Effects of compression ratios

[karolko]

Hello Again,

i have been posting like a mad man trying to learn all i can from everyone else's experiences.

So here is this weeks question:

how will increasing compression from 8.5:1 to 9.0:1 effect things like spool-up time, the converter you can run, the octane you are limited to, effects on knock, responsiveness on the street, turbo psi limitiations... everything.

As you probably know i am at a higher elevation (3500 feet) so this will also play a roll in my specific case.

i know there are some of you with real life experience with this stuff, and i feel that it has not been discussed as well as it should be.

thanks boys

 

[Alky V6]

If your choice is between the two. I would go with the lower CR. I've played with a wide range of CRs with my engine and learned that the higher CRs are going to put a limit on your max boost. If you plan on never running a high boost and the turbo you're using will not have a tendancy to want to run and work best at a high boost, then go with the higher CR understanding that you will need to stick with a certain boost limit. I did that with my last engine and a darn boost spike that went past my boost limit took out the engine. I like having the lower CR and not worrying if I have a stray boost spike.

You probably wouldn't even be able to tell the difference between the 8.5 and 9.

My last engine had a CR of 11.3 to one, with a boost limit of 22 psi. Blew it up due to a short spike to 28 psi.

My present engine runs with a 9.24 to one CR, with a boost limit of around 40 psi. Once I get the new turbo on, I'll be exploring that boost region, but the math says that's where the limit is going to be.

The torque converter stall between the 2 engines was around 2800 with the earlier and is 2440 with the later, at zero boost and the same T/C. So the CR change did drop my stall speed. Alcohol likes CR.

Engine is not as crisp off boost, but a change in the engine from 9.3 to 11.3 CR is expected. Especially with alcohol.

The small cams that are often chosen by most will develop a lot of midrange cylinder pressure. The lower CR will help protect against detonation in that rpm range.

The lower CR will give you much more cushion/protection against a bad tuneup.

If you need to increase track performance just use more boost.

My car is performing the same with the lower CR and 24 psi boost as the earlier engine did with 20-22 psi boost.

 

[karolko]

Hello Don,

Well for one thing, i will not be running 40psi. 30ish max at the track with c16.

the main reason i wanted to run a tad bit higher compression is just to help with streetability with a slightly larger turbo on my 3.8

my goal is to have a 70 or slightly larger turbo on the street that is as fun as a 67.

My hopes were that the little bump up in compression would be what i would need to help achieve this goal, while not having any determental effects. (i.e. detonating to early).

 

[John Wilde]

We have been doing it for years I would not worry about it.

 

[bison]

I wouldnt go any less than 9:1. If you plan on nothing but c16 then id go 9.3-9.5:1. It increases driveability and lowers stall requirement slightly. Cam selection becomes important as you go up in CR. Higher CR and boost =winning more races. All the high hp TSM and TSO guys have higher CR than stock

 

[d90dan]

I've always had 8.0-8.5CR on my street car. Higher on the race car. Spool up is noticeably quicker with the higher CR.

Are you getting this motor built now? If so my advice is to go over your entire combo with your engine builder. Tell him the octane your going to run, turbo and max boost, converter stall, engine management, cam. Then he can suggest the best compression for your setup.

What it comes down to is like Don said, more compression = lower max boost.

 

[Alky V6]

If this is a class competition car, then by all means go with the higher CR.

If this is a 95% street car on pump gas, then I think you'll have less problems with 8.5. Personally I'd shoot for 8.25.

 

[karolko]

i have heard the remark that higher CR makes for faster spool up times. can anyone explain to me how this actually works? my machinist was baffled when i stated that. he said that the only thing that CR would help in would be when i am not in boost.

 

[Alky V6]

The difference in driveability, and I'm talking just normal street driving will not be a big deal with a .5 point change in CR. Spool up may increase slightly due to a slight increase in combustion temps, exhaust temps, and stall speed with the higher CR. Now, if you want the most durable setup that will give you a spool up that no other engine combination can compare to, USE LOW CR AND NITROUS ASSIST.

