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MG Midget and Sprite Technical - boost

Hi there

Does anyone have experience to what kind of boost a 1275 is still reliable. I don't want to go to the limit, I run about 5 psi now and consider to put on a smaller pulley

(PS I use a intercooler, ign and fuel controlled by Megasquirt)
a.o. arnold

Hi ... no takers so far...

I have no experience of forced induction (except on current Germany 2-seater which I've not dinked with), but know that Tamsin used around a bar on here turbo 1275... so it would seem you have plenty of room to upgrade...

Anthony Cutler

Hi mate

I am running 22psi with a static cr of about 8.4/1

No problems. I also run programmable ignition but not using Megajolt however the curves are straightforward with no tricky bits to overcome detonation, it was not necessary and may I add I also run regular 95 octane fuel!
Bob Turbo Midget England

I run 9psi, but max boost measured in the intake does not tell the whole story. The CR and efficiency of the head/cam and intake gas temp also effects how much power and therefor the reliability.

Running distributor, carb, no intercooler.
Trevor Jessie

Thx guys

Well, 22 PSI not for me... I am happy to have a quick and reliable car. So I should also tell you that:

- CR is 8,5
- inlet temp are low due to intercooler (max 40 C)
- the ign map attached (I hope)

a.o. arnold

Bob, is this really boost, or pressure above vacuum? 22 PSI is very high as far as I can see

Reading Vizard mentions something like 7 PSI, which is about 0,5 bar... That would ask for 7 x 1,1 = 7,7 degrees less of total advance. I thought the midget has 35 degrees stock, so 28 degrees max. I run at the moment max of 26 degrees.... Anyone??
a.o. arnold

My megajolt map is similar to yours at full load.I run 8:1 CR with a mild cam,so no big overlaps.Inlet temp also similar.I run 17 psi boost and everything seems fine when I pull the engine down.
steve cowling

Hi Arnold yes it is pressure above atmospheric so if it were expressed in terms of vacuum I would say it was about 37 psi! :)

My timing is pretty basic i.e. 32 degrees at full mech advance but I have not as yet spent time on a rolling road to see if any improvements can be made to overall power.

Naturally reliability of any engine effectively diminishes with power output but that is always a price we must endure.
Robert (Bob) Midget Turbo

>>I thought the midget has 35 degrees stock

1275 has 13 deg max mech advance (stamped on lower dissy plate; x2 to crank degrees) and around 7 deg static... giving 26+7 = around 33 deg. Often 32 is quoted.

If the boost in you map is measured in the manifold, it would seem that part-throttle boost at max revs of 36 is a little high.

Anthony Cutler

Hi Guys

Thx for you input. I am still uncertain what to do. Easy enough is to raise the boost, modify the spark curve, but high revving pinging I am not gonne hear for sure.

I don't want the destroy the engine now that I have things more or less in order. I feel as if some more boost is possible.

What would you do? 6, 7 PSI or just leave it.

PS I have no forged pistons.., maybe using a ping sensor is an idea?

a.o. arnold

Hi Arnold
You say your car is 1275, or has it been bored out larger eg 1380?? a big factor in raising the boost is engine balance and most importantly strength I have a nitrus hardened crank, main centre strap, Omega forged pistons, 11 stud head all balanced and lightened without this I would not recomend raising the boost more than 7psi.
I ran my car standard at 7psi before the modifiacations and trust me it would not have lasted very long, it now feels very strong and powerful and I only raise the boost to 15 psi when I need to suck a Saxo up my exhaust!
So basicaly anyone can raise the boost to silly levels but how long do you want your engine to last, do you use your car on the track or just for fun ?
Rob Newt

Hi Rob

The engine is a 1293 cc, normal pistons, CR about 8.3. I use the strongers ARP studs and a MG Metro head. The cam is stock.

- Datsun 5 speed gearbox
- Megasquirt EFI incl ign control
- free flowing 2" exhaust, turbo mufler
- Mercedes 230 Kompressor supercharger
- bypass valve
- intercooler (inlet air temps are only 40 C as I can see in the log files)
- crank venting in sump
- evacusump

I run, as mentioned, now 1,33 bar max (see in it in the log files), and consider raining it. As you say 7 psi is the limit, I think I leave it...

What would you recommend me to modify to raise the boost to 7-9psi
a.o. arnold

Hi Rob not sure you have a nitrus hardened crank but I assume you had the crank Nitrided? which is a good idea. 11 stud head is pretty good however balancing is not such a big deal on an engine that is making low speed power rather than 9000RPM screamer. I like centre strapping as well.

I am curious where you draw the line between boost at "silly" level and boost at I assume good levels?

The problem with boost is that although boost may be high when measured at the turbo/plenun but that boost is NOT at the engine so to apply maximum boost to the engine you need to have the throttle fully open.

If you reduce the boost available to say 7 psi but then give it full throttle it might be no different potentially than allowing 18psi of boost but only opening the throttle by 50%. So things are not always easy to understand!

Arnold I would recommend Metro Turbo pistons as a norm (they are very good) and do what you can to make the engine as good as possible. If you are not confident about hearing detonation then you need to be very carefull. If you claim you can not hear detonation (should it occur at 5000rpm and 15psi boost) then how would you be confident you would hear it with only 7psi of boost?

Under these conditions you will need to use a rolling road to be certain.

We can all speculate about a "reasonable" boost charge but frankly that is meaningless. the more boost the more unreliable the engine will become. I am prepaqred to rebuild my engine should the worst occur, if you are not prepared to do that then perhaps 7 psi is more than enough.

I too have an intercooler and consider it a must above all else. The difference in charge temp is staggering.
Bob Turbo Midget England

I posted in the same thread in the supercharger forum. In short, my understanding having played with other boosted cars and in planning the supercharger setup for my Midget, is that boost pressure doesn't kill engines. Detonation, cylinder pressures, rpm and power output are the factors that kill engines. Boost pressure contributes to these, but if you can control detonation (due to intake temperature and cylinder pressures) and your engine is built strong enough to handle the power output and rpm, then boost pressure is largely irrelevant.

My engine has been built to take ~15psi, from a supercharger setup that I have built using an SC14 Toyota blower. I need to have some new pulleys made up, as on its first install it made 25psi by 2000rpm! One thing that I got told is very important with boosted A Series engines is to make sure that the pistons are good quality ones, and have small round oil holes rather than slots, as pistons with slots apparently always collapse sooner or later under any real boost. My engine was built with the advice of someone who has built a lot of high power A Series engines, both n/a and with forced induction. My engine has 9.2:1 compression ratio and very good pistons (but not forged), a cam ground to suit, ARP studs throughout and is very carefully balanced. The head is fairly heavily ported and larger valves are fitted. Everything was crack tested before it was used, but the standard rods and crank were re-used, and I am not using a centre main strap.

This thread was discussed between 05/07/2011 and 14/07/2011

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