6 replies to this topic

[psychobob]

I have a low compression spi 1275 on a hif44 in my hornet. after checking engine numbers, I've found it is a low compression motor. would it be feasible to plum in a relatively low pressure turbo off something modern and plumb it in? I'm thinkimg a fairly low power hike of about 20bhp. is there any sense in this or am I just being an idiot?

[psychobob]

I should have reread that post before posting it! sorry about the syntax!

[Gremlin]

You have to more research and calculations, search around for how much boost you can sensibly run with different cr's in the state your engine is in and work from there

[Cooperman]

Take off the head and accurately measure the compression ratio.

Then work out the percentage increase in CR which the current set-up (i.e. piston type, existing CR, etc) could take

Multipy that percentage by 15 and that is the amount of boost in lbs. which your engine could accept.

So, for example, if you currently have 9.4:1 and your engine could take 10.6:1 with its standard pistons, then the percentage increase is 12.75%.

Take 12.75% of 15 = 1.91. 

So the theoretical maximum boost is 1.91 lbs, so to allow for a tolerance, the boost at peak revs needs to be a maximum of 1.8 lbs.

It is hard to say what sort of power increase this would give.

If the existing CR was to be lowered by opening out the combustion chambers in the head, which good gas flowing would do, then a higher boost pressure could be run.

[Captain Mainwaring]

Take off the head and accurately measure the compression ratio.

Then work out the percentage increase in CR which the current set-up (i.e. piston type, existing CR, etc) could take

Multipy that percentage by 15 and that is the amount of boost in lbs. which your engine could accept.

So, for example, if you currently have 9.4:1 and your engine could take 10.6:1 with its standard pistons, then the percentage increase is 12.75%.

Take 12.75% of 15 = 1.91. 

So the theoretical maximum boost is 1.91 lbs, so to allow for a tolerance, the boost at peak revs needs to be a maximum of 1.8 lbs.

It is hard to say what sort of power increase this would give.

If the existing CR was to be lowered by opening out the combustion chambers in the head, which good gas flowing would do, then a higher boost pressure could be run.

 

 

Not doubting you but slinging numbers into your formula consistently comes up with low boost figures...if I turn them into barG then they'd be too high...

[coopdog]

It's not as easy as putting a turbo on,

There is a lot more to it an it's expensive

[Cooperman]

I was working on the basis of standard and well-used pistons. I certainly wouldn't want to put a lot of additional pressure on them at high revs. which is why I've gone for 10.6:1 as a maximum calculated C.R. and hence low boost pressures. Sorry if I misled anyone with this example, but I was being cautious.

Whenever I have been involved in forced induction engines I have used new pistons designed to take higher pressures. The Omega version for forced induction is ideal and then a much higher boost pressure can be used. For example, with Omega pistons and a geometric C.R. of 8:1 a boost pressure of 7.5 lbs/sq.in would be fine giving a calculated C.R. of 12:1

Even with low boost, the gain will be in the mid-range torque, which is what is needed for road engines.

I once had to rebuild a turbo 1275 engine which had been built with standard pistons, no lowering of static compression ratio (it was 9.7:1) and a boost pressure of 7.5 lbs. The distributor was also unchanged so the advance curve was wrong (yes, really!) and it lasted 4 miles before it blew up. It had been a 'cash-in-hand' deal, so the so-called expert who built it would not help in any way. I asked for the calculation sheets and, guess what, none existed. No surprise there then.

My task was to lower the basic CR partly by grinding 3.5 cc from each combustion chamber, re-bore it and fit Omega pistons suitable for a turbocharged engine, and acquire & fit a suitably re-curved distributor. With forced induction the advance curve must be slower than with a NA engine. Aldon are very good with this.

After the calculations I calculated that a maximum of 6 lbs boost was OK and got a company to set the turbocharger to give this and provide a calibration certificate confirming that.

It worked very well, giving c.125 bhp at 5700 rpm IIRC. The mid-range torque was simply amazing and a cross-pin diff was fitted to cope with this. 

Properly built forced induction engines are not cheap and must be built for the job. Just bolting a turbocharger onto a standard engine will probably 'end in tears' sooner rather than later.