questions regarding compression ratios and turbos..

[Eddie N]

to all,

it is my understanding that lower compression ratios are better for either turbo chargers or super chargers because a lower compression engine has less of a tendency to knock under boost because cylinder temps are lower...

but, lower compression means less horsepower to start with before turbocharging..

so here is my question.. what would make more sense and produce more power, a lower compression high boost motor, or a higher compression lower boost motor?

assume that the motor is being built from the ground up and that all outside variables remain constant between the two motors....

my guess is that low compression/higher boost is the way to go but i just wanted to get some more knowlegable opinions on this..

thanks !!

- eddie -

 

[Mike Brunton]

As long as your compression ratio remains over about 8:1 (ideally over 8.5:1) you will not lose alot of low-boost (read: low RPM) driveability and torque.

The tension forces arising from pulling the piston down from TDC and the compressive forces coming from pushing it up from BDC are much MUCH bigger than the compressive forces produced from the air/fuel mix detonating... so a short stroke motor with low compression and high boost will see a lot less stress than a long-stroke motor with high compression and low boost.

If your CR drops below about 8.5 or 8, the car is going to be kind of sluggish off boost and it's not going to get very good mileage.

 

[1TONY1]

Low compression and higher boost. About 8 to 8.5:1 compression. On my Formula I lowered it to 8.5 for the Procharger. Boost runs max of 13-14 psi but with that compression and the tune up it would be safe at 18 or so.

 

[red98GTS]

Great question and some great answers! I have had about thirty years of supercharging experience on my own cars and none in the turbo department. Paxton, GMC 6-71, etc. I have built my own engines specifically for the blowers that would go on them, and found years ago that there were many opinions on how to go with the compression ratio. From my own experience, I preferred to build in about 8 to 8.5 to 1 and no less. The bottom end and non-boosted driving on the street really suffered, unless you spin them up so fast that even with a 7-1 c.r., you are almost into boost often. Of course, there are loads of factors to consider....weight of car, size of engine, etc etc....tons of variables, but I still like the moderate compression ratio stuff. See my supercharged and nitrous fed T-bucket and imagine that in a 1,500 lb. vehicle, which it is. Yikes!
pete the ex glass guy and hot rodder

 

[red98GTS]

Oops! Here they are: www.geocities.com/MotorCity/Speedway/5567

 

[Eddie N]

<BLOCKQUOTE><font size="1" face="Verdana, Arial">quote:</font><HR>As long as your compression ratio remains over about 8:1 (ideally over 8.5:1) you will not lose alot of low-boost (read: low RPM) driveability and torque. <HR></BLOCKQUOTE>

so a compression ratio under 8.0:1 will allow the motor to run more boost, but will have a lumpy idle and be short in the torque department (and have bad gas mileage)?

now, for instance, if the motor made enough naturally aspirated torque with the 8:1 CR, would the car would make up for it with top end HP on high boost?

for example, the viper makes plenty of NA torque.. if somehow the compression ratio was lower (which would lower the torque but would still provide enough grunt), the car would be able to stand more boost and still be able to get off the line quickly, but would really shine up top with high trap speeds.. correct? im just using the viper as a very hypothetical example..

and as an aside, a shorter stroke means higher rev, which would equal more power under boost..

after rereading this post, it doesnt read too well.. what im trying to say is this:

a well built free breathing V8 makes enough torque to make launching diffcult.. if you sacraficed some torque to the engine gods by means a lower compression ratio and a shorter stroke, then slapped on some turbos, you would still have enough torque to get off the line quickly, and onces the turbos spooled you would take off like a bat out of hell down the quarter... is this a safe assumption all things being equal?

overall, thanks for the info.. that explains why the high revving low displacement short stroke engines in imports can run 30 pounds of boost,yet the viper guys get nervous above 7 or 8 psi..

arent you working on a twin turbo C4 project?

- eddie -

 

[Eddie N]

tony and red98,

thanks for your replies!! i must have typed up my reply while you both were typing yours..

