need help with stroker, turbo combo

v6cobra

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im currently in the process of rebuilding a cobra motor and was looking to turbo. i was also thinking of a stroker kit to go along with it. im hoping to run around 20 psi of boost with the kit but im not sure what compression i should run because i am still hoping to use 93 octane pump gas. the options i was looking at for the kit were a 23 dish piston with would give about a 8.68:1 and a 17 dish for about 9.19:1. I will still have a stock bore of 3.552. any info would be appreciated on this. let me know guys :shrug:
 

IUP99snake

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Id go with the 8.68:1 for Turbo or Top Mount. 9.19:1 would be more for a centri blower.

I think that's a good idea. With the amount of boost you plan on running, and with a turbo, a lower compression won't hurt. Good point about the centri blower, Ironhand.

However, a 9.19:1 isn't pushing it too much. I certainly wouldn't go further than 9.5:1 though.

Since the compression is a constant, the one thing you can't change about the motor, (no matter what RPM range you're at, it's low compression) I think it's better to go with a little higher compression. You can compensate with all the other variables, including timing, fuel map, fuel type, meth, boost level, etc.

A higher compression engine, N/A or boosted is always going to be more efficient. In this case, you'd theoretically have as much as 2% more power with the higher compression option, with all else held constant. Even if you have a higher risk of detonation, that can be managed at the RPM levels most at risk using meth injection especially.

Don't be afraid of the compression!

I have 10.5:1 compression!! Granted, I have a centri car and I only make 10-12 PSI of boost. The higher compression makes the engine more efficient and I have better throttle response at low RPM before the boost builds up. It's MUCH better for around town driving.

Homer
 

v6cobra

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I think that's a good idea. With the amount of boost you plan on running, and with a turbo, a lower compression won't hurt. Good point about the centri blower, Ironhand.

However, a 9.19:1 isn't pushing it too much. I certainly wouldn't go further than 9.5:1 though.

Since the compression is a constant, the one thing you can't change about the motor, (no matter what RPM range you're at, it's low compression) I think it's better to go with a little higher compression. You can compensate with all the other variables, including timing, fuel map, fuel type, meth, boost level, etc.

A higher compression engine, N/A or boosted is always going to be more efficient. In this case, you'd theoretically have as much as 2% more power with the higher compression option, with all else held constant. Even if you have a higher risk of detonation, that can be managed at the RPM levels most at risk using meth injection especially.

Don't be afraid of the compression!

I have 10.5:1 compression!! Granted, I have a centri car and I only make 10-12 PSI of boost. The higher compression makes the engine more efficient and I have better throttle response at low RPM before the boost builds up. It's MUCH better for around town driving.

Homer


thanks for the info. im relatively new to the whole world of boost so ive got a lot of questions lol. do you still run pump gas on your setup? i am still partial to the higher compression rather than the low. so theoretically it IS possible to run the 9.19 with 20 psi on 93 pump gas, with the needed timing and fuel map adjustments, but of course the higher risk of detonation? what if i dropped the boost by 3-5 psi, will that run a little more on the safe side? my goal is to hopefully put down at least 700 to wheels.
 

61mmstang94

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With the increased boost you'll be able to run with lower compression you'll easily make up that little 2% loss from the lower compression pistons and your tunes on pump gas since you'll be capable of running more boost and it will be safer. You'll be happy and just fine with the 8.68 pistons for a street car running pump gas.
 

Ironhand

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thanks for the info. im relatively new to the whole world of boost so ive got a lot of questions lol. do you still run pump gas on your setup? i am still partial to the higher compression rather than the low. so theoretically it IS possible to run the 9.19 with 20 psi on 93 pump gas, with the needed timing and fuel map adjustments, but of course the higher risk of detonation? what if i dropped the boost by 3-5 psi, will that run a little more on the safe side? my goal is to hopefully put down at least 700 to wheels.

700rwhp+ is done all the time with 03-04 4.6Ls with a single 76mm Turbo. 15-17lbs of boost and 15-17 degrees on timing on 93 octane and 8.5:1 compression.

Since you are using a turbo it is actually more effecient to up the boost to make up for the lower compression because the turbo will make more then 2% more power with the increase in airflow ie boost or the additional timing u can safely put into the car due to a lower static compression ratio.

Yes it is possible to run 9.19:1 with 20 psi on 93 pump. However that will come at a cost. Cost of extremely low timing, sluggish spool, and poor throttle response. In those guidelines, 15psi with more aggresive timing with produce more power then your 20psi. Mainly because all of the timing u would be able to add at the lower boost pressure.
 

v6cobra

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700rwhp+ is done all the time with 03-04 4.6Ls with a single 76mm Turbo. 15-17lbs of boost and 15-17 degrees on timing on 93 octane and 8.5:1 compression.

