maf conversion injector question

87-t-top-gt · Feb 8, 2012

Ok, so I had every intention of staying speed density when I purchased my 87 gt. Well, I've read enough threads on some cams working and those same cams not working at all for others to decide to hell with it. I know theres hundreds, if not thousands, of people who pulled off an SD cam swap but I'm just not patient enough to want to mess with it just to end up going maf anyway.

Anyways...lol. My question is injector size. I've got some ported gt40ps and a matched tubular intake and am thinking e303, b303, or xe270hr. 70mm TB, 75mm MAF sensor. I have FRPP shorties for the heads and pypes cat-less "beast" manifold-back system. But regardless of the cam i choose, something in that mild range, I was thinking I'd need 24 lb injectors. Any thoughts, or better yest as I'm sure it too has been done hundreds of times, any experiance with +/- this combo? Thanks guys in advance
-Dave

jrichker · Feb 9, 2012

Remeber that larger injectors require a matching MAF or cutstom dyno tune chip to make the computer provide the correct fuel flow.

Fuel injector sizing & injector photos

Revised 11-Dec-2011 to add larger injector sizes to injector table

Injector HP ratings: divide flow rating by.5 and multiply the result by the number of injectors. This uses a 100% duty cycle. These ratings are for naturally aspirated engines at the flywheel.

Example:
19/.5 = 38, 38 x 8 = 304 HP
24/.5 = 48, 48 x 8 = 384 HP
30/.5 = 60, 60 x 8 = 480 HP
36/.5 = 72, 72 x 8 = 576 HP
42/.5 = 84, 84 x 8 = 672 HP

The preferred duty cycle is about 85% maximum, so for a safety factor multiply the final figure times .85.

19/.5 = 38, 38 x 8 = 304 HP x .85 = 258 HP
24/.5 = 48, 48 x 8 = 384 HP x .85 = 326 HP
30/.5 = 60, 60 x 8 = 480 HP x .85 = 408 HP
36/.5 = 72, 72 x 8 = 576 HP x .85 = 490 HP
42/.5 = 84, 84 x 8 = 672 HP x .85 = 571 HP

Remember that the above ratings are at 39 PSI. Increasing the pressure will effectively increase the flow rating. Example: a 19 lb injector will flow 24 lbs at 63 PSI, and a 24 lb injector will flow 30 lbs at 63 PSI.

See http://users.erols.com/srweiss/ to get the calculators used in these examples.

Here's the duty cycle explanation. Duty cycle is how much of the time the intake is open the injectors are turned on. The 85% figure means that for 85% of the time the intake valve is open, the injectors are spraying. The idea is that you want some percentage of the duty cycle left over so that you have some room to grow the process.

If you are at 100% and you need more fuel, all you can do is turn up the fuel pressure. That means the whole fuel curve from idle to WOT is affected. Maybe you are already too rich at idle, and turning up the fuel pressure makes it worse. If you had some injector duty cycle left to play with, a custom tune could use that where it is needed. That would not over richen the whole range from idle to WOT.

If you did turn up the fuel pressure, you might be able to change the injector duty cycle to get the air/fuel mixture ratio you want since the injectors will have extra fuel delivery capability.

With larger than stock injectors or higher that stock fuel pressure, you will need an aftermarket MAF that matches the injector size. The MAF “lies” to the computer to get a fuel delivery schedule that meets the engine’s needs and isn’t too rich or too lean. The best strategy is an aftermarket MAF and a custom tune to insure the best air/fuel ratio over all the RPM range.

Don't forget to increase the fuel pump size when you increase injector size or significantly increase the fuel pressure

Diagram courtesy of Tmoss & Stang&2birds

87-t-top-gt · Feb 9, 2012

Awesome man, much more info than I was expecting. Thank you, I appreciate that!

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