Power increase with higher compression

[afret]

If compression is increased from 11:1 to 12:1 about how much power gain could be expected for a 500 horsepower or so 428?
Would a camshaft change be indicated for the one point compression increase if the cam gave close to optimal 1/4 mile performance with the lower compression?  Thanks!

[jayb]

If the cam is really optimized for the lower compression ratio, IMO it will have too little duration for the higher compression ratio.  Have you done a DCR calculation on your existing combo?  If not I can do one for you; I have a spreadsheet set up for doing them.  I need bore, stroke, rod length, deck height of the piston, dome or dish volume of the piston, head gasket thickness, chamber volume, and the ADVERTISED duration of the cam, plus the intake centerline that the cam is degreed at. 

If you bump the compression one point, and increase the cam duration to match, I think you will pick up 20-25 horsepower on a 500 HP engine.

[e philpott]

Jay ... how bout the other way ?? ..... take a combo that is optimised for 12.0 to 1 and then drop compression 1 point of compression to 11.0 to 1 ... how much do you think it would lose on a 550 ish hp combo ??

[afret]

Did the DCR calculations and it would increase from 8.1 to 8.9 with the same cam.

[jayb]

Did the DCR calculations and it would increase from 8.1 to 8.9 with the same cam.

Are you running it on the street, Earl?  8.9 might be a little much for pump gas.  However, there is definitely power there without a cam change, because if you can run good gas at the track, a bump of that much in DCR will definitely yield power without a cam change.  Maybe 20 HP?  It's a guess, really.  You'll probably still get more if you change the cam too.

If you run it on the street like that, you might be able to run pump gas if you keep your foot out of it.  I've always kept to 8.5:1 for DCR when the engine is going to run on pump gas, and never had any troubles, but with my new high riser I'm going up to 9.3:1 and am hoping it will run OK around town if I'm easy on the throttle.  We will see.  Another alternative for you might be to go up just a little on the cam, so that you end up with 8.5:1 DCR with the new pistons.  That might be the safest way to go.

[jayb]

Jay ... how bout the other way ?? ..... take a combo that is optimised for 12.0 to 1 and then drop compression 1 point of compression to 11.0 to 1 ... how much do you think it would lose on a 550 ish hp combo ??

I'm thinking about the same 25 HP if you drop the cam duration to match, but if you don't, you might lose more.  These variations all sound like good fodder for some future dyno tests...

[e philpott]

that would be a good test session .... I have always questioned the DCR's after running jr dragsters for the last 10 , the low comprssion flat head engines just love the big cams despite 6.5 compression and I had a simular expeirence on my own Fairlane , my 416 with 10 to 1 had a Comp drag race FB294B flat tappet and I had everything Comp recomended , 3500 stall , 3500 plus pounds vehicle ect and was not happy at all with performance , I then purchased a Comp FB304B cam wich Comp recomend's 12.5 to 1 , installed a 5000 stall torque converter and low gear , I have everything but the 12.5 compression and it rips , the big cam is way faster than the smaller cam regardless of the lower than recomended compression

[afret]

Thanks for the info, Jay.  Not a street car so race gas only.  Would be interesting to run it with the same cam at the track after the compression increase and then swap in a bigger cam at the track and see what happens.

[mammyjammer]

This is a very educational site for me!!! I had never heard  the term DCR until this thread.

Does this page:

http://cochise.uia.net/pkelley2/DynamicCR.html

offer accurate information detailing exactly what DCR is?

[Barry_R]

DCR can be interesting to use as a directional tool, but I personally feel that folks get way too fixated on it. 

DCR calcs are focused on generating a "user readable number".  But they leave a huge amount of highly influential characteristcs unaccounted for - chamber design and shape, port flow efficiency and tuning at various RPM levels (based on flow, runner length and cross section), rod length and resultant in cylinder pressure rate variances (although I'm not all that big on the rod length thing either...), not to mention vehicle characteristics like weight and gearing.  By DCR the fuel tolerance for a flathead or a 392 Hemi is the same as a Yates headed small block or a mod motor - yet we all know that is not valid.

Use the DCR as a good cross check on rational behavior.  I generally cam for displacement and usage RPM.  DCR just keeps you from doing something stupid by accident - well it slow me down - but trying "stupid" is part of my game at times...at least I know when I'm doing it....

[jayb]

This is a very educational site for me!!! I had never heard  the term DCR until this thread.

Does this page:

http://cochise.uia.net/pkelley2/DynamicCR.html

offer accurate information detailing exactly what DCR is?

Yes, that site provides an accurate way to calculate DCR, at least as I calculate it.  I agree with Barry that DCR is a directional tool and that other things have to be considered when deciding on your cam and compression ratio, but on the other hand I've had great luck with various engine combinations using a max of 8.25:1 DCR for iron head engines and 8.5:1 for aluminum head engines.  These include stock wedge chambers, Edelbrock CJ chambers, Blue Thunder chambers, and SOHC hemi chambers.  None of the engines built using these guidelines knocked when running on the street, although it must be said that all were put together with reasonably high stall speed converters and aggressive rear gearing.  Nevertheless, I think those DCR limits are a really good rule of thumb for building a strong performance engine that will also live on pump gas.