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IT7 12a HP ???

Old 08-24-08, 05:28 PM
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IT7 12a HP ???

I just brought a '81 RX7 to Ontario, Canada to race at Mosport. We don't have an "IT" class here and was wondering what kind of horse power out put a IT7 engine (12a)will put out. It has been balanced and blueprinted with modified carb, RB header and 2 1/2" exhaust straight back to a single resonator.

Ball park ??
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Old 08-24-08, 06:15 PM
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The big difference from IT sounds like the carb. Just how modified is it? If it is close to stock with just the extra junk removed you should be able to get about 125-130hp to the wheels. More carb mod's could get you up to about 140 with a stock port and what you listed.
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Old 08-25-08, 06:45 PM
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I had a 1983 engine balanced and built by a reputable builder (now apparently out of the biz? Stan ...) After breakin on an engine dyno, it put out 147hp at the crank, with a stock, but jetted carb.
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Old 08-27-08, 03:38 AM
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OK ! ..........I dug up more info........I have a Sterling carb ! and the engine was balanced and blueprinted by ISC Racing in Florida.
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Old 08-27-08, 06:17 AM
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Originally Posted by dyoungre View Post
I had a 1983 engine balanced and built by a reputable builder (now apparently out of the biz? Stan ...) After breakin on an engine dyno, it put out 147hp at the crank, with a stock, but jetted carb.
It is my opinion that 150 crank HP / 115 wheel HP is the limit of the stock carb even with a fresh engine and free exhaust.

Of course if the carburetor is bored out bigger, and the manifold is ported and matched you can get more however.

There is quite a bit of pressure drop from the stock carby in the upper rpm region. I was pulling as much as 3psi pressure drop across my carby even at wide open throttle with my (small) streetported 12A. I've since put TII throttles on it and I have about 1/20 of the pressure drop I used to have at the same rpms.

I'm pretty sure my new intake setup should be good for another 20% or so power above my previous dyno run of 115rwhp.
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Old 08-27-08, 09:58 AM
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You could also have bigger horses down under so you end up with fewer...

Really at these power levels you will see quite a variety of numbers depending on what brand of dyno is used. The best way to know if you have enough power is to put it on the track in the class it runs and see if you can keep up on the straights. If you are confident you are exiting the turns well and everybody runs away from you then worry about it. You've got some good parts in the mix, just use the dyno to make sure everything is the best it can be with tuning and go have fun.

If you do something that you think will give you more power go back to the same dyno. If you don't there is a good chance you will be comparing apples to oranges with the numbers you get.
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Old 08-27-08, 10:43 AM
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Good news on the Sterling! You've got a great carb to work with.
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Old 08-29-08, 06:15 AM
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According to Paul Yaw, he was hard pressed to get 155 at the crank with a stockport, no matter what he did to the carb.
jgrewe is right about dynos and such low HP #s. At around 120-140 HP =/-, a 4-5% variance between dynos can really screw you up to where you can't tell what's a better jetting direction. Always better to use the same machine to compare, especially when tuning.
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