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Old June 1st, 2014, 09:18 AM   #1
subxero
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replacing exhaust cam with intake cam

There has been mention of this in a few different threads but no real info.

So the idea is that the intake cam has slightly more lift than the exhaust cam, running 2 intakes will give you more exhaust lift.

Have seen things said like the following
"yeah it might give you some gains" "no because timing will be off"

Has anyone or does anyone know of anyone who has done this and the actual outcome?

I'd like to get some solid feedback before i consider trying it.
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Old June 1st, 2014, 10:30 AM   #2
Racer x
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I did it. And it made a huge difference with nitrous. But without nitrous it did not do much. There is a lot more lift with the intake cam then the exhaust cam and making the change to two intake cams has potential. I have not dialed them in yet.
But I know what I need and did move them in a good direction. I need to dyno it again to see what difference it made. But my feeling is it will make a difference when the cams are adjusted with a different center line.
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Old June 1st, 2014, 12:19 PM   #3
micoulisninja
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Quote:
Originally Posted by Racer x View Post
I did it. And it made a huge difference with nitrous. But without nitrous it did not do much. There is a lot more lift with the intake cam then the exhaust cam and making the change to two intake cams has potential. I have not dialed them in yet.
But I know what I need and did move them in a good direction. I need to dyno it again to see what difference it made. But my feeling is it will make a difference when the cams are adjusted with a different center line.
in an exclusively N.A. engine there will be little to no gains by doing that (I've tried it in every modifying stage of my engine) and results are....
1. imperative to alter (and experiment with) lobe center...as arough guide I recommend somewhere between 104-106 for intake and 105 (as is the ex cam adjusted from factory) for the intake cam at the exhaust...
2. if using OEM primary pipes with two intake cams you will see some little gains mostly in the low-mid range (about 1hp up to 9k) but no max hp gains... BUT peak hp will be broader in the high rpm range, the dive after 12k is smaller and if you give the thing a little compression by taking off the gasket under the block or miling the head for 0,2mm torque will be improved in the areas it suffers the most...
3. if you use a full exhaust system you will have a LOSS in the low rpm range because of the lack of backpressure and torque diagram becomes a "hill and valley" kind of thing UNLESS you increase base CR over 13:1 which saves the situation quite a bit. high rpm peak hp will have little gains but again the rpm band it is delivered broadens
needless to say that in both cases muffler make no to minimal difference performance-wise...
4. I could provide more data if you had EFI since all P.C., bazaaz etc, help a lot by refining afr ratio but instead I think a have a couple of things to point out that I had mentioned long ago in a similar post a couple of years ago...

I. it the intake of the 250 engine that lets the engine starving, not the exhaust... small throttle bodies(carbs), tiny ports, intake valves and intake cam DURATION @0,05"(the reason I don't mention overall duration is that it can be quite "deceiving" in many cases as to how "mild or wild" the cam profile really is AND is totally misinformative as far as valve acceleration and -most important- decceleration values it stresses the valve to). Considering you use oem valve springs there are certain limits of course...anyway ...if you make a regrind intake cam of about... say 234@0,05" (still "mild" profile, but easy to work on any engine stage) and port the intake you will FEEL the difference... combined with higher CR, it comes really close to the 300's performance...see why the 300 for only 50 more cc has bigger TB, Intake ports and valves but no change exhaust-wise...
II. irrelevant to your question but I believe useful piece of info is another easy DIY work that helps a lot the engine's responsiveness and low rpm torque. Moving the rotor sensor by 2 degrees by making the holes with which it is bolted on the shell oval wider by about a little less than half an inch (1mm)
Sorry I am not providing any photos but I don't keep any...
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