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How low of a compression Ratio with long rod motor?
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Author:  Bohmer2 [ Mon Sep 11, 2006 7:20 am ]
Post subject:  How low of a compression Ratio with long rod motor?
I am thinking about building a long rod motor since I have a set of 198 rods and several 225 blocks laying around. The question I have is: I am hoping to eventually EFI and turbo the motor so after a conversation with Dennis at MMW, I am trying to figure out if it is possible to get the compression ratio on a long rod motor low enough to run under moderate boost? Would lowering the CR down so that the motor can run under boost minimize the gains from the long rod 2.2 combo?

I have read with flat top 2.2's the compression ratio is up around 12, and with dished 2.2's you can get it down to 9.5 or 10's. I am assuming to run the motor under moderate boost I will need to get it down to somewhere between 8 and 8.5?

So I am thinking there are several options to try to get the compression down (thick head gasket seems like the cheapest but most risky/dangerous idea). Would the Aussie head gaskets be an option.

Custom pistons seems like a more expensive option but would it be possible to get the CR down.

But what about head work to open up the bowls, could this be used to get the compression ratio down?

Finally, I am wondering if it would be an option to O-Ring the block and use the 2.2 dished pistons with compression ratio around 9.5? Would O-rings be a less expensive option than custom pistons?

The purpose of this all is to get a street driven slanted e-body together that is different, fun, and can be run occasionally on a road coarse/drag strip. I just dont want to be the convetional everday dime a dozen slant six powered e-body!!!
Brian
Author:  LUCKY13 [ Mon Sep 11, 2006 8:38 am ]
Post subject: 
I think it would be worth the money to get a set of custom pistons made, get the CR around 8 1/2 : 1. They should cost around $650.00 for a set of forged that way, maybe a little more depends on who you get to make them.

I am not sure what to say about Oring the block, it will help it hold better but if you have a problem then it will kill the piston instead of blowing the head gasket. Some think of the head gaskets like a fuss. But pinging & detination is what blows head gaskest & not so much the boost, if it is tuned right with a good gasket & ARP head studs it should not blow gaskets unless the tune is off. But better to blow a gasket than burn a piston is the idea. Then if you blow a gasket you know there is something wrong with your tune & need to fix it (timing - fuel).


But of course Oring in the block is going to hold more Horse Power
( boost has noting to do with it) but it will be less forgiving with mistakes in the fuel & timing.

Jess
Author:  CStryker [ Mon Sep 11, 2006 12:39 pm ]
Post subject: 
That conversation got me thinking as well... I have a friend who has a 5.0 in a cougar which runs about 9.5:1 compression w/ the stock HO cam related parts (modified computer) and he runs about 12 lbs boost. What makes that motor so much more forgiving than our slants? Or is his just waiting to blow up?
Author:  Dart270 [ Mon Sep 11, 2006 2:20 pm ]
Post subject: 
You really will not hurt anything with 9.5:1 and 12lbs. The Drakes have run over 14psi on over 9:1 comp and Tilley has run at least 17lbs on 9:1. I think with a stock ~60cc head, flattop 2.2 pistons get you around 10.5:1 since most blocks have 0.180" deck ht and the 2.2s put you around 0.040" deck. Dished 2.2s should get you to a safe range.

You can also get a copper gasket up to 0.093" thick from Coxes. The good Aussie composite head gasket is 0.044" compressed.

Lou
Author:  DionR [ Mon Sep 11, 2006 4:58 pm ]
Post subject: 
Seems to me that the Drakes are running alcohol, and if I remember correctly, alcohol will support a higher CR.

If it was me, I would stick to 8.5 or so. Non-intercooled 2.2's are easy to get detonation at 8 psi, and the /6 is similar enough that I would expect a 225 with 8 psi and 9.5 on pump gas to ping and rattle itself to death. I could be wrong, maybe high octane or race gas would make the difference, but for me I am going to stick to the mid 8's on anything I build to run on the street with boost.

About the Cougar, I would bet he runs a pretty high test fuel and has retarded the timing a fair amount. Both would make it live, but he is giving up some power, and maybe spending a bit of money if he needs anything higher than 93 test.
Author:  CStryker [ Mon Sep 11, 2006 5:40 pm ]
Post subject: 
He runs premium pump gas, but he also has twin air-air intercoolers (one for each turbo). He modeled his after another guy's build, and the other guy had 450 rwhp on a dyno test. Not sure what his spark map looks like.
Author:  slantzilla [ Mon Sep 11, 2006 8:44 pm ]
Post subject: 
2.2 turbo pistons on a virgin block will give a deck height of -.028" on average. I have had mine in 2 different blocks and it was that number on both. IIRC, Doug came up with the same number on one of his.

Not sure how many lbs. of boost spray relates to, but I have split a hyper 2.2 piston in half with a 175 shot.

