust as restrictive, yet it's larger than the factory sidepipes on my 69 390 hp. Anyway, when I get it up to 6000 rpm, it should still scare the neighbors! Mark

My error............1971 was the first year that the open chambered heads were used.
The LT1 cam is marginally more tolerant of lowered compression, such as yours, than is the 30-30.

This is the cam card for your cam. As you can see, it is the 346 cam ground with zero advance, since the ICL is coincident with the LCL:

http://www.compcams.com/Company/CC/c...px?csid=3&sb=2

"LT1 replacement" is often used interchangeably with "30-30 or 3849346 replacement", but they are quite different.