Cam Selection Vs Compression

[blandy 29]

Been looking through cam specs and through all the spec sheets on various cams none mention the desired compression ratio.

 

I completely understand the valve clearance etc but how do you decide what compression ratio is needed vs the cam choice as looking through threads ive seen a couple where compression ratio was to high or to low

 

i.e for say a cat cam 110 in a gti6 engine they give the info

 

camshaft data:

lash ramp : hydro hydro

duration @ 0.1mm : 282° 275°

duration @ 1.0mm : 246° 239°

valve lift : 11.50mm 11.50mm

cam lift :

lobe angle : 108° 108°

timing @ 1.0mm : 15° / 51° 48° / 11°

valve lift @ TDC : 2.95mm 2.45mm

[petert 586]

There's no "correct" static CR but there are desired dynamic CR for various applications. eg road, hot street, race etc. Typically an inlet cam with that much duration will need at least 11:1 and that's about as big as you can go with hydraulic grinds on an XU10J4(RS). They'd be better on 106/109 centrelines however. Download a dynamic CR calculator and have a play.

[blandy 29]

Cool thanks I just grabbed those figures as was the hottest cam they offered on hydraulic lifters although I no doubt guess solids will probably be the way to go with a track engine

[welshpug 1,657]

I've got a bigger hydraulic cam than that in mine, newman ph4.

 

seems to work with the compression I ended up with but no idea if its the ideal!

[petert 586]

Given the PH4 is only 2.75mm @ TDC compared to 2.95mm @ TDC, both on a 108 centreline, Id say the Catcam 110 is bigger. What is the duration @ 1mm of the PH4 inlet?

[welshpug 1,657]

no idea actually, that's not on their data sheet.

 

I used Newman as they were cheaper and not heard any issues with valve train wear from them.

 

I'm over 12-1 on 2188cc.

[petert 586]

It probably goes ok.

 

Also keep in mind that volumetric efficiency effects dynamic CR as well. Australian V8 Supercars make 650+hp from 5000cc, 10:1 and 7500rpm. You can never get a peak under the bonnet. They guard their inlet designs so secretly.

[welshpug 1,657]

they're on E85 too which must make a difference as well.

[petert 586]

With only 10:1 on a N/A engine, I doubt it has any benefit other than to improve the image of the Series.

[blandy 29]

So much more involved than building diesel tractor engines.

 

Interesting reading.

Would you generally pick a cam then spec the pistons etc to suit?

[blandy 29]

Maybe a better question would be what cam for 240-250 bhp Pretty much playing with ideas as don't want to buy any components until I can work out every detail.

 

Been looking on cat cams, piper and Kent etc and they appear to list more as street, rally, race etc there's a set of piper bp320 on eBay for cheap money but I'm guessing there far to wild.

[TAG 25 1 Cars]

I think you need to give more information on what it is exactly you a trying to achieve, what is your intended use for the engine? Does the engine need to stay at 2.0l, or is an increase in capacity an option?

[blandy 29]

It's a circuit engine, and I do already have a 88mm diesel crank that's intended for 2.1 so no restriction on capacity as such.

[petert 586]

87mm x 88mm and those cams would achieve your goal, at 7500ish. Even more if you dry sump. Aim for 11.7-12:1. Use the PAC single springs but make sure you shim them correctly.

Edited May 6, 2017 by petert

[blandy 29]

Sorry do you mean the solid piper cams or the hyd cat cams?

 

Ive now got a project engine to start playing with so keen to nail a decision on what cams to go for, Being as the cars only purpose is trackdays would solid lifters be the appropriate thing to do? Im leaning towards hydraulic lifter option for the simplicity/price but on the other hand also only fancy spending out once so if solids are going to be more beneficial in the long run then again it makes sense to go down that route.

 

Also im guessing on the rs engine standard valves general suffice at these power levels?

[petert 586]

Setting up solid lifters costs money, in addition to their capital outlay. Probably better using hydraulics, spending the money on a dry sump system and keeping the revs under 7500.

[blandy 29]

Makes sense to me thank you. cat cams 110 it is then

[dcc 855]

Peter, if you had say 13:1 cr, a long stroke and solid buckets, what spec cam would you consider and why?

 

Assume lift and duration arent limited by the piston spec.

 

Would be more for mid range than any sort of peak figures, just interested to see your opinion.

[TAG 25 1 Cars]

Peter, if you had say 13:1 cr, a long stroke and solid buckets, what spec cam would you consider and why?

 

Assume lift and duration arent limited by the piston spec.

 

Would be more for mid range than any sort of peak figures, just interested to see your opinion.

13:1, forged pistons, you'd be duty-bound to go full retard on the cams, surely?

[petert 586]

I wouldn't do 13:1 without E85 (or similar octane). You'd need to have an inlet closing point at least 80 deg minimum ABDC to control the dynamic CR, otherwise you'll be controlling detonation with a very tame ignition advance curve.