[DeTomaso] Intake Reversion

Don Thomas don.thomas.ieee at gmail.com
Wed May 1 23:03:15 EDT 2013


*See this: http://www.holley.com/TechService/FAQ.asp?category=Carburetor *
**
**
*QUESTION* How do I tell what size power valve I need?
*ANSWER* To properly size a power valve, take a vacuum reading at idle and
if it is above 12" for a standard transmission a 6.5" will be safe to use.
For automatic transmissions take a vacuum reading in gear at idle and if
the vacuum is below 12" divide that in half for proper size. Example 9" of
vacuum in gear at idle will require a 4.5" power valve.There is a video on
this procedure on Holley TV. Click Here To View.<http://holleytv.com/?p=452>


On Wed, May 1, 2013 at 7:49 PM, Don Thomas <don.thomas.ieee at gmail.com>wrote:

> Check or change out your power valve.
>
> A good starting point for the power valve is -2Hg's below the vacuum at
> idle.
>
>
>
>
> On Wed, May 1, 2013 at 7:36 PM, John Taphorn <jtaphorn at kingwoodcable.com>wrote:
>
>> Sean
>>
>> I am skeptical about the reversion theory at that RPM.  I didn't pick up
>> weather it occurred at steady throttle or acceleration; yet, it strikes
>> me as the engine is going lean at that transition point.
>>
>> JT
>> On 5/1/2013 6:44 PM, sean mundy wrote:
>> > The engine is installed and ran a chassis dyno.  While sitting on the
>> dyno you could
>> > see the car bucking and moving a little bit back and forth on the dyno
>> wheel because of
>> > the surging/hesitation I mentioned.   I know spacers of various
>> thickness are used to increase the hp
>> > and also insulate the carb from the engine heat.  The reversion
>> specific spacers I've seen
>> > online seemed to be designed specifically for reducing the effect of
>> exhaust gases that are pushed
>> > back up into the carb because the intake/exhaust overlap is too long.
>>  The spacers I've found
>> > range from 70-150 dollars so I don't want to waste money if it's all
>> hogwash.
>> >
>> >
>> >> From: cengles at cox.net
>> >> To: seanmundy at hotmail.com
>> >> CC: detomaso at poca.net
>> >> Subject: RE: [DeTomaso] Intake Reversion
>> >> Date: Wed, 1 May 2013 18:35:04 -0500
>> >>
>> >> Dear Sean,
>> >>
>> >>
>> >> In my limited engine building experience, the engine dyno is the
>> >> place to determine the benefit of a spacer. The match between an
>> engine and
>> >> a spacer is idiosyncratic and must be tested to determine if it works.
>> Some
>> >> engines will benefit and some won't.
>> >>
>> >> It sounds like your engine is now installed. If so, then if you
>> >> want to test the spacer angle, then it would be off to the chassis
>> dyno.
>> >> If the chassis dyno showed improvement in your HP/torque numbers, then
>> it
>> >> would be worthy to road test to see if you problem is gone.
>> >>
>> >> In my limited reading about engines, I can't recall that
>> >> reversion can be quickly and simply fixed with a spacer, but I am a
>> rookie.
>> >>
>> >>
>> >>
>> >> Warmest regards, Chuck Engles
>> >>
>> >>
>> >>
>> >> -----Original Message-----
>> >> From: detomaso-bounces at poca.com [mailto:detomaso-bounces at poca.com] On
>> Behalf
>> >> Of sean mundy
>> >> Sent: Wednesday, May 01, 2013 5:08 PM
>> >> To: detomaso at poca.com
>> >> Subject: [DeTomaso] Intake Reversion
>> >>
>> >> Hey all, I recently had my new 408 Cleveland tuned and dyno'd at the
>> shop.
>> >> 438/411 hp/trq rearwheel.
>> >> Very happy with the numbers but having some issues with how the motor
>> runs.
>> >> After driving it 50 miles I noticed at anywhere between 2-3k RPM in
>> any gear
>> >> there was a surging and hesitation in the engine.
>> >> I went back to the shop and we worked on the carb (750 Holley) by
>> >> increasing the primary jets and adjusting the pump squirters.
>> >> We put it back on the dyno and ran it several times. The tech said he
>> >> couldn't get good A/F numbers because the sensor placed in the tailpipe
>> >> couldn't get past the baffling in the stock muffler. It pulls very
>> good in
>> >> the top end but he noticed the 2-3k RPM surging and hesitation that I
>> >> mentioned.
>> >> The tech told me that he thought since my vaccuum reading was so low
>> (10-12
>> >> in)
>> >> at low RPM that I was probably experiencing "reversion".
>> >> He said that the lobe separation overlap on my cam specs were probably
>> the
>> >> cause of this and also a contributing factor was my single plane
>> manifold.
>> >> Basically he made it sound like I was stuck with what I got.
>> >>
>> >> My cam specs are:
>> >>
>> >> GROSS VALVE LIFT .617 .617
>> >> DURATION @ .006
>> >> TAPPET LIFT 286 300
>> >> VALVE TIMING OPEN CLOSE
>> >> @ .050 INT 10 46
>> >> EXH 60 8
>> >> SPECS FOR CAM INST. @ 108.0 CENTERLINE
>> >> INT EXH
>> >> DURATION @ .050 236.00 248.00
>> >> LOBE LIFT .363 .363
>> >> LOBE SEPARATION 112.0
>> >>
>> >> I've done some reading on the internet about reversion and it seems
>> like a
>> >> real thing.
>> >> I read that installing a carb spacer designed to help eliminate the
>> phenomen
>> >> might work in my case.
>> >> Wondering if anyone else has experience with this and what if any
>> solution
>> >> worked for you.
>> >> Thanks, Sean
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