DCR and Boost, pump gas.

[redrider2911]

Ive done quite a bit of searching and havent found an answer. Im working on building a 4.6 stroker now and plan on adding 8 PSI of boost. With a quench height of .04 and polishing the combustion chamber, I dont believe I will have a problem running pump gas with a dynamic compression ratio around 8.7:1 (I still need to do some recalculating after choosing a final cam but that should be close) BUT what will happen when I add 8 PSI of boost to that? Is there another formula to figure out the compression ratio change when going from 12 PSI of suction to 8 PSI of boost in the intake? Any help is much appreciated.

Also, if I build this engine and run into predetonation because of the higher DCR I will be running; is it simple enough to just retard the cam timing? Or will that cause problems with the intake valve opening to late too?

[CobraMarty]

with a dynamic compression ratio around 8.7:1

What would your static CR?
What cam specs?

[Retlaw01XJ]

Car Craft covered the 'effective compression ratio with boost' in an article about a year ago. Plug that same question into Google and you'll get some good hits. Here's one from KB:
http://www.kb-silvolite.com/article.php ... ad&A_id=57

I never heard of changing cam timing to combat detonation while under boost. My first thought is it won't make a significant change.
My turbo experience consists of the early turbo V6 Buicks and the Saab 900's. Buick used a knock sensor which would retard the ignition timing, and the later Saabs used a boost-control (APC) which I feel is the better method. You should also consider water or alcohol injection while under boost to reduce charge temps.

[CobraMarty]

Great chart!

Final compression ratios above 12.4 to 1 are not recommended for use with "premium pump gasoline." The higher the final compression ratio the higher the octane rating of the gasoline must be in order to help prevent detonation and serious engine damage.
Final Compression Ratio (FCR) = (Boost / 14.7) + 1) x CR
Boost = Maximum Boost
14.7 = Psi. at Sea Level
CR = Engine Static Compression Ratio

Compression Boost (in pounds per square inch)
Ratio

Static CR 2 4 6 8 10 12 14 16 18 20 22 25


6.5:1 7.4 8.3 9.2 10.0 10.9 11.8 12.7 13.6 14.5 15.3 16.21 17.0
7.0:1 8.0 8.9 9.9 10.8 11.8 12.7 13.6 14.5 15.3 16.2 17.0 17.9
7.5:1 8.5 9.5 10.6 11.6 12.6 13.6 14.6 15.7 16.7 17.8 18.6 19.5
8.0:1 9.1 10.2 11.3 12.4 13.4 14.5 15.6 16.7 17.8 18.9 19.8 20.9
8.5:1 9.7 10.8 12.0 13.1 14.3 15.4 16.6 17.8 18.9 19.8 20.9 21.9
9.0:1 10.2 11.4 12.7 13.9 15.1 16.3 17.6 18.8 20.0 21.2 22.4 23.6
9.5:1 10.8 12.1 13.4 14.7 16.0 17.3 18.5 19.8 21.1 22.4 23.6 24.8
10.0:1 11.4 12.7 14.1 15.4 16.8 18.2 19.5 20.9 22.2 23.6 24.8 26.0
10.5:1 11.9 13.4 14.8 16.2 17.6 19.1 20.5 21.9 23.4 24.8 26.2 27.6
11.0:1 12.5 14.0 15.5 17.0 18.5 20.0 21.5 22.9 24.5 26.0 27.5 28.9

So for me with stock CR 8.7 and boost of 6 pounds -- right at 12.4 max allowable for premium pump gas.

[redrider2911]

I must have messed up on my previous calculations.

Bored .060 over - 3.935"
.043 head gasket
.003 above deck
Piston dished to 30cc
stock cumbustion chamber 56.5cc I will be pollishing the cumbustion chamber plus LS1 tuliped valves so it should add a little volume.