14 pounds of boost on a V8 must have put down some serious horsepower..

i understand the main differences between superchargers and turbochargers, but im just now beginning to weigh the advantages of both in certain applications, but i will admit, i know more about turbos..

- eddie -

 

[red98GTS]

Eddie.....I wish I knew more about turbos but basically all the same rules kind of apply...I always had my head under the hood of a supercharged car since hot rodders like them so much on the older V-8s. I was always concerned with what would happen if I put the blowers on a car with already high compression....head gaskets, detonation, having to back off on the timing, octane woes, and so on. So there's a happy medium there somewhere. It's amazing just how much better an engine can feel with only 5-7 lbs. of boost...most of the guys with Vipers will be (should be) very happy with something like Sean's new do-it-yourself supercharger kit. Once they get a taste of boost....there won't be any going back! Ha! I guarantee it! I used to give "rides" in the T-bucket but nobody would ever ride with me again after the first one!! pete the ex glass guy

 

[Mike Brunton]

<BLOCKQUOTE><font size="1" face="Verdana, Arial">quote:</font><HR>Originally posted by Eddie N:
so a compression ratio under 8.0:1 will allow the motor to run more boost, but will have a lumpy idle and be short in the torque department (and have bad gas mileage)?

now, for instance, if the motor made enough naturally aspirated torque with the 8:1 CR, would the car would make up for it with top end HP on high boost?
<HR></BLOCKQUOTE>

Eddie,

Yes, this is correct. Compression ratio has alot to do with efficiency in an engine. You want to get that air and gas in there, then squeeze it as much as you can (sort of like compressing a spring), so that when it lights off, the maximum amount of energy is absorbed by the piston. Only something like 15% of the air/gas mixture's energy goes into pushing the piston, so you can see if that mixture is being squished into a smaller area, it will be more efficient in transferring energy to the piston. The problem is that as you squeeze it, the heat goes up and the likelihood of it detonating early goes up too. If you squeeze even more air and gas in there, the chances of early ignition are even higher! On the other hand, if you lower the compression ratio to account for this, you gain the ability to run more boost, but the engine is less efficient and gets worse mileage and loses off-boost power. There is some sort of happy medium in there, and that is a compression ratio of between 8:1 and 8.5:1. At that CR, you're not going to lose all that much efficiency (mileage isn't going to be too bad), and you will retain enough torque - especially on a Viper - to get good launch power. You really want to size the turbos so that they are producing boost at about 1/3rd of the redline of the car - so on a Viper that would be about 2000rpm. So, you're only off-boost RPM's would be around 750 (idle) to 2,000 - which isn't too much, and a reasonable CR will make you happy in around-town driving.

But I'm rambling here

If I were TT'ing my Viper (and god willing, I will be after I get my SRT), I would be most concerned with the strength of the rotating assembly and how true and square the motor is. Additional loads from combustion chamber pressure aren't really too bad (something like 20% more stress on the piston at double the horsepower of stock - there's a lot of math as to why that is, but I'm too dumb to go into it all), but in a turbo, you really want to have everything square, lined up, centerlined, etc.

On the TTC4 I'm doing, it will probably run 22-24lbs of boost without trouble. I'm hoping for around 18-20lbs which should be able to run on pump gas with no problem, and should make upwards of 800-900HP at the rear wheels. One of the good ways to make horsepower in a turbo car (and people don't realize this often enough) is to REV it! Making pretty big torque at 7,500RPM is preferable to HUGE torque at 3,500RPM, because the HP numbers at 7,500 will be outlandish... and I'm shooting for over 1,000RWHP, which means I'll need to make good high-RPM torque and rev this sucker to well over 6,000 - over 7,000 and as close to 8,000 as I can.


overall, thanks for the info.. that explains why the high revving low displacement short stroke engines in imports can run 30 pounds of boost,yet the viper guys get nervous above 7 or 8 psi..

arent you working on a twin turbo C4 project?