You will have the best of all worlds. The lower CR will be the best protection against borderline tuneups, occasional bad pump gas quality, will keep combustion temps down (very good way to combat detonation), and will give you the widest tuning window to work with. The nitrous, even a small shot, will give you a spool up quality that no other tuning method could ever achieve.

Don't make your engine less durable for a slight spooling increase. Use nitrous! Ask anyone who is using nitrous now. Nothing can give a quicker spool up. You want to switch to a monster turbo? No worries. Nitrous will spool it NOW!

 

[Black Power]

DonWG speaks the truth. Nitrous is rockin' for turbo buicks.

 

[RUQWKNF]

Big can of worms with this one.....

Good thread though.

I agree with all that has been said. Higher CR will help spool up slightly. It's not the cure all. N2O is the quickest method for spooling any turbo, that is true, but it brings more tuning issues into the picture. Namely, if your chip (assuming that you're using a stock ECM) is not burned for N2O use, it will have too much timing and could cause some dangerous lean conditions which could pop a head gasket or two. If you intend to run N2O, you must have a proper tune to compensate for the sudden lean spike that occurs.

The only thing I can say, that will help explain how the CR helps spoolup is,
higher CR creates more exhaust velocity and higher EGT temps at lower rpms.
These are the main two factors that contribute the most too help spool up a large turbine wheel that are engine specific.

And the current 70 series turbos that are out there, have large turbine wheels, as compaired to what 95% of the Turbo Buick world normally runs on a street car. The only other factor that can greatly contribute to quicker spool, other than N2O, would be reducing the engines accessory drive rotating mass. IE: Billet aluminum crank pulley, water pump pulley (the extreme being eliminating the belt driven water pump and going with an electric water pump) PS pump pulley, alternator pulley, light weight 9" or 9.5" torque converter vs the stock 12" D5 converter. (easily worth 30#s right there) aluminum driveshaft, and light weight wheels.

Given, the wheels are slightly different than accessory drive and drivetrain rotating mass, but it has been said that removing 1# of rotating mass is the same as removing 7#s of static weight. In the past, I have seen gains from doing all of these items above on street cars. Especially the torque converter, light weight wheels and billet crank pulley. Those 3 items significantly helped with helping the motor rev quickly. With the tuning features that a Turbo Tweak chip have (namely allowing you to pull fuel out on spool up) can help your spool up even more.

Me personally, if you know you will never see any better gas than 93 octane, and will be street driving 95% of the time, I would suggest staying with 8.5:1 CR. If you plan on running straight 100-103 octane unleaded all the time, or alky injection under boost, then by all means, go with 9.0:1. Just remember, that you will limit yourself to the maximum allowable boost that you can physically run before you start popping head gaskets on our 4 bolt per cylinder style heads. I've had friends pop head gaskets at 30psi on C16 on a motor that had 9.0:1 CR, given the tune could have been bad, or detonation could have loosened up the gaskets in a previous pass, but it's just some things to consider. Also, you shouldn't have to run 30-35psi of boost to reach your goals to begin with. If you are having to run that much boost, you definitely have the wrong turbo on your combo.

The ideal selection with our little 231-235 motors, would be to have a turbo that only has to run 24-26psi to reach your goals and spool up as fast as a stocker. If your goals are mid to high 9's on a 235CI, you would need to run a larger than 70mm size comp wheel, as very, very few people with full weight (3650 lb+) Buicks, have accomplished this goal without the help of N2O. Sure, spraying a 235CI production motor with a good cam and maxed out aluminum heads would easily run 9's, but, just how long will the head gaskets stay in one piece?????? It sure doesn't take much to pop a head gasket, especially with the unequalized distribution of air to each cylinder factor working against us. Then you throw N2O into the mix and you have no way of controlling how much nitrous each cylinder takes, unless you plumb up a direct port N2O kit.

