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Guest gliding_serpent

9.5:1 compression ratio with E12 head, 91 octane fuel?

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Guest gliding_serpent

Lots of questions from me recently as I am having my 73Tii (dual webbers) engine rebuilt and am in the planning and learning stages. I am about to spring for some pistons. I have an E12 head, requires to 90mm, likely won't need head leveling, but I am having that double checked by the machinist. I am primarily looking at the 90mm IE cast pistons that give a 9.5:1 euro spec compression ratio, but Jaymic in England is trying to source and price some 90mm OEM's.

I have been trying to read on the subject, but my concern is that 91 octane is all that is available locally. I suppose I can mix airplane fuel with pump gas to generate something higher, but I have minimal interest in that. I would also like to avoid the mixed opinions on efficacy that comes with octane booster.

So my question is, who has experience with 9.5:1 on 91 octane in their 02? How far did you need to retard the timing (I know every engine will be different)? Do you get ping/pre-ignition running a hill? Did your level of timing generate compromise in idle? I read of a few people talking about ping with less than 93, but some of these people are not willing to compromise the extra power of advanced timing. Again, my main question is can I go 9.5:1 on 91 octane and retard timing to the point of no ping without compromising idle? I plan on putting the KF injection back on eventually, so I also want to consider it's function to.

Thanks for any help. I have been learning a lot from this forum, and the BMW '02 restoration guide by Mike Macartney is great also.

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I have exactly the same setup, but with a 2 bbl Weber 32/36 carb and my car performs just fine on midgrade gas. It took a little tweaking of the timing but now I only get light pinging when on an upgrade in too high a gear--and none at all on level ground or even under hard acceleration.

Your setup might benefit from a recurved dizzy especially if you're going to a hotter than stock cam. I'm sure foks on the board can recommend dizzy overhaulers. I used the guy up in Minnesota/Wisconsin--cannot remember his name--and am real happy with his rebuild.

cheers

mike

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Guest gliding_serpent

thanks for the reply. "Dizzy" is a new concept to me (I assume you mean distributor) so will need to make it my next level of research as I am on a steep learning curve.

I am trying to keep things as stock as possible since I want to pop the KF injector back on at some point after the rebuild (just no $$$ for that now). I will be keeping the stock cam.

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Your machinist can set the valves a bit deeper into the head

(it doesn't take much)

to lower the compression ratio pretty easily.

But make sure everything's measured carefully-

a "9.5:1 ratio" is surprisingly variable depending

mostly on head work and a little bit on how the block goes together.

It's worth pre- assembling the block and making sure the 'pop- out' is

consistent across all 4

(it usually is)

and cc'ing the head to make sure the volumes are the same

(they usually aren't.)

These are the things that will make "9.5:1" work.

hth

t

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I have an Eric Kerman built engine with 9.5:1 CR, stock cam (I have an automatic), 38/38 Weber, Tii dizzy and exhaust manifold, and I run 91 octane gas. The difference in price between mid-grade ($3.55) and premium ($3.61) is so little I don't worry about it.

I mean, 72¢ more for 260 miles of driving is no biggy.

Bob Napier

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Why dont you go for a lower comp ratio? I have run flat top pistons about 8.5:1 with a 296 cam on twin webbers with low octane fuel without it pinging.This was only a stop gap motor while another was being built but it performed fairly good.Jetting and timing have to be right .By lowering the comp ratio you actually slightly raise the torque figure as well even though the power drops off a bit .Well thats what we found after some testing on the dyno.I think that running 9.5 on 91 may be a bit of an issue in the long run if your pushing the car a bit , especially with twim webers and a few miles between tune ups. You would get away with it if you went fuel inj?

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Guest gliding_serpent

I guess the trick is finding the right pistons. I can get any compression I want if I go forged, but I guess I am educating myself on the options. I would prefer to stay with cast options, and I would prefer to not have to mod my head to lower compression. I am also thinking ahead for when I eventually get the KF rebuild and bolted back on. I suppose it all depends on if, and how much leveling my head needs.

I spoke with a very nice Cali gentleman and he suggested that he did well with 9.5:1 on 91 with stock cam without pinging issues (timing at about 33 degrees). The trick was to have your timing right, and thus your distributor in tip-top shape. Advanced Distributors may now be part of my increasing rebuild budget.

Like anything in life, I can push things all I want with timing, but at a cost to reliability. I once tried to have this general type of discussion with folks from the tuning crowd on a different forum. My conclusion was that the more output your engine had above stock, the less you believe that those mods will in any way compromise reliability. My opinion was that you can only have two of the following three: power, reliability, low price.

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I have 9.5:1 piano top pistons in my 74 tii. The engine was rebuilt to Euro specs (9.5:1), I run premium 91 gas and I have no pinging what so ever.

G-Man

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I have been running 93. But went out of state in the car recently and only had 91. I had no issues with the 91, but I was also a lot higher up in altitude in Eureka Springs vs Dallas Texas. Not sure if it makes a difference or not. I run the same set up you have described

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