NS Dev
|
| posted on 2/4/10 at 02:09 PM |
|
|
Yea, was worried about the detonation issues!
I am still not clear on the facts there though....
one of my big questions!
Why, on a supercharged engine, running the same boost throughout the rpm range, will it detonate worst at lowest rpm?
Indeed does that happen, or not?
My gut feel is that it will, and that its because the piston speed will obviously be low, yet the flame front travel speed will be much the same as at
higher rpm, therefore peak pressure will build early and cause det....................
Is that anything like right?
If so, presumably 8.5:1 CR, plus a gobload of excess fuel "should" help?
On the plus side, just talked to my mate who has the big turbo xe engine, and I don't need 20psi, he made 425hp on 15psi last time on the
rolling road, using the same cams and pistons, valves etc etc that I will be using.
Retro RWD is the way forward...........automotive fabrication, car restoration, sheetmetal work, engine conversion
retro car restoration and tuning
|
|
|
|
|
flak monkey
|
| posted on 2/4/10 at 06:34 PM |
|
|
Yes, that thinking is correct Nat. Essentially at higher rpm you have less chance of having time for detonation to happen. Hence why most detonation
is experienced at cruising rpm.
Low compression, lots of fuel (you'll want to run around 12-12.3:1 AFR under boost anyway - any more is just wasting fuel.) You'll also
need to back the timing off, you can run negative advance at idle if you need to. If none of that solve it then you may need to look into water
injection as well.
David
Sera
http://www.motosera.com
|
|
|
NS Dev
|
| posted on 3/4/10 at 09:36 AM |
|
|
OK sounds like a plan!
Will seek to avoid water injection as I'm trying to keep the installation as simple as possible. Being for a grasstrack car its going to get
covered in mud and in a total mess so the less to go wrong the better!
The 15psi news is good in that at lower revs the boost should be more sensible anyway.
Retro RWD is the way forward...........automotive fabrication, car restoration, sheetmetal work, engine conversion
retro car restoration and tuning
|
|
|
alistairolsen
|
| posted on 3/4/10 at 10:20 AM |
|
|
quote:
Originally posted by NS Dev
Why, on a supercharged engine, running the same boost throughout the rpm range, will it detonate worst at lowest rpm?
Indeed does that happen, or not?
Becuase the relatively low speeds involved mean that VE is very high. At high rpm you are using boost to make up for low VE and to overcome it. At low
rpm you get a much greater cylinder fill for a given pressure.
Also at low rpm the piston speed is low, so it hangs around at the top of the bore for much longer. Det is related to PCP(peak cylinder pressure( and
this is higher at low speeds for a given fill.
try to imagine in your head the relationship to both time and angle and how this is affected by ignition advance.
Certainly be an interesting project, its not done a great deal in the vaux world. IMO however unless you need lots of towrque at 1500rpm Id still be
going for a turbo.
My Build Thread
|
|
|
NS Dev
|
| posted on 3/4/10 at 11:59 AM |
|
|
I do need lots of torque low down, 75mph 1st gear and huge startline grip on autograss tyres!
Turbos on autograss cars never seem to work, even wrc spec engines with anti-lag always get beaten by the decent v8's and the twin bike cars.
Retro RWD is the way forward...........automotive fabrication, car restoration, sheetmetal work, engine conversion
retro car restoration and tuning
|
|
|
NS Dev
|
| posted on 3/4/10 at 12:01 PM |
|
|
quote:
Originally posted by alistairolsen
quote:
Originally posted by NS Dev
Why, on a supercharged engine, running the same boost throughout the rpm range, will it detonate worst at lowest rpm?
Indeed does that happen, or not?
Becuase the relatively low speeds involved mean that VE is very high. At high rpm you are using boost to make up for low VE and to overcome it. At low
rpm you get a much greater cylinder fill for a given pressure.
Also at low rpm the piston speed is low, so it hangs around at the top of the bore for much longer. Det is related to PCP(peak cylinder pressure( and
this is higher at low speeds for a given fill.
try to imagine in your head the relationship to both time and angle and how this is affected by ignition advance.
Certainly be an interesting project, its not done a great deal in the vaux world. IMO however unless you need lots of towrque at 1500rpm Id still be
going for a turbo.
With a supercharger though surely the VE is pretty constant across the rpm range?
Agreed on the piston speed, but not sure about VE, the usual rules go out of the window when you have forced induction/scavenging
Retro RWD is the way forward...........automotive fabrication, car restoration, sheetmetal work, engine conversion
retro car restoration and tuning
|
|
|
alistairolsen
|
| posted on 3/4/10 at 12:37 PM |
|
|
I dont mean the VE of the FI unit, I mean in terms of an N/A engine.
if you take an xe and try to feed in 1 bar, you will get twice as much air into the cylinder as you would do at atmospheric pressure....
but:
at 2000rpm the n/a engine will have a VE of say 90% so now you have 180% of the cylinder volume under boost.
If youre doing 7k, the VE is waaay off, probably as low as 40% and you now have 80% or the basic cylinder volume under boost.
This is why the honda boys can run 20psi on standard compression, because they only run high boost at high rpm to make up for the massive drop in VE,
theyre not overfilling cylinders at very low RPM.
Big boost at low rpm has always been difficult and always will be. for cheap bottom end strength Im inclined to suggest the saab unit as its
significantly better than the vauxhall one!
My Build Thread
|
|
|
MikeRJ
|
| posted on 3/4/10 at 03:32 PM |
|
|
quote:
Originally posted by NS Dev
Agreed on the piston speed, but not sure about VE, the usual rules go out of the window when you have forced induction/scavenging
The engines VE characteristics are pretty much the same whether normally aspirated or supercharged i.e. obviously the numbers will be higher for FI,
but the shape will be similar.
|
|
|
Neville Jones
|
| posted on 3/4/10 at 04:41 PM |
|
|
Nat,
For the pressures you're talking, an intercooler would be an essential.
If it's for grasstrack, can you run on methanol?
This can cool the charge to the point of icing, and solves a lot of the detonation problems. Add a little nitro to stabilise things, and you'll
have a winner. 
Otherwise, a mister spraying the intercooler will help a lot.
Cheers,
Nev.
|
|
|
NS Dev
|
| posted on 5/4/10 at 09:24 AM |
|
|
quote:
Originally posted by alistairolsen
Im inclined to suggest the saab unit as its significantly better than the vauxhall one!
This is what I thought, but experience has taught otherwise.
Yep the saab crank is stronger, but then an XE crank is unbreakable anyway (like rods wrapped round it and it is still in one piece) and the rods are
fine for 450hp+, they are great in compression, its only revs (i.e. tension load) that breaks them, and they are fine to 8000rpm as std
anyway..............(well thats the limit on my locost and they've not snapped yet, and they didn't in my old autograsser either)
Retro RWD is the way forward...........automotive fabrication, car restoration, sheetmetal work, engine conversion
retro car restoration and tuning
|
|
|
T66
|
| posted on 7/4/10 at 01:18 PM |
|
|
Saab T5 - 9000 engine...
Good for 450bhp+ on stock internals.
http://www.ericsaabsite.com/
If working on a low budget, big bhp can be had for not a lot of bucks.
As you say Nat, lag may be an issue for a grass car though....
You will see my T7 Saab soon....
|
|
|
