Ignition targets
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From: Chicago Bears
perhaps minor but "every bit counts"...the 928 with those long hoses running from the air filter up to the radiator will have some pressure drop especially at high airflow demand (WOT/higher RPM).
say 14.5 psi is ambient, by the end of those hoses, through the filter and down into the valley you might be getting 14? at the throttle plate.
say 14.5 psi is ambient, by the end of those hoses, through the filter and down into the valley you might be getting 14? at the throttle plate.
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From: Chicago Bears
It's November, so, how's it coming along?
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From: London, UK
Well, that's good feedback.
The point I would mostly like to make about runner length though is that it should not impact peak numbers by itself, all other thing being equal (particularly cam profile and cam timing), but rather how the engine accelerates through the rev range. From what I have read on here and in the WSM, the Helmholtz effect as varied through the flappy's hysteresis control affects the shape of the curves but peak torque remains the same figure whether the flappy is open or closed.
(By the way, when the WSM shows that peak torque rpm varies by a whopping ~1000 rpm with the flappy open/closed while peak hp rpm remains the same tells just much how remarkably restrictive up top the stock intake can be).
The interesting power figures you mention are I think mostly due to engine compression, cam profile and timing, exhaust flow, peak runner flow, though not runner length which is the dimension on which the Helmholtz effect will have a varying impact.
It is a little annoying to read that you think your maps are **** even though you reached some pretty decent numbers on your special GTS engine, as it does not tell if any differential approach and testing has been done by varying cam profiles, cam timing and ignition timing to find the sweet spot for the S62 ITBs to shine through. If you put hours of testing by adjusting such variables until reaching the numbers you mention please stress this otherwise I would tend to think you have reached some kind of lucky result without being actually sure how you actually got there. I'm not saying that gives any less credit to the results reached by the ITBs over the standard intake but it becomes a bit difficult to say for sure they are of the optimum length on any of the engines they have been tested so far, again all other things being equal.
At this point my feeling, my two pence worth obviously, is that the much better than stock dyno results you got on several engines with the S62 ITBs is mostly due to their most likely superior (static) flow capacity over the standard runners, which flow only about 245 cfm as stated on the 928M website. I believe you could gain more engine "top end liveliness", i.e. reducing the time needed for the engine to accelerate between peak torque rpm and peak power rpm, with trying shorter runners. This involves though that your ignition map is already right on the money otherwise you may not be able to reach conclusive results by varying your ignition profile in a differential way that makes any scientific sense.
The point I would mostly like to make about runner length though is that it should not impact peak numbers by itself, all other thing being equal (particularly cam profile and cam timing), but rather how the engine accelerates through the rev range. From what I have read on here and in the WSM, the Helmholtz effect as varied through the flappy's hysteresis control affects the shape of the curves but peak torque remains the same figure whether the flappy is open or closed.
(By the way, when the WSM shows that peak torque rpm varies by a whopping ~1000 rpm with the flappy open/closed while peak hp rpm remains the same tells just much how remarkably restrictive up top the stock intake can be).
The interesting power figures you mention are I think mostly due to engine compression, cam profile and timing, exhaust flow, peak runner flow, though not runner length which is the dimension on which the Helmholtz effect will have a varying impact.
It is a little annoying to read that you think your maps are **** even though you reached some pretty decent numbers on your special GTS engine, as it does not tell if any differential approach and testing has been done by varying cam profiles, cam timing and ignition timing to find the sweet spot for the S62 ITBs to shine through. If you put hours of testing by adjusting such variables until reaching the numbers you mention please stress this otherwise I would tend to think you have reached some kind of lucky result without being actually sure how you actually got there. I'm not saying that gives any less credit to the results reached by the ITBs over the standard intake but it becomes a bit difficult to say for sure they are of the optimum length on any of the engines they have been tested so far, again all other things being equal.
