Hi,
I currently own two '71 LT-1's and performance between the two is night and day. I've driven several C3's over the years and the best i can do to describe the difference in performance between the two cars is that one has the power similar to that of a big block while the other feels closer to the base engine small block or L46. I believe the more powerful of my two cars is how the LT-1 should respond, and i'd like to see what i can do to improve performance on the other one.

The quicker car has 79k miles (smog equipment is long gone), and i was told it had a rebuild at about 60k and that the rebuild was to factory spec, however i'm just going by what i was told. The other car has 103k original miles with smog still intact. It may simply be that the latter is just showing her age, but that being said I haven't really done anything to the car since i got it a year ago and was wondering if there's something i can (short of a engine rebuild) to improve performance.

I did have the car looked at by a Corvette restoration shop when I first got it to give the whole thing a quick look-over. I did ask him about the engine said he wouldn't do a rebuild...he said no need, she sounds and runs great, but i didn't get into much more detail than that with him regarding performance and where it is vs. where it should be.

I'm looking on advice and where to start. Keep in mind i'm a novice when it comes to working on these things so assume i know nothing if you respond to this thread. I've done a lot of cosmetic stuff, interior work etc, but it's time i start learning how to tinker more under the hood. First thing i need to read up on is adjusting the lifters. Not sure if that will affect performance at all but i hear it's something that should be done on these cars. Aside from the smog and mileage difference both cars are basically the same...pretty original cars

Thanks,
Paul

 

maybe someone has changed out the cam, lifters adjusted improperly, carb out of adjustment, timing not correct or dist vac/mech advance not working are few things to check out...good luck.

 

what type of adjustment should i be making to the carb, and how can i tell if it's necessary?

Thanks,
Paul

 

If I were you, and wanted the aged car to perform as good as possible, short of a rebuild. I would compare compression readings, or leak down tests, manifold vacume readings, distributor curves on them both. This may reveil some compression loss, or tuning variation. One car has 19,000 the other 103,000. You are in an ideal situation, having 2 exact LT-1s, you can compare everything about them. If I may say I raced go carts for 10 years, stock 5 hp briggs. All my motors were built with torque in mind, usually with the same machine work and valve timing. ( per WKA rules) That being said there were always good ones and bad ones, but once in a while there came a Beast. You may have that Beast, and the other one may never run as good. Just compare things.

 

Check the gearing, maybe the one that you feel under-performs is geared higher. Gearing can have a huge impact on seat of the pants feel.

 

i go with the Carb and distributor, I'm betting on the secondarys not opening properly or the advance on the distributor not working properly. check out Nelson specialties for ignition work they are in Charlotte and do ignition systems for NASCAR. teams. also it may sound strange but make sure your throttle is opening fully, i once fixed a problem like this by removing the floor mat, believe it or not! good luck

 

I agree, first insure the pedal is opening the carb so the secondaries are engaged. The pedal assembly bends over time not allowing full throttle and if you have mats the issue can be amplified. Have a friend watch the carb as you depress the pedal.

Second, Leave the carb alone at this time and go to your timing. The carb should always be the last item you touch.

There is a good thread for timing for all in on the forum. I changed out my springs and reset the timing and it made a huge difference. Lars has a great paper taking you through the steps.

If no gain in performance after this step I would check the compression.

If the compression is fine more than likely there has been a modification to your engine in the past.

I always look at the easy fixes first.

 

The first Corvette I ever owned was a 72 LT-1 with C-60 air conditioning. I'll never forget the owner's remark that it was probably the last Vette he would ever own and turn a profit on during ownership (he was replacing it with a shiny new 86 C4 Vette.) The Vette ran like "dogsh**" at high RPM when I bought it. I proceeded to adjust the valves, trash the poor ignition components: wires, plugs, cap, rotor, and points. I rebuilt the distributor and freed up the sticky advanced weights, lubed the tach drive gears. I then went through the Holley carb with a "trick kit" and went to the drags to tune it up. It ran 14.30's while throwing fan belts. I replaced the alternator pulley with a normal LT-1 deep groove and attended Vette Magic #16. The LT-1 ran a best of 14.06@99.xx MPH (I remeber that MPH figure because I wanted 100MPH very badly and never made it.)

The former owner came up to me during the event and remarked, I never knew that Vette was that fast (his 86 was running 14.40's.) My remark back to him after skinning knuckles and bleeding for 2 weeks solid while dialing the Vette in: IT WASN'T! the thing ran like a complete POS when I bought it because of neglect (yes it was shiny, and the oil was clean but, mechanically it was pure DOGSH**!)

One really bad potential of poor LT-1 performance would be an original nylon timing gear loosing a tooth on the cam gear and retarding the cam timing causing a great loss of bottom end power. Over 100K miles and 40 years of time can cause things to fail.

