'69 L71 435 Tri-Power Question
Thread Starter
Pro
Joined: Jun 2004
Posts: 561
Likes: 1
From: No longer in Heaven
'69 L71 435 Tri-Power Question
My car drives great. Had the engine rebuilt last winter and it dyno'd at 496 hp and 512 torque.
My question is at what point do the two outside carbs actually begin functioning? Obviously when I pop the throttle they kick in, but for instance, when I'm cruising at 80 mph/3500 rpm,
is the middle 2 bbl only feeding the engine until I step on it?
If I was cruising at 100 mph/4500 rpm,
would they be feeding the beast?
My question is at what point do the two outside carbs actually begin functioning? Obviously when I pop the throttle they kick in, but for instance, when I'm cruising at 80 mph/3500 rpm,
is the middle 2 bbl only feeding the engine until I step on it? If I was cruising at 100 mph/4500 rpm,
would they be feeding the beast?
Team Owner
Joined: Jan 2006
Posts: 37,637
Likes: 3,118
From: Crossville TN
I'm sure some tri-power experts will provide better detail, but the outer carbs are vacuum operated. They will begin to open when the 'signal' vacuum level allows it. There is a specific set-up procedure for adjusting that tri-power setup...I believe that it is in the Chevrolet Chassis Service Manual for your year car. At normal cruise speeds (including legal highway speeds), I doubt that the outer carbs are cracked at all. At 100mph they are likely beginning to open. I much preferred the solid, progressive linkage on a tri-power setup. If adjusted so that the outer carbs wouldn't begin opening until the throttle was at 60% or more, the center 2-bbl would be all that was ever used, unless you were running at elevated speeds or at WOT (when all carbs were fully opened by the throttle linkage).
Burning Brakes
Joined: Mar 2006
Posts: 915
Likes: 2
From: Lima, OH -- 69 L71 427/435hp
Here's worth printing and reading:
http://www.corvettefaq.com/c3/tripower/
Outboards open dependant on speed and load conditions; via loss of manifold vaccuum which in turn opens outboards.
Mine seems to usually opens up past half throttle.
From experience, if you have a vaccuum leak around the base of the carbs it will not allow them to open properly, if at all, but it would also idle a bit rough.
If you want to check when they are opening and by how much, put a paperclip or similar on the actuator rod on the back of the diaphrams to see how much (if at all) they moved under different conditions.
http://www.corvettefaq.com/c3/tripower/
Outboards open dependant on speed and load conditions; via loss of manifold vaccuum which in turn opens outboards.
Mine seems to usually opens up past half throttle.
From experience, if you have a vaccuum leak around the base of the carbs it will not allow them to open properly, if at all, but it would also idle a bit rough.
If you want to check when they are opening and by how much, put a paperclip or similar on the actuator rod on the back of the diaphrams to see how much (if at all) they moved under different conditions.
Thread Starter
Pro
Joined: Jun 2004
Posts: 561
Likes: 1
From: No longer in Heaven
Here's worth printing and reading:
http://www.corvettefaq.com/c3/tripower/
Outboards open dependant on speed and load conditions; via loss of manifold vaccuum which in turn opens outboards.
Mine seems to usually opens up past half throttle.
From experience, if you have a vaccuum leak around the base of the carbs it will not allow them to open properly, if at all, but it would also idle a bit rough.
If you want to check when they are opening and by how much, put a paperclip or similar on the actuator rod on the back of the diaphrams to see how much (if at all) they moved under different conditions.
http://www.corvettefaq.com/c3/tripower/
Outboards open dependant on speed and load conditions; via loss of manifold vaccuum which in turn opens outboards.
Mine seems to usually opens up past half throttle.
From experience, if you have a vaccuum leak around the base of the carbs it will not allow them to open properly, if at all, but it would also idle a bit rough.
If you want to check when they are opening and by how much, put a paperclip or similar on the actuator rod on the back of the diaphrams to see how much (if at all) they moved under different conditions.
This is a good piece of info that all 3X2 guys should have in their files.
Race Director
Joined: Oct 2000
Posts: 15,065
Likes: 79
From: central FL, near the beach.
imo they should not open at 100 mph steady. if they did, the gas tank would empty pretty quick as they have only power jetting, not cruise. even 120 should be closed, but at some point they will crack open. With an A/F gauge u could see this on the gauge. it would go rich.
Race Director
Joined: Nov 2002
Posts: 12,142
Likes: 54
From: Taylor Michigan
Race Director
Joined: May 2006
Posts: 16,528
Likes: 53
From: Dayton, Ohio
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Racer
Joined: Apr 2007
Posts: 364
Likes: 1
From: Dane Wisconsin
Burning Brakes
Joined: Mar 2006
Posts: 915
Likes: 2
From: Lima, OH -- 69 L71 427/435hp
Most enthusiasts know the result of over-carburetion--poor engine response, slower acceleration and poorer elapsed times. Holley and Chevrolet engineers point out that trying to supplant the vacuum system with mechanical linkage will probably lead to the problem of over-carburetion.
...so you have an inside scoop above the engineers of Holley and Chevrolet? I'd like to see numbers and proof that it really does to be honest, because if it does I may try it myself.
I for one just don't see how mechanical linkage would do better...please explain more if you could.
