additives are snake oil.
Low HP L71
Thread Starter
Intermediate
Joined: Jul 2016
Posts: 31
Likes: 0
Low HP L71
All
All suggestions gratefully received - have the car booked into a rolling road but not for another month.
I recently bought a 69 L71 that was rebuilt maybe 5K ago, its beautiful. Its very stock apart from an L88 cam, ign at 12 BTDC static, transistor ignition. It has a 4.11 too.
But driving it....I was expecting tyre lighting at pretty much any gear....its certainly not 435HP+. It drives well, no obvious issues bar lacklustre performance.
I am now pretty sure the secondary carbs are opening after a couple of O rings and some adjusts, but its still flat. Even when revving from idle statically the pick up is pretty lazy, its far from hot / sharp / peppy.
Added to that the viscous fan (or the air pump not sure which) makes so much noise that the engine note / induction roar is barely audible, and it has a very muffled tailpipe exhaust system, both effects further reducing the impression of performance.
It idles lumpily but consistantly at 750 easily, maybe lower too.
Any ideas - what so you think about it having L88 ignition timing rather than L71? Should timing go with the cam?
Thanks!!
All suggestions gratefully received - have the car booked into a rolling road but not for another month.
I recently bought a 69 L71 that was rebuilt maybe 5K ago, its beautiful. Its very stock apart from an L88 cam, ign at 12 BTDC static, transistor ignition. It has a 4.11 too.
But driving it....I was expecting tyre lighting at pretty much any gear....its certainly not 435HP+. It drives well, no obvious issues bar lacklustre performance.
I am now pretty sure the secondary carbs are opening after a couple of O rings and some adjusts, but its still flat. Even when revving from idle statically the pick up is pretty lazy, its far from hot / sharp / peppy.
Added to that the viscous fan (or the air pump not sure which) makes so much noise that the engine note / induction roar is barely audible, and it has a very muffled tailpipe exhaust system, both effects further reducing the impression of performance.
It idles lumpily but consistantly at 750 easily, maybe lower too.
Any ideas - what so you think about it having L88 ignition timing rather than L71? Should timing go with the cam?
Thanks!!
Safety Car
Joined: Jan 2009
Posts: 3,736
Likes: 56
From: anchorage ak
I'm not really a big block guy...yet..I have one in the garage an plan to build it one day. I would think that a decent set of aluminum heads and a camshaft to match would get you the low end you desire, but for the life of me the 4.11's that you have combined with the bbc torque should boil the tires at will, can you post both the camshaft specs and the engine specs from the rebuild, ie compression ratio and what head. Alot od times when camshaft size is increased and the compression ratio doesn't match you end up with an engine that makes alot of power but above 3k to 3.5k rpm. Do you still have three duece (I think that came on the L71) also fuel additive would only make a difference if you had to retard the timing due to the compression and the additive would let you bump the timing back up, I'm guessing your compression is low for the size of the cam.
Last edited by bluedawg; Jul 21, 2016 at 06:14 PM. Reason: It all makes sense now and means absolutely nothing.
Burning Brakes
Joined: Jul 2016
Posts: 767
Likes: 74
From: Blaine MN
Hi, I'm also a recent tri-power owner. I don't know the background of what you've done to the carbs so far, but timing it mostly determined by cam. I'd look at the accelerator pump squirters, both of mine were partially clogged (68 L71). On a cool day I can spin 295/50's from a 10mph roll, I'm still hoping for more as I tune.
Burning Brakes
Joined: Jul 2016
Posts: 767
Likes: 74
From: Blaine MN
Race Director
Joined: Sep 2005
Posts: 18,640
Likes: 121
From: Farragut,TN
Corvette Stories
The Best of Corvette for Corvette Enthusiasts
Top 10 Most Expensive Corvettes Ever Sold on Bring A Trailer
Brett Foote
10 Things Every Corvette Owner Needs (2026 Edition)
Michael S. Palmer
8 Most "Only Corvette Owners Understand" Quirks and Problems
Pouria Savadkouei
10 Reasons the C6 Z06 is Still A Performance Benchmark After 20 Years
Joe Kucinski
How Much Horsepower Every Corvette Engine "LOST" in 1972
Joe Kucinski
Top 10 DOs and DON'Ts for Protecting Your Convertible Top!
Michael S. Palmer
Top 10 Most Explosive Corvettes Ever Made: Power-to-Weight Ratio Ranked!
Joe Kucinski
150 hp to 1,250 hp: Every Corvette Generation Compared by the Specs That Matter
Joe Kucinski
8 Coolest Corvette Pace Cars (and Replicas) of All Time
Verdad Gallardo
Burning Brakes
Joined: Jul 2016
Posts: 767
Likes: 74
From: Blaine MN
Le Mans Master
Joined: Mar 2001
Posts: 5,738
Likes: 644
From: Lincoln, CA
Couple of thoughts, Base timing is low. Most BBC's seem to like about 18 without the vac advance hooked up.