SCR=9.21:1
Add a 232-4 Comp cam to that where the intake valve closes at 56*
DCR=7.88:1

Now I want to run as high of compression ratio possible because my plan is to have the compression for low end torque and have the turbo pick up the slack in the higher RPMS. So if I range the amount of piston dishing I do, I have the option to go all the way up to a SCR of 10.94 and DCR of 9.33 using the stock piston dish of 13.5cc.

All though the chart provided is useful I dont believe it is complete. As far as I understand DYNAMIC COMPRESSION RATIO is the final deciding factor in a n/a engine and then DCR plus boost in a force induction engine. I have found that they say a DCR of 8.5:1 is the highest you would want for pump gas, 9:1 is possible with an efficient combustion chamber but pushing it. So my original question, what does Boost do to DCR? Can I use that same formula? But then what are acceptable numbers on TRUE FCR?

Retlaw, by retarding the cam your intake valve will close later thus shortening the compression stroke and lowering the DCR

[Retlaw01XJ]

Retlaw, by retarding the cam your intake valve will close later thus shortening the compression stroke and lowering the DCR

Understood. But I wouldn't recommend it for your build since retarding the cam will move the power band higher, reducing low-end power which isn't your goal. Crunched the numbers in the calculator and retarding the cam 2 degrees would reduce your 7.88 DCR to 7.78.
Retarding 4 degrees results in a 7.67 DCR.
...It's not a significant reduction in DCR and the cam timing change alone will hurt your low end performance.
You could get the same DCR reduction (7.67) by dishing the pistons an extra 3cc. BTW, are you using forged pistons?

So my original question, what does Boost do to DCR? Can I use that same formula? But then what are acceptable numbers on TRUE FCR?

DCR under boost....
DCR is a mechanical formula that doesn't take variables such as air density (altitude, weather conditions) or cylinder filling (efficiency) into account.
The charts use (I believe) a simple formula.
For a build with 7.88 DCR:
atmospheric pressure (sea level) = 14.7 lbs
boost pressure = 8 lbs, for a total 0f 14.7 + 8 = 22.7 lbs.
Now take the ratio of boosted/normal, 22.7/14.7 = 1.54
and multiply your DCR by that ratio, 7.88 * 1.54 = 12.1 boosted DCR.

I know you're asking what the boosted DCR limit is for street use. I don't have that answer, we'll have to research that!
You are primarily concerned with detonation control (under boost). Some items having a huge effect on that are:
- boost pressure (adjustable)
- ignition timing advance (adjustable)
- fuel mixture (adjustable)
- intake charge temperature. Intercoolers and water/alcohol injection will help here. Immensely.
- load on engine. at 15 lbs boost, my '79 turbo Regal was fine in 1st and 2nd gear, but was more likely to detonate in third gear.

You're off to a good start with a tight quench which will help prevent detonation.

[redrider2911]

Im looking at Silvolite 2229c or Engine tech P3071 pistons. Honestly what ever I can find cheaper and preferably in a complete rebuild kit. I read somewhere max dishing was 30cc?

I plan on running an intercooler, which brings me to another question. Would I be better off running a full size intercooler in front of the radiator with all the long tubing involved; or a smaller intercooler like a stock one from a 3000GT, mount it above the engine against the bottom of the hood where I will have a hood scoop, and a fan mounted to the bottom of the intercooler pulling cool air in? I want as little turbo lag as possible. Will be using this jeep offroad.

For fuel and ignition I plan on eventually running Mega Squirt, figured it was the most accurate way to tune the thing without risks of lean fuel misture at certain RPMs and load. This is all going in a 98 XJ with OBDII.

Right now I have he 232 comp cam from a prvious build. My brother knocked over the parts tray when I had the lifters set out in order. Either way I should probably get new lifters so I am thinking of buying a whole cam kit. Is there another cam you think I should look into? I have plugged all of my specs and estimated numbers into desktop dyno, I have to say that almost no matter what I do I should be more then happy with the power gain.

[Retlaw01XJ]

Im looking at Silvolite 2229c or Engine tech P3071 pistons. .

