The Basket
Senior Master Sergeant
- 3,712
- Jun 27, 2007
it is...especailly in due to its testing against a Spitfire and well it comes out.
I thought the Gloster fighter was ignored after its cancellation....obviously not.
Saburo Sakai Zero vs Bf-109
- Thread starter Chiron
- Start date
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it is...especailly in due to its testing against a Spitfire and well it comes out.
I thought the Gloster fighter was ignored after its cancellation....obviously not.
claidemore
Senior Airman
arggghhh
claidemore
Senior Airman
Duh, hit the submit buttom by accident before I was ready to post.
First chart shows 362 and 378 for a P40N. (available in the Technical/Flight Test/P40 Performance Thread on these forums.)
2nd chart shows 366 for E and 370 for F. Note the speeds for D,E and F are calculated.
First chart shows 362 and 378 for a P40N. (available in the Technical/Flight Test/P40 Performance Thread on these forums.)
2nd chart shows 366 for E and 370 for F. Note the speeds for D,E and F are calculated.
Attachments
Hi Claidemore,
>Duh, hit the submit buttom by accident before I was ready to post.
Has happened to me, too - I think it must be because the submit position is in the same position as the attach button on some other forums
>First chart shows 362 and 378 for a P40N. (available in the Technical/Flight Test/P40 Performance Thread on these forums.)
>2nd chart shows 366 for E and 370 for F. Note the speeds for D,E and F are calculated.
Interesting to see that. However, it's not actual test data, and the P-40 tests I'm aware of (and those that Peril, who built the site containing these summaries, was aware of at that time) yielded somewhat lower speeds. I see that Peril has found more data than we had a couple of years back, so I wouldn't rule it out that he has some tests showing higher speeds now.
However, I still consider my analysis to be quite accurate, and I'm sure that if such a test indicating a higher speed actually turns up, there will be a good engineering reason to explain the difference between this and the various slower tests.
Regards,
Henning (HoHun)
>Duh, hit the submit buttom by accident before I was ready to post.
Has happened to me, too - I think it must be because the submit position is in the same position as the attach button on some other forums
>First chart shows 362 and 378 for a P40N. (available in the Technical/Flight Test/P40 Performance Thread on these forums.)
>2nd chart shows 366 for E and 370 for F. Note the speeds for D,E and F are calculated.
Interesting to see that. However, it's not actual test data, and the P-40 tests I'm aware of (and those that Peril, who built the site containing these summaries, was aware of at that time) yielded somewhat lower speeds. I see that Peril has found more data than we had a couple of years back, so I wouldn't rule it out that he has some tests showing higher speeds now.
However, I still consider my analysis to be quite accurate, and I'm sure that if such a test indicating a higher speed actually turns up, there will be a good engineering reason to explain the difference between this and the various slower tests.
Regards,
Henning (HoHun)
claidemore
Senior Airman
Henning,
The higher speeds listed (calculated) are right from Curtiss, so i suspect, as Peril points out, that they are exageratted. When you look close at that second chart, it looks like the data is typed onto a printed form. There are too many other documents with P40 E speed listed as 340 mph to throw them all out for one that lists it higher.
I'd say your analysis is correct too.
An interesting sidebar; on Wiki, the last page of Spit variants, shows very slow 'max' speeds for Mk IX and Mk XIV. People do tend to pick and choose the figures they like the best.
Claidmore
The higher speeds listed (calculated) are right from Curtiss, so i suspect, as Peril points out, that they are exageratted. When you look close at that second chart, it looks like the data is typed onto a printed form. There are too many other documents with P40 E speed listed as 340 mph to throw them all out for one that lists it higher.
I'd say your analysis is correct too.
An interesting sidebar; on Wiki, the last page of Spit variants, shows very slow 'max' speeds for Mk IX and Mk XIV. People do tend to pick and choose the figures they like the best.
Claidmore
Hi Claidemore,
>An interesting sidebar; on Wiki, the last page of Spit variants, shows very slow 'max' speeds for Mk IX and Mk XIV. People do tend to pick and choose the figures they like the best.
Absolutely true, and especially dangerous in Wikipedia were it's hard to find out who exactly put the numbers into the article!
I really take my hat off to Peril in that regard - when I sent him the results from my analysis, I warned him "I'm afraid these figures don't look flattering for the P-40", but he simply said "That's allright, I just want to know how good or how bad it was, as accurately as possible!"
