Now that I have a drivable project, I'm trying to iron out the final kinks and get pointed in the right direction. The car is running and driving and seems to be doing great. I'm pretty much just breaking the engine in now and I'm about to try to get just a little more out of it.

The car is a 1988 Rx7 convertible. I swapped in the engine and transmission from an 88 Trans Am GTA, but I pulled the engine almost immediately and rebuilt it. It needed a rebuild... really bad. I didn't really think a lot of my decisions through before acting on them, I pretty much just built this thing on impulse.
The pistons are flat top forged pistons. The heads were freebie 993's that I threw on there because they lowered compression. I was going to throw a turbo on this and make some stupid power (hence the larger chamber heads, but I learned that was also a bad idea. Something about quench and how they're shitty heads, so I gave it up but they're still on there) The block is still the same ol' L98 block with the same L98 roller cam. I wanted it drivable, so I scored a cheap Holley Street Dominator intake and an Edelbrock 1406 and threw it all together to get it driving. I immediately hated it. Mostly because of the 700r4. Maybe it was the 4.10 gears coupled to an 8.5:1 CR engine through a 700r4, but I hated it. I found a T5 and replaced the 700r4 promptly (which also cemented the fact that I'm definitely not putting a turbo on it now). So now it's more fun, but it's still kind of a slug. Yeah, the car really only weighs like 2800 lbs, but I still need a little more.

Ok, let's sum it up.
88 Rx7
L98 block with forged flat tops, 993 (76cc) heads, Edelbrock 1406 carb, Holley street dominator dual plane intake, World class T5 from an 88 Camaro.

I'm about to toss on a set of junkyard vortec heads. So that should raise compression to 9.8:1. So this is going to force me to change the intake obviously, so I'll probably use the Jegs 513002 or whatever intake works with these vortec heads. I guess my question is really, have I overlooked something again? I'm getting kind of tired of overlooking something stupid and having to completely reinvent the wheel each time I try to do something. I plan on using some 1.6 rockers, so I know I'll have to cut the valve guides and drill the pushrod holes, put the new heads on, new intake manifold, and slap the carb back on. I also know I'll need different valve covers too. I'm just hoping that I'm not going to end up with an insufficient amount of carburetor or the fuel pump (mechanical, btw) doesn't flow enough fuel or something. I kind of want to eventually go back to EFI, but the TPI intake (as cool as it looks) sucks. I just don't know if EFI is going to happen based on MPFI intake availability for the vortec heads. Sure, I could drop another grand on some kind of TBI style EFI system, but really what would I gain with that (aside from a little extra reliability).

I don't really have that much experience with carburetors. Sure they're simple enough, but I'm just not as familiar with them as I am EFI stuff. Anyways, I'm just looking for someone to back me up on what I'm doing here.

 

#1. This is a forum for Third Gen Camaro/Firebird, not Mazda RX-7 cars. But given you are using a GM Firebird engine see #2 below.

#2. You have a lot going on with that engine, and there are any number of potential issues with the way it runs now (or maybe it runs fine but you just aren't happy with the current power output). You say it's "kind of a slug", which is relative because what is a slug to you might be a hotrod to someone else. The point in this that readers don't have anything concrete or definitive to define your problem or how to solve it. We have no idea what horsepower this thing is putting out and don't have any point of reference to determine if you actually even have a problem. Right now it looks like you are just guessing at everything and throwing parts at it to see what happens.

HOWEVER, I noticed you mentioned having little experience with carburetors and installed an Edelbrock 1406. Like any carburetor, if you want to get maximum potential out of it you have to properly tune it. I would start there. Pick up the manual that came with it (or find the PDF online) and read it front to back. Then read it again, and teach yourself how to tune it. It's a pretty easy carburetor. Mine ran decent new out of the box, but properly tuning it made a world of difference.

 

Related specifically to your engine development, the Vortec headed 350 is a proven performer.

