[Diy_efi] TPI cam limit.

[Joe Boucher]

I'm going to roll all the replies into one here.

I left out the comment about the 205 degree intake lob was for a TBI system.

I have thought of the restriction idea also, plus I have wondered if using a
small reservoir as a capacitor downstream of the restriction might help.
The problem with these ideas is the effect of the changes on off idle
response.  Also, how about a mechanical switch.  When the throttle is closed
at the idle position, the switch through a relay and a false manifold signal
is sent to the ECM?  Raising the idle, messing with the spark advance and
enriching the mixture are ideas I hadn't thought of.

I wasn't planning on going too wild (215, 220) as I understand the Chevy TPI
doesn't like to run much over 5000 to 5500 RPM with the stock runners.

I have ordered a book on how to make canoes out of carbon fiber for the
carbon fiber manufacturing techniques.  Maybe a custom manifold later on?

Thanks,

Joe B

-----Original Message-----
From: diy_efi-bounces at diy-efi.org [mailto:diy_efi-bounces at diy-efi.org]On
Behalf Of John Gross
Sent: Saturday, November 26, 2005 10:08 PM
To: diy_efi at diy-efi.org
Subject: RE: [Diy_efi] TPI cam limit.


One major reason a lot of people have problems with larger cams on EFI
engines is because people aren't willing to raise the idle speed of the
engine.  You can't have an engine with a stock 600 rpm idle and a cam with
lots of duration and a narrow LSA.  On a 383 LT1, with 224/230 duration and
a 113 LSA, I had a hard time getting the idle under 900 rpm.  At 900 rpm,
however, the engine would idle smoothly, not jump around or hunt, both at
temp and cold.  BTW, that was a MAP car.  On the engines we build a work, I
can't tell you the cam numbers, but there is definitely non-stock duration
on the cams, and the engines absolutely will not idle below 1300 rpm.  They
find a comfortable idle at 1600-1700 rpm, however.

Long story short, placing a "cam limit" on an particular engine when talking
about idle quality is really only valid if you're talking about trying to
retain stock idle RPM.  If you're willing to raise the idle speed to help
retain the idle quality, you'll quickly run out of manifold at speed on a
TPI before you have idle problems.



>From: "Becker, Damon (Damon)" <damonb at avaya.com>
>Reply-To: diy_efi at diy-efi.org
>To: <diy_efi at diy-efi.org>
>Subject: RE: [Diy_efi] TPI cam limit.
>Date: Sat, 26 Nov 2005 20:41:52 -0700
>
>Filtration can help, along with vacuum manifolds (pneumatic receivers)
>and such.  You can also "tune around" this type of thing by allowing for
>a flat spot in the idle fuel tables.  If none of this works, then you
>can go with a hybrid controller (hybrid speed-density and alpha-N).
>Camming an engine can frequently require the transition to an EMS to
>correctly control the engine, depending on how your original ECU was
>setup.
>
>If you are in the Denver area and need help with this, I am more than
>happy to help you out.
>
>   _____
>
>From: diy_efi-bounces at diy-efi.org [mailto:diy_efi-bounces at diy-efi.org]
>On Behalf Of Joe Boucher
>Sent: Saturday, November 26, 2005 7:10 PM
>To: diy_efi at diy-efi.org
>Subject: [Diy_efi] TPI cam limit.
>
>
>I saw a reference just a few days ago claiming the practical cam input
>lobe timing limit at .050' lift is 205 degrees.  Primarily the vacuum
>signal jumps around too much.
>
>Is there a similar limit on the MAP driven TPI systems?
>
>Thanks,
>
>Joe B


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