Compression Ratio’s

[bogbeast]

I am thinking about the spec of my next engine and I thought I’d start with the basics.

Is anyone aware of any formulas or principle that allow you to work out the highest compression ration you can run for a particular grade of fuel (i.e 98 octane). Are there any other engine specifications to take into account ? (I am aware of valve lift etc)

Cheers.


>>> Edited by bogbeast on Friday 25th October 00:11

[shpub]

I am thinking about the spec of my next engine and I thought I’d start with the basics.

Is anyone aware of any formulas or principle that allow you to work out the highest compression ration you can run for a particular grade of fuel (i.e 98 octane). Are there any other engine specifications to take into account ? (I am aware of valve lift etc)

Cheers.


>>> Edited by bogbeast on Friday 25th October 00:11

Not the way I'm afraid as you need to to a top down approach. The 520 engine is running close to 12 :1 CR and needs direct mapped ignition and injection to run rich to stop pinking and pre-detonation. needs two fuel pumps to maintain pressure. needs oil changes almost everytimne it is used as the excess fuel contaminates the oil. The oil is a special import form the US. Ok for a competition car but very inconvenient for anything else.

Start with what power and torque do you want? What characteristics? How much is the budget?

>> Edited by shpub on Friday 25th October 07:41

[dannylt]

You certainly make it sounds pretty over the top Steve! Motorbike engines regularly have 12:1 compression and higher. My Red Rose 4.5 is supposed to be 12.5 or so, and runs happily without burning oil, contaminating it, or running rich or anything nasty like that - it all comes down to other compromises made, especially with the Rover V8. The is the "Engines" section now though, not "Rover V8"

As for twin fuel pumps - the standard Bosch will quite happily supply enough fuel for 450+ bhp - there must be some other reason you need two? Swirl pot for cornering? Safety?

However, higher compression will lead to risk of pinking/detonation, but as Steve says, this is also a factor of ignition & fuelling, as well as port & combustion chamber design. Higher compression leads to potentially higher efficiency, but this is all wasted if you then have to compromise the ignition/fuelling.

At Steve says, I guess it all depends on what sort of engine you want first, with what sort of driveability and power and state of tune.

danny


>> Edited by dannylt on Friday 25th October 09:17

[2 sheds]

some of the hotter TVR rovers engines from late eighties ran 11.5 -1 , and are still running but with modified ignition, to cope with the lower octane fuel, you can always fit fully mapped ignition, but i wouldn't recommend going over 10.5:1.
Tim

[shpub]

The is the "Engines" section now though, not "Rover V8"

As for twin fuel pumps - the standard Bosch will quite happily supply enough fuel for 450+ bhp - there must be some other reason you need two? Swirl pot for cornering? Safety?

However, higher compression will lead to risk of pinking/detonation, but as Steve says, this is also a factor of ignition & fuelling, as well as port & combustion chamber design. Higher compression leads to potentially higher efficiency, but this is all wasted if you then have to compromise the ignition/fuelling.

At Steve says, I guess it all depends on what sort of engine you want first, with what sort of driveability and power and state of tune.

danny


>> Edited by dannylt on Friday 25th October 09:17

You are right this is for a Rover V8 and for other engines this is less/different type of an issue as pinking etc is coaused by all sorts of other factors.

The Fuel pumps already have a 2 litre swirl tank fed from two tanks and the problem is caused by dips in fuel pressure causing leaning and detonation. When you spend a lot of money on an engine and the engine builder says do this and that guy is John Eales then you are foolish not to. It is primarily to support sudden enrichment when blatting the throttle. One pump was definitely borderline so fitted two.

The point is that deciding how close to the edge needs a lot of careful consideration as while it is possible, you wouldn't necessarily want to do it.

[350matt]

Compression ratios quoted as high as 12:1 etc are often in reality not that high as this is based on a static calculation and in reality when the engine is running a lot depends on where the camshafts are timed and how long they stay open for this is the major influence in dynamic / engine running compression ratio.
Depending on what engine you're going to be working with also has an effect as most 2 valve combustion chamber designs aren't as effective as 4 valve and this in conjunction with the ECU, spark hardware etc. limits the amount of advance you can run without det
These days it isn't uncommon for road engines to be running 11+:1 mainly due to better ECU control and combusiton design.
As a rule of thumb go for at very least 10:1 and depending on all of the above push it higher if you can, you can always fit a thick head gasket if it all goes wrong!

Matt

[RichieG]

Look for some David Vizard information, as suggested before you have to look at 'Dynamic' and 'Static' CR, thus bringing in the question of what cam you want to run and timing that effectivly.

[james]

dannylt said:
As for twin fuel pumps - the standard Bosch will quite happily supply enough fuel for 450+ bhp - there must be some other reason you need two? Swirl pot for cornering? Safety?

A competition car will generally have 2 low pressure fuel pumps filling a swirl pot, and then a single high pressure pump emptying it into the engine, so 3 is more normal in this instance. I can only assume that Steve is just running the 2 low pressure pumps directly into the fuel rail. If he's doing a lot of track work though, a swirl pot is usually recommended.

James

[GreenV8S]

As for twin fuel pumps - the standard Bosch will quite happily supply enough fuel for 450+ bhp - there must be some other reason you need two? Swirl pot for cornering? Safety?

One thing that would affect this a lot is the required fuel pressure. I understand there are advantages to running at higher pressure (lets you get away with smaller injectors, which work better at light load). No idea what sort of pressure Steve's running, but this could vary a lot between cars. So just because the pump is enough to feed a 450 BHP engine in one case, doesn't mean it'll cope with Steve's.

[2 sheds]

In my Tuscan we used a single Fuel pump from a Marcos GT racer, this was fine until the fuel pressure regulator jammed shut, offering the injectors 9 bar, which was nice.

[bogbeast]

Thanks for all the posts..

The engines going into my Ultima. I have approx 10K to spend and was looking at an ally 383 Chevy small block.

I'd prefer to keep it normally aspirated hence to get the best from it I was looking to run as high a compression as possible using a fully mapped injection/ignition system to minimise the detonation. I had heard that thermal barrier coating across the piston and valve head can also help a lot. As far as heads and cam are concerned, I haven’t got any definite ideas. I would be keen to hear from to hear from anybody (particularly our American members !) with any experience in building a similar engine.. I am looking for 600+ brake…

[ByronTVR]

Hello,
Since you're thinking for horsepower more than 600 bhp's and you'll have a fully mapped ECU then ... you should supply your car with a fuel pump from a Mercedes 600SL i think it will do the job.
Otherwise if you want to spend more money , better check for after market fuel pumps...they sell different models , depending the use..i think they have till 1000bhp's + fuel pumps.

ByronTVR!

>> Edited by ByronTVR on Saturday 26th October 22:08

[350matt]

Regarding the 'thermal barrier' treatment I'd avoid it like the plague, we've tested a varient of this at work and what happens is the ceramic coating gets hot and then stays hot, which then b#ggers your airflow and conseqently power.
Anyway with 383ci capacity 600Bhp should be attainable with conventional tuning, just need to make sure it can breathe properly, as most Yank engines have pretty poor port and chamber designs. However the parts are available to achieve decent power figures without supercharging, although it might work out cheaper to go that route....

Best of Luck
Matt