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MG MGB GT V8 Factory Originals Technical - Compression ratios in small V6

I would like to address this to Dann of British Car Conversions. Recently I read in the "V6 questions" that you claim to be able use the aluminum heads from a front wheel drive engine on a rear wheel drive vehicle. I'm puzzled about all of the banter that was issued about it. Could you spell out the reason you came to the conclusion that you achieved "10.666" to 1 compression ratio in you racing engine? I think I've seen the car at several autocross events. I think it was a medium dark green. Kev
Kevin

I meant to ask how many sized engines does GM make in the 60 V6.
Kevin

Kevin,

DON'T POKE THE BEAR !!! Just kidding !! Can't wait to hear this answer.... Go for it Dann. Alan
Alan

Kevin, I received your email and I decided to answer your questions here. That was our car. It is green with amber racing stripes. My daughters name is Amber. :-)

Anyway what you read in the columns was one persons failure to communicate with the other. One part of the parties got strung out on an irrelevant issue over CR, but it seemed important to him. I thought rather than debate the issue to much I decided not to explain my findings that I would let him find out for himself. Anyway that's all behind and forgotten on my side of the fence.


Most information that is given out from dealers is derived from manufacturers info. and is written as basic information so as to give fair warning that compression ratio's can exceed manufactures specifications for emission output, octane requirements, ECU errors and diagnostics, timing problems and more, when dealing with part changes not recommended. A tuner / builder mostly ignores this information a deals with it through experience.
The General Motors Corporation world wide manufactures or manufactured these engines in a 60°V6 FWD configuration. The 2.8 to 3.4 engines had 9:5 to 1 compression ratio's according to my sources which is the Chevrolet Hobby Shop. I used to be an engineer there.
2.5 LB8 145 HP 2490 cc
2.8 120 HP
3.0 LW9 170 HP 2986 cc
3.1 LG8 175 HP 3136 cc
3.4 LA1 180 HP 3350 cc
3.4 200 HP 3350 cc
The 3.4 rear wheel drive has the same configuration as the LA1 but less compression ratio (9 to 1), iron heads, and 160 HP.

I derived my CR like this with the aluminum head to the Camaro 3.4 head swap. All was figured on clean surfaces.

3350 cc divided by 6 (cylinders) = 558.33 per cylinder

CC in piston dish and deck height + space above upper compression ring + . 015 deck height = 20 cc

Compressed head gasket of 6 cc =6 cc

Head squish area= 30 cc

30 cc + 20 cc + 6 cc = 56 cc

Divide 588.33 cc by 56 cc = 10.5 to 1

I added for carbon build up which is minimal and rod stretch to achieve a real figure of at least 10.6 to 1 compression ratio. To achieve a higher ratio would require a shallower piston dish or close off the squish area. The latter being to expensive to consider without good head flowing knowledge and a good flow bench. The engine revs very easily to above 7200 RPM Hope this helps,

Dann BCC


Dann Wade

Kevin, That last engine on the list was a DOHC version.

I thought something was odd about that ratio I posted before. I was trying to recall the formula from memory. :-P

I forgot to add the squish to the cylinder volume. I made the same mistake before. GEEEEEZ! I hate getting old. Anyway see below:

Cylinder volume 588+ 56= 644 divided by 56 = 11.5 to 1.

No biggie.... the Dodge Viper and a lot of late model performance cars have a CR of 11.5 to 1 or more.
This is even better than 10.6! The gasoline octane figures at the pump are within a percentage of the stated figure anyway. We have tested gasoline for specific gravity and almost always supersedes the rated figure. Actually in the real world the final compression ratio has to be figured by the profile of your cam and intake/manifold efficiency but mostly at the top end. I'm using a mild Crane street cam and it's a little bumpy. In the final analysis it runs well on high test pump gas with no knock and without additives or drastic timing programs. Sorry about that. Dann BCC
Dann Wade

Alan, you can have fun with this! I even misquoted my own figures again!

Kevin, the real figures one more time are these.

558+56 = 614 divided by 56= 10.96 Darn.... now I'm disappointed, I wanted 11.5.

Oh well, Dann BCC
Dann Wade

Dann
I would really like to have some of that stuff you keep smoking. Sounds like you need another internet humiliation. Stange talk for a guy who needed to contact another website to find out how to wire the computer for the Ventura 3.4 to Berretta project, saw the post and had to laugh. Engineer? You can't figure out the pinouts on a Motorola 80868 I dunno.. Scrathing my head ... must have got that from you... Dann.....CU Alan
Alan

Alan,

The Flames pages are located in another forum on the MG BBS site. This is not the place. I come here for useful information. Dann just provided some. If he did or didn't know some electrical connections for an ECU, he sought out information. Do you have a problem with that?