Were I doing a serious snail power moder, I would not bother with cheap pistons. You'll be happier in the long run getting a set of custom forged and end the worries. :D
Author:  zedpapa [ Tue Sep 12, 2006 6:36 pm ]
Post subject: 
if you are planning to go with efi, then you can get away with higher compression and boost. also, is E85 available in your area? it has an octane rating of over 100. however it takes about 30% more fuel to make the same power. but it is cheaper.

if it were my motor, i would shoot for 9:1 comp., build the efi system, add the turbo system, add boost, and see what happens.

zedpapa
Author:  Bohmer2 [ Wed Sep 13, 2006 6:32 am ]
Post subject: 
The plan does include going with MPEFI, I am not sure which controller I am going to use, the FAST system sounds like it has more flexibility than a Megasquirt but costs considerably more. I have been reading on the MS II and it sounds like it may have more flexibility. But with the turbo, and I am thinking the FAST system would be a worthwhile investment to help keep the motor together.

I guess I need to start tearing down blocks and looking at the heads I have laying around and see what I can find out about these blocks to get an idea of what I will need to get 2.2 pistons down to the 9.0 CR range.

I appreciate everyone's input on this. If anyone has more I would appreciate hearing it.

Brian
Author:  Thropar [ Wed Sep 13, 2006 12:53 pm ]
Post subject:  stock rods
Hey Brian, why not stick with the stock length rods and build your turbo motor? There are several people in Australia running std. rod 225's with boost and getting a LOT of power out of them (350-600hp). The stock 225 is already set up with compression perfect for boost, even after cleaning up the deck and head for straightness.

If not, I understand. You get a better rod ratio and also a nice big bore for some inches. Piston availabilty is also a + (forged). I'm interested in seeing what comes up for this build if anyone has the 198/2.2 low compression answers, because I was trying to figure this out a few months ago.
Author:  LUCKY13 [ Wed Sep 13, 2006 1:30 pm ]
Post subject: 
Venola offers a good forged piston for the factory rod.


http://www.venolia.com/pistons.html


Also BS3 (Big Stuff 3) has a very good EFI system you can get for around $1900.00 and this comes with a Wideband and a lot of extras and ajustability. It has one of the best injector driver cuircuts out there which gives better low RPM & idle controll of big injectors to give better A/F, cleaner burning & smoother drivability.

The MSII can be very good for the money also, it has changed alot lately & I wouldnt dismiss it because of features because it can do as much as other & has new stuff added all the time here lately.


I wouldnt go over 9:1 or at the most 9 1/2 :1 on the slant. If we had a Aluminum head it may be different. 9:1 I think would best best, still have some off boost power & take a fair amount of boost also, A good compromise for all around street setup that could make alot of power if you wanted to add the fuel to support it.



Jess
Author:  Bohmer2 [ Wed Sep 13, 2006 2:52 pm ]
Post subject: 
Quote:
Hey Brian, why not stick with the stock length rods and build your turbo motor? There are several people in Australia running std. rod 225's with boost and getting a LOT of power out of them (350-600hp). The stock 225 is already set up with compression perfect for boost, even after cleaning up the deck and head for straightness.
This had been my original plan, to build a stout stock motor with boost, but the reason I brought this topic up was to see if would be possible to get the benefits of the long rod motor and run a turbo. It just seems to be that the torque and hp curves of a long rod slant could be extended further with the use of the turbo/mpefi combo and still be very street friendly.

In terms of cost, I think it would be far cheaper to run a stock rod motor on boost. At the same time I and those I tell would be the only one's to know its a long rod motor, so its not like it would be built to just show off.

Jess,
Thanks for the info on the BS3 and MSII, looks like I need to get back into my MPEFI research and figure out the capabilities of these systems again.
Author:  Bren67Cuda904 [ Wed Sep 13, 2006 2:53 pm ]
Post subject: 
My engine builder is planning (hoping) to stroke my motor to 4.5 and use the 198 rods. We will be using custom forged pistons. We still have to measure to see if we can fit it all in there. He mentioned some special oil ring thing that might give us the room. We're going with the largest bore possiable and avaiable ring size. Sonic testing is being done to find this point. We are leaving enough for nitrous though.
I am really getting a little crazy with this motor, but if I know I didn't go all the way, I wouldn't be happy.
Author:  gmader [ Wed Sep 13, 2006 2:57 pm ]
Post subject:  Doug's article
Doug's article says that the KB 268 gets you right where you want.

http://slantsix.org/articles/stroking/stroking.htm

Deck Heights: 1.594
Dish Volume: 18 cc
Compression Ratios: (w/ .045 head gasket): 8.73 C/R

So, I think that you would want a forged version of a dished Turbo 2.2 piston.

Is this wrong?
Author:  Matt Cramer [ Wed Sep 13, 2006 3:56 pm ]
Post subject: 
Quote:
That conversation got me thinking as well... I have a friend who has a 5.0 in a cougar which runs about 9.5:1 compression w/ the stock HO cam related parts (modified computer) and he runs about 12 lbs boost. What makes that motor so much more forgiving than our slants? Or is his just waiting to blow up?
Forged pistons. The HO 5.0 had forged slugs from the factory.

And that boost level isn't that much higher than what you can run in a stock slant six. The real key is avoiding detonation.
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