Underdrive crank....Do I need overdrive Alt?
Thread Starter
Burning Brakes
Joined: May 2014
Posts: 884
Likes: 128
From: Rhine Georgia
Underdrive crank....Do I need overdrive Alt?
Running a underdrive crank pulley. I'm getting 12.8 at idle with all accessories on. Dual Fans biggest draw. 14.2 at idle with everything off. I've had to charge 2 year old battery maybe 3 times when noticed it feeling weak/dragging while starting car. I always blamed it on not driving enough. Question is.....Would a overdrive alt pulley help/solve or does my current setup sound normal?
Le Mans Master
Joined: Oct 2006
Posts: 7,123
Likes: 433
From: Gladstone MO
C3 of Year Finalist (appearance mods) 2019
Why the underdrive crank pulley? That slows down all of the belt driven items- water pump, power steering and alternator. I'd have to see dyno sheets to prove that underdriving the accessories would be worth the potential gain in power.
Melting Slicks
Joined: Jul 2006
Posts: 2,292
Likes: 494
From: Salmon Arm, BC
I had an under drive crank pulley on my other car, with a stock pulley on the alternator, and never had any troubles with it....normal driving will keep your battery topped, you just need to do it more often
Thread Starter
Burning Brakes
Joined: May 2014
Posts: 884
Likes: 128
From: Rhine Georgia
Why the underdrive crank pulley? That slows down all of the belt driven items
Melting Slicks
Joined: Sep 2004
Posts: 3,227
Likes: 671
From: Northeast
When I went on the dyno I asked the operator how much HP I could expect from the underdrive system and he just smiled. I said "3" and he smiled again.
For me, my system has worked fine, not going through batteries, charges enough at idle to keep the gen light out and looks great. I agree with SSstocker.
For me, my system has worked fine, not going through batteries, charges enough at idle to keep the gen light out and looks great. I agree with SSstocker.
Last edited by BKarol; Sep 3, 2017 at 05:31 PM.
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Race Director
Joined: Apr 2007
Posts: 11,152
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From: South Western Ontario
I wouldn't expect that alternator to keep up with electric fans on the car.
Besides changing it, the best you can do is change the pulley ratio to limit the alternator rpm to around 1800rpm at your max engine rpm and see if it helps.
Besides changing it, the best you can do is change the pulley ratio to limit the alternator rpm to around 1800rpm at your max engine rpm and see if it helps.
Team Owner
Joined: Sep 2002
Posts: 38,369
Likes: 2,085
From: California
Convert the alternator to a more modern like 63 amp internal regulated alternator! Simplify the wiring with like a Mad kit!
The more modern will support a better charging at lower rpms!
For pulleys if if you have a 30% increase in horsepower let's say theoretically a 30% increase in rpm capability, then go for a 30% change of your pulley system! If everything is 10% then match everything at 10%! Or 20%! I have a chart of stock pulleys somewhere to walk up the percentage! In many performance cars to save money, they invented deep groove pulleys, instead of engineering the accessories, and belt speeds! The older alternators put out at running rpms, plus the cars didn't have all kinds of battery capable draining mods like electric fans, electric water pumps, killer stereos, CD boxes, etc. if you have added such, mod your system too match!
The more modern will support a better charging at lower rpms!
For pulleys if if you have a 30% increase in horsepower let's say theoretically a 30% increase in rpm capability, then go for a 30% change of your pulley system! If everything is 10% then match everything at 10%! Or 20%! I have a chart of stock pulleys somewhere to walk up the percentage! In many performance cars to save money, they invented deep groove pulleys, instead of engineering the accessories, and belt speeds! The older alternators put out at running rpms, plus the cars didn't have all kinds of battery capable draining mods like electric fans, electric water pumps, killer stereos, CD boxes, etc. if you have added such, mod your system too match!
Last edited by TCracingCA; Sep 5, 2017 at 10:12 PM.
Melting Slicks
Joined: Jun 2016
Posts: 2,979
Likes: 332
From: Woodinville WA
3.7 hp at peak and 1.7 ft lbs across the entire RPM range on a 525 hp engine is the loss from spinning just the water pump.
A great article for addressing the losses and then gains when using an underdrive pulley for just the water pump, anyway: http://www.hotrod.com/articles/hrdp-...ps-suck-power/
I can't remember if it was this article or a different one that found that the HP and torque output was almost identical between an electric water pump and a high flow water pump that is underdriven by 20% with an underdrive crank pulley.
If a high flow water pump can flow 20% more at a given RPM -then if you underdrive it by 20% you still have the same coolant flow with less HP loss. -That seems like a winning combination, IMHO. (Especially if you're getting rid of an old heavy, iron water pump at the same time.)
Adam
A great article for addressing the losses and then gains when using an underdrive pulley for just the water pump, anyway: http://www.hotrod.com/articles/hrdp-...ps-suck-power/
I can't remember if it was this article or a different one that found that the HP and torque output was almost identical between an electric water pump and a high flow water pump that is underdriven by 20% with an underdrive crank pulley.
If a high flow water pump can flow 20% more at a given RPM -then if you underdrive it by 20% you still have the same coolant flow with less HP loss. -That seems like a winning combination, IMHO. (Especially if you're getting rid of an old heavy, iron water pump at the same time.)
Adam
Melting Slicks
Joined: Jun 2016
Posts: 2,979
Likes: 332
From: Woodinville WA
Regarding HP / Torque increases with underdrive pulleys:
GMHTP July 2003 Volume 9, # 4: "Project Blue Thunder" article installed a 20% underdrive pulley on a 1989 Firebird (305 TPI) -on a SuperFlow chassis dyno: car showed a 17 hp improvement and 13 ft lbs at peak hp and torque.
Here's a dyno showing the before and after hp / torque curves for a WS6 LS1 Transam, too: http://www.ws6.com/mod-2.htm 0.7 hp increase at peak and 5.1 ft lbs at torque peak, but average HP was up 4.4 HP and average torque by 5.8 ft lbs.
I don't have any more data points but it makes sense that V belt and older accessories might see a bigger improvement, but as has been pointed out here, 10SI and I think 12SI alternators don't respond well to low RPMs -so upgrade to a CS alternator or go with an alternator overdrive pulley.
Good to see what you can get vs. what it costs you, anyway.
I've seen a few comments that underdrive pulleys are worth roughly 0.1 sec in the quarter mile; no idea in what parameters that applies but seems to be a general rule-of-thumb.
Adam
GMHTP July 2003 Volume 9, # 4: "Project Blue Thunder" article installed a 20% underdrive pulley on a 1989 Firebird (305 TPI) -on a SuperFlow chassis dyno: car showed a 17 hp improvement and 13 ft lbs at peak hp and torque.
Here's a dyno showing the before and after hp / torque curves for a WS6 LS1 Transam, too: http://www.ws6.com/mod-2.htm 0.7 hp increase at peak and 5.1 ft lbs at torque peak, but average HP was up 4.4 HP and average torque by 5.8 ft lbs.
I don't have any more data points but it makes sense that V belt and older accessories might see a bigger improvement, but as has been pointed out here, 10SI and I think 12SI alternators don't respond well to low RPMs -so upgrade to a CS alternator or go with an alternator overdrive pulley.
Good to see what you can get vs. what it costs you, anyway.
I've seen a few comments that underdrive pulleys are worth roughly 0.1 sec in the quarter mile; no idea in what parameters that applies but seems to be a general rule-of-thumb.
Adam