2.7L Race Motor -> 2.5L Race Motor- need help with some info
 

2.7L Race Motor -> 2.5L Race Motor- need help with some info
I have done a search on 2.5L race motors and found some interesting information. Here is my problem- I have a very nice 2.7L race motor with all the upgrades (racing valve springs, cams, J&E's 9.5:1). My racing group VARA will only allow 2.5L race motors for vintage racing. I also have a 2.2l T motor in my garage I was going to rebuild into my new 2.5 race motor with a 2.7 crank and 10.5:1 J&E's 85mm pistons.
I need some advise on what to do?

I now understand I can find a 66mm crank (counterwieghted) and install that with 2.0L rods into my current 2.7L race motor to make it a 2.5L (race legal). Anyone know what the compression would be?
OR:
Continue with my current plan of rebuilding the 2.2L T motor to a nice race 2.5 with a 2.7 crank/rods and 85mm J&E pistons. What valve sizes would you recommend with this motor?

If I convert my current 2.7 race motor to a 2.5, the cost will be very reasonable because all the machining/upgrades have been done to the case etc. I then could use the 2.7 crank/rods on my 2.2 and have a spare 2.5l motor.

HMMMM.... this board has me thinking about two different 2.5L motors now... What would be the performance difference between the two motors? Anyone have experience with what I am trying to do? Any help would be great because I am truly stuck on what to do.
Thanks in advance-
Chad

From what I've read, the 2.5L short stroke seems to be more desireable for racing due to higher RPM limits. If I'm correct (if not, others please do correct me), Porsche used a "long-stroke" 2.5L race motor for a very short time due to reliability. Plus if you have a really nice 2.7L, why change it? I say two separate motors are better than one.

Short stroke 2.5 or a "Stroker" 2.5... Such a tough decision.. :)

HP and Torque-wise, I doubt you will find much difference between the two assuming that they have the same porting, valve sizes and cams.

The stock "street" 70.4 mm crankshaft will have vibration issues at high RPM's (7500 - 8500 RPM). Starkey describes the issue in his R-RS-RSR book. I don't have it near me, but I believe that it manifests itself as cracked or loose flywheel bolts. Porsche used specially modified cranks that were also carried over to the 2.8RSR engines. I believe that Warren (Early S Man) has described the specifics of the change in the past, so a search of Pelican's history will most likely turn it up.

Good info John. We usally do not go over 7500RPM on the race motors. We like like them to last a few seasons. So If I were to go with the 70.4mm crank and keep it below 7500 RPM things should be ok?

7500 RPM is the rev limit for stock 2.4S's -- right? Given that, I don't see why it should be an issue. I'd have to check because I seem to remember reading somewhere (Excellence?) that one of the teams (Brumos?) came up with an alternative solution. I think it was in the article on the Brumos 911ST from a couple of years ago.

Plavan,...John:

All things being equal, the long-stroke 2.5 has greater "area under the curve" and will pull harder out of corners. Its a significant difference. Peak power is about the same.

The 70.4mm crank tries to loosen its flywheel bolts at precisely at 8000 RPM. One can secure this with lighter flywheels (changing the vibratory signature of the whole mass) and torquing the bolts to 150-160 lb-ft. We also carefully balance the motors and hand-fit the crank to the flywheel in use.

If you stay below 8000 RPM, this combination is quite durable and long lived. Of course, its assumed that all case mods are done and the crank is crossdrilled. The 2.4S is mechanically safe beyond 7500; its simply no benefit to do so as it well beyond its power peak.

Crossdrilled crank.... Can you explain. I never heard of this.

Plavan:

This is one of several oiling system modifications needed for high-RPM 911 engines. This involves drilling some holes in some of the main bearing journals for increased oil volume and pressure at the center of the crankshaft and enlarging the main bearing galley passages to suit. The main bearing shell is also opened up.

911 crankshafts are end-oiled and the oil get centrifuged out at high (7400+) RPM. To make these motors live under such conditions, cross-drilling and other mods have been SOP for 25+ years.... :)

Steve-
I should of clarified. I have heard of crossdrilling cranks (Chevy, Ford) just not on 911 cranks. In all the books I have read I did not realize that Porsche did this to their race motors, but it makes perfect sence. Now you have given me even more great info to digest. Seeing that I will not be going over 7500RPM I may not apply it to this motor.

