HP500 cam installation
 

Does anyone know the correct angle to degree the intake lobe on a 95 HP cam? Was installed at 108 ATDC and am seeing moderate reversion using the stock gill exhaust.

Re: HP500 cam installation
 

cdscarab
The ICL on the Hp 500 cam is 105* ATDC, your cam was installed 3* retarded.
Bob

Re: HP500 cam installation
 

Do you think cam time has much effect on reversion.I always thought exhaust design and cam overlap were the mane issues.

Re: HP500 cam installation
 

When the cam is advanced you will tend to see a little more reversion.
The cam in the carbed HP500 was a little on the mild side, so If you install that cam say at 109 to 110 degrees ILCL, you will see a small increase in performance from about 4000 rpm on up!

Re: HP500 cam installation
 

HARRISONMIRAGE
It has been my experience that altering the overlap crossover point +/- 4* in either direction will do little to eliminate the effects of exhaust reversion. That is subject almost entirely to overlap duration. I do believe that advancing the cam can have an effect on intake reversion, due to the earlier intake valve opening point in the exhaust cycle and the corresponding earlier exhaust valve closing points.
Bob

Re: HP500 cam installation
 

Just to be clear, intake reversion is a separate phenomenon from water reversion on the exhaust side.
Bob

Re: HP500 cam installation
 

Thanks for the input guys, talked with Mark at Precision Marine this AM and apparently the reversion problem is a combination of the narrow LSA and the Gill exhaust. He says this is the reason Merc. went with the CMI elbow tops. Now the question is what do I do for an exhaust system, or what could I do to modify the gill risers to eliminate the reversion without going completely dry? I wonder if anyone has ever experimented with welding shrouds over the water ports to direct the water rear ward? This would impart additional momentum to the water making it harder for the exhaust to pull it back in. Any thoughts on that?

Re: HP500 cam installation
 

go with the dry pipes!! gotta love that sound! :D:D

Re: HP500 cam installation
 

A great deal of research has been done on the automotive side on reversion control from the point of charge contamination and efficiency, rather than water ingestion, mostly in the area of header design and reversion damming. Given the design parameters of a marine wet exhaust I haven't seen an engineering solution yet that works for boats.
Bob

Re: HP500 cam installation
 

Do you think water reversion is worse if the duration at low lift is fairly large(more overlap @ .006).I put a zz502 cam in my motor that had the intake lobe retarded 4 degrees.This opened my lsa to112.I instaled the cam at 110 ilcl.The duration is 224/234 wich gives 5 degrees overlap.The duration at .006 is up around 300.I have run cams with more overlap at .050 that have had less reversion.Could this be due to the fact that there is more overlap at low lift with the zz cam.My last cam had 9 degrees overlap and I was using silent choice.I modified my risers to be dry to the transom with the zz cam and it still suffers from reversion if I let it idle to low.

Re: HP500 cam installation
 

Harrisonmirage,
Part of the confusion is in the calculations. Overlap is a direct function of duration and lobe separation. The zz502 cam has a LSA of 110* and that is ground into the cam and cannot be altered without regrinding the cam. You cannot open or close your lobe separation. The lobe separation angle is the fixed amount of degrees between the intake and exhaust lobe. If a cam is ground with a 110* LSA and the intake centerline (ICL) is set at 110* ATDC and the exhaust centerline (ECL) is 110* BTDC, the cam is straight up. If you advance the ICL closer to TDC by 4* the ICL becomes 106* ATDC, the ECL moves away from TDC a corresponding 4* and becomes 114* BTDC. No matter how far you alter your installed ICL, the lobe separation and duration remains the same, as does the overlap for this cam. Given 224*/234* @ .050” on a 110* LSA, the overlap for the zz502 cam will always be 9* @ .050”. When you advance or retard your cam all you are basically doing is moving your overlap crossover point in relation to TDC.
We always use seat-to-seat figures (adv duration) to calculate overlap because it will give a more accurate representation of the entire lobe.
The differences between adv duration and .050” figures give the indication of lobe intensity, which may alter overlap by inch/degree calculations but not by degrees of rotation.
Bob

Re: HP500 cam installation
 

When I say I retarded my intake lobe I meant I had it reground,thus changing the lsa to112.

Re: HP500 cam installation
 

Back to my original question would a cam with more seat to seat overlap have more reversion.Most people seem to be interested in .050 figures only but I would think seat to seat must have an effect.I would also think a motor with weak intake vacuum at idle would have less reversion as well.I like to here everones opinion.I don't count sheep at night anymore I visualise cam lobes.