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zambon · Post by **zambon** » Sun Mar 19, 2006 6:03 pm

I have been collecting parts for my 3l supercharging project for some time now... So far I have an Eaton MP90, 164 runners and fuel rail, 30lb injectors, and an unfinished Megasquirt.
Next I intend to buy pistons, but I am unsure of what compression ratio I should target.
I think that the 90 cubic inch blower will produce, at minimum around 12 psi of boost. It would also go higher with the use of small pulleys. I plan to use water injection to help control detonation in this engine, I also would consider a water to air intercooler.
With that in mind, what should my compression ratio be? One number that has been suggested by a knowlegeable person is 7.5 to 1. When I have mentioned this number to rice boys that I know, they told me that that would be unnecessarily low. What does this collective think?

Barry · Post by **Barry** » Sun Mar 19, 2006 8:34 pm

James,I would tend to agree that the old school thinking of 7:1 is,well,old school..
I would look at at something like 8:1 to 8.5:1 cr..I dont know what your fuel quality is like,but on our crap we have here Id run 5:1cr!!!!
No seriously,8.5:1 would be my target cr in this case..Greg???

*could I do with your tow rig today-got too move a few cars out of my place,starting to look like a junk yard..*

Micke · Post by **Micke** » Sun Mar 19, 2006 11:46 pm

I don't have experience about this but here's something which might help.

Greg Gordon · Post by **Greg Gordon** » Mon Mar 20, 2006 9:40 am

For your project I would go with 7:1, perhaps 7.5:1. Here is why:

The M 90 is a big supercharger. It will put 17 pounds of boost into a 3.0 with no problem. Less then 12 pounds of boost and the numbers for the M 90 will be terrible because it's RPM will be way below it's good volumetric efficiency range. Especailly at low RPM which would defeat the idea of using a Roots blower in the first place. If you wan't less then 12 pounds of boost you need a smaller supercharger.

All this means that you will need at least 12 pounds of boost, and may want 17. Well with U.S. spec 93 octane fuel, and only 91 octane in some areas. You will need comression down near 7:1 to get away with 17 pounds of boost. You might be able to get away with 8.5:1 with 12 pounds but that would really be pushing it. Besides, you will want to increase boost at some point.

Remember that there is no off boost driveability issue here so that's not a factor. The supercharger will automatically and instantly add boost when needed (assuming you are using a rotory bypass valve, not a plunger type but that's a whole different topic). The only real drawback with going down to 7:1 is that the car will be a slug if you remove the supercharger for some reason.

zambon · Post by **zambon** » Mon Mar 20, 2006 10:39 am

Should I build the engine with high RPM capability? I know that roots superchargers are generally known for their low rpm power. Would it be counter productive to use high rev valve springs and balance the bottom to go safely to 7500 rpm? Since the MP90 is so big, could it accomodate that? I should have no problem with fuel delivery since my injectors are 30lb/hr.
Thank you for the advice,
James

Greg Gordon · Post by **Greg Gordon** » Mon Mar 20, 2006 5:36 pm

You can go to 7500 RPM with the M90. Would it be productive or not...I don't think so. Max boost on a STOCK M90 (some of the modified ones can deliver more) with a 7500 RPM redline will be about 12 pounds. I think 17 pounds and a lower redline is a better option, but that's me.

If you want 17 psi AND a 7500 rpm redline you need a bigger blower! Like everything else superchargers operate efficently within a certain range, get out of that range and things don't look so good.

I say ballancing is always good. I don't know about the high rev valve springs, I like to stay near the stock rev limits.

Daniel · Post by **Daniel** » Mon Mar 20, 2006 6:31 pm

Doesn't all of this depend on the drive ratio to the blower ? (naturally aspirated dummy asking questions again here)

A couple of quick calcs says a 90ci blower on a 3.0 engine needs to run at about 2.2 times engine speed to produce 17 psi boost and that's way too many revs for the blower (16500 rpm on the blower at 7500 rpm engine)
12 psi can be had by running the s/c at 1.8 times engine speed which also keeps the rpm within the MP90's 14000 rpm limit.
8.5:1 cr with 12 psi gives an equivalent cr of 15.4:1 and on a 3.0 engine, this boost gives an equivalent engine capacity of 5.4 litres (thats how much air is being shoved in every 2 rpm)
All of this is from my basic calcs and I may have compl;etely the wrong idea (see above comment about n/a dummy) but I think it's pretty close and may help.