Such an open-minded stance with regard to the performance of one's favourite aircraft is something I really admire, as I think it's very easy to sub-consciously tend toward the more optimistic values within the range of uncertainty that inevitably results from the often incoherent and contradicting data we're dealing with.
Regards,
Henning (HoHun)
>An interesting sidebar; on Wiki, the last page of Spit variants, shows very slow 'max' speeds for Mk IX and Mk XIV. People do tend to pick and choose the figures they like the best.
Absolutely true, and especially dangerous in Wikipedia were it's hard to find out who exactly put the numbers into the article!
I really take my hat off to Peril in that regard - when I sent him the results from my analysis, I warned him "I'm afraid these figures don't look flattering for the P-40", but he simply said "That's allright, I just want to know how good or how bad it was, as accurately as possible!"
Such an open-minded stance with regard to the performance of one's favourite aircraft is something I really admire, as I think it's very easy to sub-consciously tend toward the more optimistic values within the range of uncertainty that inevitably results from the often incoherent and contradicting data we're dealing with.
Regards,
Henning (HoHun)
kool kitty89
Senior Master Sergeant
The 365 mph figure fot the P-40 (first series, no letter) Tomahawk I was actual test data as listed and that figure is a bit more than I would have thought even with the lighter armament and lack of armor. (which was not included standard for this model)
And on the other chart only Military power is listed (except for the P-40N) which is ~1,150 hp for most models. And the power figures of the M and N (which have the same engine) are listed at different ratings at different altitudes. And in standard procedure the full 1,480 hp WEP of the V-1710-81 was only available between 3,000-10,400 ft.
And on the other chart only Military power is listed (except for the P-40N) which is ~1,150 hp for most models. And the power figures of the M and N (which have the same engine) are listed at different ratings at different altitudes. And in standard procedure the full 1,480 hp WEP of the V-1710-81 was only available between 3,000-10,400 ft.
kool kitty89
Senior Master Sergeant
That site is interesting, I hadn't realized those V-1710-39/73 engines had those kind of WEP ratings, albeit only at low altitude. (though I should have expected this seeing the performance of the P-38's engines) The Merlin, 1710-33/81 were as I expected. The 1710-81 really traded low alt performance for medium altitude performance, and even then at lower max power. The higher blower ratio had the low altitude disadvantages of overboost, high manifolt temp, and thus detonation problems, but the medium and high alt performance was much improved, this engine obviously not being meant to ever use turbocharging. (while the other designs reflect the intent of turbo use, at least for their sister designs)
From those figures I'd think the P-40K would have the best initial climb rate and performance below 10,000 ft, the P-40F the best performance above 20,000 ft, and the 1710-80 powered M/N to have the best climb from 5,000 to 15,000 ft ant the highest top speed. All of which are pretty much accurate.
From those figures I'd think the P-40K would have the best initial climb rate and performance below 10,000 ft, the P-40F the best performance above 20,000 ft, and the 1710-80 powered M/N to have the best climb from 5,000 to 15,000 ft ant the highest top speed. All of which are pretty much accurate.
Hi Koolkitty,
>The 365 mph figure fot the P-40 (first series, no letter) Tomahawk I was actual test data as listed
It's Curtiss data which usually indicates higher speeds than the P-40 achieves in service tests.
>And in standard procedure the full 1,480 hp WEP of the V-1710-81 was only available between 3,000-10,400 ft.
Actually, the peak power would be only available exactly at the full throttle height. Below it would be slightly less due to the drop in charge mass caused by the higher air temperature, above it would be decaying because the supercharger wouldn't be able to achieve the full boost pressure. At the top end of the range, power would have dropped to the same value as the engine achieves at the same rpm without emergency boost.
Regards,
Henning (HoHun)
>The 365 mph figure fot the P-40 (first series, no letter) Tomahawk I was actual test data as listed
It's Curtiss data which usually indicates higher speeds than the P-40 achieves in service tests.
>And in standard procedure the full 1,480 hp WEP of the V-1710-81 was only available between 3,000-10,400 ft.
