I don't think you should have a problem since you're using aluminum heads. Assuming 96 octane in Iceland is the same as it is in the US. I know Europe 98 octane in is about like 93 US octane. Which you can run 93 US octane with 10.5 CR & Aluminum heads with appropriate valve overlap.

 

Hi, i have i found out that you guys are the best source of information i have ever found, so one more question. Im putting to gather a chevy 383, flat top pistons, 6 inch rods, aluminum heads,2.02 in.valves and 1.60 ex. and intake runners 180cc. makes 10,5:1 compr. I was planning to use comp cam 280. 231@0.50 in and out 480 lift, i will use 1.6 rocker ratio so that makes the lift about 510 and 0.50duration 233�. So i started to think about detonation im going to run this engine with 96 octane, you think i might need more cam duration ? or am i just worrying to much ?
Thanks again guys for your time i really appreciate your help.
Regards. Kristofer.

Built my 383 based upon a zero deck block, Eagle internal balance crank and 5.7 rods, Speed Pro 12cc dish pistons, TF 23* 64cc heads, CC XE274 flat tappet hydraulic cam duration @ 0.050 - 230 intake, 236 exhaust with 0.490 lift with CC 1.52 rockers. Static CR calculates out to 10.30, static CR to about 8.2 dynamic CR. Timing is 34* all in by 2600 RPM, vacuum adds an additional 16* @9" using manifold vacuum. Engine produces 13" vacuum at idle.

I have been using 89 octane BP pump gas since I built it, about 2 years ago. This is in a 3300# C3 Corvette with a ST10. Car is mainly a street car/cruiser.

 

Should work out fine. Engineer your squish at 0.035" to 0.045".

 

The engine in this link was built with 10.5 on compression and the dynamic was 8.3 and no issues on pump gas and it has AFR's, I would think you should be fine with your set up.
Good luck Carl

 

I might be out to lunch here but I don't think changing the rocker arm ratio gives you any additional duration at .050 you said that you get an additional 2 degrees with a 1.6 ratio arm right?

Like I said I might be wrong here. Maybe someone else has input.

But other than that I think your set up is nice, I would go with something a little nastier but that all depends on application and preference. I built a similar motor using an XE268H (Duration @ 0.050": 224° / 230°. Max Lift w/ 1.5RR: .477" / .480") but after installing it and driving for awhile I wished I would have gone with the Xe274H (Duration @ 0.050": 230° / 236°. Max Lift w/ 1.5RR: .487" / .490")

or even the XE284H (Duration @ 0.050": 240° / 246°. Max Lift w/ 1.5RR: .507" / .510" ) which would have taken some other modifications like a different torque converter and gears. I still might do that.

 

I might be out to lunch here but I don't think changing the rocker arm ratio gives you any additional duration at .050 you said that you get an additional 2 degrees with a 1.6 ratio arm right?

Like I said I might be wrong here. Maybe someone else has input.

But other than that I think your set up is nice, I would go with something a little nastier but that all depends on application and preference. I built a similar motor using an XE268H (Duration @ 0.050": 224° / 230°. Max Lift w/ 1.5RR: .477" / .480") but after installing it and driving for awhile I wished I would have gone with the Xe274H (Duration @ 0.050": 230° / 236°. Max Lift w/ 1.5RR: .487" / .490")

or even the XE284H (Duration @ 0.050": 240° / 246°. Max Lift w/ 1.5RR: .507" / .510" ) which would have taken some other modifications like a different torque converter and gears. I still might do that.

changinng the rocker arm ratio actually does give you a slight amount more of duration- it all depends on the steepness of the ramp (acceleration rate) but a few degrees is the general rule of thumb.

 

Is there a formula for that?

 

um... not really some say that the difference between a 1.6 ratio and a 1.5 ratio's duration is about 7%... the same as the lift increase, but the lobe is not a flat slope, is curved- so making that kind of generalizaton doesn't quite work out... older cams tend to have more added because they have lazier ramps, the more agressive the ramp the less duration is added.

 

The more I think about I guess it would make sense, it would vary quite a bit though based on ramp, how much lift was gained with the different ratio, and how long it took that change to come back to zero.

That is why I like this board it gets me thinking about things like that.

 

If you kept the same valve lash for a 1.6 as you did with a 1.5 you do pick up a couple of degrees of duration. Look at it this way. We all know that by loosening or tightening lash you can change a cams duration. Working backwards .015 lash at the tip of the valve of 1.5 rocker is .010 clearance at the pushrod side of the rocker. .015 lash at the tip of the valve of a 1.6 rocker is .009 at the pushrod side. So inversely .009 clearance at the pushrod side of a 1.5 rocker is like setting your lash at .014. So effectively you've tightened your lash by .001, not much but still a couple of degrees.

 

you will pick up some duration at 0.050" since the cam gets to 0.050" quicker with 1.6's. However, the seat duration should change very little. And the seat duration, (intake valve closing point) is what you use to calculate dynamic compression.

Valve overlap doesn't effect cylinder pressure since the engine isn't even making compression on that part of the cycle. The cam spec that effects cylinder pressure is the intake valve closing point. No cylinder pressure is made until the intake valve closes, and a long duration cam tends to have a late intake closing point.

