My 360 has a lot of initial because it has the MP 292-508 cam. It didn't like starting hot, so I put an ignition kill switch in it. I get it spinning, then flip the switch on.
RPM drop and electric fans!
- Thread starter FDNY340
- Start date
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Crappy weather here. Tomorrow gonna be better so I'm gonna play around bit.
Cool. Keep us posted. Was kinda nasty here today and supposed to be a wash out Tuesday.
SRT_DSTRHOLC
"There is never enough horsepower...."
Fix the charging and fan draw, then the timing, then the carb
I have a retard too, but it sits behind the wheel.
FD,
That is a very sizeable cam. And it is in a 340, not a 440. Less cubes to soak up the extra duration/overlap. Post #61 is a 5 min test that will TELL you the optimum idle timing. It is worth doing just to give you a number. It may require more idle timing [ say, 44* ] than it does for WOT. You may have to compromise & settle for, say, 35*. Locked timing would help. You would need a high hp, 3.0 +, starter for consistent starting. [ the factory starter is about 1hp ] Or instal a sw as per post #76. You might be able to find a programmable ign system that handles all of this.
Now that we have the cam specs, the problem is the cam [ size ]. Cams are power shifters. More cam takes power from the low end....& transfers it to the top end. That is the cause of your problem. Not making enough HP at idle speed to take the alt load.
The 40 amps of fan load loads up the alt, which loads the engine. The engine hp at idle is reduced because of the cam, engine struggles with the load...& drops 400 rpm. Nothing to with the alt per se, wiring, etc. A different alt may reduce the rpm drop slightly but I doubt it would be anything like recovering 400 rpm. Increasing the idle timing will help. because it makes more hp ] , but I doubt it would recover the full 400 rpm.
If there is room, I would instal a clutch fan.
That is a very sizeable cam. And it is in a 340, not a 440. Less cubes to soak up the extra duration/overlap. Post #61 is a 5 min test that will TELL you the optimum idle timing. It is worth doing just to give you a number. It may require more idle timing [ say, 44* ] than it does for WOT. You may have to compromise & settle for, say, 35*. Locked timing would help. You would need a high hp, 3.0 +, starter for consistent starting. [ the factory starter is about 1hp ] Or instal a sw as per post #76. You might be able to find a programmable ign system that handles all of this.
Now that we have the cam specs, the problem is the cam [ size ]. Cams are power shifters. More cam takes power from the low end....& transfers it to the top end. That is the cause of your problem. Not making enough HP at idle speed to take the alt load.
The 40 amps of fan load loads up the alt, which loads the engine. The engine hp at idle is reduced because of the cam, engine struggles with the load...& drops 400 rpm. Nothing to with the alt per se, wiring, etc. A different alt may reduce the rpm drop slightly but I doubt it would be anything like recovering 400 rpm. Increasing the idle timing will help. because it makes more hp ] , but I doubt it would recover the full 400 rpm.
If there is room, I would instal a clutch fan.
Jeremiah (jd)
Well-Known Member
A few things need to be addressed:
1) You need a higher output alternator. Most alternators are tested at room temp. A typical unit loses 20% efficiency at operating/underhood temps. 70A nominal output isn't nearly enough. With two electric fans, MSD ignitiin, headlights, fuel pump etc. I was melting the varnish off of the windings on a 95A Powermaster. Do ensure that yoour operating RPM is within spec (i.e. <18k RPM). Powermaster sells a larger pulley for higher rpm apllications. A 5.5" crank pulley will also help keep the RPM down.
2) #6 power cable is undersized. I've attached a chart to help select the cable length. If your car has a floating ground system make sure to use adequately sized cable for the entire length of the power run.
1) You need a higher output alternator. Most alternators are tested at room temp. A typical unit loses 20% efficiency at operating/underhood temps. 70A nominal output isn't nearly enough. With two electric fans, MSD ignitiin, headlights, fuel pump etc. I was melting the varnish off of the windings on a 95A Powermaster. Do ensure that yoour operating RPM is within spec (i.e. <18k RPM). Powermaster sells a larger pulley for higher rpm apllications. A 5.5" crank pulley will also help keep the RPM down.
2) #6 power cable is undersized. I've attached a chart to help select the cable length. If your car has a floating ground system make sure to use adequately sized cable for the entire length of the power run.
Did you install electric fans because stock clutch fan / shroud would not cool it ?
I ran a similar cam in my 73 340 Dart Sport and had no cooling issues using stock clutch 7 blade fan/ shroud . Even in 100* + weather. And both my current cars use factory setups with aluminum 2 core 1” radiators .
68 Barracuda 408 221/229 cam
69 340 4sp Swinger 4” crank , W2s , solid roller 246/250 108 .648 lift .
just a thought …
I ran a similar cam in my 73 340 Dart Sport and had no cooling issues using stock clutch 7 blade fan/ shroud . Even in 100* + weather. And both my current cars use factory setups with aluminum 2 core 1” radiators .