You also don't know how "original" the rebuild was on the lower mileage Vette and wheather everything is stock, I would never install a factory LT-1 grind in even a stock rebuild. Maybe the other Vette has seen some "improvements" you don't know about.

My life expectancy for a good running LT-1 would run around 60-100K miles depending on the "tune-up" and use of the Vette. Mine tend to live shorter lives as they see 7K RPM's on almost every drive

My 70 LT-1 was running off the last time I drove it, 2 hours of valve adjustments and a little carb adjusting, it's back running like a Swiss watch again. If you don't like working on cars, you should own another type of Vette than one with solid lifters, this goes for big blocks too, that's why they made hydraulic lifters.

 

 

The better performer has a 3.55 rear end while the "slower" one has a 3.70.
The accelerator cable on the slower car was also toast and i recently replaced that.....it frayed to the point that last time i drove it before putting it away for the winter the throttle would not pulling back and car was stuck at high idle. Not good! With all the snow i haven't had the car out since. Something simple like that may also have an impact. Next up will be a timing adjustment and will read up on holly adjustments should i have to go there next.

Don't mind...just have to learn...and I'm thinking nows the time

 

to start after timing turn air screws on side (one on each side) in all the way and lightly seat, DO NOT screw in tight, and then back out 1 and 1/2 turns...this is the basic starting point setting, also check the dist weights as mentioned and be sure they move freely. good luck. 3.70 gear car should run slightly better on acceleration.

 

Thank you.

I was also going to pick this up..
http://www.ecklers.com/corvette-serv...nual-1971.html. Let me know if there's another good book to have around.

 

I tend to agree with Blue L48, you have two LT-1s of the same year, not a '70 and a '71. Personally I would try and get some comparitive baselines.

First what is the general health of the motors? Without resorting to removing major components, I would first do a compression test on both motors.

Now one motor may have a higher compression reading on average compared to the other, for example 160 lbs compared to 180 lbs, but this can be due to several factors: worn rings, leaking valves, different cam, worn cam lobes, or just plain assembly tolerences. Don't be surprised if the rebuilt motor may have lower cranking compression - that motor was apart - cam change, pistons?

I would then squirt oil into a few cylinders to see if compression increases in either motor to determine if there is any cylinder leakage at the rings. I would assume the higher mileage motor may see an increase. If you have one or two cylinders that are more than 15/20 percent lower, and the oil squirt test does not increase compression, I would try a leakdown test to determine of you have any leaky intake or exhaust valves.

Next I would hook up a vacuum gauge. This easy test is underrated and often overlooked, but is very easy to do and will give you an indication on the overall internal functioning/health of the motor. You want to see a steady needle. Differerent needle readings between the two motors could be many things, one of them being different cam timing, from the cam profile itself.

If these tests all look good. Then I would start to really focus on the ignition, have it recurved if necessary, and then how the carb is functioning. You can really wake up a small block by doing these two things alone.

I think you really need to find out first if you are comparing two healthy LT-1s, or a tired LT-1 and a freshened LT-1 that may have had some minor mods.

I'm no expert, but this is what I would do. Good luck in your detective work.

 

I agree with all the above about tuning, after performing a compression test, look at the cyl head casting marks, the heads may be differnet, the PO may have changed them, or they were swapped in the past with better or worse heads by accident or mistake, heck, they may not even match. "Camel back" heads had a casting mark in front that looked like a two humped camel and had large 2.02" intake valves. My old 69 C10 had these and she hauled A**, especialy after I added an OE HEI distributor. Air flow, compression,fuel mix and timing are going to make ALL the diference in the world. Make sure your base tuning (timing and ignition parts) are ok, cam timing (chain not worn), then keep going...

 

I have one of those for my '69 and an not impressed at all. It misses alot things (espically wiring) because it is trying to cover alot of different cars.
I wish I still had my Helms manual from back in the '70s but it went with my first '69.
I apologize that I cannot suggest a better book, but just know that one isn't what I expected.

 

Verify that the throttle is opening fully with the accel pedal. Also, verify that the choke lockout lever (prevents secondaries from activating when the engine is cold) is adjusted properly and not limiting secondary action. That should be your first thing to do. {Note: Holley carbs may not have that device...but might have something similar.}

Next is to recurve the distributor for performance, rather than timing it as the owner's manual recommends. Full mechanical timing of 32-34 degrees advanced should be in completely by 3000 rpm (2600-2800 rpm would be better, as long as you get no pre-ignition knock).

If that doesn't "wake it up", check compression [to verify that some low compression heads haven't been installed sometime in the past], and do a leak-down test to verify ring and valve sealing integrity.

