Hi all,

I'm rebuilding an 89 roller 305 TPI and the whole bottom end is together, I'm at the point of putting the fuel rail back on and because his is a custom build I'm wondering if the factory 19lb injectors are going to cut the mustard or not. This is what my car will have when the project is done...

3.23 LSD rear
14x7 rims with 235/65/14 Kuhmo tyres
aluminium drive shaft
shift kitted 700-R4
Hedman's 1"5/8 headers and 3" exhaust all the way to the exhaust tips
true 9.1CR (I think that was the number for 91 octane fuel)
081 heads with 1.90" intake with port work done
Electric cooling fan
MAF TPI
ported plenum
computer tuning
No air conditioning, no smog pump, no cat
boxed LCAs
subframe connectors

The Laser cam I'm running...

Adv. duration @ 0.006 262/267
duration @ 0.050 lift 209/215
lobe lift 0.325/0.325
gross lift @ 1.5 0.487/0.487
lobe seperation angle 112

When I got the custom cam done I told Laser cams I want to do about 13.5s down the quarter mile and so they made me the above custom cam. I understand that to do this you need to be pushing around 300HP which 19lb injectors won't do. Of course fuel pressure has a role to play here, but I don't wish to have the fuel pressure up so high that it puts unecessary strain on the fuel pump and causes it to wear out. Because my goal is to have power and economy I guess having the fuel pressure high would steal some economy. Am I right? I'm going to convert my factory regulator into an adjustable one.

I figure I'm going to need 24lb injectors which will support 307HP at 80% DC. I'm going to be doing PROM tuning so no worries there.

So what do people think? Stay with 19lb injectors or go larger? The more I search on this topic, the more I get myself confused!

 

I'm in a very similar situation. You are right in your statement that 19# injectors (at 43.5 psi) are really too small to support 300 FWHP. There are a couple of guys who are doing it, but they are running 54 + psi, which like you've said, is a bit hard on a stock fuel pump.

I think that 24# injectors are too big for a strong (streetable) 305 like you are building. You would need to get a new PROM just to get it to run well enough to drive it... Lowering fuel pressure enough to flow even close to the proper amount will cause a poor fuel atomization issue, and probably other issues as well.

I'm going with 22# SVO's on my engine. From what I've read and been told, they are plenty big enough for 300+ FWHP. I plan to lower the fuel pressure to ~40 psi get it to run, and then I'll learn to burn a custom PROM in order to bump it back up to ~44+ psi.

 

Thanks for your thoughts.

I've come across several calculators for figuring out what injector size you need, and it seems to confuse me more. Like how do you know what your volumetric efficiency is etc?

You're probably right, 22lb is likely what I want. But because the fuel is squirted directly into the cylinder, how is atomisation an issue? It certainly is with a carb setup. I will be doing programming so compensating for 24lb injectors should be OK.

I've had a play with http://www.bgsoflex.com/pwcomp.html and with 80% VE, 6000RPM, 13.0 AF ratio (rich) with an injector size of 22lb we get an 85% duty cycle which according to various things I've read is on the bleeding edge of what you want, which implies 24lb would be better.

I dunno!

 

I've seen several of those injector size calculators too, and most of them give basically the same ballpark results.

The thing to remember about TPI is that it will not produce good power at 6000 rpm. The intake system is tuned for a much lower rpm power band. Stock 305 HP and Torque peaks are 4000 rpm and 3200 rpm with an auto trans. Sure we all want to rev higher than that, and we will, but don't mislead yourself into believing that your motor is going to be much different, even with a different cam, and better flowing heads, making power above 5000 rpm will be tough. TPI just doesn't like to do it...

Fuel injectors are flow rated at 43.5 psi, you don't want to use a lower fuel pressure because the spray pattern will suffer. Running them at 46 psi (where you'll get more flow and maybe better spray) and getting better performance is very common with these cars.

I live in California so staying semi smog legal is an issue for me, you may not have to deal with that. Still getting basically 1 FWHP per cube is a pretty realistic goal. Getting that much at the rear wheels would be pretty tough without using a power adder or NOS.

Turning a 13.5 1/4 mi. with a NA 305 will be a challenge in a car that weighs ~3400 lbs., plus the drivers weight... good luck. I'm thinking that a 13.9 would be reasonable to shoot for in my vert.

 

Thanks for that.

I've decided to go for 24lb injectors after talking to a local guy who does computer tuning with cars for a living. 24lb is what he recommended due to what he's seen with other cars. I know Tuned Port 335 is using 19lb injectors, which is why choosing the right injector size is so difficult, there's always more than one way to solve the problem and I always to do it the best way.

 

You might also want to think about getting an adjustable fuel pressure regulator to bump up the pressure going to the injectors.

 

You definitely want to install an AFPR. Its a great tuning tool.

 

Yeah I'd thought about that, so I'm going to modify my factory regulator. I've modified my schrader valve to take a 100psi fuel gauge too.