68 Barracuda 408 221/229 cam
69 340 4sp Swinger 4” crank , W2s , solid roller 246/250 108 .648 lift .
just a thought …
Greetings!
Got out today and played a bit. Timing at where the idle was although all over the place hoovered around 14degrees and seemingly into the curve already. Pulled dist cap and rotor and there were 2 light silver springs. I swapped one out to the heavy spring and that stabilized the initial timing. I couldn't do the test you mentioned in the prior post as I was alone, but I gave it some advance to 21 degrees initial and that helped tremendously with the idle and fan combo. As soon as I have an extra set of hands will continue with your method.
So... a new question would be now is there anyone that makes bushings for the MSD to accomodate smaller advance curves? MSD's smallest is 18 degrees in the kit. Motor was dyno'ed at 35 degrees so where ever I land with initial, I will have to adjust range to work.
Thank you!!
Yes. You can search ebay for the larger bushings. I think the .394 is 14 degree and the .405 is 10 degree. Although some distributors require filing the slot a little larger to accommodate the larger bushing. I did mine with the .394 anyway, just to assure there was no binding whatsoever. The .405 bushing comes with flats ground on the OD, but that's still not enough in some cases and you have to file the slot some.
Thanks RRR
FD.
Below are some examples of what increasing idle timing did. Many used vac adv connected to manifold vac [ MVA ] to get the extra advance at idle. I doubt MVA would work with your combo because the vac is likely to low to pull in the plunger on the VA unit. The engine will not care where/how the extra timing comes from....as long as it gets it. Locked timing is one method.
Below are some examples of what increasing idle timing did. Many used vac adv connected to manifold vac [ MVA ] to get the extra advance at idle. I doubt MVA would work with your combo because the vac is likely to low to pull in the plunger on the VA unit. The engine will not care where/how the extra timing comes from....as long as it gets it. Locked timing is one method.
Vacuum is one of the issues Im trying to address. No ports on manifold, spacer or Carb.
Was going to drill and tap my spacer. Can you suggest another method??
And thank you for reinforcing the "why" behind the "way"!!
What on EARTH intake doesn't have vacuum ports? Not even one?
Ahhhhhh disregard that. Got confused with the BBC with the Victor manifold in the boat!!!
Stupid Chevy stuff. Just had that on the dyno last week.
I would find what the minimum timing it will tolerate at idle first by just by moving the distributor. Make sure the distributor is not adding timing at idle speed through mechanical advance. You could easily add vacuum to the adapter or even the base plate of the carb. Closer to the throttle plates the stronger the signal. Holley? What model?
Here is a problem you can run into with vac. advance and your camshaft. Because your vacuum is low at idle the canister may not fully apply advance or if it does it might be tetering on the edge. This can lead to the "circle of death" as described in an earlier post. An engine with "no power at idle" is unstable and if loaded will drop in rpm. That will cause vacuum to drop. Then the vacuum advance retards timing. That causes rpm to drop and so on.
You could match the vacuum canister spring rate or preload to the low vacuum signal at idle to solve that problem. That could cause another problem. When the rpms raise above idle the vacuum will also increase. Vacuum for motor with a stock cam at idle will raise slightly when the rpms are raised. Vacuum at idle with a cam like yours will raise significantly with rpm. With the softer spring in the vacuum can you might end up with a delayed drop in timing when the engine is loaded to the point of detonation. Sort of a catch 22. Sometimes limiting the amount of timing that the vacuum advance adds can fix this but sometimes not. Beware and watch for signs of detonation or ping when tuning. Sometimes a vac advance is a good fit and sometimes it is not.
Yeah I was thinkin single plane of SOME kind.
Oh and for what it's worth....I run the vacuum can on my truck on MVA. That engine has a pretty lumpy old school Crane cam in it. It only has between 6-8 HG at idle, but it still manages to pull in all what the vacuum can has.
Somebody Mention a Boat?
Balanced with Forged pistons, special Marine cam, head milled .080ths with mild port job.
350 chevy sized prop on the OMC Outdrive to capture the added power.
Ran out 7,000 rpm all day long, smooth. 47 mph (on the water) at full trim.
Did not have to touch the factory ignition or factory carburetor, was the right camshaft for the right application.
☆☆☆☆☆
Balanced with Forged pistons, special Marine cam, head milled .080ths with mild port job.
350 chevy sized prop on the OMC Outdrive to capture the added power.
Ran out 7,000 rpm all day long, smooth. 47 mph (on the water) at full trim.
Did not have to touch the factory ignition or factory carburetor, was the right camshaft for the right application.
☆☆☆☆☆
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