Larry,
The more overlap a cam has the more lumpy the idle will be, because (as you
say) both valves are open at the same time. The overlap also causes raw fuel
to go out the exhaust port, as it comes in some goes right out... and this is
why hot cams are NOT smog able. Raw gas out the tail pipe is a big No No....
for some people.
The reason why overlap is desireable on a hot cam is because at higher RPM you
actually get more fuel in the cylinders during the compression stroke which
means higher speeds. Without overlap you get more fuel in at lower RPMs which
means more lowend power, it's a trade-off.
Duration comes into play by getting more fuel in at any RPM, the longer the
valve stays open the more fuel comes in.
Now before you can cam just any engine you must know the piston compression
because of overlap. Since overlap lets some gas out the exhaust that means
when the piston starts the compression stroke the exhaust valve is still
open... so you are actually loosing some of your piston compression. So if you
stick a hot cam in a 8:1 compression engine your result could very well be 7:1
compression, or lower HP then you what started with before you spent $200 for
that hot cam that got you nowhere. If your engine is stock you should only go
with a very mild cam, at the most.
Hope this helps,
BTW... from what I've read the 1500 isn't much of a high RPM engine... go with
a pumped 1275 or a stock Japanese duel cam and save yourself allot of
trouble... after you get it fitted in of course. And for you Vega Cossworth
lovers my 170 HP 18RG will eat your lunch.... after I get it running
Shields up!
-Drew
Larry Macy wrote:
> OK I got a question. As most of you know I am in the process of doing a
> rebuild on a 1500 Midget engine. I have been thinking about putting a cam
> in the engine, for a bit more performance. I have run into some
> questions I have no idea about. All of the rebuilds I have ever done have
> been stock. Whether it was a 7490 cu. in. Waukesha, a D-9 Caterpillar
> diesel, or the 1500 that is now coming apart on me, it was always stock.
>
> I am getting a lot of info on cams that I do not know how to interpret. I
> am looking for a mild street cam. I understand that for the most part a
> cam shifts the powerband, while adding a bit more ponies. (at least I
> think that is what I understand).
>
> There seems to be 3 relevant things on a cam, but I don't quite get what
> they mean, or how they relate to performance. Now bear with me.
>
> We have duration, which is how long the valves are open.
> We have lift, which is how far the valves open.
> We have cam lobe lift, which I don't know what the hell it means.
>
> For instance I have an Kent cam that has
> 295 duration
> .382 in lift & .378 exh lift
> .261 cam lift.
> Power range 2500 - 6500
>
> An APT cam that has:
> 278 duration
> .256 lobe lift
> 231 cam duration @ .050 lift
> Power range 2000 - 6500
>
> And Ted Schumacher that will grind a cam for me, but I forget the exact
> specs.
>
> And Elgin Cams, Which Mike G posted the other day, but I don't have handy.
>
> I know that all that stuff is supposed to make the engine breath better,
> but what the hell does it all mean??
>
> I know that the longer the duration the longer the valves stay open, I
> know that the greater the lift the more fuel/air charge gets into the
> cylinder. I know that if the lift gets too much you need stronger
> springs. I know that there is such a thing as valve overlap that is
> supposed to make the charge mix a bit better. (for those of that don't
> know, that is when both valves are open at the same time, one closing and
> the other opening, I think it happens on the intake stoke when the
> exhaust valve is not quite closed and the intake is beginning to open,
> but I forget a lot of the theory as to why that is a good thing).
>
> Ok put it in simple layman's terms for me. I want a cam that will give me
> a bit more oomph, but that I can tolerate in a car that is still a daily
> driver. I do spend a bit of time at stop lights so the less "lumpy" idle
> the better. What cam do I choose, and why should I choose that one, or
> should I stay with the stock one??
>
> I am going with the 9:1 pistons, balancing, and debating lightening the
> flywheel. I do not want a race car, I want a daily driver with a bit more
> oomph. I have a Weber DGV (water coke) and headers. Someday I will do the
> dual SU's but not today.
>
> Remember this is a street car, not a race car.
>
> Help
>
> Thanks
>
> Larry
>
> BTW I think my sig line really fits here ;-)
>
> Larry Macy
> 78 Midget
>
> Keep your top down and your chin up.
>
> Larry B. Macy, Ph.D.
> macy@bblmail.psycha.upenn.edu
> System Manager/Administrator
> Neuropsychiatry Section
> Department of Psychiatry
> University of Pennsylvania
> 3400 Spruce St. - 10 Gates
> Philadelphia, PA 19104
>
> Ask a question and you're a fool for three minutes; do not ask a
> question and you're a fool for the rest of your life.
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