[DeTomaso] Intake Reversion

[Sean]

The surging/hesitation happens at constant speed and acceleration. After 3k rpm things start to smooth out.  I will read up on power valves but I'm surprised the dyno techs didn't even mention that as an option. They were spraying a light mist of carb cleaner or something over the carb while it was running at 2k in 3rd gear. The motor didn't change in the way it was running. Somehow that led them to come up with the reversion theory.  Most things I've read so far seem to say the phenomenon happens at higher rpms

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On 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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