Maybe you could seek out some new knowledge. Manners perhaps?

Regards,

Brian C
Brian Corrigan

Alan,
I have to agree with Brian. You're comments have become boorish.
David
David

Hit the button too quickly.

Alan, you're acting a like a child and your comments have become boorish.
David
David

Alan, I 'll come down to your mud sucking level for just one time.

Excuse me gentlemen this asshole deserves whats coming.

I've had it with you! I had the materials right in my hands when I checked it out! The Venture NOT Ventura has an Anti theft system that can't be defeated by any normal means. You are an obstinate old ignorant SOB retired moron that has nothing to do but bug me. I'm sure your wife knows this and if I had you in my view I would whip your dumb ass. Don't even think about going for a reply, I won't answer it. Dann Wade
Dann Wade

Friends and members, I send you my apologies for exposing you to the rhetoric that I imposed on you. We have been here for years with our inputs and have all been able to get along and I don't want to give anyone an impression of me that I don't deserve. I have always been here for you to help solve problems and I've learned from you also. I think it makes for a well informed community when we can share these columns on the common grounds that we have. I feel as if I have hurt these columns by my last remarks. It isn't that I'm trying to protect my business interests at all. I grown to be very good friends with a lot of the readers of these threads and aficionados of the conversion group along with the regular MG owners. If I get booted from these columns it will be of my own doing. I did not have to go after "Alan " in that manner in these columns although I still feel how I addressed him was within reason considering the way he has addressed me. Please forgive me for not apologizing to him. "Alan' has appeared just recently. I am suspicious that there is no real "Alan," that it is a character created by someone who wishes to undermine my efforts to bring good reliable information to these columns. Anyway I would like to try and bury this whole thing.
Thank you,
Dann BCC
Dann Wade

I'd say that was probably the right thing to do.
So Dann, what happens when you swap the heads the other way around? Put you in the ballpark of a blown application maybe?

Jim
Jim Blackwood

There should be a section just for Dann and Alan, and both need manners lessons.
Totaly out of place answer Dann.
Jim, I am surprise at your support for that kind of reply. It's best to ignore, and move on.

George Stanly

George, my response was aimed at the apology. You surely didn't miss that did you? Anyway, I *was* moving on. I'm really more interested in the technical discussion than the politics. However, often you can't have one without the other so I guess it's best to learn to live with it. Of course, I also have been known to complain when it gets out of hand.

So moving on... How about that CR?

Jim
Jim Blackwood

George you are exactly right. The disagreement is far to petty to contend with. Thanks for your comment. My apologies to you directly sir. Lets move on.

Dann BCC
Dann Wade

Jim, thanks for the faith and thanks for a very good observation about the head swap! I hadn't thought about that before! A very simple and inexpensive solution to use a turbo on the FWD engine. I haven't got the exact cc for the dish pistons in the FWD engine but I'm betting it's a winner. This is a perfect example of what I was trying to say about exchange of information and learning . I will get back to you and this column as soon as I can check it out.

BTW the Beretta and I think the Gran Am had Turbo version of the FWD. I am sure Bill G. would find this interesting. How about it Bill? Iron heads, FWD short block, Turbo. I 'll bet your ears are tuned in. This is worth checking out. Everyone who has a 60°V6 could be interested in this.
Dann BCC
Dann Wade

Has anyone actually cc'd both types of heads?

I've read figures of 52, 53, and 54 cc for the iron head.

I've read that the aluminun head is "about 1/2 the iron head". Not too specific.

I'd like to get some accurate specs and play with them in a CR calculator.

I would like to point out that there is a big difference in 10.5:1 and 11.5:1 CR's.

I'm also building a 355 with about 10.3 static CR, but it'll come out to about 8.1 dynamic CR due to cam selection, quench, etc, which should make it streetable on 93 octane pump gas.

I think a 11.5 SCR would pose some serious problems with pump gas - not sure.

Glenn

Glenn, I checked the cc on the 1992 aluminum Lumina heads that I have here which cc out the same as 2002 Venture heads at 30 cc dead on. I just checked out the iron heads and came real close to 56 cc. The squashed head gasket is 6 cc. I don't have a piston on hand to check the dish for the 3.4 or 3.1 FWD engine at the shop as I write. A RWD Camaro 3.1 piston dish is real close to 9cc. I would assume the 3.4 would be more.This will give you a starting point. Have fun.

