[BoostedCutlass]

Ive done the thread search...... Ive seen the opinions....
I would like to know what methods are being used to up the CR. I know some are milling, some are using gaskets, and others are using custom/different pistons. A lot of great info on this topic, but not many specifics. Would anyone care to share their methods in detail? Especially Mr. Wood?
TIA

 

[bass]

If you want more absolute HP, lower the CR not raise it and run more boost. Raising the CR increases the low speed torque before the turbo spools but you will run into the knock limit earlier.

 

[BoostedCutlass]

That makes me wonder why some of the top names in the TR community choose to go the other way? I agree that knock retard will be an issue when running pump gas, but some of us can and do run alternatives on the street. My original question wasnt whether or not to increase compression. I am looking to see, with some detail, how people are doing it and the success they are having with their method (head gasket life, pushrod problems, etc.)
Mike

 

[Orlando_87GN]

That makes me wonder why some of the top names in the TR community choose to go the other way?

Not to continue off your topic, but the answer to that question is easy: Because probably the car is no longer a 'street' car and they don't mind running C116 fuel all the time.

 

[BoostedCutlass]

Orlando,
I agree with you that exotic fuels are not generally accepted as 'streetable'. Thanks for youre input. Any ideas towards my original question?
Mike

 

[Toadie's T-Type]

Boosted... I don't think there are many methods out there that can be done to increase comp ratio that you haven't mentions.
Course. Me having just woke up a bit ago. I haven't got my brain in gear yet.
Bringing the piston dome closer to the head increases Comp. But then you run the risk of valve train and piston collisions. eek!
Possibly even if you have them notched.
Piston height can be accomplished in many ways.
Different pistons. Different Connecting rods.
Different Crank strokes. (Not all applicable on a turbo V-5 to my knowledge.) dunno
Milling the heads raises Comp. But as with dome height.... You mill them to much. Then you run the risk of valve/piston collision. eek!
I would think that using the 2 head gaskets per side method would actually lower compression ratio.

Anywho Boosted..... I hope my Rambling on has helped you in some way or another.

 

[BoostedCutlass]

Thanks for you input Toadie.

I guess I need to rephrase my quesion.
Those that have chosen to increase their compression ratio, for street and/or strip, what method did you use and what are some of the pitfalls to using the choosen method? This would only apply to non stage 2 applications. I am building my second(read: backup) engine. I have always been curious about increased compression and its effects on turbocharged engines. I believe that an increase from stock to between 9 and 10 to 1 are possible and still be usable in daily driven applications. Obviously, high boost(25+ psi) might not be an option. I do think that a large turbo capable of high airflow at moderate boost levels(16-20 psi) can lead to an engine that makes great power and have good longevity.
Sorry, I get a little long winded sometimes. I know that there are some that have or are doing this and I would like to hear about their experiences.
Thanks again
Mike

 

[Steve Wood]

Sorry to call B.S. but the boost/compression discussion is generally that with someone citing Ricardo from 75 years ago.

Like Conley, Duttweiller, Billy Anderson, et al, I prefer more compression than less.

One of my cars is basically a stock short block with 9-1 cr. I ran 16-17# on 93 octane and up to 24# spraying alky.

My GN is heavily modified with a roller cam, front mount, etc. I have run as high as 19# on 93 but more typically 17# and 24# on alky. I have not been over 28# on C16 but I had no detonation at that point. It runs 9-1 as well.

How much more boost do you want than that? I can make more power than a stock block will handle very often.

All these learned comments about hp don't hold up on the street, or the track, imo. While peak hp may be debatable, I have not seen it win on the street or the track. Peak Hp may not win races as we don't run at a fixed rpm.

Cal, are you around? What do you believe from your experiences? You have built enuf serious cars for both street and strip to have a good idea.

The easiest way to get more compression during a rebuild is to do a clean up on the heads and block and then use a single shim head gasket which is only about .019" thick.

Try to recall that we run fairly high rear gears and we have to pull a bit broader rpm band than a low geared chevy. Also remember that we need torque at lower rpm to spool the turbo and launch the car. Most of us drive our cars on the street and it ain't much fun to have a doggy car in normal driving.



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[Boosted Bob]

Cutlass you need to decide what CR you want ultimately. For a half point bump in CR, easiest way is with a thin HG. I dont recommend milling the heads more than is required to get them flat, as they need all the strength possible to resist flexing. If the engine is out for a rebuild, decking the block a small amount is a good way to go.
If you do it right, and mill the intake properly, 9:1 compression with normal pistons is pretty easy. Just be prepared to get different length pushrods (may or may not be necessary, depending on how far you go) and learn how to set preload and you will be good to go powersix

 

[Steve Wood]

Yes, don't forget shimming the rocker shaft or getting the correct length pushrods...



