Max compression ratio for supercharger

madjak · Postby **madjak** » Fri Dec 19, 2014 7:05 pm

I have a high hp built engine with 11:1 compression ratio, high flow head, big cams etc. At some point I'm thinking of either installing itbs or go all out for a rotrex SC. Target hp would be around the 250-280 mark with a blower.

What compression ratio would be considered the maximum you could run? E85 would help somewhat but I would think not enough to make 11:1 work. Would also install a mid sized intercooler to keep temps down.

What would be the ideal compression ratio for a built engine and is there a range that works well or a specific ratio? 9:1?

Magpie · Postby **Magpie** » Fri Dec 19, 2014 7:12 pm

Drum E85, 11:1 you should be able to used forced induction. With drum E85 you could easily go to 13:1 naturally aspirated.

Postby **NitroDann** » Sat Dec 20, 2014 9:39 am

I've just built and am tuning an 11.2:1 turbo engine.

madjak · Postby **madjak** » Sat Dec 20, 2014 3:12 pm

Was it running 98 or e85?

Does it mean you have to pull the timing a lot to stop det? I really don't have much experience with FI. I suppose it all depends on boost levels.

My motor is set up with peak power at around 7800. Are big duration cams ok to run with a sc?

greenMachine · Postby **greenMachine** » Sat Dec 20, 2014 5:55 pm

Don't forget that a supercharger heats the air, and intercooling becomes more important than for a turbo - unless it is a centrifugal blower, in which case they have the same (I think) needs.

Pushing the CR to its limit could be called 'courageous', you might need to get some serious advice from someone who knows about this stuff, and make sure they know exactly what setup you will be running. Internet forums have their uses (and I am sure you have searched on all the relevant places here and in the US and elsewhere), but nothing beats sitting down with the expert and working through what you are proposing to do, and how to address the issues. Even if you have to pay them for their time.

In any event you will need to watch your IAT like a hawk, avoid knock like the plague, and trust your fuel supplier not to change specification.

:mrgreen:

PS I think I saw one of the ECU people have a smart fuel sensor that can detect variations in the fuel used. I would think something like this would be a must for doing what you are talking about.

Magpie · Postby **Magpie** » Sat Dec 20, 2014 6:18 pm

Hence my advice to use drum E85, quality is almost guaranteed. However if you change brands you will need to retune.

The decision for mine has been made on the brand of E85 and whilst a fuel flex sensor will be installed it will be tuned on pump 98 and brand X E85. The quality of pump E85 will not allow a tune to be pushed because it can be from 70 to 85%, as per Australian standards.

madjak · Postby **madjak** » Mon Dec 22, 2014 7:57 pm

Yeah I'll talk to my tuner about it all. You can get the Haltech flex sensors for the ethanol content. At the end of next season I'd look at pulling the engine down for a rebuilt, so putting in some lower compression pistons then wouldn't add much cost to the install.

I would look at using a Rotrex... so a C30-74 or something like that.

In terms of intercooler, given I can find one the right size, it could sit next to my half width radiator and get clean air in and out. Not sure if this has been done before in a MX5 but it probably means I could run a small intercooler given the better airflow? Is bigger always better... I assume it all comes down to airflow multipled by area. Looking at it, a 300x300 sized intercooler would probably fine given most guys run a long skinny one in front of the radiator.

Postby **NitroDann** » Tue Dec 23, 2014 4:20 am

Yes e85.

The higher the boost the further your power band will move up the revs.

speed · Postby **speed** » Tue Dec 23, 2014 8:44 am

NitroDann wrote:The higher the boost the further your power band will move up the revs.

Interesting.

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speed · Postby **speed** » Tue Dec 23, 2014 8:50 am

madjak wrote:Yeah I'll talk to my tuner about it all. You can get the Haltech flex sensors for the ethanol content. At the end of next season I'd look at pulling the engine down for a rebuilt, so putting in some lower compression pistons then wouldn't add much cost to the install.

I would look at using a Rotrex... so a C30-74 or something like that.

In terms of intercooler, given I can find one the right size, it could sit next to my half width radiator and get clean air in and out. Not sure if this has been done before in a MX5 but it probably means I could run a small intercooler

Wish I had the sense to have had a custom radiator made to enable 2.5 inch piping either side. Oh well, could always re do it

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Sean · Postby **Sean** » Wed Dec 24, 2014 10:32 pm

madjak wrote:Yeah I'll talk to my tuner about it all. You can get the Haltech flex sensors for the ethanol content.

Tip on the flex sensor - It's a GM part, and significantly cheaper if you buy it with the GM part number rather than the Haltech part number and sticker