Too much of a good thing...need more

RPMClassic · 09-12-2003, 08:01 AM

Looking to extract more power from my 2.5 shortstroke motor. Thinking about additional head flow work, more radical cams then the factory s that are in there and ditching the factory heat exchangers with sport exhaust for a dedicated race exhaust.

Seeing that its alot easier to spend some one elses money then your own...fire away. What should I do?

Thanks.

Tim Walsh · Tim Walsh's Garage

Well first of all is it a flat out race motor? and what are full specs now? 11.0:1 CR dual plugged? Are you running webbers?

RPMClassic · 09-12-2003, 08:14 AM

Webers, twin plug, CR 9:8:1.

Tim Walsh · Tim Walsh's Garage

hmm.. are you running race gas? if so I think you could go up a fair amount on the CR and squeeze a few more HP out of it that way.

Hotter cams would work for a few more ponies if you don't need the low end torque. I think there's a hp/rpm analysis in the archives. Search for it and it will show how steep 906 cams are and even how bad S cams are when it comes to broad HP.

what size ports are you running right now? I don't think you'll need much over 36(stock S) but with hotter cams it might help.

btw I'm just an amatuer and all my advice/stupidity should be taken with a grain of salt.

RoninLB · 09-12-2003, 08:24 AM

Hi RPM.. glad to see you around here again.

I bet TimT would have something to say on this one.......Ron

RPMClassic · 09-12-2003, 08:38 AM

I run pump and cam2 and the car loves it. I don't think going over 10.5 CR is something I want to do - just for reliability. As I already bought new Mahle P&C's( ie threw myself in front of the bus $$), I am not eager to get rid of them. My current thinking is that careful headwork,different cams and headers/exhaust will be a good stop gap for me. The previous new motor work won't go to waste and I can still enjoy it for another season or two - before building a 2.9

In the meantime Im going to buy waynes books today and get reading! Keep the ideas coming. Car will be under the knife in 2 weeks. Thanks. Jim

Tim Walsh · Tim Walsh's Garage

AHH understand about the CR. I don't think half a point is going to get you much since CR has diminishing returns. It may only be good for about 5 hp. You can get that compression from shaving the heads so you wouldn't have to touch your P&C'. Just be careful about the port sizes with your heads. If you go TOO large you'll lose gas velocity and therefore HP. John has a great spreadsheet out there that helps with port sizes and such. For a racecar hotter cams will probably go a long way but watch out for valve clearance.

Wayne's book is a great tool and there's lots of ideas for extra HP.

RPMClassic · 09-12-2003, 08:51 AM

I am not sure what the vodoo is all about with shaving heads.....but on the suggestion of a few people I have asked some machine shops about this procedure - and every shop discouraged it and/or flat out refused to do it. Hopefully waynes book will shed some light onto this.

It seems that most builders or car owners don't bother with the 2.5's because of the cost compared to swapping in 3.0/3.2/3.6 etc - but it is a fun little motor.

Tim Walsh · Tim Walsh's Garage

Ya shaving the heads isn't the ideal solution but it's a cheap way to gain a little HP at the expense of having to watch for cam alignment and several other things. Hopefully tyson will chime in here as I think he's done this with "Scruffy". I guess I wasn't that clear, for 5HP I'd skip the head shaving. For my 2.4T engine I was planning on 2.2T pistons and shaved heads. I brought this up to my machine shop (speedwerks) and he said that it wouldn't require much shaving to go from the 8.5 to 9.5 which on the CR scale translates to more HP than going from 9.8 to 10.8. He didn't see any problems with it but if you go too far with it you run into loads of problems from cam alignment to chain length, etc.

For the CR you're taking about it's not worth it. I think you'll get the most HP out of new cams but I'm guessing you're running a rally car? The factory normally used S cams in their rally cars so I don't know how much top end vs. torque would be appropriate in your case. It will definitly net you 10-20 hp I think. (factory S at 170 and 906 at 210 with a 2L )

RPMClassic · 09-12-2003, 09:08 AM

Most of the shops with experience building these motors seem to think that a strong 240 HP is very attainable with a properly done 2.5. As you noted, not at a full loss of low RPM driving though. Careful headwork and cam selection seems to be the ticket - not huge sewer pipe ports all around...just big where it needs to be.

The one shop that does incrdible headwork in my area has a 6 month wait...so I am going to have to use fed-ex to get these babies off to someone that knows what they are doing.

Tim Walsh · Tim Walsh's Garage

I've had good luck with speedwerks where david brown does very good work just don't let him go overboard. He's a factory trainted machinist and I belive he cut his teeth on 934's and 935's. It's the end of the racing season so he's a little slower than normal. I'm not sure where you're at but he's near highpoint, NC, give him a call and see what he thinks.