2.2 twin plug or 2.5 low compression?

[gestalt1]

Hi all, I have a few options for my engine build in the 911r clone. the car will be a street/track car and for the engine i'm starting with a stock sandcast case, i have 2 66mm CW cranks and will be using RSR sprint cams. i have 90mm mahle cylinders and 75 2.7 s heads. i just bought from a fellow pelican a set of 9.5:1 CR JEs so i could build a 2.5ss but the CR would only be about 9:1 CR with the 66mm crank. I also just bought a set of 84mm JEs with 10.7 CR on a 70.4mm crank, with my 66mm crank they would be about 10.2:1 CR. with these pistons i would need to find cylinders and twin plug the heads etc.. I think the high CR twin plug 2.2 would be more in character for the car but the extra expense might get me to only a little more power than the low CR 2.5 that i can run any octane gas without worry.

What would you build?

[Steve@Rennsport]

JMHO, but if you are stuck on using RSR sprint cams, you need CR.

If it were my engine, I would rethink your parts list and use some high-compression 90mm (10:1) pistons to do this right. Twin-ignition without a doubt.

RSR cams don't "light up" until 5500 RPM in those size engines and for any street use, you really need displacement.

Naturally, close-ratio gears are a MUST.

[gestalt1]

Thanks Steve, It seems that the 2.2 with 10.2:1 CR and the RSR cams and twin plug would work as a combo, it would not have much low end but would be great on the track. I agree on the 2.5, the RSR cams would not be a good match - maybe a Mod S cam or E cam would be better as a street engine 2.5. I'm thinking this would be a 200hp engine with good torque and a 7000rpm redline. the 2.2 I would need to upgrade the valvetrain and connecting rods for high rpm to get approx 220hp with less low end torque.

I could also find a good 70.4 crank and build the typical high compression 2.7rs engine - would rsr cams work well for that? I really am having trouble understanding the drawback to rsr cams over S cams - the lobe center is wider on the rsr cams so i would think the low end torque would be better than with S cams - right?

[Steve@Rennsport]

Anything smaller than 3.4 litres with RSR cams is going to struggle below 5K and show some real bad manners between 4K & 5K. Its a decent race cam but a poor street one unless its a 3.5 or bigger.

RSR sprint cams have much more duration & lift than S cams so its not a good choice for a street car.

We recently did a 10:1, twin-plug 2.5 that made 261BHP with a VERY good powerband and no bad manners. Its all in how the parts are selected as everything must all work together.

[tadd]

Steve:
Why wouldn't a sprint cam 2.2 work well?

I've ridden in an SWB with a 906 (all but for 69S heads) and the port size reduction (36 vs 38) takes all the mid range flat spot right out. You loose about 10 hp on top, though. Ive had a few emails with another who has done the same and they told me that they have driven thru london traffic and said it was doable. Supposedly this was the 'trick in the day'.

I've also been told that swapping to 40 mm carbs can make a huge difference as well. Didn't Tony Adamanz (sp?) run 40 mm solex on his 2.0 trans am car and it was a monster?

I would think the extra 200 cc could only improve things and the sprints have less LC than the 'bad' 906 cams. At 101* it just needs lots of compression and a good bottom end to spin it, right?

Kenik has a bud with a 2.8SS with sprints and he says it's magic on 40 mm SC ports.

What am I missing?

t

[Steve@Rennsport]

Its all about personal expectations and tolerances,.....

Some folks LIKE a high-RPM engine that stutters & stammers until its on the cams,...some people prefer something thats more civilized and pulls itself up into its powerband without coaxing and drama.

JMHO, but the novelty of a high-strung race engine wears off during street use and especially when some late-model machine drives away leaving one "waiting for the elevator" before the rush arrives. Ultra-light weight and close-ratio gears makes such machinery more tolerable, but its no fun if you've experienced such things.

It comes down to what a car will be used for and personal tolerances,...

This is a VERY complex subject frought with a ton of variables and can/will be debated for as long as I'm kickin'.

I'd encourage anyone to call me for more details.

[BURN-BROS]

I have one customer that runs a 2.5SS with 906 cams,10.3/1, weber 46's, GT exhaust. Absolutely a great an exciting weekend toy, but it's not for everyone. Choose your cams wisely as the grind you want may not be the one you really enjoy.

IMHO I recommend a cam a step up from an "S" for the short stokes like a DC60. This gives the customer the occasional jaunts to 7800- 8000 rpm. Run as much piston as you can afford to help the lower rpms as well as a comp ratio of 10.5/1.

As Steve mentioned, the sprint cams are very aggressive. They are also a very old grind. There are more modern alternatives that do not give up so much at low rpm. Example; 906 cams can have as much as 40hp less than a ModS @ 5000 rpm....balance your build and you will get more enjoyment.

The 70.4 cranks are not too happy above 7500 rpm. Choose a cam that compliments that crank like a ModS.

