[Gvtec]

If you'll humor me for a second, lets pretend I have a JDM B18C "R" and would only run 110 octane fuel. Rebuild bore pending, I would like to slap in a set of the high compression Endyn units and with the aid of a thin head gasket, achieve a peek CR of ~14:1. Now to the meat of thing: how would one determine the static timing advance setting? With CR peeking this high, I'm getting the feeling a J&S should be added to this pretend scenario buildup.

Do you believe hitting +210 on stock cams is out of the question, or do I need to layoff the Wild Turkey before bed time?

*EDIT* -Does anyone have the link to one of the Honda sites that allows one to calculate static CR, based off a drop down menu with Honda spec parts? I thought I had it book marked...

 

[SurferX]

C-speed has a good calculator.

http://www.c-speedracing.com/howto/compcalc/compcalc.html

I think you'll need to be looking at a custom ECU program with ignition timing maps for that setup. I'm not sure how changing the distributor timing affects your custom ECU timing maps though.

 

[MikeSarrGSR]

Oh my gawd man, at 14:1 and stock ITR cams and say a 84mm bore and spotmatched head you would be in the 220-240 range fully tuned on a bad day. upped displacement needs less cam to move air as you well know, the upped compression will lift the effective output. 14:1 seems a bit parasitic, I say you could see this power mentioned at 12:1-12.5:1 on 100 octane fuel.

 

[MichaelDelaney]

I don't think Gvtec was thinking about boring out the engine. 14:1 CR will be too much CR for stock cams I would think. You'll lose some juice up top in the upper rpms compared to a lower CR in the correct cylinder pressure range.

shorter cam durations tend to be more tolerant of timing advance but I certainly don't see the timing exceeding the middle of the stock range.

 

[jonny-vtir]

JDM land runs 100 octane fuel off the pump as their premium grade, just so you know. And I would probably go with a set of CTR piston (12.1/1) and a 2 layer headgasket for a 12.4/1 CR setup. Throw in a Hi-Tech exhaust system and cam gears + a V-AFC and you would be looking at 200+ BHP which is a slightly more realistic goal, and it wouldn't be as risky (just in case you decide to build this monster).

 

[Gerson_911]

so for Gvtec's hypothetical setup, what cams would be formitable if the R's would make him loose top emd power??

 

[MichaelDelaney]

cams that don't have the correct exhaust opening duration for the amount of air being stuffed in would take a nosedive up top in the upper rpms.

A wild cam spec with a later ABDC intake valve closing point works well in a low rod ratio B18 when you get to 14:1 CR. If the volumetric efficiency lags behind where the peak torque is made by more than 1000 rpm and you are losing power from a loss of cylinder pressure at 10-24 ATDC crank angle, then you have too big of a cam for your engine combination. We like to see efficiency and peak torque to happen at around the same rpm. Beginners really don't understand the relationship between cam spec, ideal cylinder pressure, and static/dynamic CR. They just go for the wilder cam spec blindly thinking it will make power guaranteed. It's another case of buying the "next part" without thinking ahead about the package and how things work together in a package not on their own.

You guys should do a search using the term "dynamic cr" in MD Tech Corner and locate the website which has the calculator I linked to. You plug in the numbers to find dynamic CR. You want the same amount of dynamic CR or a little higher than what you see with the stock cam and stock CR. Getting too much cylinder pressure for a the cam you have can stiffle power as easily as having too little cylinder pressure on a wild cam.

 

[MichaelDelaney]

here's a quick tuning tutorial:

if you got bit by the hypemeisters and bought a cam that is too big for your package, you can regain some lost cylinder pressure by advancing the intake cam gear severely. Remember, you lost your cylinder pressure due to reversion up the intake port with a wild cam with a later ABDC intake valve closing point...dynamic CR. So to get some of that back, you close the intake valve earlier as the piston is coming up and generate the cylinder pressure you need. Sometimes though, you just don't have enough clearance on the intake cam advance to get back what you lost...then you need to upgrade the pistons for a higher static CR.

That leads you to another compromise. You gain back the lost cylinder pressure from reversion up the intake port with pistons by adding a bigger dome. This reduces the combustion chamber volume though and you reduce what we call capture volume from the intake stroke (how much volume or displacement in the chamber there is to fill). Elementary thinking says more air in = more power. So when you reduce the amount of air you allow in from a bigger dome, you may reduce the power potential. So engine building is an endless compromise. Stuff more air/fuel in with cams, lose cylinder pressure from an intake valve that is open too long on the power stroke, get it back by going to a big dome, lose capture volume. Thank goodness the amount of capture volume lost is negligible in terms of the gains you get from a better more complete burn...flow quality issues.


BTW, for those of you rebuilding your engines, get the machine shop to measure your piston to valve clearances and how much play you have with the cam gears ahead of time. They can precisely tell you how far you can advance the cam gear safely with a safe margin of clearance. You can then advance the cam gear at a dyno tuning session with confidence of your margin before you approach piston slap...no guessing.

complex but I hope that helps you in the way you go about thinking when you shop for cams.

 

[MichaelDelaney]

Now the headporting at the valve angles and bowl blending can really limit how much reversion you get up the intake port...so that is another building trick to help you choose a wild cam with confidence for the package...

 

[MichaelDelaney]

sometimes you have to cut your losses. If the cost of getting a wild cam leads you to having to redo the bottom end, you should sell the cams and get a less wilder one that will work with the static CR you have....
Unless you have a bottom end that is on it's last legs and you have a lousy compression test number and need to upgrade the bottom end anyways....

Similarly, if you got 14:1 CR and bought a cam that was too small for the new pistons..time to sell the cams and get a wilder spec with a longer duration or try a tighter valve lash first to see if she'll like more cam.

 

[Gvtec]

Thanks for the info, MD.

Becasue this is a budget minded project, I believe a stock engine with a thin headgasket and fresh rings are going to be the limits, until the rest of the car is brought up to speed. Maybe one day it'll have those RWs & cams in her...