Sorry it's so long winded, just some of my thoughts.


Patrick

 

[karolko]

Pat, thank you for the very informative response,

I have already considered a nitrous assist setup for the GN, and i really like the idea of it.

To make things more specific to my application, and what you have stated above, i have all RJC pulleys, and the torque converter is a precision industries vigilante 3200 stall single disc l/u (don't know what size that is, but), wheels and what nots,

I am running a fast XFI setup, that will be tuned by Cal, so poor tune will be out of the question.

you stated the ar would run lean during those few seconds? what if you are running a wet system? then it would run lean at all would it? also, would i have to change my fuel and timing numbers in the XFI if i were to run a nitrous setup, or would the wet system be independant of the regular fuel setup?

i know it is illegal, but do you guys run nitrous assist on the streets?

you stated this:

"If you plan on running straight 100-103 octane unleaded all the time, or alky injection under boost, then by all means, go with 9.0:1."

I might be acting retarded, but are you saying that if i run 93 oktane AND akly on the street, then i will be okay? i do have akly,

Okay one last question for now, i think i might know the answer to it, but i jsut want to make sure, when driving the car, there will be no difference in off boost performance with a larger turbo than a smaller one right???


thanks for all your help.


Well

 

[Alky V6]

Very good info Pat. When I brought up nitrous, I was only suggesting it's use to spool up the turbo. In my case, with the T76 and a 224 cid engine, 16 psi boost is achieved in around .4 seconds. That's with a 200 shot of nitrous and methanol. With a small single shot wet system, used only for spooling assist and not left on for the complete run, problems because of the nitrous will be nill. The FAST system can be setup to retard the timing at activation of the nitrous. Keep the nitrous system on the fat side and you'll be OK. There are some tricks that can be used to avoid any lean spots on activation of the system. The main fueling map will not need to be changed. Just have the timing retard set and with the nitrous system properly jetted to give you a safe mixture, even at elevated bottle pressures, and you'll be set. Any rotating mass lightening tricks will be very low on the 'to do list', if you use nitrous. You may even just scratch that off the list.

 

[Alky V6]

Nitrous is the great equalizer in the world of turbocharging. All the tuning parameters that were so important to get that elusive quick spool quality will be so, so unimportant.

Say goodbye to:
Getting the perfect torque converter stall speed.
Special air/fuel ratios for spoolup.
Special timing strategies for spoolup.
Searching for the big hp turbo with the small wheel weight.
Searching for the perfect compromise for the turbine housing a/r ratio.
Rotating mass lightening tricks.
Concerns of the downpipe affecting spoolup.
Searching for the best compression ratio compromise.
Searching for the best camshaft compromise.

And anything else you can think of that could possibly affect turbo spoolup time is no longer a concern.

 

[karolko]

I'm sold. Totally. I am going to end up running 9.0:1 compression and when i end up going to a larger turbo, i will use the nitrous assist to help me get up there, and yes even on the street. oh boy i am going to get into soooo much trouble around here. watch out black and blue!!

i was originally thinking of using nitrous a while back, but now with the XFI, it is going to make things soo much easier and nicer.

Thanks boys. i am sure i will come up with more questions very very soon. in fact i already have one i am dyin gto ask, but i must search first to avoid be scalded by the "use the search function" groupies

 

[bison]

I am running a fast XFI setup, that will be tuned by Cal, so poor tune will be out of the question.

you stated the ar would run lean during those few seconds? what if you are running a wet system? then it would run lean at all would it? also, would i have to change my fuel and timing numbers in the XFI if i were to run a nitrous setup, or would the wet system be independant of the regular fuel setup?

i know it is illegal, but do you guys run nitrous assist on the streets?

