Injector flow rates between displacements

[someguy360]

Hi All,

Just wanting to make sure I cross my t's and dot my i's for my new build for first start and this is one thats been on my mind for a little bit.

I've just built a fresh 355 - 10.6:1 comp, 236/246 (108LSA) hydraulic cam, EFI Hardware 4 barrel TB with dual plane edelbrock intake etc.

I'm still using my existing throttle/intake setup and fuel system from my old cammed/warm 304 which had Bosch 775 Blue tops, Walbro 255lph pump, Raceworks regulator which is good for first start because it's what I know and have already tuned for. My question is, I know the injector flow rate needs to be changed for the increase in cubic inches, and I believe the base fuel pressure needs tickling too, but being a fresh flat tappet cam I don't want to go making too many changes to the tune as it really needs to start and run straight away at a level I can break the cam and rings in for 30 odd minutes, I'm also mindful of going too rich on the AFR for tht first start too because I don't want to wash the rings out.

Will I be ok starting it on the 304's injector flow rate and fuel pressure just for the purpose of sitting it at 2000RPM for 20-30 mins to perform the break in, at that point I'll then dump the oil and load up the 355 base map I've been working on, but I'd rather not risk making any changes to the tune for the initial run, if I sit there trying to get it running, or get it running and it's super rich I'm going to run into issues.

Correct me if I'm wrong but I figure leaving the injector flow rate, fuel pressure, VE table etc as is will just cause it to start and run a bit on the lean side, which is going to cause less issues with no load on the engine than it would being too rich.

Any advice on that one would be much appreciated.

[v6bucket]

Someone may correct me, you new motor is 17% larger, a little bit bigger cam, so I think you'll need more fuel.

[antus]

The flow rate and pressure should match the injectors, not the engine. If you need more fuel, you might need larger injectors. Having said that, you can fudge the flow rate to get it to inject more fuel, but it's a hack and might cause problems elsewhere, its not the correct way to do it. You havn't told us what computer, if its MAF it should measure the extra air the engine sucks in pretty well out of the box, if its MAP your VE table will probably need adjusting as the pressure the VE is mapped against will be flowing a different amount of air to the engine. so the tune will be out. If you are on MAP and just need to make it run OK, you might be ok with just the injector rate, but you'd want to remember what you have done, and plan to back out of these changes and do it properly when you replace the TB etc. Changing cam, TB (+IAC) and injectors will throw a lot of things in the tune out.

[Deuce]

The flow rate and pressure should match the injectors, not the engine. If you need more fuel, you might need larger injectors. Having said that, you can fudge the flow rate to get it to inject more fuel, but it's a hack and might cause problems elsewhere, its not the correct way to do it. You havn't told us what computer, if its MAF it should measure the extra air the engine sucks in pretty well out of the box, if its MAP your VE table will probably need adjusting as the pressure the VE is mapped against will be flowing a different amount of air to the engine. so the tune will be out. If you are on MAP and just need to make it run OK, you might be ok with just the injector rate, but you'd want to remember what you have done, and plan to back out of these changes and do it properly when you replace the TB etc. Changing cam, TB (+IAC) and injectors will throw a lot of things in the tune out.

It's MAP, Delco 424 I think. He is swapping intake over as is from the existing motor, so no curve balls there.

I would say leave the tune as is, except for lifting the injectors vs displacement value to suit.
Then everything else can be tweaked later, or during the run in procedure if time allows.

[pman92]

Kinjflow calculator:
viewtopic.php?p=21675#p21675

Put in the details for your existing setup and verify it pretty well matches what you already have.

Then just change to suit the new 355 displacement, and it will give you a good idea of how far to move it.

Leave fuel pressure and everything else the same