Best compression ratio neccesary for .420 cams

How much compression do you need for the .420 Cams? Will 10:25 to 1 work okay? What is the power band in a 1098cc does that usually make?

10.25:1 should be OK. But to get the benefits you'll have to do some major port work. Also most of you power and torque are going to be really high up in the RPM range. I would try something like Web #118 or #119 if it's for a streetbike.

Later, Doug

Or the all time favorite, the APE K410

kzzone.com/cams.html


Jay

Jay...or anyone who might know, what determines what compression ratio you should shoot for for any particular set of cams? For example, you are running a 1327 motor with 10.25-1 compression with .425 lift cams...is that enough compression? Say you wanted to run bigger cams...say .465-.480 lift. Will the 10.25-1 be ok or would you have to run 12-1 compression. I know to run the bigger cams, which have smaller base circles, the head will have to be set up differently, but I have heard both sides-run what you want and it will be fine and that the bigger cams won't work right unless you have high-compression because of the lift. I have talked to guys running the big lifts with street compression and they say they run great, but have been told by others that the motor will be a dog because you need the compression to use all the gas being delivered by the bigger lifts. Any comments, anyone?

This gets complicated

It's not really the valve lift that determines suggested minimum compression ratio, it's more a case of valve timing, specifically overlap (the period of time measured in degrees that both valves are open at the same time). Generally speaking, the more duration the cams have, the higher compression ratio required, and the higher in the rpm band they make power.

"But". (again generally speaking) most, if not all, the cams made for the KZ have increased duration to go along with the increased lift, so most of the time if you are looking at big lift you are also looking at big duration and more compression.
Using APE's cams as an example, the 410 should work ok on stock compression, but it would work a lot better at 10:5:1, while the 435 pretty much calls for 10.5:1 minimum.

Engine size makes a difference as well, if you call the 410 cam a "stage one" cam for a 1015 motor, then a 435 would be a "stage one" for a 1348 motor.

Simple answer

Ask the manufacturer, they will know what cams to use for your particular motor and planned use (street, drags, road racing, etc) and can tell you the minimum compression ratio for those cams.

The most common mistake made by backyard engine builders is using too much cam, it drives the cam manufacturers crazy.

If the manufacturer is no longer in business, then ask Jay, I think he knows all the cams ever made for the KZ

Your turn Jay

Post edited by: steell, at: 2007/10/20 08:33

compression ratio is a factor from volume of the head (cc,s ) and the dome of the piston, the valve overlap can play a small part but the tighter the chamber the higher the comp ratio.
I dont think I have ever installed a cam that made any real changes in the compression ratio, and changing the degree of the cam made no differance either, but it does make the motor respond

I knew that duration would move the power band up to some degree, but didn't know how the compression would effect the whole package. I know where you place the cams would have an effect, of which I know nothing at this point, but I have heard lots about how compression works into the equation and didn't really understand that part. Why does the compression need to be higher with longer duration numbers? Does the longer duration bleed off enough compression you need the higher number to basically keep compression after some is lost with the higher duration? I was alway told that you can't run .60-.480 lift cams on the street because you would need to run 12-1 compression, which would overheat, but a few guys were running these cams with 10.25-1 and they said they weren't having any problems and was wondering why the big difference.

Steell;
Thanks for saving me all that typing :laugh:

Jay

Post edited by: APE Jay, at: 2007/10/20 18:17

it sounds like u have the idea racer. as the cams duration increses ,it reduces compresion because both valves are open together longer. so if u dont have much compression to begin with (stock motor)and u add a lot of duration (wich comes with lift)u will not benifit.as the duration increases so does your power bads rpm range, so a lot of duration is not desired for the street engine guy that wants low end torque.Did i say that right Steel.

