2.7RS vs. 2.7CIS....and 2.5SS rpms

[berettafan]

i know this must be a dumb question but where is the hp difference (60+) in the 2.7RS vs. the CIS middie motor? same crank, same heads but for 1mm port diff. on intake IIRC and the RS even has low compression. RS was S cam was it not?

part 2 of my question is why was the 2.5 long stroke motor problematic (per Frere) but the 2.7RS was not? The crank was mentioned as the issue in Frere's book yet both used the same.

Lastly, would a 2.5SS need to be rev'd to 8k to make power?

Trying to do some VERY preliminary math on a 2.5SS vs. a 2.7RS spec (both would be carb'd). Starting point is a 2.4 (not a 7R however).

THanks,

Erik

[Eagledriver]

The reason the Carrera RS motor makes so much more power is the cams and intake tract. The CIS has mild cams and a restrictive intake tract. All the power advantage is above 5000 RPM.

The 70.4mm crank (2.7) is fine up to about 7500 RPM. You can get good power out of your 2.7 and have a reliable engine (relatively) if you keep to this limit. Race motors of that vintage often went to 8000RPM and the motors wouldn't last doing that. One of the main problems is that the flywheel comes off when subjected to high RPM consistantly.

-Andy

[Steve@Rennsport]

Originally Posted by berettafan

part 2 of my question is why was the 2.5 long stroke motor problematic (per Frere) but the 2.7RS was not? The crank was mentioned as the issue in Frere's book yet both used the same.

Lastly, would a 2.5SS need to be rev'd to 8k to make power?

Trying to do some VERY preliminary math on a 2.5SS vs. a 2.7RS spec (both would be carb'd). Starting point is a 2.4 (not a 7R however).

THanks,

Erik

Erik,

The 70.4mm crank has a second order harmonic right at 8K that shakes off the 6-bolt flywheels. Keep RPM's below that and its plenty strong.

Now,....these engines need some oiling modifications to be reliable above 7500 so that should be part of anyone's build when high RPM is anticipated.

[berettafan]

Thanks Steve, that explains it perfectly.

[Plavan]

The 2.5L SS makes plenty of power below 8K rpm. You can do a search for "Plavan Dyno" and probably pull the results up.

[BURN-BROS]

Originally Posted by berettafan

i know this must be a dumb question but where is the hp difference (60+) in the 2.7RS vs. the CIS middie motor? same crank, same heads but for 1mm port diff. on intake IIRC and the RS even has low compression. RS was S cam was it not?

part 2 of my question is why was the 2.5 long stroke motor problematic (per Frere) but the 2.7RS was not? The crank was mentioned as the issue in Frere's book yet both used the same.

Lastly, would a 2.5SS need to be rev'd to 8k to make power?

Trying to do some VERY preliminary math on a 2.5SS vs. a 2.7RS spec (both would be carb'd). Starting point is a 2.4 (not a 7R however).

THanks,

Erik

Hi Eric,

The 3 engine you are comparing are vastly different than one another in regards to tuning.

If you compared a 2.5SS and the 2.7RS with the same cams, comp, etc, you will get a better answer to what I believe you are searching for.

Search for members that have 2.2S motors that have bumped displacement up to 2.5 and get feedback from them.

Cornpanzer wrote a recent article "short stroke shootout" in the latest Excellence and I believe that article is online on their homepage. It may give you a better understanding.

[berettafan]

Aaron that article was interesting and added fuel for the inner debate i'm having.