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Old 05-28-2010, 01:42 AM   #1
83flathood
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Default Compression Ratio

Hi all have a 93 + b230ft motor i want to get going a little bit.

I was thinking a bigger turbo than stock and an ipd cam. My shop teacher also suggested i raise the compression ratio.

What do you guys think should i raise it a bit? any big trade offs?
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Old 05-28-2010, 01:47 AM   #2
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I wouldn't. There are better things to spend your money on IMO

Last edited by Tom Wiley; 05-28-2010 at 01:48 AM.. Reason: can't read
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Old 05-28-2010, 02:05 AM   #3
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Raising it is supposed to give you better off-boost torque, but it is said to lower the detonation threshold. That being said, people +T the normally aspirated B230F's all the time, and they've got a 9.8:1 CR vs. your 8.7:1.

I'd say raise it to 11:1 and keep stock boost level with some headwork, get some bigger injectors, and drive lightly while the ECU learns the new setup.
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Old 05-28-2010, 01:12 PM   #4
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Raising the SCR from where that motor is now is not necessarily a problem, IF you go about it intelligently.

True: raising the SCR does tend to lower the detonation threshhold. Which is why that motor is about 8.7:1 from the factory, rather than the higher than 9:1 ratio the NA motors were built at.

If you want to raise the SCR 'some', AND raise the detonation threshhold at the same time, then the way to go is to do the things needed to make your motor 'tight squish'.

....and the way to do that is to measure the piston deck height [done with the cyl head removed] to see how positive the deck height is. That motor ['93 and newer B230FTs] is very likely to have positive piston deck height of about 6 thou [0.006in]. i.e., the pistons top out at TDC about 6 thou above the block deck.

You have to measure and confirm the piston deck height. You do NOT ASSume what the piston deck height is: you measure it.

THEN, you order the correct thickness Cometic MLS HG that will result in giving you a piston to cylinder head clearance somewhere between 30 thou and 35 thou.

...usually a 36 thou MLS HG will do the job quite nicely.

Read up on the subject of 'tight squish'. It works.

You'll also need a way of adjusting/correcting the camshaft timing. i.e., an adjustable or re-indexable timing gear for the camshaft. Otherwise, going tight squish via a thinner HG will retard the camshaft timing, which is not helpful usually.

At least one heat range colder spark plugs is also required. I run two heat ranges colder.


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Old 05-29-2010, 12:14 AM   #5
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I'd say raise it to 11:1 and keep stock boost level with some headwork, get some bigger injectors, and drive lightly while the ECU learns the new setup.
Whoa now. I ran a NA with SCR over 11:1 for a while and it was fun.

But, when I raised the SCR on my last b230ft build I tried to stay ~9:1.

I wouldn't recommend going past 10:1 on anything not using race fuel or water injection if using a turbo as well.
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Old 05-29-2010, 12:22 AM   #6
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The other consideration is that while you can increase the SCR quite a bit if at the same time you bring the dynamic CR back down via more cam, you can only go so far if you have no means of adjusting your timing curve. You need to increase the turbo size at the same time as well since the additional overlap will hurt performance if you are not reducing backpressure at the same time.

it's a tricky balancing act when take to the extreme, but something in the 9-9.5:1 area with soemthing like a K cam and an 0.63 hotside is a good combo
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Old 05-29-2010, 04:27 AM   #7
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Whoa now. I ran a NA with SCR over 11:1 for a while and it was fun.

But, when I raised the SCR on my last b230ft build I tried to stay ~9:1.

I wouldn't recommend going past 10:1 on anything not using race fuel or water injection if using a turbo as well.

N/A race engines that are able to breathe well but use more extreme cam timings can use even higher arithmetic CRs... around 12.5:1 is common... but that has nothing to do with a turbo application. Where cam timing has far less influence on VE such long duration intake periods aren't needed, and a late closing exhaust valve is definately not required, so the CR should be correspondingly lower.

Even on a low-boost application I'd be reluctant to go very much above 9:1


At the other extreme a really serious race turbo motor might rely heavily on boost for cylinder filling at higher rpm, and a much lower arithmetic CR will give a smaller reduction in cylinder pressure per degree of crankshaft rotation for the same peak cylinder pressure, and therefore a greater effective pressure and at larger crankpin angles producing more torque.

These sort of engines can run CRs as low as 6:1, but in combination with a high boost threshold off-boost performance is very poor.