You will run a dry setup with the FAST. It will be very tunable with the small hit and the FAST. Any a/f you want can be achieved. Forget about a wet system. Your adding a lot of variables that way. Just select the correct size injector. 83's with alky will be fine. Nitrous is not illegal and is run on the street by thousands of racers in the U.S. Canada is not so lucky with their stupid laws on it.

 

[bison]

i have heard the remark that higher CR makes for faster spool up times. can anyone explain to me how this actually works? my machinist was baffled when i stated that. he said that the only thing that CR would help in would be when i am not in boost.

CR increase will increase power all over the power band under all conditions as long as detonation is avoided. If it didnt matter the hardcore racers TSM, TSO, etc. would run low compression. Believe me, it helps a lot going from 8.0:1 to 9.0:1. On c16 your still good to run high 30's boost till you hit the detonation limit on a good tune. Others that blew head gaskets at lower levels had a tune issue or some other problem. Like wrong cam or cam improperly installed. You wont be going near those levels most likely. 30 psi is no problem on 9.0:1 and c16.

 

[Alky V6]

I can't blame you for wanting to go with a CR that will mean top efficiency. Heck I was exactly the same way with my first two engines. Back when I was deciding what CR to use on those engines, I heard advice much the same as I've given here on this thread. If I recall correctly some of the advice I came across regarding CR was indirectly from Kenny D. Back then, I all but ignored it. My mindset was much the same as you. Putting together the most efficient engine I could. I told myself I'd deal with the limitations when the time came. That just showed my inexperience. When it finally did come to tuning the engines I realized just how limited I was and how little room for mistake there was. I wised up with the latest engine. After getting this engine dialed in, I'm not missing the compression at all. With a change from 11.25 to 9.25, it only took a few more psi of boost to match performance.

So, now I have the same performance with only a few more pounds of boost, but on top of that, I have the option of going with another 16 to 21 psi boost if I wish to take the fuel to the limit. I did not have that option with the higher static CR.

Do some research into the difference of static and dynamic compression ratios.

Something else to think about. And this is one that has stuck with me from way back when I was putting together my first engine. This was one of the pieces of advice I chose to ignore back then, but realized, even back then, that this was a very good point. I think this is the one that came from Kenny.

Intake boost pressure is basically a measure of air/fuel mixture density before compression. If you run a high CR that puts a limit on how high you can go with boost pressure, then you're also limiting the mixture density you can run in your engine. In other words, you have put a limit on the dynamic displacement of your engine. Sure, you can get a fraction more out of that smaller density of mixture by compressing it further during combustion, but is the juice worth the squeeze?

With a lower CR, you have the ability to go further with the dynamic displacement of the engine before coming to the limits of the fuel due to the limiting dynamic compression ratio.

So, would you rather have a higher CR engine where the limiting dynamic CR is reached at a lower mixture density before compression? Or a lower static CR engine with the same dynamic CR limit as the higher CR engine, but at a much higher mixture density?

The idea behind making big HP is cramming as much mixture into the cylinder as possible. The saying goes, there is no replacement for displacement. In our case, dynamic displacement. The saying does not go, there is no replacement for CR.

 

[Alky V6]

It's accepted that engine efficiency improves 16 percent as compression ratios increase from 8:1 to 18:1. So what is the engine efficiency improvement with an increase from 8.5:1 to 9:0? Less than 1 percent?

So I ask again, is the juice worth the squeeze?

Good luck with your project. I hope the best for you.

 

[bison]

The saying goes, there is no replacement for displacement. In our case, dynamic displacement. The saying does not go, there is no replacement for CR.

Except boost and CR on a good tune.:biggrin: I would understand if he said he was going to run over 9.5:1. With his goals i doubt he would ever be able to achieve enough boost to hit the detonation limit which would still be in the high 30's with c16 and proper tuning. I dont think the wastegate would ever be able to deal with the backpressure of a 3 bolt setup without throwing A LOT of pressure behind it to hold it closed. 30 psi on 9.0:1 is very common on any hard running engine these days.