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Old 05-22-2007, 02:45 AM   #1
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Default Are our internals horsepower limited or boost limited?

To better phrase that, if some are safe running 12psi out of large-ish turbos like T3S60s and stuff, am I safe running 15-17psi for short bursts out of a T25 as long as I can keep the intake temps down (using WI)? Are the rings in the Miata motors limited by actual pressure or power?
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Old 05-22-2007, 03:08 AM   #2
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You are really limited by the quality of your tune up to about 270-300hp. That is what it seems like. Remember that there are people that blow up their motors at less boost and less hp.
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Old 05-22-2007, 03:59 AM   #3
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boost indirectly limits how much you can safely run through the motor. Two things cause great stress for the motor given it is tuned right: high hp AND high rpm. The rods are the first to go.

You will be pushing the envelope on stock rods at 275+whp but any increase in power will results in added stress to the motor in any case.

Of course proper cooling, etc have to be present to support the power level you are running.
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Old 05-22-2007, 04:11 AM   #4
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Quote:
Originally Posted by Lex View Post
Two things cause great stress for the motor given it is tuned right: high hp AND high rpm.
Actually I believe the two would actually be torque and high rpm. Horsepower is a function of torque and rpm whereas torque is a calculation of how much power each stroke of a piston is putting down, which in turn estimates the ammount of force pushing down on the piston (which pushes on the wristpin, which pushes on the rod, which pushes on the crank). High rpm causes a slightly different sort of stress.
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Old 05-22-2007, 04:19 AM   #5
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Quote:
Originally Posted by akaryrye View Post
Actually I believe the two would actually be torque and high rpm. Horsepower is a function of torque and rpm whereas torque is a calculation of how much power each stroke of a piston is putting down, which in turn estimates the ammount of force pushing down on the piston (which pushes on the wristpin, which pushes on the rod, which pushes on the crank). High rpm causes a slightly different sort of stress.
of course hp is derived from torque but I mentioned hp since it is directly related to torque and is always the higher curve past 5252 RPM.

and high RPM simply caused the moving parts to be accelerated faster and stress is thus applied and removed at a higher rate causing .... well more stress on the metal.
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Old 05-22-2007, 04:37 AM   #6
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you have a good point. I can see how a constant torque would create far greater stress at 7k rpm as opposed to 4k rpm due to the rate at which force is applied and removed. So maybe hp is a better general indicator of the capabilities of a motor given a good tune?
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Old 05-22-2007, 07:25 AM   #7
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I'd venture to guess that 90% of snapped rods in Miatas are from knock. Not many people run race gas in their Miatas, especially non-built ones, but I bet you wouldn't see the same failures at 300whp on race gas as people wouldn't be knocking.

Frank
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Old 05-22-2007, 08:39 AM   #8
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Originally Posted by fmowry View Post
I'd venture to guess that 90% of snapped rods in Miatas are from knock. Not many people run race gas in their Miatas, especially non-built ones, but I bet you wouldn't see the same failures at 300whp on race gas as people wouldn't be knocking.

Frank
True nock and shock laoding like the ones who where running 270+ whp and put in a wet shot of NOS.
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Old 05-22-2007, 11:19 AM   #9
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well the torque and hp and boost and knock are all going to have an effect on.... cylinder pressures. while power can influence them, it wont have nearly the effect of detonation, preignition, or knock. compressing an explosion too early is not good for rods.

but there are also reports of missed-shift rod failures. which makes me wonder if the high vacuum forces are slightly stretching them and weakening them.
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Old 05-22-2007, 11:28 AM   #10
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high vacuum at high RPM is very detrimental to rod bolts due to stretching - this is why it is recommended to change the rod bolts when doing an engine build.

However, like it was said, a lot of the failures are due to a poor tune or something going wrong. You don't need race gas to run 20psi on the Miata motors, just enough ignition retard and fuel.
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Old 05-22-2007, 11:46 AM   #11
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ignition retard whats that we wanna advance ours
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Old 05-22-2007, 12:16 PM   #12
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Quote:
Originally Posted by Lex View Post
However, like it was said, a lot of the failures are due to a poor tune or something going wrong. You don't need race gas to run 20psi on the Miata motors, just enough ignition retard and fuel.
unless you're ignition is behind LPP, then output falls off with diminishing returns.


Is this where lowering compression will add power...because now more boost can be added without detonation or pre-ignition?
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Old 05-22-2007, 02:15 PM   #13
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boost effectively raises compression ... it has a similar effect to running a higher static compression ratio. So you have to make a compromise at some point between boost, compression ratio, and ignition timing. I run 7 degrees advance timing at my maximum torque point and boost to be on the safe side. If you really want to run high octane, run some natural gas as fuel Or you can do some work on the combustion chamber to remove hot spots.

But realistically, a 9:1 compression ratio will let you run lots of boost ... easily over 20psi on 94 octane pump gas and well into the 300whp territory. Also throw enough fuel in there to keep combustion fairly cool. The stock rods will go before you've maxed power due to detonation.
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Old 05-22-2007, 02:47 PM   #14
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this thread is making me question my supertech 8.6:1 piston purchase.
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Old 05-22-2007, 02:53 PM   #15
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8.6 compression is not that low - it should be ok on and off boost. A slightly higher compression helps spoolup time and torque off boost while the slightly lower compression will give you a bit more of a safety margin and may allow you to use cheaper gas depending on your tune and amount of boost you run.
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Old 05-22-2007, 02:57 PM   #16
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Quote:
Originally Posted by Lex View Post
8.6 compression is not that low - it should be ok on and off boost. A slightly higher compression helps spoolup time and torque off boost while the slightly lower compression will give you a bit more of a safety margin and may allow you to use cheaper gas depending on your tune and amount of boost you run.
I went for the forged lowered comp pistons because I track the car, and want the extra safety. I thought it was good insurance, and I want to avoid rebuilds.
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Old 05-22-2007, 02:59 PM   #17
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Quote:
Originally Posted by Lex View Post
You don't need race gas to run 20psi on the Miata motors, just enough ignition retard and fuel.
That's exactly why I run race gas, to not have to retard as much..

**** it, if 93 is 3.65, and race gas is 4.85, I know what I'm doing, drinking one less bottle of gray goose per month.
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Old 05-22-2007, 03:01 PM   #18
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Quote:
Originally Posted by hustler View Post
I went for the forged lowered comp pistons because I track the car, and want the extra safety. I thought it was good insurance, and I want to avoid rebuilds.
Unnecessary cost. The pistion is unlikely to wear out and require replacing, rings on the other hand ... But really, do you think you'll own this car for 10 more years?
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Old 05-22-2007, 03:02 PM   #19
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ignition retard sucks ***, i know why the WI love it sooo much now.
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Old 05-22-2007, 03:03 PM   #20
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what is your guys' minimum ignition advance under boost?
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