- eddie -

[/B][/QUOTE]

 

[ronviper]

I have had lots of turbo cars, supra's and buicks, Ken Duttweiler is a genius with turbo cars. He likes low compression and high boost, he dyno's his projects and has tried all different compression ratio's but comes back to 8.0 to 8.5 to 1. Presently i have a stock block 87 gn with steel heads 8.0.1 compression that will run low to mid ten's depending on whether i run 25 or 30 lbs of boost. The viper with double the cubes should have no problem with torque the draw back is the six speed trans and suspension. The viper will make tons of torque and horsepower will cann't use it effiently.

 

[meggleto]

Eddie,

This is another one of those questions where it depends on what you plan on doing with the car.

Lowering compression will allow higher boost and the potential to make more peak power and more top end power. Fuel economy for street driving will go down.

Raising the compression will allow you to make more low end power but will reduce the maximum boost pressure allowed and reduce the peak power potential. Fuel economy will increase.

So if you want to have a daily driver and part time roadrace/autocross vehicle you would opt for the higher compression lower boost vehicle. It would get better gas milage. It would have more torque at low RPMs for powering out of corners and still have an excellent peak power number.

If you want a drag car then you go with the low compression engine. It gets poor gas milage but you're not driving it daily so you don't care. It has torque like a Honda when at low RPM or low boost but at the strip you'll be at the high RPMs (except for the launch) and you would use NOS off the line to get you going so you don't care about the poor low end power.

For a real life example my autox car (Talon) had a 7.8:1 compression ratio and got 24mpg on the street. It ran 18psi on 94Octane pump gas with no knock at 20degs timing at full throttle. It was a dog off boost and coming out of corners at the autox and roadraces. I wanted a better autox car so I raised compression to 9.0:1 and now the car gets 30mpg on the street. It has good torque down low and can now power out of corners. I can only run 15psi on 94Octane gas if I want zero knock at 20degs timing at full throttle. To make up for the loss in top end power I had the head ported which made the car even faster than previously on the top end. (Note you can't exactly compare the psi numbers because psi is a variable that's affected by how restricted the intake system is and I had the head ported at the same time compression was raised.)

I would quote qtr mile times but I haven't been to the track yet after the compression ratio increase. Before I raised compression it ran 12.8sec. With the ported head and compression ratio increase I'm realistically expecting 12.4 but hoping for 11.9999999. Without the ported head but increasing the compression ratio I would have expected to be running 13.0's.

In summary....

Drag car....low compression with higher boost

Daily driver and "part-time" roadrace/autox/drag car....higher compression(8.0:1 to 9.0:1) with lower boost.

 

[Motor City Mad Man]

<BLOCKQUOTE><font size="1" face="Verdana, Arial">quote:</font><HR>Originally posted by meggleto:
I'm realistically expecting 12.4 but hoping for 11.9999999. Without the ported head but increasing the compression ratio I would have expected to be running 13.0's.


<HR></BLOCKQUOTE>


Meggleto, I am hoping to see those 11.999999's out of my Viper this year too. Wish the track conditions at Milan didn't **** so much.

 

[meggleto]

Motor City Mad Man

With your VEC, Hi-Flow Cats, Headers, Exhaust and Lightened Flywheel you have a pretty good shot. If that doesn't do it then a set of cam gears or tires stickier than the Pilots should get you kicked out for not having a roll cage. I should probably try some sticky tires too. Once I finish trashing these stank Kumho Ecsta's I'm going to step up to some good tires again which should drop me from 1.8 60ft to 1.7 60fts. That's what I get for trying out different tires. I lost .1 in 60ft time going to the Kumhos.

Milan is terrible for traction but it's the only dragstrip within 90 miles. If I get within 12.2 I'm heading to Norwalk. I usually get .2sec faster times there because they actually prep the track and I shave .1sec off the 60ft. I used to get high 1.6's there before I got the Kumho's.

 

[TurboSnake]

I find that 9.0:1 is a great C.R. for forced induction applications,especially with alum. heads.
Your off-boost performance will not suffer and you will make good power when under boost.