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not enough power!!
hi, i just got my new exhaust put on which includes headers and 2.5 inch piping right to the back with 2 thrush mufflers (one in the middle of the car and one at the end, and is very noisy) i also have cold air intake with a cone airfilter and ported out the intake manifold. It has a 12a carburated engine. I am still not satisfied with the power i did not notice much of a difference with the new exhaust and feels like the car does not have enough torque. Could i be loosing back pressure? Any suggestions?
Thanks, Jason |
How did you port the intake manifold?
I hop you didn't open up the primary port runners to the size of the ports in the engine. If you do that, you introduce tons of turbulence which kills top-end power and you lose the port velocity and the important port mismatch, which kills low-end and midrange. |
by saying that poting the primarys u mean the two middle ports on the intake manifold? no the ones on the outer edges of the manifold right?
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isn't it something like if you change the porting, not only is it killing the velocity into the intake, but also taking away from the all important atomization of the fuel which kills power because your not getting a complete combustion of the fuel? Correct me if i'm wrong anyone.
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Cone airfilter on a carb... what?!
And I agree, you probably fuxed up that intake manifold. |
the 79 rx7's had bigger ports on the intake mainifold that matched the size of the engine intake ports then later they wanted more economy so they made the intake ports on the intake manifold smaller, so all i did was make them the same size as on the engine block.
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I've found that there's nothing that controls noise better than the stock muffler or Racing Beats Prima Flow muffler. Adding a header to a rotary increases noise level. Racing Beat also offers presilencers. If ur using the stock carb, u might want to check into a performance intake and carb setup. I personally prefer Racing Beats Holly kit. They claim that with there intake and carb kit and exhaust system, u get a 25% increase in power. Whatever setup u chose to use, u'll have to replace ur fuel pump with one that flows at least 6 psi.
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i have a pipe running from the carb to infront of the rad with a cone air filter on the end
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the 79 rx7's had bigger ports on the intake mainifold that matched the size of the engine intake ports then later they wanted more economy so they made the intake ports on the intake manifold smaller, so all i did was make them the same size as on the engine block. When I installed my RB Dellorto manifolds, they don't match the stock intake ports on the engine either. And remember, this is a Racing Beat intake manifold designed for maximum power. In the Racing Beat instructions, they specifically and strongly warn against porting the intake manifold to match the engine! They state that this will have negative effects on performance. I think the people that posted above are correct. |
Originally posted by buckfutter125 then later they wanted more economy so they made the intake ports on the intake manifold smaller, so all i did was make them the same size as on the engine block. Hate to say it but your manifold is pretty much garbage now unless you're good with JB Weld to get it back to the way it was originally. |
I have a spare manifold off of an 83 GSL Ill sell :).
~T.J. |
i guess i was told some false info on porting the intake but its no problem because i got a whole shit load of rx7 parts so i will just put my original intake manifold on.
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Here's the article I was talking about... glommed it from the RX-7 FB mailing list...
I purposely make the manifold runner smaller than the port runner on all engines that I build. A small offset makes very little difference in the forward flow, but it makes a huge difference in the reverse flow. This is a big deal because the charge in the manifold is moving in both directions during each cycle. This is due to residual exhaust pressure in the chamber, and the fact that the intake port closes well after BDC. At low rpm, a large portion of the intake charge gets spit back into the manifold during the period from BDC of the intake cycle, to intake port closing, which is the end of the intake cycle. Anything that you do to increase the reverse flow potential of the induction system will reduce low speed power. If you don't believe me, and want a visual representation, all you need is a 12A with a stock manifold. From under the hood, open the throttle all the way until the secondaries open. (Don't blow it up.) You will see a large cloud of gas vapor spit out of the secondaries, BUT NOT THE PRIMARIES! The stock primaries in the manifold are smaller than the ports in the motor by nearly a half inch on both the top and the bottom. This limits the reverse flow, and that is why the primaries do not blast a cloud of mixture out the top of the carb. This spitback at high rpm is due to residual exhaust pressure, so you can see that discouraging reverse flow can help throughout the powerband. (Hey, I thought that was a trade secret!) (stuff about Lake Cities manifold deleted) As a side note, do not EVER port match the primaries of a stock 12A manifold. You will lose low, AND high speed power. The reverse flow will increase, and the forward flow will decrease. Here's why. By making the port taller, you will increase the velocity differential between the "long side" and "short side" of the port. This will lead to separation of the airflow on the short side, (It's bad enough already!) and the majority of the port will be a big swirling vortex, rather than a nice gradual flow path. A drastic area change should never occur in the middle of a turn., Or in the words of a very bright friend of mine (Who designed the water cooled engine at Moller.) "You cannot diffuse and turn at the same time!" I have made nearly 190 horsepower with the stock manifold and quiet street exhaust without enlarging the primary runners at the exit. On an obnoxious race motor, I have made in excess of 210, again without changing the height of the primary runner exit. Have you guys seen the bumper sticker that says "Friends don't let friends drink and grind." Paul Yaw |
I always see your car at Canadian Tire, and I saw it at 7-11 by R.E. yesterday, loud!
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hey, WINNIPEG85GSL what color is your car maybe iv seen it around. Anything done to the engine? Anyways my exhaust is too loud and i think part of the reason i am loosing power is due to not enough backpressure so I am putting a muffler at the back instead of a cherry bomb.
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Backpressure never makes power, on any kind of engine. Especially rotaries!
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Oh no! Backpressure once again rears it's ugly head. Peejay's right, any extra backpressure just means more work for the engine. What you want is for your exhaust to be as free flowing as possible for a given, predetermined header length and pipe diameter. The reason you don't wanna go too big is because you'll lose velocity.
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my tender blue car has been off the road for the past couple months cause of a .. ............ :eek: 302 swap
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Not enough power? What were you expecting from the mod?
The best prediction for the new system would be an increase of around 20hp, if its substandard may be 15hp. the best impact on performance would be about .5-.6 of a second over a 1/4 mile. That is a 3% improvement. You could not measure this subjectively. Expecting a significant improvement may persuade you there is no impact when in practice the potential has been acheived. Without a dyno, the solution is to compare the average of three runs before and after the mod. The points on manifold porting and backpressure are interesting, the Forum is a great place to learn. The real qustion is what to do now. I suggest some timed runs and compare it with published data. Any improvement over say above .4 second shows the potental of the mods has been treached. the level of noise is a different matter. |
this is kind of off topic, but any suggestions for a muffler to put at the back of the car i now have 2 cherry bombs and want to take the one off the back and replace it with a muffler. I was looking at racing beat but want something a bit cheaper. I mainly want a muffler that will quiet it down a bit without taking away from the flow too much.
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Quiet, cheap, free-flowing, long-lasting. Pick two.
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I've got a Canadian Tire $39.99 special muffler, a presilencer off of an '81 Hon-Duh Accord, and a straight pipe on my car, and it's nice and quiet... until I step on the gas. I quite like the sound of my car. Perhaps I'll get it on tape, or video some time...
Jeff |
what kind of muffler did you buy from canadian tire?
I work at canadian tire in the parts department so if i can find a mufler that is decent from them i could get it for next to nothin |
Sure is a lot of Canadian chatter going on here. Keep looking up guys :) Your America's Junior!! J/K, just saw that on "the simpsons" a week ago and Homer said that! But thought it was funny :D But seriously, I love going to Canada.
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