Ordering pistons what compression?

[Alky V6]

I like 9.1 to 9.5-1. I have run as much as 10.5-1 but as Don hit on, the higher compression will make your tuning window smaller. I tune a local turbo car with 12.2-1 and 22psi of boost and it's very touchy. We are trying this because of the small turbo limitation in the class keeps the boost under 25psi. I feel I have to pull the timing back so far that I could make just as much power with 10.1 and more timing. I'm not sure going higher than 9.5-1 on gas has any gains because you have to run less timing to keep cylinder pressure in check.

My answer. 9.1-9.5-1

That was with C16???

 

[Alky V6]

Norbs, what intercooler efficiency are you planning on again?

 

[bison]

Sounds reasonable, Bison posted that ( you) Allan should be running 10.25, or your combo will need 40 psi boost to make best results, do you think this is safe?

That's not what I posted. I posted that on his current tune 10.25:1 would make more power. His CR and and cam will make peak power with boost in the low 40's with the CR he's running. 10:1@ 30psi is no problem.

 

[bison]

Just drawing some lines on this graph it looks like about 3% more power going from 8:1 to 9:1. You can see that the increase in thermal effeciency (power) gets lower and lower the higher you go. But, the risk of detonation gets higher and higher. I'm sure compression ratio has some effect on spool up and converter also, but I don't know to what extent.

Thermal effeciency is direcly related to hp: HP = TE x FUEL FLOW (PPH) x 7.466
For a 1000hp engine 3% is 30 hp. What do most v6's in the 1000hp range pick up with 1 lb. of boost?

I scanned this from the book "Engineering Fundamentals of Internal Combustion Engines" by Pulkrabek.

That may be true on a naturally aspirated engine but if you're looking for power on a turbocharged engine you're looking heavily at the ex stroke and the cam profile/timing to clean the cylinder out.

 

[Alky V6]

I recently finished tuning on another methanol burning Buick V6 with even less static CR than mine. 8.9:1. First time out with the car it did 5.90s in the 1/8 with 18 psi boost, had nitrous to spool a 91mm, but didn't really need the nitrous to spool it. It was using a T4 turbine housing. And, that was just on the primary fuel system. We didn't even kick in the secondary injectors during those tests.
High static CR is not all it's cracked up to be.

 

[Dusty Bradford]

That was with C16???

If I'm racing a motor it's on C-16 or Q-16. Either way I'd still run the same compression on a pump gas motor. A pump gas motor won't be pushed to the edge or it wouldn't be on pump gas. I'd much rather drive around town on a 9.5-1 motor than an 8.5-1 motor.

The cam is something else that has to be taken into consideration. You can have an engine that is 10.5-1 with a cam the bleeds off cylinder pressure and pumps 210psi.........another can be 9.5-1 with a different cam and pump 230psi.

 

[Alky V6]

If I'm racing a motor it's on C-16 or Q-16. Either way I'd still run the same compression on a pump gas motor. A pump gas motor won't be pushed to the edge or it wouldn't be on pump gas. I'd much rather drive around town on a 9.5-1 motor than an 8.5-1 motor.

The cam is something else that has to be taken into consideration. You can have an engine that is 10.5-1 with a cam the bleeds off cylinder pressure and pumps 210psi.........another can be 9.5-1 with a different cam and pump 230psi.

Just pray you guys don't have to eventually deal with the same octane of pump gas us in California have to deal with.

 

[Alky V6]

I built a n/a 360 Chrysler engine for a fella in a Dakota. The head supplier told us, "10:1 CR. No problem." WRONG. The thing had a transitional knock that was a pain to tune out. The customer went through a few tuners before he got it to stop knocking and the power lost from whatever they had to do was noticeable. The first tuner he was using was very well known for tuning on Dakotas. He was somewhere in Florida. California pump gas must really be crap compared to everywhere else.

 

[Dusty Bradford]

I am about to order some pistons for my stage project, what compression is the best to choose, I don;t really think I;ll be going over 30 psi boost though on c-16 maybe 23 psi on alky and 93. looking at a 230 cam about or so in size.

Just pray you guys don't have to deal with the octane of pump gas us in California have to deal with.

If his original question was "What compression ratio is best to use on 87 octane" I would have a different answer. The lower compression could help widen the tuning window. But for his application the higher compression will work just fine.

 

[turbobitt]

11:1 flat top pump gas 460 ford jet boat making about 500 hp @ 5200 rpm with aluminum heads and no problems. Engine is always at high rpm. (About 32-34 degrees timing)

10.75:1 Mopar 340 W2 iron head motor with no problems (About 36-38 degrees timing)

11:1 Mopar 572 Aluminum head Hemi with no problems (About 32-34 degrees timing)

All run on 93 octane pump gas

AG.