Thanks
Racer
Joined: Apr 2007
Posts: 364
Likes: 1
From: Dane Wisconsin
not saying it would do better as a matter of fact it does have a SMALL bog right off the line but it still blows the tires loose so if you roll into it it does not bog and stays hooked.... not saying it is better just that i did not agree with this statement
"manual linkage will not work properly with the Vette tri-powers. Performance would be horrible"
is it perfect no not even saying that could it be better sure it guess so if it did not have any bog but to say it will not work and performance would be horrible is not true... im doing it now.
"manual linkage will not work properly with the Vette tri-powers. Performance would be horrible"
is it perfect no not even saying that could it be better sure it guess so if it did not have any bog but to say it will not work and performance would be horrible is not true... im doing it now.
Racer
Joined: Apr 2007
Posts: 364
Likes: 1
From: Dane Wisconsin
yeah it is the center opens about 1/2 way then the outer ones open at 2:1 so they all hit wot at the same time i have thought about changing out the front for a different 500cfm unit to get an extra acc pump but thougth it runs well enough now
Instructor
Joined: Nov 2011
Posts: 159
Likes: 1
imo they should not open at 100 mph steady. if they did, the gas tank would empty pretty quick as they have only power jetting, not cruise. even 120 should be closed, but at some point they will crack open. With an A/F gauge u could see this on the gauge. it would go rich.
I dunno...I would assume at 120 mph you are under a pretty severe load with asymmetric increases in aerodynamic friction/rolling resistance which would probably start opening fore and aft carbs.
Le Mans Master
Joined: Apr 2007
Posts: 7,353
Likes: 72
From: Graceland in a Not Correctly Restored Stingray
Trips aren't as complicated as many would have you believe. Skipping past primary and linkage setup (do ask if you need help there), the 2nds should be setup to begin openning as quickly as is practical for best performance, which is done by installing the weakest secondary diaphragm springs which don't cause bog at WOT under load. Holley kit #20-13, two required. Never try to re-invent the linkage system, and, as always, verify the ignition system is right first before ever attempting to tune a carb.
Last edited by TheSkunkWorks; Jan 28, 2012 at 09:44 PM.
Racer
Joined: Mar 2009
Posts: 278
Likes: 14
Hey,
I was wondering if you don't mind giving a ball park figure on what you paid for the rebuild? I am thinking of doing it on mine, and was wondering what a ball park going rate is. Now i know there is a ton of variables but id appreciate it.
Thanks
I was wondering if you don't mind giving a ball park figure on what you paid for the rebuild? I am thinking of doing it on mine, and was wondering what a ball park going rate is. Now i know there is a ton of variables but id appreciate it.
Thanks
Drifting
Joined: Nov 2002
Posts: 1,692
Likes: 207
From: Loveland, CO
I agree with Skunkworks. Experiment with the secondary springs. It worked for me. Also, does your engine still have the stock transistorized ignition system, which came on the L71 tripower. They can be fun to tinker with to keep working as designed.
Ralph.
Ralph.
Last edited by rponfick; Jan 29, 2012 at 10:59 PM.
Racer
Joined: Aug 2000
Posts: 349
Likes: 1
From: Hofheim Germany
Hi over there!
Living in Germany, I've the opportunity to run my '69 427/400 on unlimited Autobahns sometimes. These are the moments, I can feel the two additional carbs of my Tripower joining the feed-work of my engine. I've made the experience, that the secondaries came in at about 3.500 rpm -what's about 100 mph (3.08 ratio).
I've never tried out the absolute high-speed of my Vette, because I'm afraid of any damage from the engine. Away from this, running with 130 mph at 4.500 rpm (my personal high-score) is good enough for a 40+ old car.
Greets from Germany
Gerrit
Living in Germany, I've the opportunity to run my '69 427/400 on unlimited Autobahns sometimes. These are the moments, I can feel the two additional carbs of my Tripower joining the feed-work of my engine. I've made the experience, that the secondaries came in at about 3.500 rpm -what's about 100 mph (3.08 ratio).
I've never tried out the absolute high-speed of my Vette, because I'm afraid of any damage from the engine. Away from this, running with 130 mph at 4.500 rpm (my personal high-score) is good enough for a 40+ old car.
Greets from Germany
Gerrit
Le Mans Master
Joined: Apr 1999
Posts: 6,134
Likes: 46
From: Raleigh North Carolina
Here's worth printing and reading:
http://www.corvettefaq.com/c3/tripower/
Outboards open dependant on speed and load conditions; via loss of manifold vaccuum which in turn opens outboards.
Mine seems to usually opens up past half throttle.
From experience, if you have a vaccuum leak around the base of the carbs it will not allow them to open properly, if at all, but it would also idle a bit rough.
If you want to check when they are opening and by how much, put a paperclip or similar on the actuator rod on the back of the diaphrams to see how much (if at all) they moved under different conditions.
http://www.corvettefaq.com/c3/tripower/
Outboards open dependant on speed and load conditions; via loss of manifold vaccuum which in turn opens outboards.
Mine seems to usually opens up past half throttle.
From experience, if you have a vaccuum leak around the base of the carbs it will not allow them to open properly, if at all, but it would also idle a bit rough.
If you want to check when they are opening and by how much, put a paperclip or similar on the actuator rod on the back of the diaphrams to see how much (if at all) they moved under different conditions.