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[BoostedCutlass]

Thank you Steve and Bob!
We are starting to get to an answer. I dont want to debate low vs. high CR. I tossed out an idea that I had, to inspire.
Steve, how did you end up obtaining 9:1 CR? I was looking through PAWs catalog and I saw that FM had hyper. pistons for a 350 Buick. These pistons would seem to fit a 23x engine with one important exception; the dish was about half the depth of the 23x piston. This seemed that it should raise CR to ~ 9:1. Im not suggesting hypers...... Ive read all opinions..... thats beyond the scope of what Im asking here. It is just an example.
I think that the idea of increased CR is a bit too much for the masses. Im sure that the tuning issues can be one obstacle. I see the advantages in it and Im willing to try it. I just wish to draw on the experience of others before I lay out the cash on some custom pistons or radically modified block/head work. Thats what this forum is for, correct.
Im sorry if Im beating a dead horse here, but the time has come to make an important decision and it needs to be correct. I dont wish to plan poorly and end up selling unusable components at a loss.
Thanks to everyone for the input
Mike

<small>[ March 25, 2003, 07:54 PM: Message edited by: BoostedCutlass ]</small>

 

[Steve Wood]

My GN has TRW forged pistons and the T has stock pistons. Combination of decking/milling and single shim gaskets. I think the T actually is a bit more than 9-1. The heads on it have been milled several times. I may be wrong but I seem to recall that the trws had a bit more piston volume.

Actually there are many people running higher compression these days so you would not be a rarity anymore. Tuning is not really an issue. It could become one if you got up to around 10-1 and your car started wanting more octane than 93 for "no boost" driving.

Total combination is important. When I was running a 221 deg flat tappet, I could run 20# on 93 because the the cam duration reduced the effective dynamic compression of the engine at lower rpm. Worked well, tho, because the compression made up for the loss caused by the cam at low rpm.



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[BoostedCutlass]

You guys be me to the post.....
I see where I need to go. 9:1 is relatively easy to obtian. Thats my target.
Eventually Ill end up with a T70 or 72 turbo. The idea is to create just enough boost with one of those monsters to keep in the 'efficiency range' and flow LOTS of air. Somewhere Ive seen a GN with this set up running 11.0x on only 20 psi. I think it was in GMHTP. A bit more tweaking and I know I can meet the goal in my profile.
By the way, I moved to turbo V6 power after many years of racing Oldsmobile engines. Im framiliar with the troubles of lifter preload. Its a PITA for sure!
Thanks guys, for the assistance here. I knew that the answer was out there.
Mike

 

[Steve Wood]

Just keep in mind that the boost may be less but the air flow is the same and cylinder pressure is therefore the same....Therefore an engine at 20# of boost is still straining the engine parts as much as one that makes the same power on 25#.

In theory, the air should be a bit cooler because the turbo may be working in a more efficient zone.

On the other hand, the engine is more likely to be operating in the compressor surge zone which can make part throttle operation very annoying on the street.

Lots of considerations to be evaluated as you move up.

Also, finally, remember that the engine has to be able to use the excess air flow which means great heads and cam.



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[HighPSI]

Steve Wood:
Cal, are you around? What do you believe from your experiences? You have built enuf serious cars for both street and strip to have a good idea.

Steve: You have pretty much summed everything up. I won't get into all the details about why it works good on street cars.

Mike: I have a LOT of customers driving everyday with increased compression. If you don't think my car will run on 93 your mistaken.

There are a lot of factors that come into play when dealing with knock (tuning, camshaft etc..).

Anyway..... To answer Cutlass' original question. The way I raise compression is with the piston. Pay attention to the deck height, etc.. This involves a custom piston.

 

[Steve Wood]

Cal, are you using J&Es? or Diamond. Ross, etc?

Thanks for the comment.



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[Boosted Bob]

Cal why do you choose the piston as opposed to decking the block? Is it because you want to preserve block strength? Is the piston you use still dished, or flat?

 

[HighPSI]

I use J & E dished pistons. If it is 9:1 or over, it will have valve reliefs. Since I am using a custom piston (usually 3.805 on stock blocks), I might as well chose the compression ratio vs decking, etc..