Who has a nice mild street port template for 12A?
10-12-07, 05:36 PM
#27
Port size and location determines intake velocity and rpm at which torque and hp peaks.
Venturi sizes are used to fine tune, but not radically change the above.
A monster streetport makes peak hp at, say, 9000rpms. Put smaller chokes in the carb (36mm for example) and you'll increase intake velocity and fuel atomization at the carburetor end. Some more torque at a lower rpm, and ^ MPG.
But the large intake port will inherently have lower velocity. And smaller chokes limit the amount of air that can enter the motor and it'll reach maximum airflow below 9000rpms. Revving over 6500rpms would make no more horsepower because the small chokes are flowing the maximum amount of air they can. All you're doing is spinning the motor.
Smaller ports would work much better with the smaller chokes. Want more HP? Increase both moderately thus the term street port.
The key is to match venturi size with port size for the best performance over the motors powerband.
Venturi sizes are used to fine tune, but not radically change the above.
A monster streetport makes peak hp at, say, 9000rpms. Put smaller chokes in the carb (36mm for example) and you'll increase intake velocity and fuel atomization at the carburetor end. Some more torque at a lower rpm, and ^ MPG.
But the large intake port will inherently have lower velocity. And smaller chokes limit the amount of air that can enter the motor and it'll reach maximum airflow below 9000rpms. Revving over 6500rpms would make no more horsepower because the small chokes are flowing the maximum amount of air they can. All you're doing is spinning the motor.
Smaller ports would work much better with the smaller chokes. Want more HP? Increase both moderately thus the term street port.
The key is to match venturi size with port size for the best performance over the motors powerband.
10-13-07, 12:16 AM
#28
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Exactly. No amount of tuning on a big-ported engine will prevent reversion at low RPM or reduce overlap. You should choose the port that best suits the type of driving you do, then tune to match the port.
10-13-07, 01:51 AM
#32
lol what I meant was when you use smaller jets and venturis to try to make up for large ports, you're left with an engine that has little bottom and low power up through the rev range. That's not very fun to drive.
Ask Percent how his GLC ran when we tried the Hitachi with 20mm venturis and 92 primary jets, and then tried the other Hitachi with 22mm venturis and 105 primary jets. Ask him which carb he likes better on his rather large ported 13B, and I'm sure he'll tell you the bigger of the two.
Ask Percent how his GLC ran when we tried the Hitachi with 20mm venturis and 92 primary jets, and then tried the other Hitachi with 22mm venturis and 105 primary jets. Ask him which carb he likes better on his rather large ported 13B, and I'm sure he'll tell you the bigger of the two.
10-13-07, 06:00 AM
#33
Nobody thinks of the obvious
I can't believe so few people have done what I do on all the engines I build for street use - I do a minor cleanup of the primary and exhaust ports and then use the RB street port template for the secondary intake ports. This gives driveability and fuel mileage virtually the same as a stock motor, but with a significant gain in full throttle performance. Look at the relative size of the primary and secondary throttles in the carb, and the logic of this scheme is obvious. My 12a is done that way, but with an RX-4 carb and RB header, got 130 hp to the wheels, which is at least 150 hp at the flywheel. My peak horsepower is at 7,000, compared with 6,000 for a stock motor.
10-13-07, 11:31 AM
#35
Your cars. Kidding. 
stilettoman, I did a mild port on the primaries and a large streetport (I think it's RB sized) on the secondaries in the GLC. The exhaust ports are a little bigger than T2 spec. It's using a '76 Cosmo carb which has the large 22mm venturis. The air bleeds were already excellent at 60 and 90, and I upped the fuel jets to 105 pri and 160 secondary. That carb is completely maxed out, and it is the same size as the earlier '74-'75 REPU and RX-4 carbs, except this one has a smaller vacuum secondary jet allowing faster open time (according to Klaus42 who knows about these carbs) of either 60 or 90, instead of 120 of the older carb. I also removed the spring from the vacuum diaphram thing so they pop open easier. I've tried larger fuel jets (117, 170) but they always have trouble. I can't go any smaller on the air bleeds.
As I said, this carb it totally maxed and yet it is still inadequate for the engine. Now if the ports were smaller, let's say '74 spec primary and mild streetport secondary with smaller exhaust ports, the carb would match the engine a little better. For example my REPU has '74 spec intake ports all around and the exhausts are the same size as the GLC's (to correct some stupid upside down D ports) and I'm using the '74 carb with 105 pri, 155 sec, and 60 and 90 air bleeds. This carb has a 120 vacuum secondary jet with a removed spring. Its secondaries don't hit as hard as the Cosmo carb, but the primaries have decent power in such a heavy vehicle with a stock 30 pound flywheel. If I threw in a light steel flywheel, like in the GLC, it would be a more accurate comparison.
stilettoman, I did a mild port on the primaries and a large streetport (I think it's RB sized) on the secondaries in the GLC. The exhaust ports are a little bigger than T2 spec. It's using a '76 Cosmo carb which has the large 22mm venturis. The air bleeds were already excellent at 60 and 90, and I upped the fuel jets to 105 pri and 160 secondary. That carb is completely maxed out, and it is the same size as the earlier '74-'75 REPU and RX-4 carbs, except this one has a smaller vacuum secondary jet allowing faster open time (according to Klaus42 who knows about these carbs) of either 60 or 90, instead of 120 of the older carb. I also removed the spring from the vacuum diaphram thing so they pop open easier. I've tried larger fuel jets (117, 170) but they always have trouble. I can't go any smaller on the air bleeds.
As I said, this carb it totally maxed and yet it is still inadequate for the engine. Now if the ports were smaller, let's say '74 spec primary and mild streetport secondary with smaller exhaust ports, the carb would match the engine a little better. For example my REPU has '74 spec intake ports all around and the exhausts are the same size as the GLC's (to correct some stupid upside down D ports) and I'm using the '74 carb with 105 pri, 155 sec, and 60 and 90 air bleeds. This carb has a 120 vacuum secondary jet with a removed spring. Its secondaries don't hit as hard as the Cosmo carb, but the primaries have decent power in such a heavy vehicle with a stock 30 pound flywheel. If I threw in a light steel flywheel, like in the GLC, it would be a more accurate comparison.
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