With a turbo, forged pistons would be recommended. Cast pistons will not withstand detonation as well as forged pistons. They'll break.
The IC and Megasquirt sound like a good plan. For good throttle response, keep turbo and plumbing sizes/lengths on the smaller size.
Maybe a supercharger will fit your wants better than a turbo?
Hoping someone else here will step in to answer your questions, I'm near my limit as far as specifics on boosting a Jeep!

[CobraMarty]

I've always read, the max for iron head 8.5 and alumn head 9.5 when considering boost. I would install the cam straight up, don't try and bleed down dcr with cam retard. With better design combustion chambers and tighter squish these guidelines could go up by 0.5-1.0.
I would build it at 9.0:1 with the best it can be. With premium pump gas 92-93 octane and good tune and intercooling you should see 8 pounds boost safely.
For the trails I would not run a front mount air-air IC nor a top mount air-air with scoop. Air to water yes. Your piping for every 90* bend will cost you about 1/4-1/2 pound boost, add them all up.
I like my S/C-- boost at 1500rpm, use liquid intercooling- fuel and water/methanol. You want to build for the lowest intake air temps. Shoot for 20-30 degrees over ambient.
You're going in uncharted land with your build, but it is very doable.

[SilverXJ]

I would not go as high as 9.0:1. These are not very efficient combustion chambers nor are the engines over all. If you are building for boost get a custom can and keep the compression low.

[Retlaw01XJ]

Heck, you might just want to go with a MP90 supercharger and copy Silvers' build:
http://www.jeepstrokers.com/forum/viewt ... f=8&t=3533

I love turbo's, but I'm not convinced they're the best for low rpm and off-road use....

[CobraMarty]

I would not go as high as 9.0:1. These are not very efficient combustion chambers nor are the engines over all. If you are building for boost get a custom can and keep the compression low.

Like 7.0:1 and then you could run 12-13 pounds of boost on pump gas per the chart. Doing it like this will make the engine sluggish, have poor economy, etc, when not in boost, but will make the same peak power as 8.5-9.0:1 and 8 pounds boost. I would rather have the 8.5-9.0:1 engine any day. 2 different schools of thought.
I would love to try a alumn Hesco ported headed, 4.0 or 4.6L, 9.5-10.0:1 and 8 pounds of turbo boost. This would be very doable.

[redrider2911]

Cobra. Why do you suggest a water/air intercooler over a hood scoop and air/air? I feel the air/air would cool better, be less problem prone, and cheaper.

Retlaw. My thinking is I have the Stroker for the low end RPM torque. I won't need the turbo when crawling and such. Might even turn the boost controller all the way down on those trips. I want it for street, highway, passing, and playing in the snow. I figure I will use the turbo to boost the power curve in the higher RPMs. I like the sporty feel around town and think it would be fun to confuse all the little ricers. Lol. We also have a huge snow challenge event we do every year around here. Depending on the snow consistency I would want to use the low end torque to crawl and pack snow or use the throttle and fly right through it. We wheel in really wet and also really dry and light snow. From 1-8 feet deep. I'm just picturing all the fun I will have with this turbo. Lol

[redrider2911]

Also, about the forged pistons. There are multiple guys running turbos up to 10PSI on stock 4.0s with over 150 thousand miles. You don't think a those aftermarket cast pistons will hold up?

[CobraMarty]

Air-air intercooler need just that, air flow across the intercooler. How much airflow when out crawling? The jeeps have a hard enough time staying cool when crawling. The only location for this required airflow is up front of the radiator, Now to plumb it up there you will have 3-4' of tubing on both sides of the intercooler and 4-6 90* elbows. Turbo lag, loss of responsiveness, loss of boost due to all the elbows needed to plumb it all together. Mounting the intercooler on top with no airflow will probably end up heating the charge air further.

Air-water makes the packing so much shorter and cleaner with less length, 1-2' on each side of tubing and less 90* elbows 2-3. At low and hiway speeds the water is so much more efficient.

Heat transfer is important. I could stick my arm in a 200* oven for 10-15 seconds and in 200* water for 1 second.