I have a 1987 Rx7 which i swapped my verts 91 motor into, after months of dealing with all the electronics and the car not running right i decided to ditch the F.I. and i went with a Racing Beat 6 port manifold with a dellorto 48 webber sidedraft. What power gains will i see with this setup and what kind of 1/4 mile times will i see? MY current mods are headers, borla exhaust, the webber and I have a 4.3 gtus diff. My car weighs in roughly at 2200lbs and 2400lbs with me in it. Thanks in advance I will post pics asap...Also I talked to some guy and he was running the same setup except in an s4 motor with just headers and he was running low 9s in the 1/8th...Is this possible?

 

Isn't that slow? Would that mean almost 18 in the 1/4?

i don't drag so just asking.

dg

 

where on earth did you get the idea that carb'd intake manifolds are better than Mazda's VDI system? you might make as much peak power as the stock manifolds if you are lucky, but you won't have the torque band across the full rev range. You do know that carbs are by definition restrictive right? What do you think a venturi is?

as for your 1/4 mile times, well you've got a light car but it's just going to come down to your driving, your tires, and the track conditions.

and low 9's in the 1/8 is usuall somewhere in the 14's for the 1/4.

 

you will lose power.

 

Everyone is telling me im going to gain power or what not im just asking. Im not running the webber by choice, Im forced to go this route since three shops here cannot get my car to run right with the s5 engine in the s4. I wish I could Stay F.I. but right now it seems like this s my only route. And low 9s are 14.0-.14.5 lol. And everytime I asked here about the s4-s5 swap they just say its complicated but with no explanation. Ive tried using all s4 electronics with s4 ecu and harnness and ran but like crap ilde all over the place and stuttered really bad we cleaned up all connections and rewired it and it would crank but not turn over and I had spark. Ive been car less for about 4 months and its my only car so I cant keep wasting time. Do you think I can take the whole manifold setup off and use the s5 bare block with the s4 manifold and it would work good??

 

Just saw that you were using the S4 harness but does not say whether you are using the S4 injectors?

The injectors not only are larger in the S5 but are high impedance while the S4 up to June of 87 are low impedance. So what components are you using?

Also the carburetor set up can yield good HP results as seen in this thread on a 1st gen with a SE 13B 6 port engine. :-)

https://www.rx7club.com/1st-generation-specific-1979-1985-18/racing-beat-exhaust-dyno-compairisons-823229/


You should also be able to put all the S4 intakes, etc on that S5 engine with no trouble also. We have put the S4 and S5 intakes on our 1st Gen SE Engines.

 

A Weber 48 DCOE and Dellorto 48 DHLA are two different carbs.

Anyway, you may get about 180hp out of your engine. That would work out to about 13.81 in the quarter if you go purely by the math.

Yes

 

it all depends on your jetting and tuning

 

The fact you went with the sidedraft kills your only chance of making more power than the EFI. The only place a carb will make more power than EFI is above 7000rpm with good porting on the engine. That is with a downdraft carb (IDA). The main reason it makes more power up there is the shorter intake runners which you don't have with the wrap around manifold.

 

it was an 86 harness so i was using low imp injectors

 

Now you have me confused?

You stated that you have a "87" car so why would you have a "86" harness?

So the only different thing that you changed is that you were using the S5 intake?

Verifying that you used your 86 or 87 AFM, the TPS from the S4, also the injectors from the S4? Were you using the S4 BPAV? Also if you were using the S5 intake did you have the VDI system hooked up and supply from the air pump?

 

the VDI system effectively shortens the intake runners in the high rpm range:



so with the carb you are still stuck with a restrictive venturi and intake manifolds that don't adapt to changes in rpm, unlike the system Mazda spent millions of dollars on

 

The runners are still too long for hi rpm power. Its a great system for the street, makes more power everywhere below about 7K. An IDA with 42mm venturis will make 15 more HP up around 8500 where the stock manifold is done helping. With the stock manifold there is no reason to spin the engine passed 8700, the power is falling off. The weber will keep pulling a few more hundred rpm while making more power.

 

its not all about the intake manifold.

 

Within the EP rules we are allowed a streetport and either the stock manifold/EFI or a two barrel carb on a Y type manifold which pretty much means a Weber/Delorto.

On the engine dyno, not chassis dyno, with EFI/Motec with everything tuned to ideal the engine made 15 hp less than the Weber with 42mm chokes and everything tuned to ideal(afr's ,timing,egt, etc). Back to back sessions. I'm sure there are a few things influencing the power besides the manifold but the intake system in general was the only thing changed in the tests.

Don't get me wrong, I freaking hate carbs. For the 99% of the time the engine spends on the street EFI is far superior. For the rpm range we run on the track the carb can be better. It all depends on the rules.

 

I don't know what your problem is. You should have been able to just swap the S5 block in and use all your original S4 accessories and manifolds. The only thing you have to modify is the LIM with JB weld to extend the middle part down about half an inch. This is what I just finished doing in my car and it fired right up on the first try. I will be converting over to the S5 manifolds soon. You kinda took one problem and made it more complicated. Did you ask for help on here?

 

well yesterday we got a spare 87 rx7 and i took my bare s5 block and put everything from the s4 inlcuding the lim and uim so hopefully today it will fire up with no issues...Im just going to give this one more try if not i will have to stick with the weber route. Wish me luck!!!