Blown alcohol on lower compression hemi??

We are currently running a procharger f 3 on a 540 inch hemi but are considering a switch to a roots with a big and ugly. The current set up has run as much as 40 psi with a compression of 8.5. Generally the alcohol motors are of higher compression but would we be able to run alcohol at the lower ratio or would it be to inefficient?

 

Not sure what class you are aiming for or type of car, but that combo with a 14-71 at around 20% over is almost Ideal for a 30% nitro setup. Similar to what would be run for the nitro coupe setup ProMod style. On straight methanol the car would be lazy out of the hole with the 8.5:1 compression ratio compared to same set up with 10.75:1 - 12.0:1 compression. On straight Methanol I prefer 10.75:1 and more blower. I really like the 30% set up myself.

 

I had planned on running 9.5:1 with a 14-71 HH on a fathead motor and was told by a few reputable engine builders that it would be way too sluggish. 12:1 is target CR from what I have found.

 

I was always under the assumption that for you spin the blower hard like 33-44 over that you can run 10:1 compression and make it work. The better the blower and lower overdrive the more compression it takes to make it happy.

 

Appreciate you guys taking the time to reply. Sluggish out of the hole might be a good thing as the car runs in a 10.5 stock suspension style class. With either setup we wouldnt be using all the available power as are local tracks are not on par with the ones down south that run this kind of class.

 

I'm saying sluggish as in a stone for the entire duration of the pass. You would probably get away with that kind of compression ratio on a screw motor but not a roots. To your original question, inefficiency would definitely come into play due to the high OD% you'd need to not be disappointed.

Give Jeremy at BAE a call and pick his brain on it. He is always great to deal with.

 

What kind of car, how heavy, and what class do you plan on running ?

 

Lol a stone for the whole length definitely doesnt sound like fun. Its a 70 duster, 3400 with driver, 10.5 tire and stock suspension. Im up in regina sask just running a handful of local events against similiarily equipped cars.

 

How quick did you go with the procharger setup in that car?

 

So far its been 8.5 at 171. Initial goal was 7.99 at 170 but so far the learning curve has been quite lengthy.

 

With the small tire, and stock type rear suspension, and 3400lb. weight, you are limited on what you can hit the track with at the launch. I would build the engine with 10.5 or 10.75:1 compression and modest OD on the blower to start. You can always get another set of pistons with more compression later, and up the boost later. At 10.5:1 the tune up is much less critical of mistakes. Sounds like you want to have fun racing not spending $$$$ . With a 1900lb. car 478cu in hemi at 8.3:1 and 10% under driven front discharge 14-71 we could run 7.50's all day long and be non critical on the tune up. We could either put 30% nitro in it or up the OD on the blower with just methanol and hit 6.90 easy. Yes the lighter car makes a difference but you don't need the latest and greatest Top alky combo to run quick with you car. Look at the big picture , you are not running Top Alcohol , you are running against your buddies and having fun. As long as you can keep your tires hooked the 10.5:1 combo will get you 7.50's and probably mid 180's on mph. and not wreck the household budget.

 

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Question. Have you tried going higher in terms of CR?. How id that pan out with the screw? Thanks.