[Diy_efi] TPI cam limit.

Mon Nov 28 00:56:51 UTC 2005

Good comment John. The old rule of thumb is 90000/RPM gives 
length in inches.
Not hard and fast, as cross-sectional changes in dia and shape, 
i.e. a bell mouth, will
vary results. Lines up with your findings though.
As you rev past this point, the inrush of air tends to bounce of 
the closed inlet valve
or rising piston if the valve is still open, upsetting the flow 
in the ram tube or runners.

Joe, the filter idea only averages out the pulses. As revs come 
up, it will have no effect.

Geoff H

From: "John Gross" Sent: Monday, November 28, 2005 3:59 AM
Subject: RE: [Diy_efi] TPI cam limit.

In terms of how far you can push the stock manifold before 
running out of
air, let me just say this:  My '87 vette with 128 casting heads, 
after
adding 1-5/8" primary headers and porting the heads (intake flow 
was ~240 @
0.500", exh was ~170 @ 0.500"), the engine made peak power at 
3800 rpm, and
peak torque at 2800 rpm with the stock cam.  I never did recam 
the motor,
but a large portion of the lack of high rpm performance is due 
to the 22"
long intake tract from plenum to intake valve, as well as the 
small
cross-section of the runners.
-----Original Message-----
From: diy_efi-bounces at diy-efi.org 
[mailto:diy_efi-bounces at diy-efi.org] On
Behalf Of Joe Boucher
Sent: Sunday, November 27, 2005 8:50 AM
To: diy_efi at diy-efi.org
Subject: RE: [Diy_efi] TPI cam limit.

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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