348 Throttle Body Replacment

Ultimately you will be limited by the size of the hole in the plenum. So, 65mm won't get you anywhere unless you bore out the upper plenum. We've seen that too much air flow can be a problem too, because too much air means more gas and our maps and injectors have a limit.

I'd have the stock ones reamed. Remember to have them bored out in such a way that it creates velocity - slightly funneled in the center. The velocity helps flow. Then do your intake runners. Make sure you have the later model fuel pressure regulator of 3.8 bar.

 

Does the guy in FL taper the entry and exit like Vince suggested (and RobZ used to do)?

 

Maybe we should stooge at my place fire up the tig welder with a few pounds of aluminum and make one.

 

The issue with the stock plenum is, the inlets are 58.5mm and there really isn't much material to bore them out more.

I talked to the guy at MaxBore and he can tapper port them. As Vince pointed out, the tapper is important to get the velocity up. You really want that air moving to get it past the throttle plate. When Rob ported mine he told me the limit is 57mm at the throttle plate. The reason is due to the bearing for the throttle shaft. It can't get bored any bigger because the bearing is in the way. However, with it tapered out to 62mm at the TB inlet, my 57mm tb's outflowed straight 60mm tb's.

So make sure they get tapered out to 62mm.

 

Apologies in advance for what is probably an obvious question to some (or most) of you guys, but I'm pretty uninitiated when it comes to this sort of thing. I'm trying to pick it up as I go. Most of my automotive experience is limited to simple maintenance and bolt item repairs.

As such, I have a question. Say I removed the stock throttle bodies and had the holes in the plenum tapered to 62mm as suggested previously, how many other things would I have to change before I could see the benefits? And what kind of benefits could I expect? Keeping in mind that my 348 is currently a bone stock 1990 model.

Obviously, that one thing is not going to boost the engine up to the 400hp in the other thread.

I ask because I'm still trying to decide whether to keep mine stock or mod it. She's plenty fun to drive as is, but I've been considering a few add ons lately like new headers and exhaust, and opening the throttle bodies seems like another logical step if I wanted to boost hp.

 

It's the throttle bodies that get the tapper port.

The inlet on the plenum top does not need to get ported.

The hole entering the plenum is already bigger than the inner diameter of the throttle body, even a ported tb. However, the hole in the plenum would benefit from getting a radius put on the inside entry to the plenum. The radius of the backside of the hole will help smooth the distribution of the air flow entering the plenum.

I'll start a different thread to discuss modifications.

 

Okay, look Andy, you are about to undertake a refining project of the intake system, which is worthy. You have an early car, so there are other items to attend to as a total package to make your car was good as stock can get.

1. Bore your TB's as indicated
2. Smooth out the interior of the upper plenum
3. Smooth out the interior of the lower plenum, cut down the "dams" in front of the TB ports
4. Bore out the runners - there is a lot of room in there, so open those things up
5. Make sure you have a 3.8 bar fuel pressure regulator
6. Replace your exhaust system and cats with free flow system of your choice
7. Remove the MAF screens, just don't run a KN or other oil saturated air filter
8. Get a more recent ECU chip set. Tim has a few, I have some, but need to get copies made.
9. Install 355 or 360 wheels and tires - very important to handle the better throttle response and tune out instability.

There you go. This is about the easiest and best set of upgrades you can do for the car.

The number 1 item to improve performance is the exhaust system - you will save weight and gain flow/power. Think also about removing weight. I dumped 70 lbs. of sound system crap from the prior owner. Now days with your phone, Bluetooth and some remote speakers you can have a great system for low weight and cost. Replacing your stock seats with Corbeau race seats saves weight and enhances your driver experience.

 

I know this was for Andy, but I wanted to say thanks for posting it. Very helpful.

 

The stock plenum needs to be opened up at the runners. All 8 are too narrow, plus they are really 16 holes that lead into 8 holes at head attachment. There is a lot of material that can be hogged out of them for better and greater flow.