I just had to get into this one! With the .420 u need under bucket shims and you may know it. Early style kz heads have flow problems around the valve guides and need focused porting to help this restriction. At .420 lift, the flow is limited by the stock ports flow dynamics. U can keep opening the valve but the motor will not draw and expel anymore air volume than at say .410. Reason 1 is the ports flow capability. 2 is the compression ratio in relation to duration. For example: first of all, forget overlap, the exhaust valve being open with the intake has no affect on comp. Advancing the intake cam increases the time (or number of degrees), both valves are open at the same time. Closing the intake earlier increases comp. Advancing the intake cam opens it earlier (increasing overlap), and as a result it closes earlier, trapping more comp. Two identical lift cams can be ground where one has extra duration on the opening side (increasing overlap) and still closing early saving any compression drop but performance benefits from the extra intake charge volume. The other has the duration on the closing side or "bottom", whereas the intake is open longer and compression drops "possibly" hurting low end and if it drops to far, hurting top end peak torque as well. Take a sock 150psi motor and with .410's the will be 130psi or so. The motor will run good because of the extra flow and with increase in duration the peak output rpm wil be higher. The low end will be down some because of the comp. drop. With 10.25/1 the comp. is 180psi, install .410's and it's now 150. A good place to be, you get the torque and the hp. Use any higher duration than what the /410's have and compression drops below the standard of performance plus. Ideal would be .410 cams with 180 final ratio. Then your snappy. The .420's may have the same or a little more duration than the .410's. If they have more, the comp. will drop below 150 installed with 10.25/1. Sure it will work, but it's going backwards somewhat. At .420 lift with say 150psi the flow limit of the early head style shows up like a sop sighn at the end of a T road. Your starting to hit the wall with flow. If using 180psi, it becomes trapped compression and the motor fights to breath. Sounds good, snappy, but no go. But I gotta go, so good luck and remember the name Larry Caven, it's likely he will have something to say about flow dynamics and any corrections to my little masterpiece here.

Post edited by: nads.com, at: 2007/10/20 22:36

I just had to get into this one! With the .420 u need under bucket shims and you may know it. Early style kz heads have flow problems around the valve guides and need focused porting to help this restriction. At .420 lift, the flow is limited by the stock ports flow dynamics. U can keep opening the valve but the motor will not draw and expel anymore air volume than at say .410. Reason 1 is the ports flow capability. 2 is the compression ratio in relation to duration. For example: first of all, forget overlap, the exhaust valve being open with the intake has no affect on comp. Advancing the intake cam increases the time (or number of degrees), both valves are open at the same time. Closing the intake earlier increases comp. Advancing the intake cam opens it earlier (increasing overlap), and as a result it closes earlier, trapping more comp. Two identical lift cams can be ground where one has extra duration on the opening side (increasing overlap) and still closing early saving any compression drop but performance benefits from the extra intake charge volume. The other has the duration on the closing side or "bottom", whereas the intake is open longer and compression drops "possibly" hurting low end and if it drops to far, hurting top end peak torque as well. Take a sock 150psi motor and with .410's the will be 30psi or so. The motor will run good because of the extra flow and with increase in duration the peak output rpm wil be higher. The low end will be down some because of the comp. drop. With 10.25/1 the comp. is 180psi, install .410's and it's now 150. A good place to be, you get the torque and the hp. Use any higher duration than what the /410's have and compression drops below the standard of performance plus. Ideal would be .410 cams with 180 final ratio. Then your snappy. The .420's may have the same or a little more duration than the .410's. If they have more, the comp. will drop below 150 installed with 10.25/1. Sure it will work, but it's going backwards somewhat. At .420 lift with say 150psi the flow limit of the early head style shows up like a sop sighn at the end of a T road. Your starting to hit the wall with flow. If using 180psi, it becomes trapped compression and the motor fights to breath. Sounds good, snappy, but no go. But I gotta go, so good luck and remember the name Larry Caven, it's likely he will have something to say about flow dynamics and any corrections to my little masterpiece here.

Thanks nads . that clears things up for me too.