Are our internals horsepower limited or boost limited?
#1
Former Vendor
![](/mt2011/images_td/ranks/supporter1.gif)
Thread Starter
iTrader: (31)
Join Date: Nov 2006
Location: Sunnyvale, CA
Posts: 15,442
Total Cats: 2,103
![Default](https://www.miataturbo.net/images/icons/icon1.gif)
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?
#3
![Default](https://www.miataturbo.net/images/icons/icon1.gif)
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.
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.
#4
Elite Member
iTrader: (5)
Join Date: Jul 2006
Location: Central California
Posts: 2,557
Total Cats: 5
![Default](https://www.miataturbo.net/images/icons/icon1.gif)
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.
#5
![Default](https://www.miataturbo.net/images/icons/icon1.gif)
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.
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.
#6
Elite Member
iTrader: (5)
Join Date: Jul 2006
Location: Central California
Posts: 2,557
Total Cats: 5
![Default](https://www.miataturbo.net/images/icons/icon1.gif)
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?
#9
2 Props,3 Dildos,& 1 Cat
![](/mt2011/images_td/ranks/supporter1.gif)
![](/mt2011/images_td/ranks/smod.png)
iTrader: (8)
Join Date: Jun 2005
Location: Fake Virginia
Posts: 19,338
Total Cats: 573
![Default](https://www.miataturbo.net/images/icons/icon1.gif)
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.
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.
#10
![Default](https://www.miataturbo.net/images/icons/icon1.gif)
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.
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.
#12
Tour de Franzia
![](/mt2011/images_td/ranks/supporter1.gif)
iTrader: (6)
Join Date: Jun 2006
Location: Republic of Dallas
Posts: 29,085
Total Cats: 375
![Default](https://www.miataturbo.net/images/icons/icon1.gif)
Is this where lowering compression will add power...because now more boost can be added without detonation or pre-ignition?
#13
![Default](https://www.miataturbo.net/images/icons/icon1.gif)
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.
![](https://www.miataturbo.net/images/smilies/smile.gif)
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.
#15
![Default](https://www.miataturbo.net/images/icons/icon1.gif)
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.
#16
Tour de Franzia
![](/mt2011/images_td/ranks/supporter1.gif)
iTrader: (6)
Join Date: Jun 2006
Location: Republic of Dallas
Posts: 29,085
Total Cats: 375
![Default](https://www.miataturbo.net/images/icons/icon1.gif)
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.