Actually, the peak power would be only available exactly at the full throttle height. Below it would be slightly less due to the drop in charge mass caused by the higher air temperature, above it would be decaying because the supercharger wouldn't be able to achieve the full boost pressure. At the top end of the range, power would have dropped to the same value as the engine achieves at the same rpm without emergency boost.
Regards,
Henning (HoHun)
kool kitty89
Senior Master Sergeant
According to Mustang (Allison Engine) Performance Trials 1480 hp could be attained at "part" throttle at 5,000 ft and the same at 10,400 "wide open."
And also the 10,400 ft figure for the 1710-81's 1,480 hp was only during high level speed for "additional ram air" otherwise crit alt was ~7,500 ft. (the lower figure for climb, the higher figure for level flight)
The chart http://www.wwiiaircraftperformance.org/mustang/p-51a-1-6007.jpg from the same website, supports your (Henning) statement, which makes perfect sense, and these figures are w/out the "ram" air due to the lower airspeed of climb. (1,400 hp SL, 1,480 ph 7,500 ft)
But this info on power at low alt an overboost (cleared 60" max) for the P-40E/K would be very interesting to bring over to the P-40 vs 109 thread. (which has been languishing)
Another thing to note is that (if direct copies of the British engine) the Merlins used on the P-40 would have the same -G problems which (although improved since the BoB) was still a problem for sistained -G or inverted flight. However if US injected carb's (on all Allisons) were fitted there would be n problem. (similar to "throttle body injection" this was later added to the British Merlin which fully solved the problem)
And Juha, the Finn's 75A-1 was much slower than (some of) the US's P-36 or the 75A-3/4 as those had 1,200 hp engines and the A-4's R-1820 had ~500 ft higher crit alt due to the 2-speed supercharger, allowing a higher top speed, though at lower altitudes it was slower than the 1,200 hp R-1830 of smaller diameter which gave less drag. The Finn's planes had only 950 hp and were low altitude models to boot. (there was also the A-2 with 1,050 hp, but I think the Finns had predominantly A-1's, though they received A-1/2/3 and 8 A-6's, either way the performances of these a/c would all be very different)
The A-1 had ~280-290 mph, A-2 ~300 mph, the A-3 311 mph @ 10,000 ft, the A-4 323 mph @ 15,100 ft.
And the B-239 (Brewster), though it had the same HP as the F2A-1 (950 hp takeoff 1,000 hp WEP) it had a critical altitude for this powe of less than 10,000 ft compared to the F2A-1's 18,000 ft. Plus in both cases the Finnish a/c were fitted with armor and added equipment (albeit the Brewster's Navy equipment was omitted) which made the a/c heavier. The F2A-1 has a top speed of 311 mph at 18,000 ft, the B-239 had a top speed of ~298 mph at ~9,000 ft, though this speed stayed the same to ~15,000 ft due drag reducing about as much as thrust.
Now I wonder how the P-36 would do with a 2-stage supercharged R-1830 like the Wildcat had, It would probably have been quite a bit better performing than the Wildcat as the F4F-3 topped out at ~334 mph at 21,000 ft, (while the 75A-3 was already doing more than 320 mph at half that altitude) doubtless it would have been better than the 75A-4 at all altitudes. I'd bet that it would be better than a similar (with similar weapons config) P-40 above 18,000 ft. (and above 15,000 ft for all except the F,J,M,N)
And also the 10,400 ft figure for the 1710-81's 1,480 hp was only during high level speed for "additional ram air" otherwise crit alt was ~7,500 ft. (the lower figure for climb, the higher figure for level flight)
The chart http://www.wwiiaircraftperformance.org/mustang/p-51a-1-6007.jpg from the same website, supports your (Henning) statement, which makes perfect sense, and these figures are w/out the "ram" air due to the lower airspeed of climb. (1,400 hp SL, 1,480 ph 7,500 ft)
But this info on power at low alt an overboost (cleared 60" max) for the P-40E/K would be very interesting to bring over to the P-40 vs 109 thread. (which has been languishing)
Another thing to note is that (if direct copies of the British engine) the Merlins used on the P-40 would have the same -G problems which (although improved since the BoB) was still a problem for sistained -G or inverted flight. However if US injected carb's (on all Allisons) were fitted there would be n problem. (similar to "throttle body injection" this was later added to the British Merlin which fully solved the problem)
And Juha, the Finn's 75A-1 was much slower than (some of) the US's P-36 or the 75A-3/4 as those had 1,200 hp engines and the A-4's R-1820 had ~500 ft higher crit alt due to the 2-speed supercharger, allowing a higher top speed, though at lower altitudes it was slower than the 1,200 hp R-1830 of smaller diameter which gave less drag. The Finn's planes had only 950 hp and were low altitude models to boot. (there was also the A-2 with 1,050 hp, but I think the Finns had predominantly A-1's, though they received A-1/2/3 and 8 A-6's, either way the performances of these a/c would all be very different)
The A-1 had ~280-290 mph, A-2 ~300 mph, the A-3 311 mph @ 10,000 ft, the A-4 323 mph @ 15,100 ft.