At this point my feeling, my two pence worth obviously, is that the much better than stock dyno results you got on several engines with the S62 ITBs is mostly due to their most likely superior (static) flow capacity over the standard runners, which flow only about 245 cfm as stated on the 928M website. I believe you could gain more engine "top end liveliness", i.e. reducing the time needed for the engine to accelerate between peak torque rpm and peak power rpm, with trying shorter runners. This involves though that your ignition map is already right on the money otherwise you may not be able to reach conclusive results by varying your ignition profile in a differential way that makes any scientific sense.
We fitted the shorter trumpets initially, then we strapped the car on the rollers. Countless runs and pushing the engine to the point where we were saying to each other how remarkable was that it could survive so many flat out runs on a dyno. We had access to variable length trumpets. We fitted them too. The shorter we went, the worse it got...
We were getting nowhere - perhaps 360rwhp. Thats not very good, you would have to agree. We played with cam timing too. Nothing, nada. We would run into a wall, where the engine would make noise, but not a lot of horsepower. Pretty rubbish result considering the costs involved.
We fitted the S62 trumpets as advised by the oracle and then we were able to map it to achieve 435rwhp. Trust me, it is scarily fast and the way it revs from 3500 to 7000 would leave you speechless.
When I say the map is ****, I mean that it could have been perhaps a bit more polished. After all we spent close to 9hrs on the dyno that day. I know there is more there, but after seeing the higherst number posted ever by a standard displacement NA 928 engine we called it a day and went to drink beer.
The heads are running 968 intake valves with shorter guides as well as some proper porting. The cams are Colins Stage 2 cams.
You would have to trust trust me that the S62 length is the correct length for these builds - keeping all other things equal, the simple change gained horsepower.
After all, we nearly destroyed a $25k engine to prove that we are on it, while your assumptions were somewhat off the mark
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From: Edinburgh, Scotland
We will need to see what if any adjustments are required and move forward from there.
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I have read your post again and it still leads to the questions I made earlier.
I am aware of another 928 engine that uses ITBs with much shorter runners and it makes 90 rwhp/L while yours makes 80 rwhp/L, just saying.
What is the spec of the "Colin's Stage 2 cams" you mention? Where do you have the inlet valve closing ABDC and by how many degrees did you adjust this on the dyno?
Where those cams specifically designed for your engine or rather for a stock engine using the stock intake?
Do you have a dyno sheet of the stock-ish GTS engine using GT cams and the S62 ITBs? It is closer to a stock engine so perhaps easier for me to imagine how the ITBs affect its power delivery, and as I suggested earlier the increased in power is most likely down to higher runner flow rather than runner length and shape.
There are too many things that do not add up on your 12.5:1 engine, such as varying cam timing having no effect. This should have way more effect than playing with runner length, even regardless of valve overlap. I am not doubting the results you got but the way you present them is somewhat off the mark
I am aware of another 928 engine that uses ITBs with much shorter runners and it makes 90 rwhp/L while yours makes 80 rwhp/L, just saying.
What is the spec of the "Colin's Stage 2 cams" you mention? Where do you have the inlet valve closing ABDC and by how many degrees did you adjust this on the dyno?
Where those cams specifically designed for your engine or rather for a stock engine using the stock intake?
Do you have a dyno sheet of the stock-ish GTS engine using GT cams and the S62 ITBs? It is closer to a stock engine so perhaps easier for me to imagine how the ITBs affect its power delivery, and as I suggested earlier the increased in power is most likely down to higher runner flow rather than runner length and shape.
There are too many things that do not add up on your 12.5:1 engine, such as varying cam timing having no effect. This should have way more effect than playing with runner length, even regardless of valve overlap. I am not doubting the results you got but the way you present them is somewhat off the mark
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From: London, UK
Marti,
I cant at this very moment - the car is in storage. Hopefully I would be dropping on her this coming week to swap the bellhousing and a new intermediate plate on the DD clutch. I will then measure the trumpet length, the ITB length, the adaptor plate height as well as the depth of the intake ports within the heads. Will also give you an idea with regards to the distance between the injector nozzle and top of the intake valves.
Btw, Thom - another little snippet of information for you - fitting a DD clutch to a 32v car allows for much more aggressive ignition advance mapping. We could run both more aggressive advance outright as well as they way advance ramped up.