Getting the compression ratio back up will make a considerable difference. Build on that with a cam to compliment it. At 9.8:1, something in the 220-224 @ .050" has been demonstrated to work well. I've personally run several cams on a similar platform and was happy with Comp's XR276HR. This isn't a cam recommendation per se, but more like relating what I know will work.

As for machine work on the Vortecs, keep in mind that they use a self guided rocker. Sticking with that uncomplicates things somewhat. You may want to consider replacing the pressed in rocker studs with a screw in version. Totally necessary? That's a matter of debate and is dependent on whether you want to step up later. It's not a 100% requirement that the guides get cut for clearance. There are valve springs like Comp's 26915 or 26918 that will drop in without modification. I understand that the LS6 springs are also a drop in using the appropriate retainer for the Vortec valve stem diameter.

I'm not sure of the Jeg's intake quality but for a few dollars more there's the RPM Air Gap https://www.jegs.com/i/JEGS/555/513036/10002/-1 This I know is a quality item. And it works.

Do yourself a favour, and regardless of the carb you'll use, install a wide band O2 sensor and A/F gauge. It will be infinitely helpful both in performance and fuel efficiency.

Pay attention to the ignition timing and you should be good to go.

 

Thanks skinnyz. There already is a wideband O2 installed. It was already in the car when I was still using the rotary. I (personally) tuned the rotary to almost 350 rwhp using a megasquirt. Carbs are a new world to me, so I figured I'd give it a whirl asking people who know the engines well in a forum dedicated to carburetors on these exact engines.

The car does run well, it's just not as fast as it was. I guess it could be a hotrod to some, but it doesn't have near the power it once did. Yet.

What I don't like about the carb is the guesswork. Currently, at wide open throttle the AFR dips into the high 9's, with cruising closer to 13.5/14.0. I still haven't changed over to the vortec heads yet, so I know I'll have to rejet (again) when I do that. For now, I'm looking at the calibration reference chart and I see that I can make a 4%, 8%, or 12% change in jet size. None of which come in the Edelbrock 1487 kit. Is there any way to scale that to what size jet/metering rod combo I need to get closer to 12.5/13.0?

Other info bits:
idle speed is 600 rpm
Idle AFR is 13.5-14.0
Idle vacuum is 17"
Rear axle ratio just got changed to 3.90
Ignition timing is set at 12° BTDC @ idle
Fuel pressure is regulated down to 5.5 psi

 

Carbs have accelerator pumps that squirt a shot of fuel into the air flowing into the carb in an attempt to cover the LEAN mixture that would result from suddenly opening the throttle blades when accelerating. That is probably the "dip" that you see on the AFR gauge.

 

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I wish I could say I knew more about Edelbrock carbs but having only limited experience with them, there's not a lot I can offer. Some people really like them. For me, I'm a Holley format guy. Those carbs have been part of my hot rodding for longer than I care to say.
This will bump your thread to the top again. Maybe someone will chime in but I'll be following.

 

That could very well be.
​​​​​​In addition to the wide band O2 sensor/AFR gauge mentioned earlier, I also tune with an Innovate data logger. I've incorporated a MAP sensor into my carbed setup and can observe the AFRs, manifold pressure as well as RPM. That in itself is immensely helpful and takes out a lot of the "guesswork".

 

That's why I loved the megasquirt. It had a datalog feature and I could do a drive around town, mark the times when I had problems, and tune it out once I could focus on it.

The 9 AFR isn't the accelerator pump. It's absolutely the main jet(s). I'll just have to start buying jets and rods based on a best educated guess and the chart.

On a different note, I have decided that I will be using an MPFI manifold and another Megasquirt when I put on the vortecs. I really don't feel like playing this guessing game a second time.

Thanks for the help though. I definitely want to get it running correctly as it sits. It's probably going to take a little time to gather everything I need, but I still want to drive it in the meantime.

 

We'll, should you decide to swap out the Edelbrock for a Holley/Barry Grant/QFT post up. I may be of some assistance.
Good luck with the Vortecs. I've had great results with them. Stock, modified and aftermarket.