For the heck of it lets use common language here to better portray the picture to the readers who don't have the same knowledge as other readers.

The compression ratio at tick over can be a far different situation then at 7,000 rpm. Like I said before cam profile and manifold efficiency play a good part in what the real ratio is. If you had an engine that had 11.5 to 1, which most normally aspirated drag cars reach that figure and even more, you would use a qualified racing gas or mix your own and/or deal with timing advance or both. In street applications you would use the highest octane available and use a programmable ECU or programmable ignition control system, in conjunction with a programmable engine management system or possibly chips to prevent component failures, mostly pistons/rings, because of the high CR in a non carburetor application. You can also use a programmable ignition control system or mechanical control system with a carburetor application.

With a mechanical distributor you would try to control the ignition by mechanical means. A race car using mechanical ignition control mostly depends on getting into advance quickly and trying to maintain control over the advance curve. If you used electronic programmable control you could set the perimeters much easier with much more accuracy and reliability. I had a Mini Cooper that was bored and stroked to 1430 cc. I had to use aviation gasoline to prevent pre detonation and my ignition control was marginal at best. This was in 1976 and I had the only technology that was available which was a blue printed mechanical distributor. I never suffered any component losses and I was well into 11.7 to 1 CR and I drove it on the street. I had to drive it hard and fast to make the engine comfortable. At least this was my excuse ;-) It would have been impossible to use it as an everyday car and depend on it. My point is this:

CR is very important but with modern electronic controls it isn't much of an issue. I would want to go beyond 15.5.

Dann Wade

A lot of people do not realize how large a part intake efficiency plays in C/R. A fairly extreme example is my blown 215 Olds. Here the static C/R is 8.5:1 but max boost is over 15 psi and I don't have detonation problems. How is this possible with a roots type blower? Fuel injection, intercooling and electronic ignition of course play a part, but for the real answer we have to go back to the basic engine itself. Recall that of the BOP/Rover family the Olds design has the poorest performance in terms of port flow. When you consider the effect this has at maximum output it's not too hard to see that a substantial part of the blower's output is being used simply to overcome the shortcomings of the head itself. If the blower wasn't there at all, at some point the bottom would drop out of the dynamic C/R but with the boost power output is quite dramatic. However, take the same blower and put it on a Buick or 3.5L Rover and it would be unlikely to live real long simply because those heads flow better and the dynamic C/R would go up with the likely result being the onset of detonation. Power output might be no more than with the Olds and could be considerably less. The other factor is that boost increases as engine speed goes up, meaning that it isn't possible to dump 15 psi into the intake at 2000rpm, so again mechanical factors come into play and protect the engine. So the point is, that you can have an engine with a high static C/R, which improves efficiency and economy but at the same time have a lower dynamic C/R which prevents the auto-destruct-sequence. Knowing this, it should be possible for me to build an Olds short block that will run 10:1 C/R with the Jetfire heads, drop the blower output back to about 8 psi and get largely the same results only with better fuel economy. This might increase the risk of detonation at low RPM but otherwise I don't see a downside to it.

Jim
Jim Blackwood

Yes Glenn. the cc was done at a race shop just for....
The cc ratio came to well over 13:1 this is not bull.
But could be several varialbles, and without them in front of us is hard to argue a point, you just take in what you want and go on.

I am not going to respond to any responses and get into an wasted argument. We are going with the cast iron heads.
Thanks Dann, your apology is accepted and well taken.
Yes, I missed the point Jim, perhaps I did not see it.
George Stanly

Jim,
This is off topic, but are Jetfire heads different than regular Olds HC heads? I have a Jetfire engine, but one of the heads has corrosion in the exhaust ports and a seized valve. I'm looking about for a replacement head.
Thanks,
David
David

TYPO My closing line should have been....CR is very important but with modern electronic controls it isn't much of an issue. I would want to go beyond 11.5

I hope I didn't raise eyebrows on that one :-)

Dann Wade

George: I guess sometimes concise does not equal clear.

Anyway, off topic, the Jetfire heads do have some differences from the 4bbl heads and is a different casting but the chamber volume is very close. I would try to run them as a pair either way, but don't think you would run into any trouble mixing them. In my experience Phil Baker would be the authority on this, but Dan may be equally informed. It is likely your exhaust ports can be heliarced and of course a stuck valve is repairable.

Didn't intend to hijack the thread there, but I did find the discussion of head swapping interesting and will be interested to know the outcome of the reverse swap.

Jim
Jim Blackwood

This thread was discussed between 02/11/2003 and 04/11/2003

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