And the B-239 (Brewster), though it had the same HP as the F2A-1 (950 hp takeoff 1,000 hp WEP) it had a critical altitude for this powe of less than 10,000 ft compared to the F2A-1's 18,000 ft. Plus in both cases the Finnish a/c were fitted with armor and added equipment (albeit the Brewster's Navy equipment was omitted) which made the a/c heavier. The F2A-1 has a top speed of 311 mph at 18,000 ft, the B-239 had a top speed of ~298 mph at ~9,000 ft, though this speed stayed the same to ~15,000 ft due drag reducing about as much as thrust.
Now I wonder how the P-36 would do with a 2-stage supercharged R-1830 like the Wildcat had, It would probably have been quite a bit better performing than the Wildcat as the F4F-3 topped out at ~334 mph at 21,000 ft, (while the 75A-3 was already doing more than 320 mph at half that altitude) doubtless it would have been better than the 75A-4 at all altitudes. I'd bet that it would be better than a similar (with similar weapons config) P-40 above 18,000 ft. (and above 15,000 ft for all except the F,J,M,N)
Hi Koolkitty,
>1480 hp could be attained at "part" throttle at 5,000 ft and the same at 10,400 "wide open."
The "wide open throttle" bit is the origin of the British term "full throttle height" (called "critical altitude" by the Americans).
That 1480 HP are achieved both at sea level and at full throttle height is an indication that the early-war US rating system is used which assumes a power that is independend of altitude. You'd have to increase boost pressure at low altitude to actually achieve this.
With the typical boost regulators controlling to a constant boost pressure, the engine would actually lose some power below full throttle height.
(I'm not sure how realistic the US rating method was, since the use of a higher boost pressure increased the charge temperature, bringing the engine even closer to detonation than by increased boost pressure alone. The only engine I'm aware of that was not regulated for constant boost pressure but for constant charge mass was the Jumo 213, but it had a special "swirl" throttle in the supercharger intake which reduced the inevitable charge temperature increase.)
>And also the 10,400 ft figure for the 1710-81's 1,480 hp was only during high level speed for "additional ram air" otherwise crit alt was ~7,500 ft. (the lower figure for climb, the higher figure for level flight)
I hadn't thought about the exact altitudes you gave, but you're of course correct that ram effect is important when comparing the fast P-51 to the slower P-40.
>But this info on power at low alt an overboost (cleared 60" max) for the P-40E/K would be very interesting to bring over to the P-40 vs 109 thread. (which has been languishing)
Indeed! But where did you find the information on 60" Hg? I'm afraid I can't find that bit at the moment ...
Regards,
Henning (HoHun)
>1480 hp could be attained at "part" throttle at 5,000 ft and the same at 10,400 "wide open."
The "wide open throttle" bit is the origin of the British term "full throttle height" (called "critical altitude" by the Americans).
That 1480 HP are achieved both at sea level and at full throttle height is an indication that the early-war US rating system is used which assumes a power that is independend of altitude. You'd have to increase boost pressure at low altitude to actually achieve this.
With the typical boost regulators controlling to a constant boost pressure, the engine would actually lose some power below full throttle height.
(I'm not sure how realistic the US rating method was, since the use of a higher boost pressure increased the charge temperature, bringing the engine even closer to detonation than by increased boost pressure alone. The only engine I'm aware of that was not regulated for constant boost pressure but for constant charge mass was the Jumo 213, but it had a special "swirl" throttle in the supercharger intake which reduced the inevitable charge temperature increase.)