A higher arithmetic CR would no doubt improve off-boost performance and fuel economy, but will ultimately limit the usable boost pressure. You've really got to build the motor to suit your own needs, but I can't see this being a situation where a CR of anything like 11:1 would be advisable!
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Old 06-04-2010, 05:21 PM   #8
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Old 06-04-2010, 05:51 PM   #9
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That's what a +T is. You can raise the compression around a point by putting an NA block under all that turbo stuff. Drill the return hole out, add an oil feed from the front of the block. Only $100 or so for a short block from pick and pull. Dollar for dollar, raising the compression ration for the cost of a cheap cam is a good bang for the buck (much pun intended). Though at that point you are producing enough squish to go bang.
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Old 06-04-2010, 06:24 PM   #10
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With tight squish, large duration cam, large turbine housing and header, and methanol injection I'm running 20psi on my 10:1 B230F+T with no audible detonation.
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Old 06-04-2010, 06:29 PM   #11
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Quote:
Originally Posted by Poik View Post
With tight squish, large duration cam, large turbine housing and header, and methanol injection I'm running 20psi on my 10:1 B230F+T with no audible detonation.
and better than 300 horsepower
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Old 06-07-2010, 04:20 PM   #12
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Quote:
Originally Posted by Poik View Post
With tight squish, large duration cam, large turbine housing and header, and methanol injection I'm running 20psi on my 10:1 B230F+T with no audible detonation.
Out of curiousity, what do you use to monitor/track detonation that you cannot hear?
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Old 06-07-2010, 04:25 PM   #13
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Out of curiousity, what do you use to monitor/track detonation that you cannot hear?
Knocksense

With or without ability to datalog. Your choice
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Old 06-07-2010, 04:26 PM   #14
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I was asking Poik what he uses - I am aware of Knocksense and have it installed on one of my cars...
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Old 06-07-2010, 08:39 PM   #15
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I use knocksense tied into megasquirt, so if it knocks it will pull timing.
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Old 06-08-2010, 02:08 PM   #16
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as mentioned....static and dynamic are two different things and cam choices affect it alot.
my 91 b230ft with ipd cam all stock with 8v head cranked 145psi
same engine now stroked to 2.5l.....16v head with stock cams.....rsi 16v pistons im cranking a little over 210psi
mathed out the static is supposed to 8.5-1 but cranking sure did go up.
i have some adjustable gears coming to help dial this out and will get turbo cams ground one of these days for this motor.
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Old 06-08-2010, 02:14 PM   #17
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Originally Posted by Nathaninwa View Post
as mentioned....static and dynamic are two different things and cam choices affect it alot.
my 91 b230ft with ipd cam all stock with 8v head cranked 145psi
same engine now stroked to 2.5l.....16v head with stock cams.....rsi 16v pistons im cranking a little over 210psi
mathed out the static is supposed to 8.5-1 but cranking sure did go up.
i have some adjustable gears coming to help dial this out and will get turbo cams ground one of these days for this motor.
It's worth mentioning that your numbers don't take leakdown in to account. An old motor will have much more blow by than a freshly honed and ringed block.
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Old 06-08-2010, 02:34 PM   #18
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210 on an 8.5:1 motor doesn't even make sense to me, even with next to no overlap... I have my doubts it's 8.5:1 but that's just me. Adjustable cams aren't about to "dial that out" either. The NA cams probably already have more overlap than is ideal for your setup as it is. In other words setting them up for turbo will actually make the compresison test higher.

Typically you'll use cam gears to effectively increase the LSA on a set of NA cams, which actually makes DCR higher.

Using my own twin cam motor as an example: With 960 cams at 0/4 my 8.5:1 motor cranks ~170. At -2/4 it cranks ~180. That's an increase in LSA of 6 degrees vs. straight up, and a more appropriate setting for turbo (at least in my case, as the motor makes more power everywhere at -2/6) .

I don't understand Karl's leakdown reference...
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Old 06-08-2010, 02:38 PM   #19
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I use knocksense tied into megasquirt, so if it knocks it will pull timing.
Do you worry about it not being able to detect knock above say 4000 ish rpm? I know the miata guys I know say they can't detect much above 4000 because the engine is so noisy (they also mount the knock sensor for KS on the bottom of the engine because I guess that's where mazda put it in later years that had one), so they basically completely ignore it above 4500. Is this an issue we also have on the volvo or is that part of the BP engine being a horribly noisy engine or if anyone has even looked into it.
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Old 06-08-2010, 02:42 PM   #20
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Above 5k it's a lot of static with a loud exhaust so I have MS ignore anything above that. If it's going to knock it is most likely going to knock around torque peak, which is about 4k on my engine.
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Old 06-08-2010, 02:51 PM   #21
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And yeah 210 is pretty high.. My 10:1 +T with IPD cam was 180.
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Old 06-08-2010, 03:05 PM   #22
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I don't understand Karl's leakdown reference...
If your rings are shot you won't get any compression. Blindly comparing compression numbers from a junk-yard motor to a freshly assembled shortblock doesn't make a whole hell of a lot of sense.
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Old 06-08-2010, 03:07 PM   #23
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I had 205-210 with my b230f back in the dizzay. I haven't checked it on any of my 16v motors, stopped caring about that a while ago. if it doesn't sound 'off' while cranking, and it runs out good, meh.
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Old 06-08-2010, 03:41 PM   #24
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i used adverised numbers from the rsi pistons....compressed elring and advertised 16v combustion chamber and ran it through a calculater. 2 different ones gave a close 8.5 to 1 motor. i was seeing if using a cometic and going to tight squish would work but then you mentioed that the 16v dont have enough squish area to worry about so i went elring.
the cranking does seem high but it is what it is....nothing fancy in my motor but more stroke.
initial cranking was 180psi before start and now with 600 miles its rockin the 210 number.
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Old 06-08-2010, 04:02 PM   #25
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How come nobody welds in material and creates clover leaf shaped combustion chamber squich pads?
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