So having been convinced not to change my 502 for a LS based motor I'm looking into running it with injection.

The Holley HP EFi setup looks like a great option, but I'm starting with a carb motor, so it's not just gonna be plug and play! I'm an engineer with a full CNC workshop at my disposal so I'm not scared to make a few bits up!

At the moment it's running a Holley 250 blower and I see no reason to change this for anything else. I know they do a 4 barrel throttle body which replaces the carb directly, but I don't see a need to go down that road when one larger (say 1000cfm) throttle body would do the job with less to go wrong?
I'm aware that these motors sometimes suffer from the rear cylinders running a bit lean with a single carb on the blower, so this is another argument against having single point injectors up on top. That strikes me as a solution for people who just want to quickly and easily swap their carb out.

I'm lucky that my intake manifold has spots ready for Injector bungs so I can run with 'proper' multi point injection.

So my next plan would be to machine a billet plenum to bolt down onto the top of the blower in place of the carb & converter plate, with just one large throttle body off the front, or possibly two medium sized ones if they're easier to source/fit in the given space.

Is there anything I've overlooked? I'm happy playing with engines (it's all just engineering after all!), but I'm no mechanic.
What else will the Holley HP system require?
Does it need a TPS and MAP sensor?
How will it link to my existing MSD ignition for timing?

I know I'm probably going about this in an awkward way, but I've always preferred to do things myself than just buy bolt on bits, especially considering the money that can be saved for other uses! Hopefully it should look quite neat & modern with the plenum on top rather than just a carb or carb style TB.

Any suggestions & advice welcome!

Thanks, James

with what you are getting ready to dive into from what I have been told(looking into doing this as well) none of the bolt on efi deals going to work without much custom tuning...that being said if you continue look into megasquirt, now this not a novice deal, but will work with work with some tuning. we are running it on my LS powered turbo drag radial race car...also looking into doing a EFI deal on my procharged HP500...alex hasby on here is very good on this and can help...he may have a different opinion as well...smitty(artifriends) is another with lots of experiance as well....good luck !

Thanks for the input! Why would you not expect the Holley HP system to work with it? What does a stock injection motor have that it'd need?

I've heard of the megasquirt system before and nearly built one for a honda I fitted with ITB's a few years ago. The building of the ECU wouldn't faze me, but we have no dyno available at all over here and absolutely no experienced tuners. This is why I was so keen on the self learning Holley system.

I think it was Alex's thread 'EFi for everyone' which got me interested in the Holley system in the first place, read it from start to finish.

Fingers crossed I'll find a way to make it work!

Cheers, James

what kept you away from a LSx motor ?

Do yourself a favor, download the free software from a couple of the EFI companies and look through it. Read the help menus, see if it's something you want to do. Self tuning seems to work well after the initial set up. It doesn't do everything, it won't stop you from melting the motor.

holley efi is the best bang for the buck and its fulley potted. use an efi blower manifold and fuels rails if you want too. at 600hp I think injectors in the throttle body will be fine. If 600hp is still your goal I would ditch that heat maker (250 blower) get good heads and make that power na Also if you are planning on using this to to hold you over until you can afford an Illmore why waste any money? You could have the ls set you wanted originality wanted and way less than an Illmore, I know it seems hard to go ls, but it really is simple. its just not main stream yet. but for up to 800hp in 28 footer in long run I think an ls is a good idea.

You need to run fuel through the roots blower for cooling and lubrication.

here you go kill two birds with one stone for pretty cheap

http://www.ebay.com/itm/Marine-super...s_Gear&vxp=mtr

As mentioned, you should definitely run fuel through the blower. Not only will it help to cool it, but it will help it seal better, thus making it more efficient.
Eddie

To keep it simple you could use the 4500 carb adapter for the 250, or make your own, to accept the Holley 2000cfm throttle body. Then your IAC and TPS are taken care of and your existing throttle linkage will work with little modification. You could sandwich a plate with injector bungs under the throttle body adapter piece and get some universal fuel rails for about $25-$30. With the straight rotor design, 8 properly spaced injectors above the blower will give you good fuel distribution. You could incorporate a couple of bungs up top for meth injection too. Because the Holley ecm is capable of controlling meth injection it's a pretty cheap system to set up. I'd probably trigger it off of manifold air temp as a safety for long wot blasts, and it will also give you some extra power up top too.