A 5.3 even at 100%VE will only displace 2.65 litre per rev being as it's a 4 stroke If you ran them at twice engine speed the would displace 4 litres per rev. IF my memory serves correct you multiply the cc per rev by the VE which on a big rover is not going to be over 80% so you get 2.12 litres per rev. Now divide the 4 litre by the 2.12 = 1.89 which is the pressure ratio, 1.89 x 14.7 psi = 27.8 psi, being as we are allready at one atmosphere you have to knock off 14.7 psi which gives 13 psi boost at a 2:1 drive ratio assuming 80%VE and no leakage within the blowers.
Please bare in mind that I've had a few beers tonight and all the above could be rubbish
Not quite sure what you mean by 'overdrive' etc. If you mean that the blower is running faster than the engine, that's how you would normally use them. The maximum throughput of the blower is determined by the blower's displacement and rev limit. The bigger Eatons are rev limited to about 12,000 rpm (the smaller ones will rev slightly higher). An M112 displaces a little over 1.8 liters, given an engine rev limit of 6000 rpm the highest gearing you could run would be 2:1 which corresponds to 3.6 liters per crank revolution, which is what a 7.2 liter 4-stroke with 100% VE would consume. You can get a rough idea how much boost that's going to produce from the capacity of the engine it's connected to. I.e. it would produce no boost at all on the 7.2 litrer engine, 1 bar of boost on a 3.6 liter engine, and so on assuming the base engine has 100% VE. In practice the engine VE is likely to be far less than 100% so you would expect to get slightly higher boost (backpressure) than this.
These are the maximum figures that the blower could produce, obviously you can gear it down to produce lower boost.
I see nothing wrong with the maths above so explane why you have to drive your charger way beyond what the maths would suggest.
Apart from anything else I was under the impression that it isnt the PSI thats important as the same size engine with different heads could flow less than the other. That doesnt mean the charger cant supply enough air it just means one engine isnt using the charge as efficiently so the pressure is more.
JP. wrote:
And what about Ve...... blown engines usualy have 110% - 120% Ve
I thought you based your calculations of the n/a VE, because that's a measure of the engines ability to pump air - the 120% you quote is as a result of forcing air into it - would it not be more accurate to say 120% is its NET ve.
katanaman wrote:I see nothing wrong with the maths above so explane why you have to drive your charger way beyond what the maths would suggest.
Sorry, I don't understand the question. You think my maths is giving an answer that is too high, or too low? It's only a rough approximation and likely to underestimate the actual backpressure for a couple of reasons. I assumed 100% VE at the engine intake; 80% would be more realistic. I ignored charged heating; the amount of charge heating will depend on the compression ratio across the blower but whatever heating there is, will tend to raise the pressure downstream of the blower.
the question was aimed at JP not you I dont see anything wrong with your reasoning.
Where I seem to be hanging up is the CID. I see it as an engines CID is only half per revolution because thats all the air it will use given 100% VE But JP seems to be saying you ignore that and use full CID per revolution even though a V8 only fills half its cylinders per revolution.
katanaman wrote:the question was aimed at JP not you I dont see anything wrong with your reasoning.
Where I seem to be hanging up is the CID. I see it as an engines CID is only half per revolution because thats all the air it will use given 100% VE But JP seems to be saying you ignore that and use full CID per revolution even though a V8 only fills half its cylinders per revolution.
Well I must admit I didn't quite follow JP's arithmetic, and I agree with you that there seems to be a factor of two missing somewhere. Still the basic approach is sound: you know the displacement of the blower, and the displacement of the engine (capacity * nominal VE * 0.5), and the ratio of the two times the gearing gives you the compression ratio across the blower. The final bit that I think JP is missing is to add in the effect of charge heating, which can be estimated once you know the approximate compression ratio and revs. Given the compression ratio and the heat you now know enough to estimate the backpressure at the blower outlet.
The RPI website states that supercharging a rover V8 is not a good idea. Apparently they dont last very long? Is there any truth in this?
The site does say that turbocharging is a better option, though?
Can anyone shed some light on this or point me to some reading material I can swot up on?
You haven't read the article properly. What RPI is saying is that on some installs the fuel distribution is really bad leading to weak mixtures which makes the pots run hot and crack liners. Whether this is actually true or not I have no idea. There have been loads of supercharged rover engines, even TVR had a production supercharged rover engine. While the TVR's were well known for being unreliable it wasn't because of broken supercharged engines.
In 90' there is an Uk compagny called Liechtfield (if my memory is good)who proposed supercharger RV8 kit based on Eaton M90. Supercharger was bolted directly on EFI inlet manifold with adaptator. Or the main problem was the outlet of supercharger was too small to convert correctly eight ports of inlet efi manifold. An of course there was an incorrect air distribution. The article of Chris speaks about this kit.
TVR system used an different system with an custom plenum and sprintex supercharger. The TVR kit was designed by DPR and Dennis Priddle. I have one of this special plenum in my garage.
The RPI website states that supercharging a rover V8 is not a good idea. Apparently they dont last very long? Is there any truth in this?
The site does say that turbocharging is a better option, though?
Can anyone shed some light on this or point me to some reading material I can swot up on?
Cheers,
Jon.
The RPI site reads like a general problem but is actually specific to one particular aftermarket supercharger installation that suffered from poor charge distribution. The cynic in me wonders if a company that specialises in highly tuned normally aspirated engines might have a reason to be prejudiced against superchargers in general.
GreenV8S wrote: The cynic in me wonders if a company that specialises in highly tuned normally aspirated engines might have a reason to be prejudiced against superchargers in general.
If you ran them at twice engine speed the would displace 4 litres per rev. IF my memory serves correct you multiply the cc per rev by the VE which on a big rover is not going to be over 80% so you get 2.12 litres per rev. Now divide the 4 litre by the 2.12 = 1.89 which is the pressure ratio, 1.89 x 14.7 psi = 27.8 psi, being as we are allready at one atmosphere you have to knock off 14.7 psi which gives 13 psi boost at a 2:1 drive ratio assuming 80%VE and no leakage within the blowers.
-JC.
