PaulFitzwarryne

Is it possible to modify a rotor so as to reduce compression? In simple terms, can a 12a rotor be machined to bring it to a 12at profile?

Project84

yes it is possible. Exactly how, I'm not sure. Someone in the 2nd Gen area had pics the other day of a J-Spec motor that had shaved rotors.

peejay

Yes. The bathtub can be machined out. I've heard of people taking rotors down to 7.5:1.

RotorMotorDriver

iTrader: (1)

Would you only want to do that for boosted applications though right? Why else would you want a low compression rotor? And why would you do that to begin with? I mean, wouldnt it be easier and cheaper (if you wanted T2 rotor specs) just to buy T2 rotors? I mean, I could understand it if you wanted a compression ratio that wasnt available on any rotors...But why just to make them match another rotors specs?

~T.J.

PaulFitzwarryne

Thanks Peejay for the information. I have had problems tracking down 12at rotors and housings in good condition. An engine with the stock 12a ports is an interesting project.

RotorMotorDriver

iTrader: (1)

I want to build a streetport 12A , with low compression rotors, RB exhaust, 45 DCOE, 50-75 shot N20, and a turbo now that I think about it, and 13BT rotors wouldnt fit in the 12A housings (right?), so theres another reason to do it .

~T.J.

peejay

Right.

I think RB might still have some 12AT rotor sets left... last I checked they were $450 for the set! Need to get counterweights though... probably cheaper to get a set of '79-82 counterweights and have them balanced to spec.

Actually... RB listed the counterweights needed, I never did cross-reference to see if they were ever used on a US spec car. I'd do it now but I can't access the Racing Beat website for some reason.

RotorMotorDriver

iTrader: (1)

I cant get onto RB either...Hmmm...

~T.J.

Project84

What is the reason for having lower compression on a turbocharged engine? Is it because the air is already compressed? I thought the more compression you could get, the more powerful the bang when the spark plug lights off, of course you don't want to have such a high compression ratio that the compression itself ignites the stuff in the combustion chamber. There are formulae to figure that stuff out though. Basically I'm asking the same question as rotormotordriver.

PaulFitzwarryne

The concept is to build a replica 12AT, thus 13BT rotors will not fit. At the moment I might stick to a blowthrough Nikki carb system but for the intercooler problem.

Am going for low compression with 10psi, why I am not technically clear as there must be a direct relationship between compression ratio and boost.

MotorRotorDriver- are you certain a 45 DCOE carb will fit with a turbo? Lack of space is usually why a sidedraft like a Holley or 48IDA is used.

peejay

DCOE's are sidedraft, Holley and IDA are downdrafts...

PaulFitzwarryne

Peejay you are of course right, must remember not to post after midnight!

rotarygod

iTrader: (1)

You want lower compression the more boost you plan to run. I don't see why you'd have any problems with the stock compression ratio if you are only running 10 psi. Then again adding nos changes things a bit. I'd be more concerned with breaking apex seals. The reasons for lowering compression ratio are due to thermal stresses in the engine. 10psi on a high compression engine will generate combustion chamber temperatures much higher than 10 psi on a low compression engine. You will eventually come to a point where the timing has to be retarded so much more on the high compession engine that you are making less power than the low compression engine.

All of those carbs can be made to work. You'll have to do some modifying to make them work for a blow through application though so fuel doesn't get forced out anywhere.

diabolical1

iTrader: (3)

actually, there are a many reasons for lower compression on forced induction applications. thermal stresses, emissions, slightly better tuning margins, and boost limitations are just a few.

PaulFitzwarryne

What I am concerned with is why conventional wisdom is to lower te compression rate and up the boost. What is the disadvantage od having stock cr and a lower boost. The 12at dropped its cr by over 105 but the boost was under 6psi.

Does the lower cr- higher boost give better tuning margins for the same stresses? The only reason I can recall is that when I went HPcr on a Lotus 46, low idle was dreadful[2500+rpm], with a turbo set up idle was better[1200rpm] even if power was low until the turbo kicked in.

peejay

You can get more mass into the chamber for the same pressure.