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7.5 CR pistons?

I'm going to be doing a reseal and likely top end rebuild over the course of the winter. Without changing things too much from stock, I was thinking of going with 7.5CR pistons.

First off, is that possible, without changing anything else?
Secondly, I'm looking for just slightly more drive ability down low, so that's my main reason for doing it. thoughts from those who have done it.

Should I have posted this in the engine rebuild forum , maybe?
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1987 930
1956 Chevy 3100
2009 Subaru Forester
2003 KX250 X2(I like my toys!!)
Old 12-07-2009, 06:29 AM
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Mark,

8/1 is such a good fit for a street turbo. 7/1 is more of a race car set up. The 993TT ran 8/1, the 3.6 CIS turbo ran 7.5/1 and had a 3.8 optional P&C kit that was 8/1. I would not go over .8 bar boost and a bigger inter-cooler and or more effecent turbo (K27-7200) is a good idea with it.

If your cylinders are in good shape you can get a set of J&E turbo pistons in any configuration you wish for probably a bit over $1k. Might want to check with TurboKraft or EBS in Reno. Another option is a set of fined 3.2 cylinders bored to 3.4, re-plated, w J&E's. EBS can do that for you I hear. Then you can sell you set.

Another option is to send the rods out to be stretched about 1mm. With this they offset bush the small end. This should get you to 7.5/1 and you can keep your factory pistons.

It sounds like we should be able to get factory P&C's in custom builds. If so, that might be a best practices way to go.

SC cams w a low restriction muffler and setting the CO to about 3% also helps low end. Increasing ignition off idle by modifying the Pot's retard stop is also something to look into (Andial dist re-curve and mod).

Not an expert, just my two cents.
Old 12-07-2009, 07:21 AM
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As I mentioned, I really don't want to touch much else, so getting a set of 8/1CR pistons sounds like the better bet. I already have a K27S turbo, headers and BB exhaust accompanied with a larger IC. Thanks for all the suggestions

I'll be tearing into the engine around the holidays.. merry xmas to me!
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1987 930
1956 Chevy 3100
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2003 KX250 X2(I like my toys!!)
Old 12-07-2009, 07:31 AM
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I don't think you'll notice that much from a 0.5 CR or so bump. Have you considered going with 3.4 pistons and cylinders?
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Old 12-07-2009, 07:36 AM
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You already have a lot of good mods so I expect you are running 1 bar already.

If so, w the bump in compression to 8/1 you would have to give up about 30hp on the top end from pulling back boost to .8 bar to improve the bottom end response some 5-10%.

I think bumping the base compression lowers how much boost you can run by about 3.7 lbs for each point of compression per one resource.

Thus, SC cams and larger intake ports to increase the VE would recover the HP traded up top would go well with your plan.

However, if you are on the edge as to max fuel delivery, pulling boost back a bit and increasing your aveage HP w the compression bump would be a good fit.

If you are just looking at going to 7.5/1 CR there are other ways to increase low end response like cam timing, an ignition strategy, Synapse BOV, D-WUR, and /or lower R&P.
Old 12-07-2009, 08:09 AM
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Quote:
Originally Posted by 125shifter View Post
I don't think you'll notice that much from a 0.5 CR or so bump. Have you considered going with 3.4 pistons and cylinders?
+1

New 3.4L P&C's (8.0:1 CR pistons) makes the most sense and you'll actually feel the difference over your original thought. And it makes the car so much more enjoyable to drive around town. Great combination.
Old 12-07-2009, 05:34 PM
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hmm, interesting observations. I guess if I change things I might as well change everything, however, depending on the condition of everything, I may leave all as is. I'm not sure I want to spend that kind of 'additional' 'while you were in there money'!
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Old 12-08-2009, 05:34 AM
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Definitely consider the ignition. I suggested today to get an EFI setup and just run the ignition control, with the fuel not connected. For EFI, about half the cost is in the injectors, rails, regulators, hoses, and TPS. You can have a real kick butt ignition setup for around $10. Set the idle timing to 10 BTDC and run around 28 BTDC at part throttle and you will get happy . You can then add the fuel system part if/when you want later.
Old 12-08-2009, 05:14 PM
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Yes, ignition is a solid way to add throttle response and increase efficiency on a 930.

A low compression 930 can take -40 and more at cruse (v -26 or less stock) for a bump in MPG. It can also take more advance off idle for improved throttle response. There is good opportunity hiding on the ignition side.

You can do a test by advancing your dist timing up to all the way) and test w/o going into boost to see for yourself if more advance off idle makes a difference. Again, do not go much into boost if you try this.

If you do go w an EFI ECU it can also drive an add on frequency valve(i.e. Andial Fueler Valve) so you can tune your Control Pressures for near perfect AFRs and motivate the metering plate with acceleration for a further improvement in throttle response (done it & it works well).

If a full conversion is not in the plans, a fully programmable MSD might be considered as a way to give near full control over spark and also add improvement w firing in low rpm's that can come with less than ideal AFR's that is often part of running CIS and such low compression pre boost.

The stock ignition can also be improved with a mod to the amount of retard and a dist recurve. Andial might still do this.

If the motor is coming apart anyway and the P&C's are in great shape, working the rods with 1mm in increased off set of the small ends to get to 7.5/1 is an option as noted above. I know only .5 increase in CR is not much but the next .5 is not much eather.

This would also allow you to correct for one of the weaknesses of the 3.3 which is to up grade the rod bolts. If you do not run much above 6600rpm or so it is not an issue. If you want to run at 7000rpm doing so or upgrading to stronger rods is almost a must. The case dose not have to be split to do the rods and this would also let you to do the rod bearings. I would not skip on an aftermarket rod bolt upgrade w a 3.2 or 3.3 but that is me.

Nothing wrong with a well tuned stock 930.

Just some thoughts.

Last edited by 911st; 12-09-2009 at 07:16 AM..
Old 12-09-2009, 07:13 AM
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That's a lot of info!

I think I'm going to stick to stock for now. I don't shift above 6K (Ever) so unless head studs are broken (last I checked they weren't), I wasn't planning on replacing them. It sounds like I can't just do a straight swap of piston (and do nothing else) without it being more money than it;s worth. If all is ok, I may get a new set of rings, but that too I may leave alone if all is ok. I'll know around the holidays when the engine comes apart.

I'm not familiar enough with the timing (and don't have any tools for that yet) to mess with that too much yet.
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Old 12-09-2009, 07:21 AM
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Good plan.
Old 12-09-2009, 08:03 AM
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Quote:
Originally Posted by mark '87 930 View Post
That's a lot of info!

I think I'm going to stick to stock for now. I don't shift above 6K (Ever) so unless head studs are broken (last I checked they weren't), I wasn't planning on replacing them. It sounds like I can't just do a straight swap of piston (and do nothing else) without it being more money than it;s worth. If all is ok, I may get a new set of rings, but that too I may leave alone if all is ok. I'll know around the holidays when the engine comes apart.

I'm not familiar enough with the timing (and don't have any tools for that yet) to mess with that too much yet.
Absolutely!

Go drive the thing the way it is and enjoy it. Your 20+ year old can still spank most of the new cars on the road and that's not even driving it to it's potential in stock condition. Great decision and enjoy.
Old 12-09-2009, 04:11 PM
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