Greg Gordon · Post by **Greg Gordon** » Mon Mar 20, 2006 8:05 pm

Daniel, that's pretty much correct. Boost depends on drive ratio assuming all other factors like engine displacement, supercharger displacement etc are fixed. However the drive ratio/boost calculations are complex because the supercharger's volumetric efficiency changes quite a bit throughout its RPM range.

zambon · Post by **zambon** » Mon Mar 20, 2006 8:54 pm

I was thinking that it might be better to stay with (only) 12psi and make my rev limit higher. The advantage would be that I would be able to hold gears longer on the track. Example: A corner approaches when the engine is already reving at 6500. Not enough time to shift, but I could just hold the gear for an extra second. without the high rev limit, I would have to let off the gas and coast up to the corner.
This would be less useful than the extra power that 17psi could provide, but it would be more friendly to the components that rest between the engine and the tires. I am afraid of making too much power and breaking things all the time. Just trying to consider all options
Thanks for the responses,
James

Greg Gordon · Post by **Greg Gordon** » Tue Mar 21, 2006 7:36 am

That's a great point James. For your application that may very well be the best way to do it. I tend to think it will have less power (perhaps not an extra 1000-1500 rpm combined with cams might close the gap. Plus the supercharger will be much more efficeint at 12 pounds), but it may very well be faster around a track for the reasons you described. Bottom line is I just don't know.

Many things with our cars are at least as much about personal prefernce rather then pure numbers anyway. If you want a 7500 rpm supercharged motor then build it that way. If you want a centrifugal unit or a turbo do it! If we all made our decisions on purely subjective factors we probably wouldn't be driving Alfas.

Another thing, the sound of a supercharged Alfa V6 at 7500 with the supercharger at 14000 rpm (16000 anyone? not a great idea but it's been done!) would be awesome. So again, there are objective factors here as well.
Greg Gordon,
P.S. Check out the new project, SuperSpider! http://www.hiperformancestore.com/AlfaHiPerformance.htm

SydneyJules · Post by **SydneyJules** » Tue Mar 21, 2006 2:52 pm

So Greg, is the "Not a great Idea" bit involving the belts, or the blower itself? I mean, do all of these things have their own lubrication supply? If youre gonna spin it that hard it might pay to give it its own oil cooler... and some Gilmer drives just to be sure!

Greg Gordon · Post by **Greg Gordon** » Tue Mar 21, 2006 3:23 pm

Hi Jules, Surely Glimmer drives will help. However we can only get those from Australia and shipping them here would be expensive.

By the way those folks from Australia make really nice intercoolers. I had one on My GTV6 for a day, just fitting it before sending it to a customer. I put a picture of it up on my site, it's really nice. You can see it here: http://www.hiperformancestore.com/Custo ... argers.htm

The "not a great idea" part has nothing to do with the belt, it's about the blower itself. It will probably handle it, quite a few people have demonstrated that but it's getting way outside the ideal operational envelope. In other words, if you need 16,000 rpm you should probably be thinking about a bigger supercharger.

These things have thier own oil supply. I never heard of anyone using an oil cooler, but I guess you could do that.

GTV6GPTT · Post by **GTV6GPTT** » Wed Mar 22, 2006 8:09 am

Get some GOOD engine management! So you can have good control over the engine. Run no more then 9:1 no less then 8.5:1 compression. Being a M90 drive it at a nice ratio to get around 8-10 pounds, having good CR will make up for allot. match the drive ratio to get the right CFM\CC

Superchargers are rubbish for higher boost to much heat.

! Turbo is always the better way to go in most cases !

zambon · Post by **zambon** » Wed Mar 22, 2006 11:45 am

Hmm,
Are you saying that Megasquirt is not good engine management? Greg already said that the MP90 would be inefficient below 12 pounds of boost on a 3l engine.

zambon · Post by **zambon** » Wed Mar 22, 2006 11:51 am

I think I am leaning toward Greg´s original recomendation of 7.5 to 1. I will use water injection and run between 12 and 15 pounds of boost. If I ever change the pulley to run 17 pounds, I will add an intercooler.
I am planning to run stock cams for the moment. Is this blasphemy? My reasoning is that cams are expensive and I should get over 300hp without them.
How does this setup sound?

What compression to match my MP90?