>And also the 10,400 ft figure for the 1710-81's 1,480 hp was only during high level speed for "additional ram air" otherwise crit alt was ~7,500 ft. (the lower figure for climb, the higher figure for level flight)
I hadn't thought about the exact altitudes you gave, but you're of course correct that ram effect is important when comparing the fast P-51 to the slower P-40.
>But this info on power at low alt an overboost (cleared 60" max) for the P-40E/K would be very interesting to bring over to the P-40 vs 109 thread. (which has been languishing)
Indeed! But where did you find the information on 60" Hg? I'm afraid I can't find that bit at the moment ...
Regards,
Henning (HoHun)
Hello Koolkitty!
What I meant is that Brewster's spec for Model 239 for climb rate was the same that Finns got in their tests.
In speedtrials flown by BW-366, max speed was 480kmh when Brewster's figure was 484kmh. At sea level BW-366 428kmh and Brewster's figure 427kmh. I would say, that if all manufactures specs would have been that near to service tests, buyers would have been very happy.
In test with Curtiss CUW-551 max low level speed was 429kmh, that of Curtiss CUW-557 was 425kmh at 1500m. Max speed for CU-572 was a bit under 440kmh at 3000m, max speed for FAF's few Cyclone Hawks was 480kmh at full throttle height, when Curtiss promised 520kmh, even if Curtiss figure was with S3C3-G engine, Finns thought it very optimistic..
Juha
What I meant is that Brewster's spec for Model 239 for climb rate was the same that Finns got in their tests.
In speedtrials flown by BW-366, max speed was 480kmh when Brewster's figure was 484kmh. At sea level BW-366 428kmh and Brewster's figure 427kmh. I would say, that if all manufactures specs would have been that near to service tests, buyers would have been very happy.
In test with Curtiss CUW-551 max low level speed was 429kmh, that of Curtiss CUW-557 was 425kmh at 1500m. Max speed for CU-572 was a bit under 440kmh at 3000m, max speed for FAF's few Cyclone Hawks was 480kmh at full throttle height, when Curtiss promised 520kmh, even if Curtiss figure was with S3C3-G engine, Finns thought it very optimistic..
Juha
kool kitty89
Senior Master Sergeant
Okay, if it was Brewster's figures for the export B-239, not the the US F2A-1 that makes sense.
With those figures it may have been better to put those more powerful R-1820's into Brewsters which were better matched to the engine. (designed around)
It would still have been interesting to have trialed a 2-stage R-1830 Twin Wasp in a P-36, particularly if a tight cowling with cooling fan and conical spinner were fitted. (like at the tail end of the P-42 project; the long cowling was found to not help at all and cause much cooling problems, but a normal sized tight fitting cowling with fan and spinner would be advantageous as seen in the Fw 190, XP-47N, and XP-72. (and many japanese fighter designs, and a few less common others)
With those figures it may have been better to put those more powerful R-1820's into Brewsters which were better matched to the engine. (designed around)
It would still have been interesting to have trialed a 2-stage R-1830 Twin Wasp in a P-36, particularly if a tight cowling with cooling fan and conical spinner were fitted. (like at the tail end of the P-42 project; the long cowling was found to not help at all and cause much cooling problems, but a normal sized tight fitting cowling with fan and spinner would be advantageous as seen in the Fw 190, XP-47N, and XP-72. (and many japanese fighter designs, and a few less common others)
kool kitty89
Senior Master Sergeant
I know, that's why I mentioned the climb rated HP from the P-51A chart, which did match up quite well with your statement.
Actually I just meant the ram air in high speed level flight compared to climb speed, granted the P-51 would have a small advantage given its ~35 mph speed advantage.
I got it from Perils P40 Archive Data at the bottom of the page, there are a list of .pdf files specifically for the engines. They have the overboost clearance listed in the "V-1710-39 overboost". (also has the -73 listed in this file)
In that doccument they (Allison company) also mentions that the higher blower ratio of the -81/99 engines makes them much more dangerous to overboost. They also say that boost pressures as high as 66-70"+ are not safe to use and (even if it seems fine) that the engine may fail later on under normal operating conditions in later use. (requiring a complete tear-down and rebuild to be safe)
But one question: does anione actually know what the "blower ratio" figures stand for, the're obviously not compression ratios (at 8-9:1, at least not in common notation), so what do they mean?
