big RPM drop

[N2RODDIN]

I have a 383 blueprint engine (420 hp/450 tq} it has a TH350 trans with a 2600 - 2700 stall convertor , pertronix vacuum advance dist. the timing is where blueprint suggests (32 to 34 deg. total @ 3500 rpm ) I have to idle it at about 1350 rpm , so when I put it in gear it drops to about 900 rpm (it drops an additional 200 rpm when the dual electric fans come on . What can I do to lessen this rpm drop ?

[JT56]

What is the initial timing at?

[55 Rescue Dog]

One thought is maybe a solenoid operated idle compensator like they used in the 80's when the AC came on, that could be adapted to come on when the fans turn on to kick up the idle. Are you running ported, or non-ported vac advance? A vacuum gauge gage is a great thing to use also to diagnose what is going on too. That motor should be happy idling at 700 rpm I would think. Running full vacuum advance at idle can help a lot. If you have good vacuum at idle helps a lot too. 17+ inches is good.

[Rick_L]

Look for a vacuum leak first.

[markm]

I really wonder why such a mild motor idles at 1350 to start with, my SBC drag car idles about that with a .625 roller cam. Something else is going on, like as stated a vacuume leak. Not feeling the need for vac advance, I have some strong running street cars without it.

[N2RODDIN]

at 1st I thought I had a vacuum leak ( the rpm was not steady at all) changed to a new edelbrock 750... now steady as a rock . The engine gets about 11 inches of vacuum (needle on gage is very steady) I switched from manifold to ported vacuum and it didn't like that at all "banging out" the engine runs great , instant throttle response ,great power , just a big rpm drop. The distributor is the only thing not new ..... maybe a mechanical advance distributor ? A shop in town said they didn't think it had a vacuum leak .

[JT56]

Tell us more about the dist? I run my dist locked out at 35 degrees and my car idles 980 rpm in park and around 750 rpm in gear with two fans running and electric pump. Since its a low compression motor, have not issues with starting when hot.

[55 Rescue Dog]

Vacuum advance has been used since 1935 for a reason. The only time it is not needed is at wide open, when there is no vacuum.

[chevynut]

Vacuum advance has been used since 1935 for a reason.

And what is that reason?


The only time it is not needed is at wide open, when there is no vacuum.

Not true...it's not needed with a modern ignition system.

[Maddog]

And what is that reason?



Not true...it's not needed with a modern ignition system.



LOL, N2RODDIN ignore cnutroill he doesn't know shit about building an engine. He likes to copy stuff from the internet then post it like he knows something. Short man complex.

[Rick_L]

Chevynut, you are WAY wrong on vacuum advance. Advancing the timing when the throttle is closed and manifold air pressure is high adds efficiency and driveability under those conditions.

If you are referring to computer control of ignition timing, the same principles are applied. The timing is advanced depending on MAP input.

The only reason not to have some form of vacuum advance is if you don't intend on using the engine at part throttle.

[Maddog]

Chevynut, you are WAY wrong on vacuum advance. Advancing the timing when the throttle is closed and manifold air pressure is high adds efficiency and driveability under those conditions.

If you are referring to computer control of ignition timing, the same principles are applied. The timing is advanced depending on MAP input.
The only reason not to have some form of vacuum advance is if you don't intend on using the engine at part throttle.


"Not true...it's not needed with a modern ignition system."
Nearly fell out of my chair with that ignorant answer:D ROTFLMAO:D

RICKL, don't bother he knows every thing, just ask him.

[Maddog]

Vacuum advance is what's between Cnutrolls ears, :Dstil LMAO:D

[chevynut]

If you are referring to computer control of ignition timing, the same principles are applied. The timing is advanced depending on MAP input.

It doesn't have a vacuum advance mechanism on the distributor was my point. There's nothing to hook up to vacuum because it's all done with the computer at the same time it uses the MAP signal to fire the injectors. I know timing varies with load. I guess I should have been more explicit in my response.

[chasracer]

Well - a 1350 RPM idle is a bit off the wall for a relatively mild small block. We have full-on racing engines that idle lower than that figure. First off, I would try spraying some carb cleaner around the intake and carb gaskets to see if any tiny leak is detected. Be careful with the carb cleaner as it is volatile. You can try something like WD-40 too but unless you actually spot it being sucked in, you might still miss the leak. Eleven inches of vacuum on the gauge is not great so there is something going on. As to the 200 RPM drop when the fans come on - is this an alternator or generator system? A generator I can understand. If it's alternator, then either the alternator is maxing out on amperage or there is a systemic problem in the wiring or even the fans themselves.