getting started blown tbi GMC inliner, need advice

[huffin53]

Hello, new guy here,Just beginning by build with the addition of a supercharger to my old '53 chevy truck with a GMC 270 inline 6 cylinder. I'm tryingto get some general info regarding what direction to take. These motors have shared intake ports(only 3 ports for 6 cylinders) and will be a low boost(5 to 8psi), so nothing wild. Due to this I think only a TBI system is necesarry. What I'm thinking is to have the 3 intake runners going into a shared plenum and a single butterfly throttle body. Intake will be fabricated. Behind the butterfly(plenum side) I will have the injectors mounted(maybe 45 degrees to flow into the flow to help atomization). The supercharger will feed into the butterfly of course. Basically envision a chevy tpi sytem but instead of port injection it has some injectors injecting into the intake plenum, right behind the throttle blades. My first question involves the injectors...When running as a tbi, what is teh optimum number of injectors to run...2 or 3 or 4 or? Would MS fire all equally or just one or 2 at idle, then more as flow increases or???
Next is ignition, is there a large benefit from running my spark through MS in this fairly low tech injection system? Now it has the stock dizzy with a pertronics unit for spark. I have a MSD 6AL box and MSD boost retard box as well if needed. It would be great to have full control of my spark via my laptop, but I'm not sure I need it and would add to the complexity and cost of the system...how much I'm not sure...Any help would be greatly appreciated. hans

[Matt Cramer]

Is this throttle body going to be upstream of the blower, or downstream of it? If it's downstream, I suspect you'll have fuel distribution issues.

[huffin53]

yeah, I have thought about that and was wondering if it would be better to mount the throttle body before the blower. This would solve any issues with strange vorticies coming off the throttle blade at part throttle. As well I was thinking of having 4 injectors mounted radially on the inlet tube to the plenum 45 degrees aparts from eachother( at 3,6,9 and 12 o'clock). As well this way, I know lots of people are runnign 4 injectors so should be no issue to get up and running. What do think of using MS just for fuel and running th eexisting ignition with the MSD boost controller? It might be just as well to have it control both fuel and spark as I have a customer that wants me to build him a system for a jaguar XJR six cylinder iwth supercharger...so would be basically the same system other than running EDIS on the JAg. Can MS run off the pertronix unit installed in my >GMC distributor now and use it just for signal with no advance mech or vacuum and have the MS control all other aspects of the spark? which unit would be best suited for this?

[fury fan]

Other folks will have to help with your other questions, they are beyond my expertise (as if I had any! ).

If mounting 4 injectors as you have described, I would stagger them across the tube so they are not pointing at each other (crossfire -- picture riflemen on both sides of the street shooting at a car passing between them).

If the atomized droplets collide, they’ll make bigger droplets that may fall out of suspension.

Why not use a common GM 2-injector TBI unit with fuel pressure raised-as-needed to meet your hp goal?

[huffin53]

Not a bad idea on the injectors. On the GM tbi, that was my original thinking but the TBI unit itseld is large and very unattractive if you are trying to put it inline with the 3 inch or so tubing coming form the supercharger. The supercharger will be mounted on the passenger side of motor feeding the inlet ports on the driver side. This length of tubing will give some slight cooling to intake charge. As well this wil be going in my 53 truck which is built quite oldschool in camparision with running injection. I really don't want a large flaged TBI in the midst of things. the single butterfly is quite small almost no larger than the tubing feeding it.
I think after consideration of having MS project #2 almost in parallel that will use EDIS for spark, I have decided to do the same with my GMC motor. A bit extra work, but it will have more tuning flexibility and should have better spark as well...can you still use an MSD 6AL box on the EDIS?
My thinking on the intake is to have the 3 intake runners fabbed like the stacks of a old mechanical injection setup like Hilborn with optomized length for my head for some ram effect. The ends of the rams tubes will be enclosed in a plenum box wich will be fed by the inlet tubing coming from the supercharger. Injectors staggered slightly before the inlet to the plenum. throtlte body upstream of supercharger....well making progress on the designand what i want to run with MS now! I definetly want an IAC motor for idle control so I guess this leaves me with only the MSII as my choice of MS for both units.

[Matt Cramer]

I think after consideration of having MS project #2 almost in parallel that will use EDIS for spark, I have decided to do the same with my GMC motor. A bit extra work, but it will have more tuning flexibility and should have better spark as well...can you still use an MSD 6AL box on the EDIS?

Only if you want to run three of them - although EDIS on its own is pretty close to a single MSD 6A when it comes to coil energy.

My thinking on the intake is to have the 3 intake runners fabbed like the stacks of a old mechanical injection setup like Hilborn with optomized length for my head for some ram effect. The ends of the rams tubes will be enclosed in a plenum box wich will be fed by the inlet tubing coming from the supercharger. Injectors staggered slightly before the inlet to the plenum. throtlte body upstream of supercharger....well making progress on the designand what i want to run with MS now! I definetly want an IAC motor for idle control so I guess this leaves me with only the MSII as my choice of MS for both units.

Interesting. If the tubes are long enough, you could probably get away with three (or six) injectors at the far end of the plenum, firing straight down the tubes.

[huffin53]

don't need to run three...I'm sure the EDIS alone will do the job. Ithe intake runners will be on the order fo 14 inches or so from the intake valve head to the end of the velocity stack. I was originally thinking of runnign injectors in the tubes but wasn't sure about a consistent mixture. I guess since these will be batch fired it will stil lbe acting like a tbi in a sense but with out the whoel plenum being wet. I would think one injector in each tube would be sufficient, there are plenty of lnjectors that can flow enough fuel and lots of people running huge injectors and still able to idle quite nicely. in this case if the injector is near the end of the ram tube, I would want to position teh injector conventionally shooting fuel in the same direction of airflow. With three 75# injectors that should give enough flow for 300 hp which I think is wishfull thinking, but at least I would have room to work up ...

[fury fan]

Depending on how you make your plenum vs the entry from the supercharger vs tubes going to the engine: if using a TBI setup try to avoid 90° angles that the wetflow must navigate.

Here's a great tech article on ram manifolds.
http://www.chrysler300club.com/uniq/all ... theory.htm

[huffin53]

yup, I've built a hadnfull of intakes before, you also have to watch out for large changes in velocity of your air if it has fuel in it, as in if I were to inject fuel into the inlet tube far ahead of the plenum where the charge reaches the large volume of plenum it slows down very quickly and you can encounter your fuel faling out of suspension form your air and resulting in puddling in your plenum. THis is why I was thinking of injecting right at the plenum....but I think I can inject right into the ram tubes since it will have enough length to get a nice mixture even with uneven pulses from the two cylinders pulling on one ram tube. This will work even better I belive NOT trying to inject for each cylinder pulse, but just as volume per revolution, like a tbi does.