Hi Koolkitty,
>But one question: does anione actually know what the "blower ratio" figures stand for, the're obviously not compression ratios (at 8-9:1, at least not in common notation), so what do they mean?
"Blower" is another term for "supercharger", and the "blower ratio" must be the supercharger gear ratio, with a higher ratio resulting in a higher supercharger speed so that it uses a bit more power while increasing full throttle height as well as available boost at low altitudes. It's the latter that makes overboosting dangerous, as it's easier to go beyond the maximum safe overboost accidentally when no boost pressure regulator is used. (Which is implied by the term "overboosting".)
Regards,
Henning (HoHun)
>But one question: does anione actually know what the "blower ratio" figures stand for, the're obviously not compression ratios (at 8-9:1, at least not in common notation), so what do they mean?
"Blower" is another term for "supercharger", and the "blower ratio" must be the supercharger gear ratio, with a higher ratio resulting in a higher supercharger speed so that it uses a bit more power while increasing full throttle height as well as available boost at low altitudes. It's the latter that makes overboosting dangerous, as it's easier to go beyond the maximum safe overboost accidentally when no boost pressure regulator is used. (Which is implied by the term "overboosting".)
Regards,
Henning (HoHun)
parsifal
Colonel
Perhaps this extract might help....not my words, but i knew where to look. It relates to the fitting of a supercharger kit to a chevy big block motor.
I understand only some of this stuff. I have rebuilt a Cooper "S" motor, with a supercharger, more than twenty years ago. I remember that in order to fit the blower, we had to de-tune the engine, from a compression ratio of about 9:1 in the standard Cooper engine, down to 7.5:1. We did this by fitting a different cylinder head, and oversized head gaskets. Anyway, I thought this stuff might help....
Compression
The amount of boost you can run is directly related to an engine's static compresssion ratio. When the boost is combined with the compression ratio, the result is the effective compression ratio. Typically, a 5- to 8-psi boost range (usually produced with the supplied pulleys in blower kits) will work fine for compression ratios in the 8:1 to mid-9:1 range (operating on 91/92-octane fuel). However, this will ultimately depend on other modifications to the car, manual or automatic transmission, gearing, operating temperature, vehicle load, and altitude. If detonation is encountered it can often be controlled with boost retard devices or by experimenting with different-sized pulleys.
Special Carburetion
Choosing a carburetor or fuel injectors is a crucial step when building a blower-specific engine, because under boost the engine will often need 40-50 percent more fuel and air. Unlike a normally aspirated engine that may suffer only low power from poor fuel delivery, a supercharged engine without enough fuel under power may run extremely lean and destroy the engine. Running too small a carburetor also means that you can't flow enough air to produce maximum boost.
Because more fuel is required to feed a supercharged engine, the fuel-delivery system must be considerably improved. This means large fuel lines of AN-8 or bigger, properly selected and installed fuel pump(s), an adequately designed tank, full flowing filters, and a correctly wired electrical system to operate the fuel pumps.
Blow-Through Carburetion
On a blow-through supercharger system, the carburetor can either reside in a pressurized box or utilize a special carburetor hat. Under boost the false atmosphere (pressure being blown into the carb) requires revamping many of the original carburetor designs to properly supply fuel. A blow-through carburetor generally features sealed caps on the metering blocks, the main well, and the idle well. These carburetors typically feature only annular boosters so that as the signal gets stronger more fuel flows into the engine. As boost is increased by each psi, fuel pressure must too be increased at the same rate. To do this, a special regulator is referenced to boost pressure and raise or lower the regulated fuel pressure, depending on demand.
Turn Back
Reverse-rotation superchargers are generally used in applications where there are fitment issues or on engines designed to spin opposite of most other engines. Fitment issues arise when the area behind the belt driveline is impacted. Examples are 32-valve cylinder heads and exhaust manifolds. In this case the supercharger is mounted in front of the belt line. Consequently, the supercharger must now be rotated in a reverse motion, in which case manufacturers design the inner components of the blower as a mirror image of a standard unit.
Ignition Systems with a Supercharger
On any blown engine, high-performance ignitions are required primarily to provide adequate spark at higher-than-normal engine pressures and speeds. Additionally, it is often a good idea to run spark plugs that are one to two ranges colder than normal. Rule of thumb: the more boost, the colder the plug required.
