[DeTomaso] Intake Reversion

dave londry davel at emspace.com
Tue May 7 15:34:18 EDT 2013


Hey Sean,
Please let us know how the AEM works for you.
I have an innovate which is OK, but I'd like a gauge install.
I find the innovate is crazy sensitive to exhaust reversions (sorry to 
bring the term up)
  even with the sensor bung up near the header flange.
Oh yes - I also cooked one sensor.
I didn't know that running with the sensor unpowered poisons the sensor.
dave

On 07/05/2013 12:18 PM, sean mundy wrote:
> Thanks Will for the input.   I'm not using the stock damper.   I've
> checked for intake leaks by spraying carb cleaner all over the intake.
> Nothing detected.    I just bought an AEM a/f gauge today.  I will get
> that hooked up an start the process of dialing in everything.
>
>> Date: Mon, 6 May 2013 18:51:35 -0400
>> Subject: Re: [DeTomaso] Intake Reversion
>> From: will.kooiman at gmail.com
>> To: jeffcobb1 at me.com; seanmundy at hotmail.com; detomaso at poca.com
>>
>> I agree.
>>
>> And if you are pinging at 22 degrees, you probably have a problem
>> somewhere.  For example, if you are sucking oil through a bad intake
>> gasket seal, you will ping early.
>>
>> More likely, you are misreading the timing and vacuum.  Stock damper?  If
>> so, the outer ring has likely shifted.
>>
>> Hard to diagnose over e-mail.
>>
>> --
>> Will
>>
>>
>>
>>
>>
>> On 5/6/13 1:44 PM, "Jeff Cobb" <jeffcobb1 at me.com> wrote:
>>
>>> Sorry I sent to early.			
>>>
>>> On May 6, 2013, at 12:37 PM, Jeff Cobb wrote:
>>>
>>>> Sean,
>>>> The problems that I read are that your timing only varies by 6 degrees
>>>> and does it at 5,000 not about 3,500 +- and that your dyno man came up
>>>> with a different number than you.
>>>> I thought this engine should run about 32 deg @ about 4,000+-?
>>>> The easiest thing to do is to ping test it.
>>>> The last time I did it on a Pantera was in 83 for a customer. I found
>>>> it to be so easy. Remove the firewall plate and mark and loosen
>>>> distributor, then max warm up the engine and short shift to third, then
>>>> max load it from 1,500 to 3,500  while turning the distributor to a
>>>> consistent ping then backing off about four degrees. Retest for no ping
>>>> then maybe back off another degree for good measure.
>>> Then your drive will confirm a diff or not. Then you can retest with the
>>> timing light for reference only.
>>>> Jeff Cobb
>>>> Cell 225-907-4514
>>>> 				
>>>>
>>>> On May 5, 2013, at 8:21 PM, sean mundy wrote:
>>>>
>>>>> Jeff,
>>>>> I have a stock distributor with  Accel brand Optical points eliminator
>>>>> installed .  Standard coil and no vacuum advance connected to the
>>>>> distributor.  Here is the results I got this afternoon with my cheap
>>>>> vacuum gauge and timing light.
>>>>>   
>>>>> IDLE is 1100rpm
>>>>>   
>>>>> RPM	Vacuum	Timing
>>>>> 1100	8	18
>>>>> 1500	10	19
>>>>> 2000	13	20
>>>>> 2500	15	21.5
>>>>> 3000	16	22
>>>>> 3500	16	23.5
>>>>> 4000	17	23.5
>>>>> 5000	17	24
>>>>>
>>>>>   
>>>>>   
>>>>> The timing at 4000 to 5000RPM + plus is basically the same.  The
>>>>> timing is actually rapidly bouncing between 22-24.  Not sure if that is
>>>>> my light or not.  Dyno Tech told me he set the initial timing at 22 and
>>>>> total 31. I'm not seeing those numbers with my light but not sure why.
>>>>> He said the engine didn't like a lot of advance and that is why he
>>>>> recurved the distributor and disconnected the vacuum advance
>>>>> The carb is a standard Holley 750 Vacuum Sec electric choke.  The
>>>>> power valve is standard 65 and the primary jets have been upped to size
>>>>> 72 (from 70).
>>>>> Appreciate your any input you can provide.
>>>>> Thank You!!!
>>>>> Sean
>>>>>
>>>>> Subject: Re: [DeTomaso] Intake Reversion
>>>>> From: jeffcobb1 at me.com
>>>>> Date: Fri, 3 May 2013 19:42:28 -0500
>>>>> CC: jeffcobb1 at me.com
>>>>> To: seanmundy at hotmail.com; detomaso at poca.com
>>>>>
>>>>> Your welcome Sean,
>>>>>
>>>>> On May 3, 2013, at 3:49 PM, sean mundy <seanmundy at hotmail.com> wrote:
>>>>>
>>>>> Thanks Jeff for taking the time to respond and provide so much
>>>>> detailed information.  It always amazes me how helpful everyone is on
>>>>> the Forum.
>>>>> I enjoy helping and solving problems as most people do.
