Forced induction low or high compression?

[crofty1984]

Hard to sum up in the title, but a possibly silly question:
In the distant future, I'm thinking of supercharging my Lotus Excel. (In the medium future, I just want it on the road!)

It would be a home-brew setup, but my question is:
I currently have the 160hp low-compression engine, the same Lotus slant-four as found in the early Esprit, I was thinking of upgrading to the high-compression engine for an instant extra 20 bhp and then doing some mods to that first. But which would be a better starting point for FI? If I'm better off 'charging an LC engine, then I won't waste my time with the HC one and I'll dive straight in.

What would be a sensible boost level? Off the top of my head I was thinking 1 bar as a maximum. But I'm willing to listen to advice!

[Evoluzione]

It depends on what fuel you want to use and how det resistant the chamber is, along with how good your fuel and spark control is too - the more modern and 'clever' it is the more CR you can run.

[stevieturbo]

crofty1984 said:

Hard to sum up in the title, but a possibly silly question:
In the distant future, I'm thinking of supercharging my Lotus Excel. (In the medium future, I just want it on the road!)

It would be a home-brew setup, but my question is:
I currently have the 160hp low-compression engine, the same Lotus slant-four as found in the early Esprit, I was thinking of upgrading to the high-compression engine for an instant extra 20 bhp and then doing some mods to that first. But which would be a better starting point for FI? If I'm better off 'charging an LC engine, then I won't waste my time with the HC one and I'll dive straight in.

What would be a sensible boost level? Off the top of my head I was thinking 1 bar as a maximum. But I'm willing to listen to advice!

You dont state what low is, or high is to give any sort of reference point, nor the type of supercharger or power you are after. Or fuel you will be using, type of fuel/spark control, and whether or not it will be an intercooled setup.

So rather difficult to give an answer.

[crofty1984]

stevieturbo said:

You dont state what low is, or high is to give any sort of reference point, nor the type of supercharger or power you are after. Or fuel you will be using, type of fuel/spark control, and whether or not it will be an intercooled setup.

So rather difficult to give an answer.

Very good point!
I don't have the info to hand but I'll find it and add it

[crofty1984]

A little more information after some digging:

All engines are 2.2l slant-four

Low Compression engine
CR = 9.5:1
Power = 160bhp

High Compression engine
CR = 10.9:1
Power = 180bhp

My idea was to use Eaton-type supercharger, as they seem common enough and affordable, maybe from a MINI (assuming it will flow enough) and a small intercooler/chargecooler. Though the'cooler may get installed at a later date.

The ignition and fuelling would be controlled by an programmable ECU, Emerald springs to mind (regardless of brand, I'd have a proper map made)

Standard unleaded running would be preferable. If I have no choice then Super would be acceptable.

I'd like upwards of 250bhp. I've seen chips for the turbo esprit that will allow 1.2 bar and 330bhp.

Fag packet maths goes thus:
160bhp @ 1bara
320bhp @ 2 bara (1 bar boost)
Real science doesn't work that way = inefficiencies and parasitic loss to drive supercharger about 20% = 250bhp

I am aware that for the money I'd spend I could get a different car with more power but that's not the point.

[mk2 24v]

I'd stick with the 'low' CR engine by the specs you have posted
The VAG 20v runs pretty much the same CR as standard (different engine/head setup I know), but if you have the right ignition and fuelling it shouldnt be a problem.

I'd be looking a an Eaton bigger than the m45 from the Bini though, as it would be running flat out for a 2.2 lump really

[Nick1point9]

Don't forget that cam timing will effect the dynamic compression ratio which is more important than static, so if you are going to change the cams at any point take this into consideration.

[crofty1984]

Thanks guys, I know I'm asking what seem like stupid questions, but hopefully if I get them out of the way now, I can move on to more intricate details then in a year's time, I'll be well placed to get the job done.

[stevieturbo]

As others said, likely the mini supercharger will be too small, and it's also difficult to employ effective charge cooling with rootes type blowers.

Opt for a bigger blower, low boost and the lower CR version of the engine.

If the supercharger is a no go, stick a small turbo on it. 250-300hp will be a very responsive unit as it would only need a very small turbo, of which there are many available cheaply.

You could buy a cheap TD04 off a Subaru for under £100 and adapt it. Or there will be other options, maybe not as cheap though.

[davepoth]

You can jump up a supercharger size reasonably easily - the M62 (the next size up) was fitted to Mercedes cars with the 2.3 litre engine. That one also has a natty electromagnetic clutch on it, so you can fit a switch to turn the 'charger on and off, just like Mad Max.

[crofty1984]

davepoth said:

You can jump up a supercharger size reasonably easily - the M62 (the next size up) was fitted to Mercedes cars with the 2.3 litre engine. That one also has a natty electromagnetic clutch on it, so you can fit a switch to turn the 'charger on and off, just like Mad Max.

Mad Max awesomeness, ebay be thy name...

Thanks everyone, I thought the Bini S/C might have been marginal. There's a build thread "Lotus excel project" in readers rides if anyone wants to keep up with developments.