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Jim Stabe
Jim Stabe
San Diego, Ca
(791 posts)

Registered:
02/28/2009 10:01AM

Main British Car:
1966 MGB Roadster 350 LT1 Chevy

authors avatar
Had the car retuned after cam change
Posted by: Jim Stabe
Date: February 04, 2015 06:52PM

I recently changed out the cam in my car for a milder one to get better freeway manners in 6th gear. This cam has 10 degrees less intake duration and a wider lobe separation angle of 114 degrees which definitely improved the performance at low rpm (1400-1700) eliminating the need to down shift when traffic slowed. At the same time I put a larger pulley on the blower to reduce the boost and changed out the Flowmaster muffler for a Dynomax. The old cam made massive torque (543 ft lbs at the rear wheels) and the horsepower was 450 but at only 4800 and then it dropped off rapidly. I suspected the exhaust was restrictive and then I saw on another forum the attached chart comparing the flow of various mufflers to that of an open pipe. I had a 50 series Flowmaster on it and switched it out for the Dynomax Ultra Flow Welded. The Flowmaster flowed only 53% of an open pipe whereas the Dynomax was 101%. With the smaller cam you would expect the horsepower be lower and to peak at a lower rpm but it actually raised the peak almost 1000 rpm peaking at 466 rear wheel hp at 5750 rpm (that's 536 flywheel hp). What a difference a muffler makes! After this post the name "Flowmaster" will be forever stricken from my vocabulary.

MufflerComparisionChart.jpg

Before the horsepower and torque peaks the new curves were flatter and lower as I expected but it still makes over 300 ft lbs at 2500 rpm and 425 ft lbs at 3000 rpm and it stays over 400 ft lbs past 6000 rpm (peak is 479 ft lbs at 4300 rpm). All the new numbers were with the water/methanol injection turned off and the old ones were with it turned on so they will probably be closer with the system on. So am I disappointed with the lower performance now? It still has plenty of power to scare me and I still need to be sure to have it pointed straight ahead when the boost comes on. What is nicer is the normal driving and I think it will be much easier to control in autocrosses, I just need to control the acceleration rate by shifting early.

I am going to keep the old cam because if I build another engine with forged internals ( I have a stock shortblock with cast pistons and 11:1 compression now) I would love to see what it would do with the less restrictive exhaust. My guess is well over 500 rwhp and close to 600 rwtq and I could regain some of the low end performance with a set of Rhoads lifters [www.rhoadslifters.com] The engine just won't take that kind of hp now.



Edited 2 time(s). Last edit at 02/04/2015 07:00PM by Jim Stabe.


74ls1tr6
Calvin Grannis
Elk Grove,CA
(1151 posts)

Registered:
11/10/2007 10:05AM

Main British Car:
74 TR6 / 71 MGB GT TR6/Ls1 71 MGB GT/Ls1

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Re: Had the car retuned after cam change
Posted by: 74ls1tr6
Date: February 06, 2015 09:37AM

I second the statement on Flowcrapster! Nice information Jim. I need to ask......Indy this year?


Jim Stabe
Jim Stabe
San Diego, Ca
(791 posts)

Registered:
02/28/2009 10:01AM

Main British Car:
1966 MGB Roadster 350 LT1 Chevy

authors avatar
Re: Had the car retuned after cam change
Posted by: Jim Stabe
Date: February 06, 2015 10:26AM

Never say never but I have my doubts. That is quite a drive in an open car.


BlownMGB-V8
Jim Blackwood
9406 Gunpowder Rd., Florence, KY 41042
(6008 posts)

Registered:
10/23/2007 12:59PM

Main British Car:
1971 MGB Blown,Injected,Intercooled Buick 340/AA80E/JagIRS

authors avatar
Re: Had the car retuned after cam change
Posted by: BlownMGB-V8
Date: February 06, 2015 02:12PM

Yes it is. When I made that drive I had a Parish Plastics top on the car.

Jim


MGBV8
Carl Floyd
Kingsport, TN
(4042 posts)

Registered:
10/23/2007 11:32PM

Main British Car:
1979 MGB, Buick 215

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Re: Had the car retuned after cam change
Posted by: MGBV8
Date: February 06, 2015 03:57PM

Hah, like that is going to fit his car! ;)


BlownMGB-V8
Jim Blackwood
9406 Gunpowder Rd., Florence, KY 41042
(6008 posts)

Registered:
10/23/2007 12:59PM

Main British Car:
1971 MGB Blown,Injected,Intercooled Buick 340/AA80E/JagIRS

authors avatar
Re: Had the car retuned after cam change
Posted by: BlownMGB-V8
Date: February 06, 2015 06:46PM

Yeah, guess not.

Jim


74ls1tr6
Calvin Grannis
Elk Grove,CA
(1151 posts)

Registered:
11/10/2007 10:05AM

Main British Car:
74 TR6 / 71 MGB GT TR6/Ls1 71 MGB GT/Ls1

authors avatar
Re: Had the car retuned after cam change
Posted by: 74ls1tr6
Date: February 07, 2015 12:58AM

Possible to trailer it? We can meet up on the way, and caravan out. We can't get any better than now on fuel prices? I know....even myself want to see your ride in the flesh. It would be way cool for sure.



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