A customers Peugeot I did a while ago had less power than the engine spec should have made.
I played with the mapping, the cam timing, checked all was as it should have been.
Then I noticed it had 45mm throttle bodies... This is only a 1600cc 4 cyl revving to 7800 rpm and which should be making around 160bhp.
Most of these engines (the ones that work) have 40mm throttle bodies, so I suggested we fit some to try. Low and behold we found the missing 20 bhp, from smaller throttles....
Then I found out that the 'factory' motorsport (220 bhp @ 9000 rpm) motors use 38mm throttles.....
Air speed and resonance amplitude do change with diameter, so bigger is not necessarily better every time. It's not simply a matter of pulling fuel through the jets of a carb...
Why do you think so many of the new superbikes have two throttles in tandem, one connected to the riders wrist and one to the ECU? If you watch them on the dyno the ECU keeps the secondary throttles partly closed through the lower rpms, keeping the velocity up....
I'm not saying bigger isn't better with our motors, the curtain area of two open valves is much bigger than the port area in our engines (and that is usually and indication), it's just not a 'rule' that bigger is better.
Remember, the 'area under the curve' is much more important than 'peak' figures....
The Velocity Myth
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cannondale27
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You guys are right.I shouldnt have called it a myth.Its definitely a factor at some point.We just haven't reached it yet.I remember the day I put the Cyclone airhorn on how much it smoothed out the AFR.Thats definitly velocity there.Heck we havent even found limits of exhaust pipe flow yet.So much more fun ahead!
QUOTE (cannondale27 @ Jun 11 2007, 08:18 PM) <{POST_SNAPBACK}>
You guys are right.I shouldnt have called it a myth.Its definitely a factor at some point.We just haven't reached it yet.I remember the day I put the Cyclone airhorn on how much it smoothed out the AFR.Thats definitly velocity there.Heck we havent even found limits of exhaust pipe flow yet.So much more fun ahead!
I agree completely.
How big are you guys going on these ports? It seems like such a guarded secret. I can see being secretive if we were builders of a factory race team, or paid big bucks as pro race builders but we are such a small community that sharing our porting pics may yield better results. I just finished one of my MX builds that someone else ported on a flowbench and utilized the terminator map Tim recommended for his stroker, ported, 97mm, open airbox and it was WAY lean with the flowrate he recommended. The bike ran well, but backfired on deceleration...I richened up the flowrate from 2275 to 2025 and the power difference was like going from a stock bike to a ported big bore. I need to get it on a dyno to tune it in with the proper AFR, but this tells me that this head is flowing much better than the bike he dyno'd to create that map. I think it still still needs to go richer as well.
When Tim tells you a starting point its a nice safe spot to test with. By no means can anyone build a map and have it work with everyone's setup. Main reason is the flowrate of the injectors are all different. In this case your injectors flowed less than what he worked with so you needed to add more fuel.
QUOTE (Happyboy @ Jun 11 2007, 10:32 PM) <{POST_SNAPBACK}>
When Tim tells you a starting point its a nice safe spot to test with. By no means can anyone build a map and have it work with everyone's setup. Main reason is the flowrate of the injectors are all different. In this case your injectors flowed less than what he worked with so you needed to add more fuel.
I agree, when we are modding these bikes as much as they are now there must be a custom map built. But 250 lower and still lean tells me that it might be the port job causing the lean condition...I really am not sure until I stick it on the dyno and see what the curve looks like. Not saying Timbo or others dont do great port work, but this thread was about velocity and this port job was more about shape than hogging out until maybe the thing leaked
QUOTE (wistech @ Jun 12 2007, 12:30 AM) <{POST_SNAPBACK}>
I agree that Tim made a mistake trying to give you tuning recomendations and a map for a engine he apparently didnt build or port. That would be the job of your engine builder.
Apparently you are both missing the point here.
This is NOT to point fingers at Tim, his recommendation, his work, or anything about him. This was supposed to be a discussion about velocity and porting and was never at any point about vendors porting abilities, mapping abilities, or how their maps should work flawlessly with someone elses setup
I was merely discussing flowrate numbers in relation to a different port jobs. The engine was built my me, and yes I need to take it to a dyno and get a map that will work with the setup I am running.-
cannondale27
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- Joined: Wed Mar 10, 2021 10:41 pm
I know what your saying.More fuel needed should equal more power.Could be.Tims maps are pretty close matches to motors he made now.Mapping is much easier and quicker.Only other explanation is something is restricting fuel flow in yours or popping was caused by something as simple as a exhaust leak.Hope its more power though.
QUOTE (2000ex @ Jun 11 2007, 09:07 PM) <{POST_SNAPBACK}>
I richened up the flowrate from 2275 to 2025 and the power difference was like going from a stock bike to a ported big bore. I need to get it on a dyno to tune it in with the proper AFR, but this tells me that this head is flowing much better than the bike he dyno'd to create that map.
Be sure to get on a dyno jet with a tach signal and save the run files to disk for comparison.. You might be onto something there .
QUOTE (cannondale27 @ Jun 12 2007, 12:51 PM) <{POST_SNAPBACK}>
I know what your saying.More fuel needed should equal more power.Could be.Tims maps are pretty close matches to motors he made now.Mapping is much easier and quicker.Only other explanation is something is restricting fuel flow in yours or popping was caused by something as simple as a exhaust leak.Hope its more power though.
That was exactly my point. There shouldn't be anything restricting flow, I have wide open barbs hooked up to my 250r kit and the fuel pump noise is very difficult to even hear now
It could possibly be a leak at the header, but I installed all fresh crush washers and studs. I can only hope for more power cause this thing hauls in the mail
I should at least get a 50HP club sticker in a few weeks...hopefully a 55