Topic: CB750: Which CR is best for a street bike? DP315 cam to start with.

[PeWe]

I have left my K6 head to my local tuner for installation of 5mm conv kit, 34/28 valves. He will adjust the head for the new vallves for the pistons to be used Wiseco 900cc  CR around 10.25:1.  There is a queue of jobs until mine can be done so I have time to figure out.

Maybe reshape the chambers he did back in the 1983 when he ported it matching RC forged 65mm pistons.
I have not installed a DP315 yet so I do not know the overlap and if it need extra CR, more than 10.5:1 to give its full power.

- Is 11:1 too much in a street bike with DP315 cam and std tap fuel as Shell V-Power?

I have APE Ultra crank with Carillos + HD prim chain + restored prim hub with new rubbers so it must get to be banged hard, but without overheating or pinging. Retard ignition to avoid pinging will loose the power and make a dull engine, right?

I'm sure that my 125-20 cam will like higher CR with its longer overlap so it can jump in if the DP315 get too much of the good.

[NitroHunter]

Here is my .02
The later closing of the intake valve can really drop your cranking pressure - ie, torque - at lower RPM.
The modern engine calculators found online can give you some spot-on info for determining actual CR and compression pressure, I've back engineered an engine against these systems and found them accurate.
My personal seat of the pants experience with that cam was that upping to 200+psi on the compression tester didn't rattle much and made real mid range wheelie power.
 It will run great when revved up no matter what your CR is.

[Jerry Rxman Griffin aka MuthaF'er]

Interesting that the cams will also affect the CR. Never gave this a thought. If you will you be trying multiple cams would you post up the CR differences for us?

[BPellerine]

this is something that I also learned,low revs,big cam,low compression,high revs have to happen to make it work.bill

[PeWe]

I do not have all data needed: http://www.wallaceracing.com/dynamic-cr.php

Hot cams have often spec of high CR pistons as a must and the 12.5 CR pistons must have hot cams.
More overlap less dynamic compression when it leaks out when IN-EX are open at the same time. Or is the closing of IN most important?

Megacycle 120-75 cam is often recommended with its high lift and relatively short duration and overlap keeping the compression.
I want max power from low rpms with  the DP315, higher CR will hopefully give good power during the entire rpm range.  I hoped that someone here has experience of the RC/DP315 cam or similar and compression ratio.

Maybe 10.25/10.5:1 is the way when this is a very common big bore piston for CB750. There are also 12.5:1 for racing.... But there is something between  too...

I like the good response with my RC295 copy, but I miss the disrespect for redline my Megacycle 125-20 cam showed when it passed 10.000 very easy. I would like to get both worlds with DP315 and good CR, without pinging and too high temperature.

[BPellerine]

a few have run them with 10.5 and 836,I have flat top henry abes now so I will wait before trying my 315,with 900cc.another consideration would be the fuel octane or lack of it today.bill

[PeWe]

Shell V-Power got lower octane level a few years ago, from 99 to 98+.
EU 98 octane = US 93 octane. (RON vs AKI)
I have got the feeling when reading posts about high CR pistons that 10.5:1 is not high CR, 12.5:1 is. 12.5 pistons must have long overlap/late closing of IN to not cause pinging, right?

DP315 has not that big numbers to fit a 12.5:1 setup?  High CR and retard ignition to avoid pinging must be wrong way.  Late close of IN and long overlap in combination with race fuel must be used with a CB750 with high CR pistons?

My question is the best CR for this cam and std fuel 98/93 octane (EU/US). 10.5:1 might be the way when this CR is very common.
I hoped that someone has investigated this before and found a perfect CR range.

EDIT: Satanicmechanic homepage write this ....  "10.25 is highly recommended for milder cams and/or low octane (low 90s) gas."
http://satanicmechanic.org/bigbore.shtml
This is what I assume too, but must be sure about which CR possible to use. This page has same deviation in piston diameter as many others. Call 67.5mm piston for 915cc which is wrong.
I get these cc  (((Diam/2)x(Diam/2)) x 3.1415 x 63mm stroke x 4cyl)/1000
67mm=888.44cc, 67.5mm=901.75cc, 68mm=915,16cc

[BPellerine]

one of the things that I noticed is that cam makers and sellers do not provide much info along the lines of comp required,and or displacement,be kind of nice to know what you are buying before you find out you have too much,too little cam without experimenting at 300 a pop.bill

[NitroHunter]

I do not have all data needed: http://www.wallaceracing.com/dynamic-cr.php
 Or is the closing of IN most important?
I want max power from low rpms with  the DP315, higher CR will hopefully give good power during the entire rpm range.  I hoped that someone here has experience of the RC/DP315 cam or similar and compression ratio.

With the 315's longer duration than the 295 it is a nice higher RPM cam. I think it just depends on what you want, how high you want to wring these motors to and if a particular combination is optimized to take advantage of the potential airflow. That's really where the power comes from, breathing. And compression.

The dynamic compression is calculated as the ratio of total cylinder volume (piston location in the bore) at that moment when the intake valve closes, to the compressed volume at TDC. So earlier valve closing (BDC) = more cc's to compress.

My 10.25:1 900cc flat top piston F2 motor ran OK with that cam on premium pump gas. I felt it lacked decent torque below 5000rpm but revved nicely after that. Well until the valves floated and bent each other and a call to RC ended with "That 315 cam won't work with the F2 head, the valves will hit"... No kidding.

I was able to successfully run the 315 in a fully built 970 F2 motor with RC modified head. That motor cranked 200 psi on the compression tester, of course it ran with premium gas, and on an open road it would easily pull revs past the last numbers on the tach in 5th gear, even with much taller gearing.
It ran best on ERC 110 gas though.    Slightly larger bore and much more compression made a big riding difference at low RPMs.
It took some messing around with the smoothbores but once they got close it was a real powerhouse for it's time; a no BS 150+mph bike was a bad ass hot rod back in 1979.

I also had an 836 motor with 240 lbs cranking pressure with the RC 240 reground cam, and it was run on good old premium gas too. Of course all would rattle a little under hard load depending on ambient temperature. Old school tuning says keep backing off the timing until the hard noise stops.

FYI 754 Frank had an 836cc with a 315 cam, don't remember what his CR was.

[Jerry Rxman Griffin aka MuthaF'er]

Hell Bill, it's not so much the $300 as we can always pass along a good cam. It's all the damn work that goes into it.

[PeWe]

So, it is only when IN close that matter? Combination with EX does not matter?
(overlap, when both doors are open and the wind storms right  from the carbs thru the exhaust, or back at very low)

Last cam was a RC295 copy (RC295 scratched in the end under washer) 25/55, 55.5/22 measured at 0.04",  lift 0.393/0.363
It ran very well. (comp test around 180 PSI with my 836)   I would like to get a little bit more on top too

I THINK that the DP315 will measure: 27/57, 57/27 at 0.04" lift.  I added 4 degrees on each as compensation for the 0.04"lift instead of spec 0.05"
(The "RC295" and my old AF SS-1 differ that when measure at 0.04 vs 0.05.)

IF I'll advance the DP315 2 degrees so IN close  at 55 as the "RC295",  it will get good response from low too?  The lower IN lift of the DP315 will have a negative effect at low rpms, right? 0.365 vs 0.393

It is here I want to compensate with higher CR to get better from low. I look forward to do real  tests on the road without ignition problems or add gasket thickness.

[BPellerine]

Hell Bill, it's not so much the $300 as we can always pass along a good cam. It's all the damn work that goes into it.

+1 Jerry,finding out you have to change a bunch of stuff after the fact is a lot of work,you gotta have a lot of exp to get it right the first time.bill