peejay

I'm at that special point in my engine build where I don't know which direction I should go. Crowdsourcing opinions.

Engine is a 4-port 13B. GSL-SE center housing, so the ports aren't very tall, but they ARE the same size as the runner outlets in the intake manifold.
Racing Beat Holley manifold, 750cfm Holley modified for 1:1 secondary actuation, to be used as a throttle body ONLY (so basically, 4x42mm independent throttle body)
Two large injectors in the primaries, may switch to staged 2 primary + 2 or 4 upstream injectors later
RB long primary exhaust, which has ~2.3psi backpressure peak with current half-bridge

I'm just torn right now. The secondaries WILL be bridge ported, this is not even a question. It's the primaries that I'm thinking of.

Half bridge pros:
Vacuum sourced from the primary side only is generally sufficient for power brakes
Fuel injection without the ability to set injector timing SEEMS a lot easier
It's a known quantity, I have had three different iterations of half-bridge
23Racer seems to be doing well enough with a half bridge

Full bridge pros:
I want 250 crank HP minimum. Prefer 300, since the Group B cars had 300.
I have never had a full bridge engine before.

There isn't enough meat on the GSL-SE center housing to close very late, which is fine by me. I'm not going to close the ports super late.

Jeff20B

iTrader: (13)

You can close GSL-SE primary ports at 74 spec (aka 50 degrees) if you'd like.

Sorry, that's all I can help you with.

fidelity101

iTrader: (11)

if you go full bridge, I'm gonna have to go PP.

j9fd3s

iTrader: (3)

if you keep the closing timing stockish 50-60 seems to be pretty ideal, then go for it!

the opening timing will of course make it "bridgeyer", but power should go up

peejay

I have a supercharger that I might mount later.

j9fd3s

iTrader: (3)

positive manifold pressure fixes all the problems with port overlap....

peejay

True enough. The thing that I am concerned with, though, is if the runner area is large enough to support the flow that a bridge port can provide.

This is aside from the injection characteristics, although I am hoping that having independent throttles instead of a shared primary plenum will alleviate some of the low-load issues.

lastphaseofthis

iTrader: (8)

I have an engine with the center only(84-92 6 port) closing at rx-8 spec. AKA(can't remember), is there less meat on the GSL vs the s5 centers?

peejay

There is a LOT less meat on the GSL-SE (not GSL, those were standard carbureted 12As) than any FC. The ports start out small, and poking around through the coolant passages, the walls are as little as 1/4" thick in spots.

It's a little unfortunate that I'm married to mounting the engine via the center housing now. I could mount the primary injectors upstream and use a 12A center housing, which has huge ports, but then I'd have no way of putting the engine in the car.

peejay

This is my old TII block's ports laid over a GSL-SE center. The runners aren't much taller, and you can port to coolant if you go down very much. The top is pretty thin, too.



Where I'm at, for now. I may alter that aperture shape. Still undecided on bridge or not.



End ports opened up substantially. The runners are still 4mm shorter than my TII's.

fidelity101

iTrader: (11)

go on...

peejay

If this turd doesn't make 250 at the crank, I have a little Eaton that will force-feed it a couple extra pounds. If it does make over 250 at the crank, it will be flowing more air than the blower can provide...

rx7freak13v

i too am hoping for 300 at the crank, like the old group B cars. since your already getting some of the drawbacks of a bridge, you might as well go all the way. i say go full bridge or go home lol. full cut bridge. then take a file to the rotor housings to help the air flow into those bridge ports. thats my game plan for this week (same as a week and a half ago, before i started leaning towards the pp. which you helped lean me back away from). you've never had a full cut bridgey before, you should love it then and probably not care about the extra minor drawbacks. can't wait to hear your decision

23Racer

If I can put in my $.02. My 1/2 bridge was put together to make decent torque at lower rpm and still be able to pull to the moon when I needed to. All of the following is of the "I believe" type information as I didn't do the porting or put the motor together. We are bridged on the outer S5 TII irons using a Racing Beat outline. The center is an S4 TII housing with some wacky Judge Ito extended street port. Exhaust ports are a Racing Beat Race port. S5 rotors on an S4 eccentric shaft with all the mods done for high rpm use. The car uses a stock S4 NA transmission and S4 4:10 diff.

Now that we have the tuning pretty much nailed, the car pulls very hard from about 4,500 rpm and really pulls from about 7,000 rpm up to over 9,000 rpm. It has enough power to light up the rear tires (Toyo RA1's) in second gear from about 3,000 rpm when they are cold. It is strong and flexible, just as we planned. Can I stay with a good PP, no way. I have been on track with 01Racing's S5 FC with a very good PP and while I can stay with him up to about 6,000 rpm, he just disappears above that. Granted his car is almost 200 lbs lighter and is running a 4.88 rear gear, but I can tell that it has soooo much more top end charge.

If I had to do it all over again and I will at some point, I think that I would go with a full BP and give up some of the mid-range for the possible top end charge. You can cover for lack of mid-range with gearing and a good close ratio box, but you can't tune in a strong top end pull.

Again, just my thoughts on the subject.

