2.7 Hi comp

[philippeF]

Hi

first : may you tell me if JE 2.7 hi comp pistons would be a good improvement over stock ? (2.7RS engine)


Shall i recalibrate MFI pump, or change anything else ?

2nd : what is exactly the oil bypass mod. ? pics appreciated

best regards from France

Philippe

[cstreit]

Phillippe,

It depends on the design of the piston, since there are several. Also, what are you using the motor for?

If the piston is taller than stock, it could potentially reduce your piston to valve clearance margins. Plus the increased compression could cause you to have to twin-plug the motor or run race gas. Higher compression in general will give more torque, etc.. but at a definite cost, including potentially having to upgrade other parts (cams, head work, etc..)

[911pcars]

Mahle also makes a 9.5:1 RS piston set, available only through Andial.

With JE's, you will have to check valve-to-piston clearance and probably machine add'l valve relief in the piston crown.

Unsure how much this affects the mixture requirements.

Sherwood

[Wayne 962]

Everything made by Mahle should be available through its other distributors in North America...

In general, changes in compression have no effect on fuel mixtures or injection setups. This is because you are simply changing the ratios of the compressed gases - not the intake volumes or other combustion chamber parameters. Increasing compression is one sure-fire way to increase HP on the cheap. The sacrifice is engine life, and the requirement to run higher octane fuel...

-Wayne

[philippeF]

thanks for all

it's gonna be a track car (RS replica 72's 2.4S/RS engine), i want to achieve a 9.5 CR that 's reasonnable, and no other major mods
i was told JE had a set of 9.5 CR pistons for this application ?
octane is here in france 98.
what about the oil bypass mod ?

thanks
best regards

Philippe

[Chuck Miller]

Wayne,

If it was only that easy...

When I did my rebuild in '03 I heard about the Mehle 9.5's at Andial and called the Mahle U.S. distributors in New Jersey for a better price break...

They didn't know what the hell I was talking about ... To them they did not exist.

As far as I've learned, Andial (probably through their neighbor) did a special 'hand shake deal' directly through Mehle-Germany for sole U.S. distribution of the 9.5 and 10.5 90mm RS piston sets....

They had a few sets of each in stock when I bought mine in May '03 .

BTW- With no tax + a $100.00 gift cert.. it still came to $2608.00

Ask me if after 10,000 miles those fine German pistons were worth it............ YOU BET THEY WERE !!!!!

Cheers,

[911SCfanatic]

Everything made by Mahle should be available through its other distributors in North America...

Looked into this--not true. Several distributors told me that Mahle has separate distribution agreements for foreign and domestic vehicles. After about six phone calls, it seemed that they were correct. I found that there are only 2 authorized Mahle piston distributors for Porsches...Andial and I forget the other (the information is in one of my posts in this forum). If I'm wrong, I'd like to hear who the other distributors are because the prices are killer.

[Eagledriver]

Phillippe,

The oil bypass mod is done to the crankcase so that a later oil pump can be used. The pump is from 1976 on and has a larger pressure section and a smaller scavenge section. Because of the smaller scavenge section the scavenge pump cannot evacuate as much oil from the case and the bypass for the pressure side must exit into the intake of the pressure pump instead of just venting into the case. The mod is done by drilling a hole to connect the bypass channel to the intake channel of the pressure pump and by plugging the exit of the bypass channel where it vents to the case.

-Andy

[NevenM]

Phillippe,

The oil bypass mod is easy, look at
Oil bypass modification

I'm not sure why Wayne would say a compesssion increase decreases engine life, but the bypass mod can de done with a dremmel and a pistol drill, don't believe all that stuff about
requiring a milling machine

HTH
Neven

[Eagledriver]

A compression increase will decrease engine life for the same reason it increases horsepower. It creats higher combustion pressures and temps. Also increases likelyhood of pre-ignition and detination.

-Andy

[NevenM]

Andy

Sounds a bit thin to me, yes the stresses will be increased but the correlation between stress and engine life are hardly linear

Neven

[dweymer]

NevenM- Higher compression creates more pressure pushing down on the entire rotating assembly in the engine, cylinder pins, rods, and then crank. Each of these components rides in bearings, it is the bearings that see more wear and make a rebuild come sooner, or major damage.
At least that's my understanding of why.