 

That is the order of things I would check. I have to reach down and bend my gas pedal upwards once or twice a year and this makes a huge difference. There may be minor adjustments to the intake/carb thottle bracket as well. Last year after replacing my heads and cam, I was extremely disappointed in the resulting performance until I remembered to check the throttle linkage. Sure enough, the secondaries were not opening. My bracket "hinge"s on the carb to intake bolt and it was skewed enough to prevent the secondaries from opening. Now I have huge smile on my face every time I drive.

 

Ok....thanks everyone for the advice. i've been busy with work and haven't had a chance to do any of the things suggested until this weekend. So far here's what i've done...

1)checked the accelerator cable and it was frayed to the point where i'm surprised stepping on the pedal did anything. So i put a new cable in and that problem is solved.

2)adjusted the fuel/air mixture on the holly. The car was running way too rich...in fact, the back of the car was starting to get black residue on the bumpers and body, a definite sign, and shows up real bad on a white car! I unscrewed the view port holes on the carb and backed the fuel level down to right below the peep holes just like the shop manual says.

3)bought a timing light, made some adjustments and also compared the timing to my other LT-1. The faster car's initial time is at about 10 degrees @ 800 RPM and total timing is 32.5 degrees at 2500 RPM. The slower car i set the total timing at 32 degrees at 2500 RPM and initial timing fell in at about 12 degrees at 850 RPM. Both cars seemed to be "all in" somewhere around 2500-3000 RPM. Sounds good right? I'm thinking i don't need to fiddle with the distributor any more?

Next up I'm going to do a compression test. I bought the kit to do it and read through the instructions. It says to warm the car up, disconnect a few things (don't remember off the top of my head) and then pull all 8 wires and remove the spark plugs. After burning the s** out my hands on the exhaust manifolds i figure there's got to be a better way to do this. I don't have a lift, but do you think it would be better to do this from underneath? Trying to do it from the top seems like it's going to be difficult. Do i need to remove all the plugs? Can i just test one cylinder to get an idea if this is my issue.....kinda thinking i should be testing all 8.

Now that the ground is dry here I've been taking both cars out quite a bit. The red LT-1 sure does pull a lot harder at lower RPMs (similar to the 454 i used to have)....when it hits 4000 RPM and my head starts to rattle . The white car on the other hand doesn't pull very hard at all at lower RPMs and just starts to get exciting around 4000-4500 rpms

Thanks,
Paul

 

[QUOTE=pwsusi;1577110520]Ok....thanks everyone for the advice. i've been busy with work and haven't had a chance to do any of the things suggested until this weekend. So far here's what i've done...

1)checked the accelerator cable and it was frayed to the point where i'm surprised stepping on the pedal did anything. So i put a new cable in and that problem is solved.

2)adjusted the fuel/air mixture on the holly. The car was running way too rich...in fact, the back of the car was starting to get black residue on the bumpers and body, a definite sign, and shows up real bad on a white car! I unscrewed the view port holes on the carb and backed the fuel level down to right below the peep holes just like the shop manual says.

3)bought a timing light, made some adjustments and also compared the timing to my other LT-1. The faster car's initial time is at about 10 degrees @ 800 RPM and total timing is 32.5 degrees at 2500 RPM. The slower car i set the total timing at 32 degrees at 2500 RPM and initial timing fell in at about 12 degrees at 850 RPM. Both cars seemed to be "all in" somewhere around 2500-3000 RPM. Sounds good right? I'm thinking i don't need to fiddle with the distributor any more?

Next up I'm going to do a compression test. I bought the kit to do it and read through the instructions. It says to warm the car up, disconnect a few things (don't remember off the top of my head) and then pull all 8 wires and remove the spark plugs. After burning the s** out my hands on the exhaust manifolds i figure there's got to be a better way to do this. I don't have a lift, but do you think it would be better to do this from underneath? Trying to do it from the top seems like it's going to be difficult. Do i need to remove all the plugs? Can i just test one cylinder to get an idea if this is my issue.....kinda thinking i should be testing all 8.

Now that the ground is dry here I've been taking both cars out quite a bit. The red LT-1 sure does pull a lot harder at lower RPMs (similar to the 454 i used to have)....when it hits 4000 RPM and my head starts to rattle . The white car on the other hand doesn't pull very hard at all at lower RPMs and just starts to get exciting around 4000-4500 rpms

Thanks,
Paul[/QUOTE

Yes you must remove the spark plug on each cyl you are testing. You will need a reading from each cyl so you can compare the compression of each.
As far as the exhaust manifolds being hot... just be careful...you can wait a few minutes after you shut off the enigne.

 

Weak valve springs will cause a big reduction in power, if all your efforts don't net what you are looking for might be another place to look. 100K on stock springs with high lift cam .
Looks like you are getting a lot of good advice.