 

[Alky V6]

11:1 flat top pump gas 460 ford jet boat making about 500 hp @ 5200 rpm with aluminum heads and no problems. Engine is always at high rpm.

10.75:1 Mopar 340 W2 iron head motor with no problems

11:1 Mopar 572 Aluminum head Hemi with no problems

All run on 93 octane pump gas

AG.

The highest we have around here is 91, and who knows if it's really even that. The libs here in Cali probably line up somewhere to relieve themselves in it.

 

[turbobitt]

The highest we have around here is 91, and who knows if it's really even that.

I updated my list to include ignition timing if it matters.
Allan G.

 

[norbs]

Allan its a nice list, but n0ne of those are turbo applications, I need some turbo motor data. Check out this this calculator, shows 8:1 =24.1 at 30 psi?


http://www.rbracing-rsr.com/compression.htm

 

[turbobitt]

Allan its a nice list, but n0ne of those are turbo applications, I need some turbo motor data. Check out this this calculator, shows 8:1 =24.1 at 30 psi?


http://www.rbracing-rsr.com/compression.htm

Its just a bunch of numbers that don't mean crap. My above list was to illustrate that with the right combination, it can work. Getting back to our turbo cars, many have run 9:1. My old combination was 8:1. I think that overall it is much more responsive at 9:1. Based on what I know now and see, I could see why others have chosen to go more.

Norbs, I would think that you are much better than average when it comes to tunning. I don't think you will find it hard to tune or have a small tuning window. Look at it this way, with 8:1, your going to push the timing to get back what you lost compared to 9:1. In my opinion, blows away the conservative approach theory.

I run nothing but pump gas when driving on the street. I have run as high as 15# boost without issues. Although I don't make it a habbit to beat on my stuff with pump gas I do like to indulge myself now and then.

Whatever you chose, I'm sure it will work.

 

[norbs]

Thanks for the support , I Know I won;t be hitting too much high boost around town I think 9.5 will be a good compromise, so that is what I am going to use, the 230 cam might bleed some off at lower rpm so it may help in response as you have found.

 

[turbobitt]

Thanks for the support , I Know I won;t be hitting too much high boost around town I think 9.5 will be a good compromise, so that is what I am going to use, the 230 cam might bleed some off at lower rpm so it may help in response as you have found.

Did you mean 8.5 or 9.5 ?
Allan G.

 

[Alky V6]

9.5:1. Cool. I look forward to seeing how it works out.

 

[750H.P.V6]

Norb,

9 - 9.5 to 1 works fine. My last motor had iron heads on it with 9 to 1 compression and drove nicely on the crappy gas we get here in CA. I just got the new pistons for the Stage II motor I'm building for my street car and they are 9.5 to 1. BTW, did you end up talking to Tim Cole about a cam for your combo?

Neal

 

[ijames]

The lower the static compression and the higher the boost the more peak hp you will make, and the wider the tuning window will be for afr and timing (the short explanation is that a turbo is a more efficient air compressor than a piston). However, off boost with very low compression the economy and power will suffer and spoolup will be terrrible. On gasoline much below 8:1 is a real dog to drive and you would have a hard time getting boost up in time for a normal tree, much less a pro tree .

 

[Alky V6]

The lower the static compression and the higher the boost the more peak hp you will make, and the wider the tuning window will be for afr and timing (the short explanation is that a turbo is a more efficient air compressor than a piston). However, off boost with very low compression the economy and power will suffer and spoolup will be terrrible. On gasoline much below 8:1 is a real dog to drive and you would have a hard time getting boost up in time for a normal tree, much less a pro tree .

I've built more than a few 8 to 1 motors for people, mainly street use, and performance and spool up have never seemed to be an issue. If anything, with the turbos that most people are using on the street, the spool up was too quick for my liking. One was spooling so quick, transitional knock was a problem. He was running a turbo tweak chip that had to be sent back to be re-programmed. You guys make it sound like 8 to 1 is such a pig. Isn't the stock CR 8 to 1? A lot of people have done some pretty amazing stuff with an unopened short block, yet 8 to 1 is for pigs? I don't get it.
You know, some people have no business playing with CRs above 8 to 1 to begin with. I'm sure Norbs will do fine, but you know what I mean. Sometimes you have to look out for people and set them up with something that will survive the stupid stuff that happens sometimes. Even 8 to 1 doesn't guarantee damage won't occur with a novice behind the wheel.
8 to 1 makes a pig. Amazing. I guess my customers are just coming up with better combos.