2nd does the dizzy advance the timing on it's own without vacuum? What happens if you rev it with a timing light. Does it move?
2nd does the dizzy advance the timing on it's own without vacuum? What happens if you rev it with a timing light. Does it move?
Burning Brakes
Joined: Jul 2016
Posts: 767
Likes: 74
From: Blaine MN
I hope this isn't hijacking, let me know if it is. Are you saying 18B at idle? I'm also a new BB owner and I set it to Factory Spec, am I better off bumping it up?
Le Mans Master
Joined: Mar 2001
Posts: 5,738
Likes: 644
From: Lincoln, CA
Mine had a cam as well and the difference was very noticeable.
Not sure on a factory cam but it doesn't hurt to try. I've read about some builds using 20B.
I would ask Lars for his tuning papers. It's worth the read.
Not sure on a factory cam but it doesn't hurt to try. I've read about some builds using 20B.
I would ask Lars for his tuning papers. It's worth the read.
Safety Car
Joined: Aug 2000
Posts: 3,711
Likes: 43
From: San Antonio TX
Maybe some clarity is in order. I write this only because people have differing interpretations of the data. Let me give you a scenario and you tell me how close it is to being true: You're rolling along at a steady low speed in first gear, tachometer is at 3k rpm, you floor the throttle without any delay or progression...meaning right to the floor in a split second. What exactly does the car do? I can tell you what should happen, but it may be better reading the facts from you.
Also, how able is the dyno operator at doing some tuning while your car is there? Can they check air/fuel ratio, run a timing map, adjust valve lash and such? I wouldn't do anything to the car until your car has its first dyno run. You'll just be shooting in the dark. You want to see where the car is performing throughout the rpm band to better understand where or if it's falling down. Hopefully the operator knows something about cars.
Also, how able is the dyno operator at doing some tuning while your car is there? Can they check air/fuel ratio, run a timing map, adjust valve lash and such? I wouldn't do anything to the car until your car has its first dyno run. You'll just be shooting in the dark. You want to see where the car is performing throughout the rpm band to better understand where or if it's falling down. Hopefully the operator knows something about cars.
Le Mans Master
Joined: Sep 2013
Posts: 6,192
Likes: 750
From: near the thumb in the mitten
What is total advance? I'm thinking you need about 32 degrees at 3000. Fuel pressure and volume it takes a lot of fuel for WOT. The tri-power with the vacuum actuated outside carbs can be difficult to get dialed in. I would look a shop or person who is good with the 440+6 mopars for some help. The six pack is more popular with mopars than vettes. Same stuff just a different application. Last exhaust needs to be somewhat free flowing you don't want it to choke engine.
Just read your profile GB? is that the UK?
Just read your profile GB? is that the UK?
Last edited by Kevova; Jul 21, 2016 at 07:33 PM.
Burning Brakes
Joined: Mar 2014
Posts: 904
Likes: 46
From: Crestline CA
For what its worth, the factory exhaust system is pretty restrictive. When I bought my 68 L-36 car it had a few "hi-po" parts on it. Aftermarket carb, MSD distributor and box, long tube headers and free flowing exhaust.
While getting it ready for flight judging I put it all back to factory. Changing to the stock distributor made no change in power, the stock carb actually picked a bit of power up. When I replaced the headers for exhaust manifolds and replaced the exhaust with the closest thing to factory I could find (Gardner exhaust) it lost ALOT of power. It seriously felt like it lost 100 hp(I'm sure it didnt but it sure felt like it), it used to spin the tires when I got into the secondaries in 1st and sometimes 2nd (M21 and 3.70 rear gears)but not wont even do it in first.
I don't know how your car is set-up but 435 hp in 1969 isnt the same as a modern car rated at 435 hp.
While getting it ready for flight judging I put it all back to factory. Changing to the stock distributor made no change in power, the stock carb actually picked a bit of power up. When I replaced the headers for exhaust manifolds and replaced the exhaust with the closest thing to factory I could find (Gardner exhaust) it lost ALOT of power. It seriously felt like it lost 100 hp(I'm sure it didnt but it sure felt like it), it used to spin the tires when I got into the secondaries in 1st and sometimes 2nd (M21 and 3.70 rear gears)but not wont even do it in first.
I don't know how your car is set-up but 435 hp in 1969 isnt the same as a modern car rated at 435 hp.
Race Director
Joined: Dec 1999
Posts: 11,096
Likes: 990
From: Sierra Foothills CA
OP- as already stated, optimizing ignition curve and carbs could make a big difference but that cam could still be a bit soft at lower RPMs.
For what its worth, the factory exhaust system is pretty restrictive. When I bought my 68 L-36 car it had a few "hi-po" parts on it. Aftermarket carb, MSD distributor and box, long tube headers and free flowing exhaust.