Alex
I cant at this very moment - the car is in storage. Hopefully I would be dropping on her this coming week to swap the bellhousing and a new intermediate plate on the DD clutch. I will then measure the trumpet length, the ITB length, the adaptor plate height as well as the depth of the intake ports within the heads. Will also give you an idea with regards to the distance between the injector nozzle and top of the intake valves.
Btw, Thom - another little snippet of information for you - fitting a DD clutch to a 32v car allows for much more aggressive ignition advance mapping. We could run both more aggressive advance outright as well as they way advance ramped up.
Alex
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From: Edinburgh, Scotland
Alex, I am presuming these trumpets are not unusual in design but were longer in length than the short versions you tried initially.
There are design formulas out there that give an optimised inlet length for the revs you are looking to make maximum power at.
Is sounds like the initial length you were using was too short but I never imagined that it would hurt the power that badly. It might be that the S62 trumpets are exactly the right size for your engine or it might be ball park right and possibly there is more power to come by further development.
There are design formulas out there that give an optimised inlet length for the revs you are looking to make maximum power at.
Is sounds like the initial length you were using was too short but I never imagined that it would hurt the power that badly. It might be that the S62 trumpets are exactly the right size for your engine or it might be ball park right and possibly there is more power to come by further development.
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From: Chicago Bears
There are design formulas out there that give an optimised inlet length for the revs you are looking to make maximum power at.
Is sounds like the initial length you were using was too short but I never imagined that it would hurt the power that badly. It might be that the S62 trumpets are exactly the right size for your engine or it might be ball park right and possibly there is more power to come by further development.
Is sounds like the initial length you were using was too short but I never imagined that it would hurt the power that badly. It might be that the S62 trumpets are exactly the right size for your engine or it might be ball park right and possibly there is more power to come by further development.
However, we thought we were smarter than some very smart guys and we got punished - 3hrs of wasted dyno time, countless runs and the car was nearly torched as I just hated the way it performed.
The software and hardware dimensions match, and importantly, the dyno results confirm that the intake dimensions are already optimized as is.
Pro
Joined: May 2011
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From: Edinburgh, Scotland
Ah, ok I think I am following now, Alex used some software that suggested the S62 runner would be best, but he decided to use something shorter for some reason and waisted 3 hours before finally trying the S62 runner which did indeed make more power than the runners he initially tried.
That actually sounds quite funny when I read it back.
My only interest is whether a target rev band came into the equation and if he is using the alpha N type ST - the alpha N ST is very interesting.
That actually sounds quite funny when I read it back.
My only interest is whether a target rev band came into the equation and if he is using the alpha N type ST - the alpha N ST is very interesting.
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From: London, UK
Ah, ok I think I am following now, Alex used some software that suggested the S62 runner would be best, but he decided to use something shorter for some reason and waisted 3 hours before finally trying the S62 runner which did indeed make more power than the runners he initially tried.
That actually sounds quite funny when I read it back.
My only interest is whether a target rev band came into the equation and if he is using the alpha N type ST - the alpha N ST is very interesting.
That actually sounds quite funny when I read it back.
My only interest is whether a target rev band came into the equation and if he is using the alpha N type ST - the alpha N ST is very interesting.
we tried shorter runners as we followed the mantra that shorter runners move the powerband and peak power up the rev range
that was not the case as it was proven both via the software predicted runner length as well as empirically on the dyne once BMW runners were fitted
i am using the Alpha N ST indeed
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From: Oman
spot on Marti
we tried shorter runners as we followed the mantra that shorter runners move the powerband and peak power up the rev range
that was not the case as it was proven both via the software predicted runner length as well as empirically on the dyne once BMW runners were fitted
i am using the Alpha N ST indeed
Your dyno numbers speak for themselves- no debate really needed!
The proof of a delicious pudding is in the eating. Whether or not you can get anything further with your formula is what it is- "the icing on the cake".
Regards
Fred
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From: Chicago Bears