One of the most important concerns with any supercharger installation is detonation control. This is because under acceleration, detonation can damage the piston ring lands (or other worse yet, damage rod bearings, destroy pistons, or blow head gaskets). A handy device to counteract most detonation problems is an ignition system with a boost-retard control.
Ignition timing is especially critical with a supercharger to not only keep detonation at bay, but also provide good power. For most applications, the distributor should have a centrifugal advance mechanism set up so that the entire advance is in by 2,500 rpm. Typically, 34 degrees should be a safe level of ignition lead to provide close to optimum performance[/I][/I]
I understand only some of this stuff. I have rebuilt a Cooper "S" motor, with a supercharger, more than twenty years ago. I remember that in order to fit the blower, we had to de-tune the engine, from a compression ratio of about 9:1 in the standard Cooper engine, down to 7.5:1. We did this by fitting a different cylinder head, and oversized head gaskets. Anyway, I thought this stuff might help....
Compression
The amount of boost you can run is directly related to an engine's static compresssion ratio. When the boost is combined with the compression ratio, the result is the effective compression ratio. Typically, a 5- to 8-psi boost range (usually produced with the supplied pulleys in blower kits) will work fine for compression ratios in the 8:1 to mid-9:1 range (operating on 91/92-octane fuel). However, this will ultimately depend on other modifications to the car, manual or automatic transmission, gearing, operating temperature, vehicle load, and altitude. If detonation is encountered it can often be controlled with boost retard devices or by experimenting with different-sized pulleys.
Special Carburetion
Choosing a carburetor or fuel injectors is a crucial step when building a blower-specific engine, because under boost the engine will often need 40-50 percent more fuel and air. Unlike a normally aspirated engine that may suffer only low power from poor fuel delivery, a supercharged engine without enough fuel under power may run extremely lean and destroy the engine. Running too small a carburetor also means that you can't flow enough air to produce maximum boost.
Because more fuel is required to feed a supercharged engine, the fuel-delivery system must be considerably improved. This means large fuel lines of AN-8 or bigger, properly selected and installed fuel pump(s), an adequately designed tank, full flowing filters, and a correctly wired electrical system to operate the fuel pumps.
Blow-Through Carburetion
On a blow-through supercharger system, the carburetor can either reside in a pressurized box or utilize a special carburetor hat. Under boost the false atmosphere (pressure being blown into the carb) requires revamping many of the original carburetor designs to properly supply fuel. A blow-through carburetor generally features sealed caps on the metering blocks, the main well, and the idle well. These carburetors typically feature only annular boosters so that as the signal gets stronger more fuel flows into the engine. As boost is increased by each psi, fuel pressure must too be increased at the same rate. To do this, a special regulator is referenced to boost pressure and raise or lower the regulated fuel pressure, depending on demand.
Turn Back
Reverse-rotation superchargers are generally used in applications where there are fitment issues or on engines designed to spin opposite of most other engines. Fitment issues arise when the area behind the belt driveline is impacted. Examples are 32-valve cylinder heads and exhaust manifolds. In this case the supercharger is mounted in front of the belt line. Consequently, the supercharger must now be rotated in a reverse motion, in which case manufacturers design the inner components of the blower as a mirror image of a standard unit.
Ignition Systems with a Supercharger
On any blown engine, high-performance ignitions are required primarily to provide adequate spark at higher-than-normal engine pressures and speeds. Additionally, it is often a good idea to run spark plugs that are one to two ranges colder than normal. Rule of thumb: the more boost, the colder the plug required.
One of the most important concerns with any supercharger installation is detonation control. This is because under acceleration, detonation can damage the piston ring lands (or other worse yet, damage rod bearings, destroy pistons, or blow head gaskets). A handy device to counteract most detonation problems is an ignition system with a boost-retard control.
Ignition timing is especially critical with a supercharger to not only keep detonation at bay, but also provide good power. For most applications, the distributor should have a centrifugal advance mechanism set up so that the entire advance is in by 2,500 rpm. Typically, 34 degrees should be a safe level of ignition lead to provide close to optimum performance[/I][/I]
kool kitty89
Senior Master Sergeant
I knew the "blower" was the supercharger. (when I said compression ratio I meant pressure ratio acheived by the centrifugal compressor/supercharger)
But the gear ratio makes sense, thanks.
But the gear ratio makes sense, thanks.