 

um... I hope you're not using adv. duration to figure dynamic compression... its actaully far more reliable to use the @.050" mark plus a slight fudge factor.

 

um.. yes, advertised duration is the one to use. A fudge factor doesn't account for cam intensity (ramp rate). Just blindly adding 15 degrees to the at 0.050" is silly.

for example, an old 327 cam with 222/222 degrees at 0.050, has an advertised duration between 290 and 300 (depending how you measure it). Now compare that to a compcam xtreme cam with around the same duration at 0.050" and the advertise duration with be in the 260's. So, you are saying that these cams will have the same dynamic compression. And I say, no they will not. A fudge factor is not the correct way to do the dynamic compression calculation. A fudge factor could be helpful only if you don't know the advertised duration.

 

the adv duration is basically meaningless... Its measured at different points from various manufacturers, it also is not what the valve sees. You also have to account for lash (or hydraulic preload). the calculations I use are never more than a few psi off, which I realize isn't exact but its a hell of a lot closer than all the calculations I've seen that just use adv dur. I'm not saying you have to use my ways, I'm just saying for everyone else it works better.

 

Advertised duration meaningless? I don't think so.

Why do you think fast ramps cams make more power?

Those weak old school 327 cams are down 50hp compared to modern fast ramp cams with the same duration at 0.050". The only difference is the advertised duration (ramp rate).

 

So guys, please correct me if im wrong, When running high compression motor and getting away with using low octane fuel its better to see longer advertised duration and less @ 0.50 ? or is it just matter of tighter lobe seperation ?
Thanks. Kristofer.

 

go with longer duration @ .050" ignore the adv duration.

I would also try to run a wider LSA to help maintain some streetability.

 

When trying to run high cr with a certain octane fuel, the one thing that matters is the intake closing point. Advertised duration, LSA, and cam install position all effects when the valve intake closes during the compression stroke.

If you want to make the engine a mild as possible with high compression and low octane fuel then a lazy ramp cam with long advertised duration and a relatively short at 0.050" duration would work well. Then use a wide lobe separation angle for added idle smoothness (less over lap) and this also pushes the intake valve closing point further for even less cylinder pressure.

However, a cam that this will not make nearly as much power as a modern cam. This is what the old GM 350hp 327 cam was set up like. Known as the 151, it was way low on power, compared to new cams of today. Mostly due to a long advertised duration and a relatively short at 0.050" plus the wide LSA (114 degrees) made the throttle response slower and reduced torque by a bunch.

But, don't worry about doing something like this. Your combo is ok for pump gas.

 

You can tell what the action on the opening ramps is by subtracting the 0.050" duration from the advertised duration. Back when the cam manufacturers first got together on all of them using the 0.050" figure, the difference was around 70 degrees, 35 on the opening ramp and 35 on the closing ramp on a symetrical grind for instance. Today's fast action grinds are somewhere around a 44 degree difference, 22 degrees on each side of the lobe.

When I was a kid, we very seldom experienced or heard of a cam going flat. In my opinion, this was because of the long ramps and reduced loading at the lobe/lifter interface. I've never used one of these short ramp designs and probably never will, preferring to opt for a cam with a little easier loading at the lifter.

Most of the Crane cams I use have a 56 degree difference (28 degrees on each side of the lobe) and these are plenty fast for my tastes.

 

yes, I agree 100% with that. The newest grinds (xtreme or voodoo) have ramps that are too fast. They can go flat quickly and they make noise.

I like to use the high energy or magnum series compcam. The 268H has 218 at 0.050", a difference of 50 degrees (25 on either side). However, i think compcam uses 0.007" lift for advertised duration and others manufactures use 0.005" or even less. So, not always a fair comparison in advertised. But still much more aggressive than the really old grinds.

 

I will agree that you can find the agressiveness of the opening ramp in such a manner.... or atleast get a good idea of it. I still hold that you cannot use the adv duration for dynamic compression ratios because it does not account for lash nor does it account for the fact that the air flows around the valve much differently at low lift numbers. If you run the calculation on your own engines I'm sure you'll see that adding 15* or using some other fudge factor is actaully a lot more relaible and accurate.

 

We use a lot of aggressive lobes in our circle track and the street and strip engines (nonsymmetrical lobes) so far many years of using these cams have not experianced any flat lobes, But we do use good cam cores and good lifters and good lube.

 

Yes, air flow around the valve is weak at low lifts, Except, in the case of the piston pushing air out. Pushing air out is much more effective than sucking air in.

Hold the intake valve open just a few thousands and see what happens to the cylinder pressure.

Advertised duration does account for lash. Advertised duration is measured at 0.004 to 0.007" valve lift, so there can be a fairly big difference in the measurement between manufactures.

Again, one of the main reasons a fast ramp cam makes more power is the intake valve closes much sooner which has a dramatic effect on power. The main reason after-market cams make more power than oem's.

 

here is a web site that says to use advertised duration. I have also seen a few that add a fudge factor, but i still say it doesn't account for different ramp rates. It would be nice to get a drc calculator that requires both advertised and at 0.050".


http://www.projectpontiac.com/ppsite/content/view/16/30/

 

sorry to say but no. the at 0.050" spec is measured at the valve using a standard ratio rocker arm. And therefore the valve will get to 0.050" sooner in the cam rotation with higher ratio rockers, and increase the duration at 0.050".