While getting it ready for flight judging I put it all back to factory. Changing to the stock distributor made no change in power, the stock carb actually picked a bit of power up. When I replaced the headers for exhaust manifolds and replaced the exhaust with the closest thing to factory I could find (Gardner exhaust) it lost ALOT of power. It seriously felt like it lost 100 hp(I'm sure it didnt but it sure felt like it), it used to spin the tires when I got into the secondaries in 1st and sometimes 2nd (M21 and 3.70 rear gears)but not wont even do it in first.
I don't know how your car is set-up but 435 hp in 1969 isnt the same as a modern car rated at 435 hp.
While getting it ready for flight judging I put it all back to factory. Changing to the stock distributor made no change in power, the stock carb actually picked a bit of power up. When I replaced the headers for exhaust manifolds and replaced the exhaust with the closest thing to factory I could find (Gardner exhaust) it lost ALOT of power. It seriously felt like it lost 100 hp(I'm sure it didnt but it sure felt like it), it used to spin the tires when I got into the secondaries in 1st and sometimes 2nd (M21 and 3.70 rear gears)but not wont even do it in first.
I don't know how your car is set-up but 435 hp in 1969 isnt the same as a modern car rated at 435 hp.
Stock type exhaust on a cammed big block is a major restriction.
Safety Car
Joined: Jan 2009
Posts: 3,736
Likes: 56
From: anchorage ak
The L88 cam is a solid flat tappet that does indeed have a lot of duration. It isn't very civilized down low but comes on hard up higher.
OP- as already stated, optimizing ignition curve and carbs could make a big difference but that cam could still be a bit soft at lower RPMs.
Stock type exhaust on a cammed big block is a major restriction.
OP- as already stated, optimizing ignition curve and carbs could make a big difference but that cam could still be a bit soft at lower RPMs.
Stock type exhaust on a cammed big block is a major restriction.
Team Owner
Joined: Oct 2004
Posts: 81,241
Likes: 3,063
From: altered state
St. Jude Donor '05
Time it and tune the carb where the motors happiest
Low compression doesnt help may need more initial possibly more fuel
need to find someone who really knows those systems
Low compression doesnt help may need more initial possibly more fuel
need to find someone who really knows those systems
Race Director
Joined: Jan 2000
Posts: 13,018
Likes: 2,262
From: Corsicana, Tx
2020 C2 of the Year - Modified Winner
2020 Corvette of the Year (performance mods)
C2 of Year Winner (performance mods) 2019
2017 C2 of Year Finalist
There is something WAY off.
First...an L-88 cam is not going to idle at 750 RPM or lower...at least not happily. The L-88 cams REALLY like headers and OPEN exhaust. Mufflers of any sort take a lot away from it. A 427 with an L-88 cam and rectangular port heads isn't going to be a low end rocket....but with 4.11's it should still be pretty darn strong...especially from 3500-4000 on up. That cam is going to want to make power at 6500+ RPM if things are right.
12* initial is way too low. Need to ck total timing without vacuum. With decent gas in it I would expect it to be best with no less than 36* total....and most likely 38*-42* with the old heads. The more initial you can put in it and limit the total the better. Idling with 20*+ will help response a lot.
Any idea if they took compression out of it when they rebuilt it? Sure sounds like it.
Time to do a compression test. Maybe the cam is installed a tooth off?
I'd get timing handled first....then verify carbs are opening and go from there. The L-88 cam isn't a complete deal breaker...but it's not a super nice street cam.
JIM
First...an L-88 cam is not going to idle at 750 RPM or lower...at least not happily. The L-88 cams REALLY like headers and OPEN exhaust. Mufflers of any sort take a lot away from it. A 427 with an L-88 cam and rectangular port heads isn't going to be a low end rocket....but with 4.11's it should still be pretty darn strong...especially from 3500-4000 on up. That cam is going to want to make power at 6500+ RPM if things are right.
12* initial is way too low. Need to ck total timing without vacuum. With decent gas in it I would expect it to be best with no less than 36* total....and most likely 38*-42* with the old heads. The more initial you can put in it and limit the total the better. Idling with 20*+ will help response a lot.
Any idea if they took compression out of it when they rebuilt it? Sure sounds like it.
Time to do a compression test. Maybe the cam is installed a tooth off?
I'd get timing handled first....then verify carbs are opening and go from there. The L-88 cam isn't a complete deal breaker...but it's not a super nice street cam.
JIM
Team Owner
Joined: Oct 2004
Posts: 81,241
Likes: 3,063
From: altered state
St. Jude Donor '05
First...an L-88 cam is not going to idle at 750 RPM or lower.
Id think a real L88 cam would be happier around 1000+
Oh well anythings better than the stocker (wonder if thats what hes really got)
Last edited by cv67; Jul 22, 2016 at 10:22 AM.