Eric

j9fd3s

iTrader: (3)

your next mods should be sticky tires, and then maybe a different 5th gear/miata gears, and then maybe a different diff ratio.

going from Ra1's to hoosiers is like 3 seconds a lap for us!

peejay

It's the top end charge (or lack thereof) that worries me. I don't know if the primary runners can flow enough to justify a bridge port, so you get all the negatives without the benefits.

Disclaimer: I need a streetable exhaust because I drive anywhere from 15-20k miles per year.

I can say that a half bridge does wonders for mid range torque. That's why every engine I've run in the past couple years, save one, was half-bridge. It's so nice being able to accelerate from 4000rpm, instead of the street port "nothing until 5500" powerband.

fidelity101

iTrader: (11)

I've seen your car in action, thats why I'm tempted to do half bridge myself. Some of the more open rallyXs i don't have the power to move or turn and if I go a gear lower I get wheel spin and no "acceleration"

also a different rearend ratio would help.

peejay

The 4.78s are awesome. YOU NEED TO DRIVE THIS CAR

fidelity101

iTrader: (11)

I know

I'm tempted to come down in the fall for a WOR/OVR rallyX, if I can borrow/source a trailer.

23Racer

I hope to be getting a Miata gear cluster box shortly and that should help a lot. I am also going to be using Conti CTCC tires from now on. About .5" shorter than the Toyo's and about 1" wider. That should help a lot as well.

We will see end of August when we go back to Mosport about how fast this beasty can go. This gives me a couple of months to get it finished with the new brakes, different struts and airbox.

Eric

lastphaseofthis

iTrader: (8)

Ever considered running an rx8 center with the exhaust blocked off(welded?)? lots of meat and runner area. i have an rx8 with 13b-gsl housing project in the works. 233rwhp in an rx-8 with stock ports is how much at the flywheel? 233 rwhp dyno sheet - RX8Club.com

peejay

I HAVE to run a GSL-SE center in order to be able to put the engine in the car. It has the provisions for motormounts.

It's either that or build an FC engine, and while I do have an embarassment of FC rotorhousings, I don't have enough side housings.

I happen to have two good GSL-SE rotorhousings, and a couple good 12A endhousings, and this GSL-SE center, and a 13B 4-port Holley manifold...

peejay

Well, I've never failed by overdoing something before...

MFR ports on the outers, bridge the inners, MFR exhaust ports.

bumpstart

when i did something similar i used a K marked middle plate casting ( 12at )
but suspects you have an M
,, both have the boss on each side for middle mounts ALA HB cosmo / luce
and both are tiny ports to start from ,, the K with only enough scope to get to almost an ( already shitty ) NO spec in terms of main port

.. and the bridge is limited to the height of the runners

methinks the M ( if that is your GSL-SE plate that has the injector holes ) port is just a little larger than the K
and you may find you can get later timing than i could,, if you angle the roof

however ...use the set square to find the limits up and down for the middle plate casting runner ... and only port the bridge within the scribes
else you have to stop the drill short ,, and step or angle the very top and bottom of most bridge templates

https://www.rx7club.com/1st-generati.../#post10426457



with your custom TB setup.. if you had proviso for injectors in the secondary runners
( and sequential ECU )
then you have option to reverse the TB around,, put in the mech delay .. and use the long runners ( and ex secondary mild ports ) as the primary side
and the short direct runners to the half bridge as secondaries

and build the engine from middle plate out,, allowing full angle cut on the bridge you have,, turning the wedge end of steal seals outwards towards the mild port


the results would be interesting to see ,, particularly if the manifold used has no cross cut primary to secondary



PS
when i did mine it was with knowledge that i was limited to a 12a nikki carb for legal sakes
,, and even the primary side of that carb was going to outflow just about any mild port i could achieve from the K plate i had to use
hence the bridge ( my middle plate had to remain the primary side )

idle lope was bridged , mid range was a pleasant surprise
( in a 1350 kg HB cosmo with a 12a )


in the pics from my build you will see i have used a huge exhaust port ,, because the bridge done on the K plate has a little less overlap than other bridges

peejay

Oddly enough, I was searching for results for bridge ports in HBs when I found a post you'd done on ausrotary.

It looks like our GSL-SE is the same as the 13B with the "Super Injection" intake plenum. The primary port runners are positioned slightly higher than on the FC 6-port engines. I'd like to say 5mm higher floor, 2-3mm higher roof (smaller port size).

My experience with half-bridges says that you do not want primary/secondary throttle if you want smooth drivability, the vacuum made in a bridged port with a shut throttle pulls exhaust gas up. Smooth drivability is important to me, especially with the 9lb flywheel that is going on. Thus why my un-carburetor is 1:1. Actually it's also 1:1 because that takes a lot less fiddling around when converting a vacuum secondary carb to mechanical, just need a dab of weld and you're done! I probably will run the carb reversed just to make the cable easier to mount, since Holleys like the linkage to pull "back" and the cable's coming over top of the engine. Probably the only case where RHD would make things easier.

I do plan on making an adaptor plate so I can try the "magic" manifold I had on my half bridge Turbo II.