Tri Power Throttle Response Issue
Race Director
Joined: Jun 2000
Posts: 10,046
Likes: 675
From: FL
1) He has only owned this car for a year.
2) "When the throttle is released, the rpm drops back to the idle setting slowly rather than quickly as it seems it should."
3) From OP: "The throttle returns to the idle stop quickly via the throttle spring, but the drop in rpm significantly lags (about 2 seconds). The rpm does not hang up at the high level then drop suddenly to idle; it just drops at a slow consistent rate, back to the idle rpm."
So, we have determined that the RPM does return to idle, just not as fast as the OP thinks it should.
- How much experience has he had with older cars?
- How fast does he think it should return to idle?
- What RPM is he releasing the throttle? Is the delay proportional to RPM or the same no matter what.
- Automatic or manual. Clutch in or out. OP mentions "This happens whether you're just snapping the throttle by hand in the driveway, or actually driving the car (so independent of load, throttle cable, secondaries, etc.). " but behavior will be way different if driving, then release throttle, then clutch in.
- What is "about 2 seconds"? 1.2 seconds, 2.0 seconds, 2.5 seconds? RPM dependent?
Lots of variables here and as a test engineer, they all need to be understood and minimized.
Last edited by SteveG75; Jul 9, 2020 at 02:05 PM.
Team Owner
Joined: Jan 2006
Posts: 37,637
Likes: 3,118
From: Crossville TN
The C3 Corvette is a 'sporty' production car.....not a full-race machine. It has the same basic drive train as a Chevy sedan. If you want your C3 to behave like a race car, you need to re-engineer it to act like a race car.
The tri-power system has vacuum operated outboard carbs. They are engaged at the fuel flow rate that the engine can consume without choking on it. Since they are not strictly 'mechanically' actuated, there is a short delay in those carbs getting reset to fully closed position. Most of that is by design, so that the engine does not see a NO FUEL condition at high rpm's. Lots of tri-power systems have mechanical actuating systems (ie, progressive linkage) which can close completely in the time it takes the return spring to shut them off.
Install the WOT actuation system you wish to have.....
The tri-power system has vacuum operated outboard carbs. They are engaged at the fuel flow rate that the engine can consume without choking on it. Since they are not strictly 'mechanically' actuated, there is a short delay in those carbs getting reset to fully closed position. Most of that is by design, so that the engine does not see a NO FUEL condition at high rpm's. Lots of tri-power systems have mechanical actuating systems (ie, progressive linkage) which can close completely in the time it takes the return spring to shut them off.
Install the WOT actuation system you wish to have.....
Race Director
Joined: Jun 2000
Posts: 10,046
Likes: 675
From: FL
Ok, just came back from driving my car. 496 big block, internally balanced, 25# flywheel. Takes ~1.5 sec to return to idle from 3000 rpm. You probably have a heavier flywheel stock. ~2 sec sounds normal to me. Big blocks have a lot of rotational inertia. Takes a while to slow down.
Thread Starter
Cruising
Joined: Jan 2020
Posts: 12
Likes: 0
Steve,
Thanks for the reply, good questions. A few answers:
- I have owned three muscle cars (hemicuda, Super bee and now BB vette). I'm not an expert by any means but I am familiar with old cars.
- my expectation is the rpm would return to idle in typical fashion, based on my experience, meaning without a noticeable lag.
-In the driveway, when I snap the throttle, I release it at about 2500-3000 rpm.
- I have not conducted a rigorous analysis, but I'd say the behavior is very similar whether I snap the throttle in the driveway, or drive the car, accelerate, and quickly switch into neutral. The rpm slowly returns to idle. It may be a little slower when driving the car, but it clearly happens independent of the car being in gear.
-By "about 2 seconds" I mean more than 1.2 seconds for sure, more like between 2 and 3 depending on how high I rev the car.
I've tried to minimize the variables by analyzing the issue in the driveway (independent of clutch, opening secondaries, etc.)
Thanks for the reply, good questions. A few answers:
- I have owned three muscle cars (hemicuda, Super bee and now BB vette). I'm not an expert by any means but I am familiar with old cars.
- my expectation is the rpm would return to idle in typical fashion, based on my experience, meaning without a noticeable lag.
-In the driveway, when I snap the throttle, I release it at about 2500-3000 rpm.
- I have not conducted a rigorous analysis, but I'd say the behavior is very similar whether I snap the throttle in the driveway, or drive the car, accelerate, and quickly switch into neutral. The rpm slowly returns to idle. It may be a little slower when driving the car, but it clearly happens independent of the car being in gear.
-By "about 2 seconds" I mean more than 1.2 seconds for sure, more like between 2 and 3 depending on how high I rev the car.
I've tried to minimize the variables by analyzing the issue in the driveway (independent of clutch, opening secondaries, etc.)
Thread Starter
Cruising
Joined: Jan 2020
Posts: 12
Likes: 0
7T1vette,
Great points. I agree completely that the vacuum secondaries would close slower than mechanical secondaries and I should expect some inherent lag. however, this lag I'm seeing is independent of any secondary opening (in the driveway, no load, I stare at the secondary linkage and there is no movement, I also completely disconnected the secondaries and no change)
Great points. I agree completely that the vacuum secondaries would close slower than mechanical secondaries and I should expect some inherent lag. however, this lag I'm seeing is independent of any secondary opening (in the driveway, no load, I stare at the secondary linkage and there is no movement, I also completely disconnected the secondaries and no change)
Thread Starter
Cruising
Joined: Jan 2020
Posts: 12
Likes: 0
Very interesting. Thanks for the info on your car. My Super Bee has a stroked 440 and it snaps back to idle immediately. I have no idea if there's any significant difference in the Mopar and GM flywheel systems.
Race Director
Joined: Jul 2015
Posts: 13,082
Likes: 3,399
From: SW Florida.
possible you are getting air from the 2 outer carbs and fuel from the center. maybe try blocking the top of the front and rear carbs with your hand feeling for suction. and snapping throttle and see if any diff.
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I have found with my AED carb on a 383 the problem you describe. If the idle adjustment screws are too rich the idle is slow to return to the set idle rpm. If it is too lean the idle will drop below the set rpm. I installed an afr gage about 6 months ago which helped me figure this problem out. Try adjusting your idle adjustment screws to lean your afr and see if the problem improves. It may take several attempts. If nothing changes, go back to your original setting. Good luck.