Specs for a street 2.6 SS

[rswannabe]

I've been mulling over a planned engine build and this is where I am at this time. I would be interested in people's comments, input. I have a very healthy 2.2S in my car now and what to keep/enhance the character of that motor (ie quick revving, instant throttle response, fantastic sound). Here is what I am thinking:

7R case with all the mods (shuffle pinned, oiled mods, case savers, ext...)
66mm T (non-counterbalanced) crank cross drilled for better oiling
2.0S rods
92mm Mahle RSR pistons (10.0:1 - 10.5:1)
Nickie jugs (no compression ring groove in the top for better sealing)
Twin plug MFI heads with 38I 38E ports
Stock valvtrain
S cams
Mag MFI stacks and TB taken out to 38mm
2.2S MFI pump modded by Gus to flow for 2.6 liters
Burns Bros twin plug dizzy

This would be a street motor used for occasional track days, autox, and hill climbs. It would be limited to 7,300 - 7,500 RPM. I have considered a hotter cam, but to make use of it your run more RPMs and the more RPMs you run, the shorter the life of the engine. I thought by keeping the s cams, but upping the port size I should keep a similar rev happy character to the 2.2S while still being very streetable.

Thoughts?

[kenikh]

Hey Brooke, an engine like that needs a Mod-S (DC40, GE40 same thing), instead of stock S. There's no downside at all. Hell, I'd run a GE60, but that's just me. A 2.6 on any of these cams is going to be way more torquey than you'd expect. I was just talking to a guy that runs a 2.6 w/ 10.75:1 CR on twin plugs and pump gas with no issues, so keep that in mind. Everything else sounds perfect. One more thing, everyone I talk to says go with 2.2 rods when you go beyond 2.0 displacement. Those larger pistons get heavy.

The upsell: I also have a C/W crank and 2.2 rods, all in beautiful shape for plug and play fun. Then you can rev, rev, rev.

[tadd]

I LIKE the T crank idea (since I am doing the same ). I have done all the reading I can find on this and if ballanced well are good to 7.5-7.7 no problem. There has been a lot of argument if they are cast or forged, but I have never seen anyone complain about one breaking (although there are several posts about shreaded counterweigheted cranks). As a bonus, the T cranks are darn cheap since they are unloved. Its also more mass of the back end of the car. A nice low mass clutch assembly and that sucker will rev like lightening. I personally am thinking about a tilton carbon clutch ($$$) so I can get those stoplight holeshots against those pesky camerys .

tadd

[rswannabe]

Lightning quick revs is one of the goals for this motor, hence the non-counterweighted crank and 2.0 rods. I have heard opinions both ways on the 2.2 vs. 2.0 rods issue. I know plenty of races who swear by the 2.0's, especially since you can get APR rod bolts for them. I think I would do the 2.0 rods for their lighter weight.

I would plug this motor into a 901 tranny with a stock S 215mm clutch and pressure plate, which is plenty light I think. I'm just not sure it could handle the 220 - 240hp.

As for hotter cams Kenik, that's what I keep going back and forth on. I need to drive Keith's car to see what a 2.6 with GE60's feels like. Hopefully I'll get a chance on the run down to Sonoma.

Still, and I am just speculating here, by opening up the ports a little to compensate for the larger displacement, shouldn't an S (or Mod S) cam in a 2.6 give a very similar "peaky" feel that my 2.2S has?

[kenikh]

Mod-S makes a 2.0S supposedly feel similar to a 2.4S in terms of torque, due to faster cam ramps resulting in less overlap, with essentially the same lift. I think the Mod-S cam is the perfect cam for this motor. Camgrinder sells them for just a little over $700 w/ MFI drive. After I figure out what the cams are in my bench motor (if they aren't Mod-S), I am buying a set from him. I'd say the GE60 or RSR cams would be more successful in giving the peaky feel.

[tadd]

RSR... RSR... RSR...

rswannabe:
I feel for you man... I have bounced back and forth on cams. Me, I am going RSR or 906. I prob won't decide till they are in the mail .

Although I am totally up on ramping the valve open faster to get more area under the curve all the while netting less overlap, I have grown into a thought that some overlap can be a good thing when using hi-comp, ~11:1, since the porsche heads have almost no swirl detonation is going to be more of an issue at low rpms. Once you start spinning the motor, even in a 'clean' chamber, turbulance will occur.

Becides, think of how long your valves and guides will last with nice gentle ramps, soft factory springs, modern guide materials and a shiny set of Titanium retainers: even at 7.5-8k.

I am in agreement with you 2L rods with ARP should be great. What I really want are Porsche 906 Ti rods though.

tadd

[kenikh]

Originally posted by tadd
RSR... RSR... RSR...