LOL,...Porsche did not do these sorts of things from the Factory,.....race engine builders like ourselves did way back in the SCCA, IMSA, & Trans-AM days,.....:)

(Now, I've REALLY dated myself)

Geez Steve, your old. :D

You're telling ME???? ROFLMAO,...:)

This is a very timely thread...

I have a 2.7 in my 914/6 tarmac rally car, Otto (Otto's Venice Beach) did most of the machine work for me using a 2L sand-cast aluminum case, 9.5 JEs, S cams, Webers yada yada... (when I started this, Tom Gould was still at Otto's and I only finished it this year :( ). I cannot praise highly enough the help I've had from Otto's and Pelican Parts (especially Tom) in getting everything together for this project over the last few years.

Anyway, the engine is great - pulls like a schoolboy from nothing and makes 180bhp on the rolling road (~230 at the flywheel?) - I love it.

The problem is that some recent regulation changes mean I must run with magnesium cases in a couple of the major events (some of you might be familiar with obrut's (Ryan's) trials and tribulations on this board re: the same issue. Which is a tad annoying :mad:

Soooo... rather than simply swap my existing cases, I've decided to build a new, more powerful 2.5L engine (looking for 250hp) ready for the beginning of 2005 (I'll run with what I have in selected events next year).

BA goes into quite a lot of detail on 2.5L engines in the June 2003 Excellence (pp 40 to 50) and I quote "... we found the long-stroke engine produced a little more horsepower than the short-stroke version, but that the short stroker had more area under the curve and was a friendlier engine". Given Steve indicates the long stroker to be more friendly, I'm now a wee bit confused:confused: Any enlightenment would be welcome...

Basically, I'm set on building a short stroke 2.5L engine that will run reliably up to 8000rpm and need some 89mm P&Cs, 10.5:1. I have heard tell that Jerry Woods can make almost any combination. Does anyone have any experience of this or can suggest other avenues of supply?

Cheers,

Mark

Mark (bumble)-
I just ripped apart and excellent 2.2 mag case (no exhaust stud breaks,pulled head studs, or any streched holes). I also found a nice early alum long block for $400 (and it was in a barn...no kidding)!!!!!
I'm still trying to decide what to do also. I did read that Excellence article over and over back in June. What I may do is call Don at EBS racing and see if he can have some custom 89mm 10.5:1 J&E's made up for me. I am just hoping that the 66mm crank in the 2.0L long block is STD/STD. I will start tearing that motor down shortly.
I have a set of 85mm J&E's that I bought used from this board. They are being shipped to me as we speak. Still unclear if they are 9.5:1 or 10.5:1 but I will send them in to Don at EBS to freshen up and check (hoping 10.5:1...keeping fingers crossed). I'm thinking that I will use the 85mm in the 2.2 mag case with 2.7 crank/rods still and keep the alum case for my "super motor" for next year..... still trying to decide though.
Kinda silly to have 3 race motors in my garage..... but that is good insurance.

Chad whether those pistons are 10 or 9 to 1 you can do a few things to raise the commpression ratio. Thinner base gaskets, Take material off the cyl heads, machine material off the base of the cylinder. When you start to assemble I always do my own commpression ratio measurements as per the performance handbook. You need some special tools buts its worth it

I borrowed the CC'ing vile though. Does anyone know where to buy a vile w stand for cc'ing purposes??

The last time that I checked, they're fairly expensive from chemistry supply stores. But here's a cheap alternative I discovered by having a couple of kids. Go to your local pharmacy and ask for a couple of syrenges for giving kids medicine. Chance are they'll give you (as in free) a couple. If not they can be purchased for a couple of bucks and they are calibrated in ml (aka: cc's). A syrenge holds 10 CC's. A stock head is about 68-70 cc's and so takes almost 7 syrenges worth of fluid to fill.

For fluid, I just used rubbing alcohol with food coloring in it to make it easier to see.

Cheap. Easy. Accurate.