Squirter/pump cam tuning?

[GTO Geoff]

Wow,
The ignorance on this site about MVA benefits is breath taking. Not sure when GM started using MVA, but it was used in the early 60s. GM was forced to switch to the useless PVA around 1968 because of stricter emissions, having used MVA prior to this. MVA to PVA, not the other way round.

MVA remained in use until the end of the carb era, used by Ferd, GM, AMC & Chrysler.

This is Chryslers version during the 1970s:

'When engine coolant temp at idle reaches 225F, the valve opens & APPLIES MANIFPLD VACUUM DIRECTLY TO THE DIST....This increases engine idle speed & provides additional engine cooling.'

Ford's version:
' If engine overheats at idle, increased vacuum will flow to the dist to INCREASE ENGINE SPEED. '

AMC:
' The coolant override switch incorporates a thermal switch which reacts to coolant temps to route either INTAKE MANIFOLD VACUUM or ported vacuum to the dist.'

GM:
' the coolant temp switch overrides the system & provides FULL MANIFOLD VACUUM to the advance unit, ADVANCING TIMING in turn lowering the operating temp.'

Anybody with half a brain should be able to see that if you change the ign timing at idle [ in this case advancing the timing via MVA ] & with no other changes, idle rpm increases, the engine is making more hp from the additional timing & it is a beneficial move.
The other obvious result is that because the engine does not have to work as hard [ it runs cooler with less load ], the A/F has burned more efficiently.

[HarrysTaxi2]

Wow,
The ignorance of one on this site about MVA benefits is breath taking.

other than quoting what others print, you show that you don't have much actual real world experience.
have you ever driven anything other than a 60's Pontiac at above idle speed?

[Right hand drive]

A quick and easy win for OEM in the early days of pollution control was reduced initial ignition timing as low as 4 or even 2*. This was to complete the combustion process in the exhaust and from memory lower HC and NOx levels, NOx being the smog producing gas. Ported vacuum held ignition timing low until a determined throttle input where it would start adding vacuum adv. This when and amount of applied vac advance was carefully controlled (timed) as NOx has the potential to increase at part throttle light load lean A/F conditions. This state of low timing and late burn put a lot of heat into the cylinder head that poorly transferred that combustion heat to the coolant. Carburetor settings accommodated for this low ignition timing. When MVA was applied all it was doing was putting the engine in a state more conducive to efficiency - what it wanted. Two things happened, more efficient and earlier combustion from earlier timing meant more heat could be transferred a lot easier to the coolant via the cylinder walls therefore cooling the engine. Second, idle rpm went up as peak combustion pressure would have been closer to the ideal 14* atdc imparting energy on the piston at the right time for efficiency. If adherence to pollution regulations aren’t high on tuning priority list then unplug MVA and turn distributor to the advance MVA was giving when operating and limit mechanical advance for best total. Remove complexity. KISS. If this is enough advance to be tough on the starter then employ just enough MVA to reduce initial for easy starts (or get a high torque starter or start retard).

So back then the reference point was very low and far from ideal initial timing in crap engines, MVA was made to look like a hero of sorts. All the Chrysler, AMC, Ford etc examples above are a moot point when understanding why MVA was used in those instances and then extrapolating that for use in high output high compression non pollution controlled engines. It’s unlikely it to create the same levels of magic if the starting point is not the same as those engines of the 70’s. MVA was used on pre pollution era low tech engines where timing required for a good idle was higher than total mechanical timing under rpm load.

If an engine tune has initial ignition timing set that gives a stable idle, maintains operating temp and good tip-in and transition (usually higher than std comp) then I wouldn’t be adding MVA whether it’s considered adding complexity or not. MVA might only add a smidge of hp at idle if everything is already well tuned so you have to ask yourself “how fast do I need to go at idle?”. When a good idle can be set with the distributor without the need of MVA then use ported to increase efficiency at cruising speeds in the range where it’s of benefit.


Over the years I have found this to be an interesting and polarizing topic on the net and forums as there are not many other topics that evoke such passion from each side. Well, maybe climate change.

[GTO Geoff]

Harry,
I have lots of real world experience, & probably since before you were born....
If YOU had any real world experience, you would know what a great tuning tool MVA is......

Quoting famous & respected people in this hobby is a way of bolstering/supporting one's point of view. Versus quoting somebody who is unknown. Apparently this has not occurred to you....

[Right hand drive]

justinp61,
When you say lean spike on the LM-2 what AFR do you have pre lean spike and what does it spike to?

[HarrysTaxi2]

[GTO Geoff]

Harry,
Just keep talking....& continue showing how dumb you are.

[GTO Geoff]

Here Ya go, stupid:

[1] '...with manifold vacuum connected there is 18in Hg because of the mild Phase 1 cam. This will provide an additional 16* to smooth out the idle & allow a leaner idle mixture.......fuel economy could improve by 10% with manifold vacuum connected. With one of my cars this simple modification lifted city economy with the air on to within 6% of what was being achieved n the highway'.

[2] '....The amount of degrees...reqd to obtain maximum smoothness & the highest idle rpm possible is the number of degrees you are looking for.'

[3 '...the final result should see the engine idling with optimum spark advance so that it idles as smoothly as possible.

[4] 'If the engine rpm increases as as advance is increased, there is not enough advance built into the engine'

[5] Modified engines.....need a considerable amount of initial advance.]

[HarrysTaxi2]

[HarrysTaxi2]