fc3sxtasy87

I just got my car tuned well, come to find out it was cause when I put my motor back together I did not re aligne my CAS . So now my car when it idles it bounces between 1k and 1.5k rpm almost like it has a lopey camshaft. could this be due to the large street port. cause I just calibrated my TPS. I dont think that is it but I could be wrong.

but with load on it it runs like a freakin champ. today I got rolled up by a mk4 supra, we were side by side and boom, the supra fully gunned it as did I, both of us stayed side by side, we blew off at the same time. he was not happy that a 7 stayed with him when he was hoping to lose me

anyways any suggestions or tips are appreciated
Thanks

Syonyk

A large street port can cause a loping idle, especially with the stock ECU. The overlap is increased significantly, and this can cause idle issues.

-=Russ=-

scathcart

Unless you port up a LOT on the exhaust port (like a race port), no street port is going to idle like that.

The increase in overlap on even large streetports is not very large at all. It tends to increase the miss at idle, but it will never make the car bounce like a bridgeport.

Check the TPS again.

fc3sxtasy87

k, will do

Sideways7

iTrader: (7)

I disagree. While it may not bounce as much as a bridgeport, it will bounce, especially with the stock ecu. 1-1.5 k is a bit much for a streetport, though. Is it a regualar, somewhat fast bounce, or a slow, uneven bounce? If its the later you prob have a vac leak.

88IntegraLS

I agree with Scathcart, you need to really add overlap on the exhaust side to get a bad idle with a large street port. You can port the intakes pretty much to the maximum, to the point where going further would leave the side seal tips unsupported, and still get an idle that fluctuates only a couple hundred rpm when not using a BACV.

scathcart

If your idle bounces with any streetport where the exhaust has not been ported upwards to any large extent, something is wrong.

I built a car with absolutely monstrous ports, leading edge of the sideseal completely unsported, about 1/5 of the corner seal supported, and ported upwards on the exhaust around 2mm. No AWS, ACV, or BAC Valve. 800 rpm steady idle until the headlights turn on, and then it drops to steady 700 rpm.

Verified there are no vacuum leaks up to 50 psi/ 26 inches of vacuum between the AFM and the engine, set the TPS correctly, and set the timing to fire at TDC leading, 15 ATDC trailing, and the idle AFR's to 13.2:1. Rock steady idle, stock ECU.

Without race porting the exhaust, or bridgeporting the intake, you simply can not get enough overlap to cause a bouncing idle, unless you have something wrong.

Not enough information for you?

Let's look at why bridgeports bounce: simply put: port overlap. The intake ports are open when the exhaust ports are open and the exhaust flows into the intake charge and dilutes the intake charge. Overlap is affected by very few factors, with the most significant being exhaust backpressure, exhaust closing timing, and intake opening timing.
The stock intake opening timing is 32 degrees ATDC. A good streetport keeping the leading edge of the sideseal supported, will open around 23-25 degrees ATDC... not all that much earlier than stock. Unsupporting the leading edge... and you'll maybe get down to 15-17 degrees ATDC opening timing. So overall, you've gained a max of 17 rotational degrees of intake timing... really, nothing at all., and still nothing that opens before TDC.
A typical bridgeport opens between 110-70 degrees BTDC. Thats 125-95 rotational degrees before a LARGE streetport. Pretty big difference.
Lets look at exhaust. Stock exhaust closes at 48 ATDC. Porting even 2mm upwards and it still only closes at 52-54 ATDC. Race porting closes the exhaust anywhere from 60 degrees to 75 degrees ATDc.... 27 degrees later than stock.
Now you also have to keep in mind rotor position when thinking about intake timing... just because the ports are open at the same time doesn't mean there is a significant path for which exhaust can flow to the intake port. When the rotor is at TDC, there should only be 0.0295" MAX between the rotor face and the rotor housing... how much can really flow in a crack 3" wide by 1/34" thick? (compared to the intake port flow)
Lastly, backpressure. Any ported engine should, in the very least be running some kind of upgraded exhaust (low backpressure)to take advantage of the new engine ports, but even with the stock levels of backpressure, which develop under 0.25 psi at each exhaust port at a 800 rpm idle, there is not a significant enough pressure to cause exhaust to flow into the intake as opposed to maintaining with the high-velocity exhaust charge out into the exhaust port (and the being drawn out on the negative exhaust pulse immediately thereafter).


I have built lots of streetported engines. None of them had an idle that bounced at all, even with the stock ECU.
I have just given you examples of both theoretical evidence and practical experience, so I can truly say,


If your streetport bounces, there is something wrong.