Since you are using a turbo it is actually more effecient to up the boost to make up for the lower compression because the turbo will make more then 2% more power with the increase in airflow ie boost or the additional timing u can safely put into the car due to a lower static compression ratio.

Yes it is possible to run 9.19:1 with 20 psi on 93 pump. However that will come at a cost. Cost of extremely low timing, sluggish spool, and poor throttle response. In those guidelines, 15psi with more aggresive timing with produce more power then your 20psi. Mainly because all of the timing u would be able to add at the lower boost pressure.


when you were talking about the 15 psi making more power than 20 with the more agressive timing, are you talking about the higher or lower compression or both?
 

Ironhand

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when you were talking about the 15 psi making more power than 20 with the more agressive timing, are you talking about the higher or lower compression or both?

Both but look at it this way.

93 octane is your common factor for making power. Static compression plus boost plus timing gives u power but within the confines of the octane gas you are using. The more boost you run the less timing you can run. The more timing you run the less amount of boost you can safely get by before detonation. Same for static compression ratio. The more compression i.e 10:1 w/12lbs the less boost you can safely use before the cylinder temps and/or pressures are the same as 8.5:1 w 19lbs. Its all a give and take setup.

The harder you spin a blower the more heat it generates. If you can take the same 15lbs and chill it down 50 degrees you can safely add alot more timing to equal more power before detonation. Its mainly about heat. Kill the heat, up the boost and add the timing. Go to far with either and you have to back one of the other two off to compensate for it.
 

61mmstang94

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when you were talking about the 15 psi making more power than 20 with the more agressive timing, are you talking about the higher or lower compression or both?

The point is that higher compression = more power at same boost level. So at 10 psi the higher comp. will make a little bit more, nothing drastic, but it will make a little more.

But the thing here is that you are running pump gas so with a higher compression you might only be able to run 10 psi (just an example), but with lower compression you might be able to run 15psi. So even though the higher comp. might make more at 10psi, that is its limit where the lower comp. will be able to run more boost safely and make up for that because the lower comp. at its 15psi limikt will make more than the higher comp. at its limit of 10psi.
 

CPRsm

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The point is that higher compression = more power at same boost level. So at 10 psi the higher comp. will make a little bit more, nothing drastic, but it will make a little more.

But the thing here is that you are running pump gas so with a higher compression you might only be able to run 10 psi (just an example), but with lower compression you might be able to run 15psi. So even though the higher comp. might make more at 10psi, that is its limit where the lower comp. will be able to run more boost safely and make up for that because the lower comp. at its 15psi limikt will make more than the higher comp. at its limit of 10psi.
Exactly. All comes down to a given cyl pressure for an octane.
I'd take 15 psi over 10 any day. I'd also take 10 to 1 over 9 to 1 without detonation, at any given boost.
 

v6cobra

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thanks guys for all your help. this info helps out a lot and it seems i still have some thinking to do but this does clear up some things. :beer:
 

dadro77

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I'm doing the same build and went with 23 cc CP pistons. I'm running a .70 mm turbo for now....
 

speedofsound

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Sounds like you got your info, but I'll throw in my old setup for kicks. 9.5:1 with twins at 15psi. I still wanted to retain some low end grunt.
 

v6cobra

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I'm doing the same build and went with 23 cc CP pistons. I'm running a .70 mm turbo for now....

how far along are you on the build? do you have an idea of about how much boost you are thinking about running?


Sounds like you got your info, but I'll throw in my old setup for kicks. 9.5:1 with twins at 15psi. I still wanted to retain some low end grunt.

if dont mind me asking what type of heads do you have? do you still run pump gas? about how much timing are you running?
 

dadro77

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how far along are you on the build? do you have an idea of about how much boost you are thinking about running?




The block and rotating assembly are at the machine shop getting the .20 over and balanced. I think I will be 14-16 lbs of boost. I just want to keep it in the 600 rwhp range for the street, then in a couple years I can up the boost and make it a weekend car.....
 

speedofsound

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if dont mind me asking what type of heads do you have? do you still run pump gas? about how much timing are you running?

Stock heads and still on pump gas. I cannot remember the timing when I had twins, but now with the F1A I'm at 16.5*.
 

v6cobra

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Stock heads and still on pump gas. I cannot remember the timing when I had twins, but now with the F1A I'm at 16.5*.

how did you like the old setup and what was your reason for switching to the F1A? roughly what did you make to wheels with the old? thanks
 

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