[NevenM]

Don

This I understand but the correlation is not anywhere near linear, eg a 2.4T at 130Hp does not last 50% longer than a 2.4S at 190Hp, In fact I've never seen anyone state "The bottom end of a T last longer" As for a compression bump of 0.5 it would make so little impact on engine longevity as to be immeasurable

Neven

[911pcars]

Neven,
I understand what you're saying, but I'll say it. Comparing the two, a 2 liter, 110 HP T engine will last longer than a 2 liter 180 hp S engine.

The longevity of an engine is due to many factors. In the case of the above engines, the compression ratio is a small factor. The bigger factors are rev limits and BMEP; the S engine produces much more cylinder pressure and thus heat and wear than the rev-limited and relatively low-stressed T engine.

Sherwood

[dweymer]

Agreed it's not linear, but still shorter. There is a good write up on this, with graphs and real data in "maximum boost" by Corky Bell. I will try to dig it up.

[NevenM]

Hi

the compression ratio is a small factor

Exactly, All Phillipe orignally asked was if it was a good idea to do a 2.7RS with a slightly increased comp ratio, IMHO Yes it is, No he does not need to recal the pump, My other advice would be to investigate the Mahle hi comp set someone mentioned vs using JE's as the Mahle Set will be superior (if not more expensive), I felt it was erroneous to bring up engine longevity in this context (With respect to Wayne)

Neven

[camgrinder]

You need to factor in Volumetric efficiency.
a 911 S at 2500 to 3000 rpm is probably making less cylinder
pressure than a 911 T at the same rpms.
Does anyone have the "compression test" reading for a stock 911S and a stock 911T ?

[camgrinder]

I did some comparing, the stock 2.0L 911 T with 8.6-1 and a 2.0L 911 S with 9.8-1 compression. Knowing the valves true closing point, and the static compression ratio, I can predict the effective compression both engines will see. Of course the T engine will have less "VE" than the S engine. But this is interesting. The T running compression is 7.3-1 and the S is 7.5-1 even though the S has 1.2 more static compression.

[Carrera3.5L]

Originally posted by 911SCfanatic
Looked into this--not true. Several distributors told me that Mahle has separate distribution agreements for foreign and domestic vehicles. After about six phone calls, it seemed that they were correct. I found that there are only 2 authorized Mahle piston distributors for Porsches...Andial and I forget the other (the information is in one of my posts in this forum). If I'm wrong, I'd like to hear who the other distributors are because the prices are killer.

The only 2 Mahle distributors for PORSCHE pistons and cylinders in North America are Andial and SSF Auto Parts in the SF Bay Area. At one time (when I worked there over 10 years ago), Andial was the SOLE U.S. importer.

The exclusivity created alot of problems for many other shops/wholesalers who felt that Andial was gouging. A few years ago, Mahle opened up a "Motorsport" office in North Carolina because they want to get a slice of the "good old boy" NASCAR market. The Porsche stuff for them is not a big priority.

Mahle then "set up" regional distributors to purchase Porsche pistons/cylinders from Mahle Motorsport in NC, even though Andial's arrangement from Mahle is that they can still buy direct from Germany and bypass Motorsport in NC (another distribution level).

This arrangement didn't exactly work out too well either. The other distributors had little to no product knowledge and not much inventory to speak of. The Motorsport office was just as bad if not worse (remember, they could care less about Porsche aftermarket sales). Supply problems were also an issue (and they always have been).

Now, it's just down to SSF and Andial. Andial does their own development work in conjunction with Mahle and thus has exclusive access to some of the piston and cylinder sets. SSF is a master parts distributor who supplies wholesalers, shops, jobbers, etc. They only sell what Mahle products are available.

Mahle is ceasing to manufacture many, many piston and cylinder sets. Once supply has run out on some of these sets, it will take a large order on someone's behalf to get them made again. Hence, a golden opportunity for JE and cylinder manufacturers.

Prices for Mahle have always been high, some due to internal and some to external forces. But, if you look how devalued the dollar is against the Euro (about 1.35 right now) you can see that it will only get worse for new inventory.

Ralph

[Chuck Miller]

Thanks for the clarification Ralph,

I guess I called N.C. not N.J. ...

However, in early '03 I did also call SSF about the 9.5 piston sets and yet again another distributor didn't know what the heck I was talking about... It seemed that Andial was the only player for those sets at the time.... and maybe still are.

Cheers,