A replacement plenum far exceeding the flow of stock leads to other problems, basically the other intake system parts cannot handle the airflow. The fuel delivery system has a limit - fuel pressure regulator, injectors and ECU. Basic on-line calculators suggest around 340 HP in our stock configuration - assuming you could tune for it with the ECU mapping (which we do not seem to be able to do right now). We Stooges have come to the conclusion that with the current ECU and stock parts, we have hit the limit. A modern ECU would give us some more capacity with stock parts, but it would also allow us to experiment with increasing intake flow with custom plenums and higher capacity injectors and (in my case) larger intake and exhaust valves, and possibly slightly more aggressive cams.

We, Ernie and I, are looking for power output more in the range of the 355, or beyond with these modifications. But regardless, it all starts with the ECU upgrade. The list above gives you a road map that I used, via trial and error, to get the dyno results I have, which is 300 HP at the hubs and an estimated 330 HP or more at the crank. More importantly, the throttle response and drivability improvement will seduce you and give you a taste of what these engines can become. You may not want to spin your engine up to 9000, but it really shines up to 8000 and can easily handle this with no internal modifications necessary.

 

Send Eric a PM. He has my chips running the last month. He seems to enjoy them

Eric+

Your set is sitting here or Bruce has them. Ask for "Beta-1" chips

When Ernie gets here going to fatten it up some more. Stock ECU mapped with higher rail pressure I think can be good for a solid 400 hp with the good injectors.

Need a trip to the dyno room

 

Did those air/fuel maps and numbers I sent help?

 

Not sure how you are going to get there. If we increase fuel pressure and use stock injectors we run the risk of damaging them. Remember duty cycles. We are strive for 85%, but I fear we are going to be pushing these up beyond their safe operating averages. They are supposed to squirt and rest 4 times per rev, but I have read that you can get to the point where they are not resting (closed state), which is when they overheat and fail.

If you are talking about larger injectors, great, but don't we have to recalibrate everything to make the maps work them? I think the 358 runs fuel pressure about 55 psi now, we have had it higher, but they are not stock injectors either.

 

Vince the stock injectors can get replace. They are 18.45lbs/hr and barely flow 194cc at 3bar.
I told you about the GenIII EV1 4 hole Bosch injectors I bought. At 3bar they flow 220cc, and at 3.8bar they flow 247cc. Dead time is [email protected]/3bar. Plus they are 16.0Ω resistance. I think they can get the job done much better than the stock injectors.

At peak torque my engine is at 6100rpm. If the engine has a peak volumetric efficiency of 100% (and that is a huge STRETCH on the stock plenum), but if it was, and if we wanted an AFR of 12.8, injector fire time is 13.8mil/sec. At 6100rpm max time for injection is 19.7mil/sec per 2 rotations of the crank = 70% duty cycle with these new injectors at 3.8bar/55.1psi. Even at the lower fuel pressure of 3bar/43.5psi they would still only have a 78.4% duty cycle = plenty to get the job done and still in the 80% duty cycle safe zone.

 

Ernie, have you installed those in your car and if so, have you monitored your A/F ratios after the fact?

 

Not yet.

I'm waiting to do a before dyno pull before they get installed. It's still winter here, I don't want the cold air affecting the test.

 

Thanks for the reply. I'd very much appreciate it if you'd keep us in the loop as to your progress. Very interesting subject matter...

 

They will be the same (383) because at full throttle the ECUs go into open loop. The open loop is a set mapping, non adaptive, and it does not read the MAF, nor the coolant temp, nor the O2 sensor.

 

I don't even want to tell you what size injector I am going to be running... LOL. I know that I won't be hitting 80% very easily, even with E85. If someone is looking for a set of stock F355 injectors, I can help you out, I have a nice set of 8 lying around the garage. (250-270cc/min high impedance 16 Ohm iirc - Factory pink top injectors).