37 replies to this topic

[I'm a Red Motor fiend]

I get around 9.3 with 2.2cc dish pistons. 8.8 with 6cc dish.

[I'm a Red Motor fiend]

You wont get a head gasket 0.025 thick. A standard gasket will be around 1.2mm or 0.047 thou compressed. Thats what I used to calculate along with a gasket bore diameter of 96mm also did the more thorough calculation using piston crown diameter and piston ring height.

[I'm a Red Motor fiend]

Forgot to add before that with the IVC angle on that cam, your dynamic comp will only be like 6.7 which is not good.

[LC-GTR-1969]

Fair enough. What timings would be more desirable? It's hard, as a lot of manufacturers don't advertise timing; only duration, lift and LSA.

So I could go better without 'bigger'?

 

See how your cam spec says 78 degree IVC (inlet valve closing)- this is a reasonably late closing, however your cam is only 225@50 thou, which is not very long duration at 50 thou... which shows that the cam has a slow ramp profile... These days they are making cams which have much quicker ramps, which will build more torque. You can tell a quicker ramp cam by apparent long duration at 50 thou, like 238@50 or more, but the advertised valve timing events could be the same or very similar to your existing cam. This means that the newer style cam has more area under the curve. Another example is that if you wanted a smallish cam, you can get cams with 225@50 thou duration, but the advertised valve timing events would be more conservative than 78IVC.

 

A cam with an IVC of 78 will require quite good compression, as when the piston swings back on its return towards TDC, the inlet is actually still open, and so some cylinder pressure/ compression is lost back through the inlet valve, hence the loss of torque. In saying that, if you already have this cam, and if the engine is in the car etc, I would just run it, why not... But if the engine was out of the car, I would definitely be talking to a grinder about optimising the cam to your comp ratio as well as head flow.

Edited by LC-GTR-1969, 07 September 2016 - 06:39 AM.

[LC-GTR-1969]

Gents,

Cheers for the feedback on both CR and cams. Three more queries to close out if I may;

1) will the difference in changes to torque and low end be dramatic? That is, would it warrant pulling out a new engine and changing fur someone who has very limited spare time?

2) are there any advantages at all to these older style slow ramp cams or are they just throwing away power?

3) where does lift sit in all of this? Should you priorities s fast acting long duration cam over lift?

Bit annoyed about the whole thing now, as I always thought it was too sedate at low range but put it down to the 110 LSA and good manners for commuting etc.

Seems like you can have your cake and eat it too now?

 

If the engine is in the car, and if you are short of time (like myself), no, I would not change it. I would wait until you need to yank the motor for another reason and then change the cam should you desire.

 

The cam you have is a mild cam, so going to another mild cam, even though better designed, it would still not be a rip snorter so to speak. The main advantage would be to low down torque. 

 

If the lack of low down torque bothers you, there are 2 things that you could do to help it...

 

1) raise the compression- raise it enough to get the dynamic comp around 8.7:1 or thereabouts. There are online calculators to help with this.

 

2) reduce the total advance in the dizzy... this can help crutch a motor which has mis-matched compression. If you are running say 12 initial and 32 total now, you could try 16 initial or 18 initial and 32 total. This can easily be done by just yanking the dizzy and not needing to pull the motor. The extra timing at low rpm helps with low end response when a cam has a late IVC. 

 

Im in a similar predicament in that my cam is old school and not really ideal, but I will only change it once I yank the motor for a secondary reason

 

The main advantage to the slow ramp cams is that they are argued to be easier on cam lobes and valve trains. Modern designs, metallurgy as well as oil technology has helped allow the ramps to be more aggressive yet still maintain reliability and durability, however. The more aggressive cam lobes can also hammer valve seats, but with modern hardened seats you dont hear many issues so basically, yeah, these days you can kinda 'have your cake and eat it too'.

Edited by LC-GTR-1969, 07 September 2016 - 08:10 PM.

[LC-GTR-1969]

What could you realistically look for in CR within the limits of 98 RON?

When factoring dynamic compression, about 8.7:1 would be conservative and safe. 9:1 is probably ok but much more and we are pushing it a bit. With an alluminium head you could go about .5 higher than cast head.

[warrenm]

10.5:1 with the right cam & ignition timing is ok on 98 fuel.