MORE RPMs - The 1947 - Present Chevrolet & GMC Truck Message Board Network

COBRO · 01-27-2005, 07:01 PM

Howdy all this is my first post and this is the best site Ive found for tall hoods. Ive replaced the 402 in my 72 C-20 with a 454 and replaced the M465 with a th-400 with a GV-OD on the end of that. The engine is a .030 over just rebuilt with an Edelbrock performer cam,Edelbrock performer manifold and a 750 Edelbrock AFB. Hedman headers and 2-1/2 exhaust to the rear bumper.I put in a B&M torkconverter and the 400 trans is a fresh rebuilt.This setup is mainly for towing a travel trailer. I was turning around 2700 rpm with the 402 and stick shift at 60mph - now after getting it on the road its turning 3000 rpm at 60 mph. The rear end gearing has not been changed - still 4.10 as it was before. I cant figure what is going on here - the converter has Appox 300 to 500 rpm higher stall speed than a standard converter but wouldnt think that would cause this. Can someone please give me a clue as to whats with the extra RPMs. TIA::::::COBRO

Stroker · 01-27-2005, 07:10 PM

The additional 300 rpm is a result of torque converter slipping and is normal for an automatic without a lockup converter. If you still do a lot of towing be sure to add a transmission cooler to prolonge the life of you turbo 400.

COBRO · 01-27-2005, 07:26 PM

Thanks for the info - I knew one always got less mpg with a auto but never was sure as to why. Yes Ive got the cooler and a deep trans pan and a external 1qt. remote filter,still want to install a fluid temp guage to keep an eye on it. Thanks again::::::COBRO

Stroker · 01-27-2005, 08:09 PM

I missed the GV-OD. I would think the .78 OD would more than make up for converter slip. The 3000 rpm would be normal without the OD. Are you sure you're getting into OD in 3rd?

COBRO · 01-27-2005, 08:19 PM

Yep its going into OD and that knocks the RPMs back down to lower than I was getting with the stick,so its still a very good deal - just had it in my mind what it would be turning not knowing that the trans would let it spin up more. The older I get the more I realise how little I know about things that I thought I knew!!!!! Thanx again:::::COBRO

69 longhorn · 01-27-2005, 08:22 PM

have you checked the speedo calibration since the trans swap? It could be that the truck is cruising faster than it reads.....?

Jimbotronics · 01-27-2005, 08:02 PM

Sounds like a nice setup Cobro!

Welcome from Florida!

krue · 01-27-2005, 09:45 PM

Welcome to the board from South Mississippi! Board rules, gotta post pics of your truck!

COBRO · 01-28-2005, 12:34 AM

Longhorn that was my first idea of what was going on but after checking milepost markers with a stop watch my speedo is within 2 mph of being right and one can get that just by putting new tires on a rig. Sure wish that was it,but with running the GearVendors OD Im still in good shape. Thanks::COBRO

69 longhorn · 01-28-2005, 12:44 AM

Here is a calculator to play with. you can factor in tire size, converter , ect...... http://www.smokemup.com/auto_math/index.php

dwaite72lnghrn · 01-28-2005, 07:00 AM

My 402BB/TH400 gets to 3000rpm at 60mph with 4.1 rear end. Let me know how that gear vendors runs, I have been looking at it but the cost is keeping me from jumping into one. My truck gets about 11mgp empty at 55mph if I could drop the rpms to 2500rpms I could probably get 11mpg at 60mph. The old Qjet ran more economically I could get up to 13.5mpg with that empty at 55mph. I am still thinking of going back to the qjet.

By the way welcome to the board from PA.

*dtlilly* · 01-28-2005, 07:07 AM

My C-30 huffs pritty good good doing 55mph with the I-6 and SM465. I don't want to change a thing with mine tho, because it could pull a house off the foundation in granny low. Welcome to the board from the shadow of Lambeau Field.

01-27-2005, 07:01 PM	#1
COBRO Active Member Join Date: Jan 2005 Location: RDM,Oregon Posts: 285	MORE RPMs Howdy all this is my first post and this is the best site Ive found for tall hoods. Ive replaced the 402 in my 72 C-20 with a 454 and replaced the M465 with a th-400 with a GV-OD on the end of that. The engine is a .030 over just rebuilt with an Edelbrock performer cam,Edelbrock performer manifold and a 750 Edelbrock AFB. Hedman headers and 2-1/2 exhaust to the rear bumper.I put in a B&M torkconverter and the 400 trans is a fresh rebuilt.This setup is mainly for towing a travel trailer. I was turning around 2700 rpm with the 402 and stick shift at 60mph - now after getting it on the road its turning 3000 rpm at 60 mph. The rear end gearing has not been changed - still 4.10 as it was before. I cant figure what is going on here - the converter has Appox 300 to 500 rpm higher stall speed than a standard converter but wouldnt think that would cause this. Can someone please give me a clue as to whats with the extra RPMs. TIA::::::COBRO

