View Full Version : '77 Corvette Ignition Application

08-17-2003, 07:22 PM
Dear MSD:

I have a '77 corvette with a 350, 4-speed. It has Nitrous (150 hp) and a blow-through supercharger (about 5-8 psi boost) carbureted. I want the benefit of the boost controllers you offer, but I don't quite understand the functionality of your ignition systems. If I use the stock HEI distributor, it has the built in coil. When I look at your instructions, it appears that you still use the same stock coil as provided in the distributor. If so, what does the ignition system do for performance...or maybe a better question, what does the ignition system do...where and how will I see the gain? Should I be using a different distributor and separate coil?

I came across one of the 6btm models (which I can use for the boost control), but what else do I need and what benefit will I see?

Thanks in advance for the help!

08-17-2003, 09:55 PM
The stock HEI system provides low voltage to the primary side (+) of your coil. As rpm increases the length of time that the coil has to fill up (saturate) with the low voltage decreases and therefore at higher rpm your coils output is decreased dramatically. The stock system only produces a single spark and at low speed where fuel/air atomization is poor this can cause a miss-fire.
The MSD 6BTM system is a capacitive discharge system that provides a high voltage to the primary side (+) of the coil. Whether you are at idle or 10,000 rpm your coil receives the same amount of high voltage resulting in a lot more voltage and current out of your coil. In addition, the MSD produces multiple sparks below 3000 rpm that prevents miss-fires and produces better throttle responce, a smoother idle and quicker starts.
You can use the same coil that you are currently using, because we are basically using the coil as a step up transformer with the MSD system. For a little extra benefit, you could use the MSD HEI Coil, PN 8225.

MSD Tech1

08-18-2003, 09:57 AM
Thanks for the excellent and quick response! Your explanation was very helpfull. If I read your response correctly, I can assume that the stock HEI distributor is okay for higher RPMs...meaning that the triggering device is not restrictive, it's more so a problem of inadequate coil saturation due to the higher RPM. Since the MSD capacative ignition device can send a high voltage all of the time, the coil is properly saturated before firing, thus allowing a full spark. In other words, no need to go to a different distributor...maybe best case is to have a bigger coil to provide a really good spark, but it sounds like not necessary?

Also, is there a way to dynamically vary the RPM limiter? I assume that the "plug in" devices are simply resistors? Can a potentiometer be used here similar to the boost controller?

08-18-2003, 11:02 AM
Your explanation is exactly right. As long as the distributor is in good working order both mechanically and electrically then it should work fine for you. As for dynamically varying the RPM limiter, you can do that, but you will need to know what rpm ranges you want to cover because the rpm resistor values go from 1897.3 ohms for a 3000 rpm module all the way to 18844.5 ohms for a 13900 rpm module. MSD does offer RPM Module selectors, that plug right into the rpm socket on the MSD that allows you to select between twelve (12) different rpm limits by simply turning a knob. Six models are available to cover a range from 3000 to 12,800 rpm in 200 rpm increments. Here is a list of what we offer.

RPM Module Selector Part No.
3000-5200 PN 8670
4600-6800 PN 8671
6000-8200 PN 8672
7600-9800 PN 8673
9000-11,200 PN 8674
10,600-12,800 PN 8675

MSD Tech1

08-18-2003, 11:55 AM
You guys are AWESOME!!! Other companies could take lessons from your excellent tech support and customer service.

Thanks for all of your help!

08-18-2003, 09:47 PM
Your very welcome!!

MSD Tech1

08-23-2003, 01:07 AM
Okay, another question...

With the nitrous, I'm getting "ping" when the nitrous kicks in. Since I have the 6BTM, is there anything I can plug into it to allow for a "relay or switch driven" ignition retard in addition to the already availabe boost retard...in otherwords, two switches...one controlled by the turning on of the nitrous, and the other controlled by the boost. I really don't want to have to replace my existing 6BTM.

Thanks in advance!

08-23-2003, 12:18 PM
Here is a way that you could do it so that when you hit the nitrous button you will get additional retard through the boost retard function of the BTM. The control knob of the MSD 6BTM is basically an adjustable resistor. The higher the resistance the higher the amount of retard that the more sees. If you use a relay in line with the control knob you could add an additional amount of resistance in the control knob feed to the 6BTM when the nitrous solenoid is activated. You will have to experiment a little to figure out how much retard you need to stop the detonation, but this should work quite well for you. I cannot post a diagram right now, but I will try to get one for you on Monday.

MSD Tech1

08-23-2003, 07:38 PM
But isn't that only the case when there is enough boost to cause the BTM to activate the retard function? When I'm not yet in boost, but into the nitrous, the BTM is not yet activated, so there is a pressure lag until the RPM level is at a point of building pressure with the blower. Therefore, the pressure sensor in the BTM will not yet activate.

Is there a way to get an immediate retard via a relay without having to wait for boost? ...or am I using the wrong model?

Thanks for the help.

08-24-2003, 10:28 PM
Yes, it would only add retard when boost is present. Usually, nitrous is applied only at wide open throttle position and with a supercharger you most likely are seeing boost already. The only other way to add the retard during nitrous would be to add a retard control like the MSD PN 8975 Digital Multi-Retard or the PN 8982, Retard Control.

MSD Tech1

09-01-2003, 01:26 AM
Normally, yes. Since this is a centrifugal blower, it doesn't build boost right away...that is why I use the nitrous to get the RPM's up so the blower can start making boost. While it is true, this is only for a short duration before the boost kicks in, there is, however, a time when I'm getting "ping" from the nitrous with no ignition retard. When the blower finally builds boost, the 6btm retards the timing enough to stop the ping. Thanks for the advice.

09-01-2003, 03:57 PM
I understand your dilemma now. The add-on unit may be your best solution.

MSD Tech1