>>>>> The dyno tech guy said that he modified the advance curve on my
>>>>> distributor.
>>>>> Maybe your dyno man is not well versed in this area?
>>>>> He said the motor didn't like too much timing.
>>>>> What does that mean?
>>>>> Are you using low octane fuel?
>>>>> Maybe he is wrong.
>>>>> I found engines run the best about 4 deg before ping under max load.
>>>>> and he set the initial at 18deg.  Not sure how that will factor in
>>>>> with your note on advancing the timing.
>>>>> Is your timing digital where you can dial in diff deg under diff loads?
>>>>> You mentioned a graph of timing/vacuum.
>>>>> Yes and I believe that all dyno workers can print you out a plot/graph
>>>>> of your engines run and your pulls so you can see what timing degrees
>>>>> at what rpm under what load against your stoic " fuel/air" ratio.
>>>>> I would take timing and vacuum readings at various RPMs?
>>>>> Yes.
>>>>> Would you take a look at it and let me know what you think?
>>>>> Yes.
>>>>> Not sure what you mean by primary venturis but we upped the primary
>>>>> jets by one size.
>>>>> Replaceable throats.
>>>>> You think going smaller will help?
>>>>> No, but paying attention to a better air bleed would help.
>>>>> Going bigger seemed to help the problem a little but not cure it.
>>>>> You may not have gone enough.
>>>>> I pulled a few spark plugs last night and they are mostly sooty/dark.
>>>>> That is ok.
>>>>> Not horrible but it seems to be running on the rich side.
>>>>> Someone on the site recommend putting in an air/fuel ratio sensor,
>>>>> great idea.
>>>>> JT gave very good info on carb flow function.
>>>>>
>>>>> Your low speed idle air jet control mixture while cruising through the
>>>>> transfer slot. The transfer slot feeds fuel to the engine when the
>>>>> throttle blades are not open enough to engage the main jets. On my car
>>>>> when cruising on the highway in fifth, the throttle blades are barely
>>>>> moved and fuel delivery is through the transfer slot. Using a smaller
>>>>> idle air jet will richen the mixture in this RPM zone.
>>>>>
>>>>> You can chase your tail on this stuff all year long. I also recommend
>>>>> that you add an o2 sensor to your header and a gauge to your dash to
>>>>> inform yourself of where the engine is lean or rich. I spent a decade
>>>>> trying to guess what I learned overnight with an air/fuel gauge.
>>>>>
>>>>> JT
>>>>> Good luck,
>>>>>
>>>>> Jeff Cobb
>>>>>
>>>>> Subject: Re: [DeTomaso] Intake Reversion
>>>>> From: jeffcobb1 at me.com
>>>>> Date: Thu, 2 May 2013 09:00:27 -0500
>>>>> CC: jeffcobb1 at me.com
>>>>> To: seanmundy at hotmail.com; detomaso at poca.com
>>>>>
>>>>> Sean,
>>>>> The vacuum is too low at that rpm and you are lean surging because the
>>>>> carb does not have a strong enough vacuum signal to work.
>>>>> You are having reversion because the vacuum/exhaust flow combined with
>>>>> that cam does not create enough vacuum and flow to suck the air through
>>>>> and past the exhaust valve. On our motocross bikes of the 70's, we used
>>>>> reed valves to contain the wild overlaps of intake and exhaust.
>>>>> A longer distance from the intake valve to the throttle plate will
>>>>> help, so the plate could help but that is just a cover up.
>>>>> A vacuum to ignition timing based on rpm graph would show any
>>>>> deficiencies in the distributor or carb function through the entire
>>>>> range.
>>>>>
>>>>> Increasing the primary jets will allow more fuel to flow if the air
>>>>> bleed is strong enough to help it flow.
>>>>> Does this carb have interchangeable primary venturis or have I just
>>>>> seen too many Webers? If so, go smaller.
>>>>> Advancing the ignition timing at that speed/load could cure your
>>>>> problem.
>>>>>
>>>>> Hope this helps.
>>>>>
>>>>> Jeff Cobb
>>>>> Cell 225-907-4514
>>>>>
>>>>> Jeff Cobb Auto Works
>>>>> 1316 South Acadian Thruway
>>>>> Baton Rouge
>>>>> Louisiana, 70806
>>>>> www.liveoakconcours.org
>>>>> Work  225-343-7525
>>>>> Fax     225-343-7550
>>>>> jeffcobb1 at me.com
>>>>> ___________________________________________
>>>>> On May 1, 2013, at 5:08 PM, sean mundy wrote:
>>>>>
>>>>> 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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>>>>>
>>>>>
>>>>>
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