yes i did use the search function but i couldnt find what i was looking for. alright mods in the sig. new mods are going to be as follows. 52mm tb, gmmg exhaust(doing away with the duals). and maybe a shot of n20 or heads and intake package. im looking at a set of heads as we speak. i have stock injectors now. im guessing they are prolly somewhere around 100% duty cycle now. i was thinking that i should get a set of 35 or 36# injectors. btw i bought a brand new lt1 4 bolt main shortblock becuase just after the cam install i spun a rod bearing. the old block had 110k on it. is there anything i can do to prevent doing that to this new shortblock? im thinking it was just from fatigue on the old block. im hoping to turn about 420 rwhp on that new block. is that do able? thanks for all the help guys.

 

Injector size has to be based on flywheel HP, not rwHP. 420 at the rear wheels works out to about 510 HP at the flyhwheel with your A4. Running NA, a 36#/HR injector will probably work, but be marginal. A 42 #/HR injector would be a better choice.

Once you add nitrous, you normally don't need to change injector size, since a wet system will add the fuel through the nozzle(s) not the injectors. A dry system uses the injectors, but the LT1 dry sytems typically raise the fuel pressure when you spray to increase injector capacity.

 

thanks injuneer. your always helpful. do you think the new stock 4 bolt main block will hold that kind of power. i know they say the stock 2 bolt can handle about 500fwhp, so i figured if im around that on a 4 bolt i should be good to go...

 

lol @ 'fwhp'

 

500HP is fine for the stock short block, particularly a brand new one. I wouldn't spray too much on top of that though... the two stock rod failures I've seen documented occured at about 600 flywheel HP.

 

lol at the guy who does not understand FLYWHEEL hp

420rwhp is a steep goal for a A4 stock shortblock.

The 4 bolt mains mean about noting to the power handling potential because as you already found it is the ROD bearings that go first not the mains.

With a 52mm TB and single exhaust I just don't see 420rwhp happening so don't worry too much about sizing injectors to that.

a 36# injector will absolutely be enough for you, honestly a 30# would probably do it, the 85% rule is a bit overused.

 

Caprice, The 420rwhp would be with a 75 shot of wet nos. N/a I'm guessing il be somewhere around 370-380rwhp

 

lol I just read it and thought front wheel hp, made me chuckle

 

30# will cover 380 to the wheels, if you think you will have more hp on motor later you might go with 36#. Honestly it takes something with some ***** and usually a 7000rpm shift point to max out 36# injectors at 350ci.

You dont count the wet shot of nitrous into that because the nitrous system adds the fuel with a jet, not through the injectors. Size the injectors for what you're going to make on motor.

If you want a good education on injectors hop on youtube and watch some flow tests, there is more to injector selection than just capacity.

 

If you aren't after 420rwHP NA, then ignore what I posted. I thought you were saying you were going to make 420, then ADD spray. Wasn't very clear. I'd still go for 32's.

Not sure what "the 85% rule is a bit overused" even means. Most injector calculators use 80% for safety. Its simple math. You have to make assumptions. BSFC can vary all over the place.... depends on the details of the combustion chamber, and how good the tune is. What happens when you have a really good density altitude, and your engine is capable of making more HP than the SAE or STD corrected dyno results say it can make? Its going to run short of fuel if you are too tight on your injector sizing. Too small and you lose. Too large and..... no problems.