I am in agreement with you 2L rods with ARP should be great. What I really want are Porsche 906 Ti rods though.

tadd

Carillo is more realistic, more modern and readily available in Ti. That said, I've read that Ti rods round ends go oblong in time, so they don't last like steel rods do.

[tadd]

but, but, but.... its titanium... !

tadd

[Henry Schmidt]

When we run a 2.5/2.6 at RPMs over 7500 the use of a modified 2.2 rods with ARP bolts seems to be the best option. I have never seen a 2.2 rod break with the failure of some other part first.
They are strong, cheap and available.
As for the cams, the magic of short stroke engines is the way they sound at 8000+ RPM. I am a fan of 906 cams and DC 60 cams on the street when you are talking about engines over 2.5.
In a 2.0 they are a little aggressive but as the engine gets larger of cam seems to get milder. We have a 2.8 MFI engine in a 68 911 street car with 906 cams and it's fun to drive.

Does it run like a 3.6 Motronic engine ? NO, but maybe that's a good thing.

[rswannabe]

Henry, I did not think ARP bolts were available for the 2.2 rods. Is this a recent development?

[Henry Schmidt]

Originally posted by Henry Schmidt
When we run a 2.5/2.6 at RPMs over 7500 the use of a modified 2.2 rods with ARP bolts seems to be the best option.

ARP does not make a rod bolt that fits the 2.2 rods. We modify the rods to fit ARP 10mm rods bolts designed to fit 2.4-3.0 rods.
This and other modifications are just some of the reasons that experience counts when building high performance engines. Anyone can sell you Carillo rods $$$$.
Although I haven't read every Porsche rebuild book I would bet this option is not mentioned in any of them.

[rswannabe]

Thanks for the feedback Henry. You are correct in that I have never heard of or read about modding 2.2 rods to accept ARP rod bolts. Its another one of those tricks of the trade that makes me second guess my desire to build the 2.6 myself.

In regards to the specs, you obviously are in favor of running bigger cams, but would you ever consider running a non-counterbalanced crank in such a 2.6?

Also, I installed your MFI fuel enrichment control. Great build quality and easy installation. Thanks. Manual adjustment of the enrichment circuit on the MFI punp has always allowed perfect cold starts on my car. However I used to have to go to the back of the car to adjust the thumb screw. Now I can do it all from the drivers seat. Ah, the joys of these new fangled modern conveniences.

Brooke

[Henry Schmidt]

A non counter weighted crank is fine for all but the most aggressive drivers.
If fact we even use them in race engines when the engine is driven by a less aggressive driver. (someone who spends most of the time under 7600).
We have some drivers who insist on running above 8000 and those engines need all the help they can find.

[Scott Clarke]

Brooke-
Sounds like a great motor. I'm wondering about the non-counterbalanced crank, though. Wouldn't it result in vibration? Would the benefits outweigh the liability of a less-smooth motor?
-Scott

[kenikh]

With a great machinist, even a non C/W crank can be balanced neutral. I think this is the goal, right Brooke?

[rswannabe]

I do not know if the non-counterbalanced cranks run rougher or not. I was thinking mainly of the lower rotational mass for quicker spin up. I have never heard anyone say T's are not as smooth as E's or S's though.

Henry or others with experience using the T cranks: Do you give up some measure of smoothness if you use one? Or, as Kenik suggests, can they be balanced well enough that there is no real difference in smoothness? For that matter is the difference in rotational mass that noticable with a 66mm T crank vs. a 66mm E/S crank (ie does the motor rev significantly quicker or is it only a marginal difference)?

Inquiring minds what to know!

Brooke

PS Scott, how is the 2.5 coming along?

[kenikh]

All of the anecdotal evidence I can find says that the spool up is faster on a 66mm non-C/W than a C/W crank, but typically you can get the same effect by lightening the flywheel. Remember, lightening the rotational mass of the engine will increase spool up response, but at the expense of smoothness on deceleration due to less mass keeping the engine spinning. Like anything, it's a trade off. Didn't Porsche's infamous dual mass flywheels try to give the best of both worlds, but ended up as failures?

[Scott Clarke]

The counterbalances on the crank are there to compensate for the weight of the pistons and rods. Yes, the non-counterbalanced crank can be balanced by itself, but the counterbalanced crank is about compensating for the weight of an entire assembly. Vibration is not only a nuisance, but would seem to affect longevity, one of Brooke's stated goals. It seems to me that a lighter flywheel would be a better proposition. It achieves the same thing without inducing vibration.

The above is a conjecture and is largely intuitive. I eagerly await the responses of those more knowledgeable.

-Scott