01-27-2005, 07:10 PM	#2
Stroker Registered User Join Date: Mar 2001 Location: Moore, Ok Posts: 1,149	The additional 300 rpm is a result of torque converter slipping and is normal for an automatic without a lockup converter. If you still do a lot of towing be sure to add a transmission cooler to prolonge the life of you turbo 400. __________________ 72 SWB parts and pieces Oklahoma

01-27-2005, 08:09 PM	#4
Stroker Registered User Join Date: Mar 2001 Location: Moore, Ok Posts: 1,149	I missed the GV-OD. I would think the .78 OD would more than make up for converter slip. The 3000 rpm would be normal without the OD. Are you sure you're getting into OD in 3rd? __________________ 72 SWB parts and pieces Oklahoma

01-27-2005, 08:02 PM	#7
Jimbotronics YA YA Join Date: Aug 2004 Location: Rockledge, Fl Posts: 1,273	Sounds like a nice setup Cobro! Welcome from Florida! __________________ Jimbo '72 SWB - 350 (frame-off resto in progress - donations accepted) '04 Chevy Avalanche '03 Cadillac CTS (wife's ride) Restoration progress - http://67-72chevytrucks.com/vboard/s...d.php?t=335707

01-27-2005, 09:45 PM	#8
krue Designated A-hole! Join Date: Mar 2003 Location: South Mississippi Posts: 36,448	Welcome to the board from South Mississippi! Board rules, gotta post pics of your truck! __________________ "If it ain't broke, fix it 'til it is!" Being stupid ain't illegal. We're Still Debt Free Except for the House!!! www.daveramsey.com 70 GMC SWB Stepside project "Green Booger" soon to be 6.0l/4l80e 93 S-10 "Poppaw's Truck"

01-27-2005, 07:26 PM	#3
COBRO Active Member Join Date: Jan 2005 Location: RDM,Oregon Posts: 285	Thanks for the info - I knew one always got less mpg with a auto but never was sure as to why. Yes Ive got the cooler and a deep trans pan and a external 1qt. remote filter,still want to install a fluid temp guage to keep an eye on it. Thanks again::::::COBRO

01-27-2005, 08:19 PM	#5
COBRO Active Member Join Date: Jan 2005 Location: RDM,Oregon Posts: 285	Yep its going into OD and that knocks the RPMs back down to lower than I was getting with the stick,so its still a very good deal - just had it in my mind what it would be turning not knowing that the trans would let it spin up more. The older I get the more I realise how little I know about things that I thought I knew!!!!! Thanx again:::::COBRO

01-27-2005, 08:22 PM	#6
69 longhorn Registered User Join Date: Aug 2001 Location: rock island,il,usa Posts: 5,382	have you checked the speedo calibration since the trans swap? It could be that the truck is cruising faster than it reads.....?

01-28-2005, 12:34 AM	#9
COBRO Active Member Join Date: Jan 2005 Location: RDM,Oregon Posts: 285	Longhorn that was my first idea of what was going on but after checking milepost markers with a stop watch my speedo is within 2 mph of being right and one can get that just by putting new tires on a rig. Sure wish that was it,but with running the GearVendors OD Im still in good shape. Thanks::COBRO

01-28-2005, 12:44 AM	#10
69 longhorn Registered User Join Date: Aug 2001 Location: rock island,il,usa Posts: 5,382	Here is a calculator to play with. you can factor in tire size, converter , ect...... http://www.smokemup.com/auto_math/index.php

01-28-2005, 07:00 AM	#11
dwaite72lnghrn Registered User Join Date: Jun 2002 Location: central pa Posts: 1,525	My 402BB/TH400 gets to 3000rpm at 60mph with 4.1 rear end. Let me know how that gear vendors runs, I have been looking at it but the cost is keeping me from jumping into one. My truck gets about 11mgp empty at 55mph if I could drop the rpms to 2500rpms I could probably get 11mpg at 60mph. The old Qjet ran more economically I could get up to 13.5mpg with that empty at 55mph. I am still thinking of going back to the qjet. By the way welcome to the board from PA. __________________ 72 longhorn c30 502BB/th400/fact air/4.11rear/ custom camper 72 bug(the better half's) under reconstruction 2009 HHR aqua blue 71 GMC k20 350/sm465 2009 Chevy hd3500 6.6 Victory Red crew cab dually Dave & Jeanne

01-28-2005, 07:07 AM	#12
dtlilly Account Suspended Join Date: Jan 2003 Location: Shadow of Lambeau Field Posts: 5,597	My C-30 huffs pritty good good doing 55mph with the I-6 and SM465. I don't want to change a thing with mine tho, because it could pull a house off the foundation in granny low. Welcome to the board from the shadow of Lambeau Field.



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