[gestalt1]

wow, great info - thanks. to clarify, this 911 is a barly street legal race car - I don't have a tow vehicle and trailer so street driving is really just to and from the track. i'm using a 901 trans with AFMSX gears and the car will weigh around 1800lbs. I do really want an engine with the character of a race engine but i don't plan on running it up to 8K rpm. Displacement is the cheapest HP and the torque only helps. I do really want an engine that accelerated strong on the strait - not just out of corners. because i'm not building to any specific race rules (i do only time trial type track days) i don't need to sqeeze out as much as possible out of a small displacement engine. I am getting a little clearer idea about RSR cams - it seems they can be made to work on a smaller engine but what makes them work (smaller ports/carbs) is also what will take away there strength. I'm thinking a Mod S cam is the best compromise weather i go with 2.2 or 2.7L.

[BURN-BROS]

One way to get an idea of the character of a build is ask for a ride in a similarly prepared engine that you have in mind. There are plenty of guys with "street" 2.5 with DC60 cams(at least here in So Cal).

Whatever you decide, Enjoy!

[Henry Schmidt]

This engine is a 2.8 short stroke, 9.5:1 Cosworth pistons, 40 mm slide slides, small port (38mm), twin plug engine built on a NOS 3.0 RSR case and has RSR Sprint cams.
We built is about 8 years ago. It runs street gas makes 300 real hp.
It's entire life (sans one test day at Willow) on the street. It starts, idles and take throttle from 1100-8500 rpm. An ideal street engine it's not but it is easy to drive and a kick in the ass when you hammer the throttle.

[Steve@Rennsport]

Originally Posted by gestalt1

I'm thinking a Mod S cam is the best compromise weather i go with 2.2 or 2.7L.

Now you're talking,....

Remember, if you aren't willing or (reliably) able to run the engine to 8K, there is no point to having a race cam since thats where the power is with these smaller engines.

AFMSX gearing is a FAR better match with Mod-S cams instead of RSR sprint cams since these have a much narrower power range.

[gestalt1]

Ok, progress - the cam spec is settled. Now - what is best for the displacement?
2.2 high compression with twin plugs or 2.5 or 2.7 with low compression single plugs? I'm leaning toward the greater displacement - much cheaper!

Also, I'm planning to have my aluminum 901 trans to be rebuilt. is there anything beyond stock needed for a 210hp 2.7? I haven't decided if i should go with an LSD but likely will.

again - thanks everyone!

[Steve@Rennsport]

Originally Posted by gestalt1

Ok, progress - the cam spec is settled. Now - what is best for the displacement?
2.2 high compression with twin plugs or 2.5 or 2.7 with low compression single plugs? I'm leaning toward the greater displacement - much cheaper!

Also, I'm planning to have my aluminum 901 trans to be rebuilt. is there anything beyond stock needed for a 210hp 2.7? I haven't decided if i should go with an LSD but likely will.

again - thanks everyone!

Personally, I would do a 2.7 and get as much CR as you can with the parts you are going to use.

LSD is a must for track use. Remember, the real value of an LSD is not wheelspin control (thats only a small part), its about stabilizing the car under hard braking where an LSD really shines. This is due to the LSD's ability to keep the rear wheels locked together under trailing throttle that not only eliminates that rear-end side-to-side motion, but really helps the transition to the throttle that gets the car out of the corner.

[Henry Schmidt]

Originally Posted by Steve@Rennsport

Personally, I would do a 2.7 and get as much CR as you can with the parts you are going to use.
........edit.......

IMHO: This is a tough recommendation considering the uncertain future of fuel availability.
The fuels of the near future will certainly have lower octane and probably be unsuited for higher compression engines without fuel /engine management. My guess is that the greenies will figure out the best way to get old cars of the road is to reduce octane.
For performance Steve is right but sometimes life forces a slightly more moderate approach.
Aggressive cams, lower compression and well tuned ignitions can be great fun.
My last street engine was a simple 2.45 liter (70.4x86) 9.0:1, Mods S cams, 36 ports, 40mm Webers and twin plug and it made 180hp at the rear wheels at 6400rpm.
It pulle d from the ground up and it will run on regular gas.

[gestalt1]

I agree Henry and I'm glad you brought up current and future fuel problems. I know that paying the extra .50/gal for 92 octane is relatively cheap but it would be nice to not worry if i fill up with 87 octane occasionally. I think twin plug does help a lot but the cost is significant. I just know that a 2.7RS with only 8.5:1 compression is a great engine, extra compression does increase torque and hp but not many complain about the stock performance. throttle response i feel has a lot more to do with correct timing and intake system than compression ratio. I'm actually not that concerned about the total hp of my engine, after all a lotus elise is 2000lbs and has 190hp - its a fast car and my 911 will weigh 1800lbs with about 200hp. (and my head won't be sticking up over the windsheild as it does in the lotus!)

[piggdekk]

Originally Posted by Henry Schmidt

The fuels of the near future will certainly have lower octane and probably be unsuited for higher compression engines without fuel /engine management.

Are You sure about this? Car makers are pushing oil companies to produce higher quality fuels to help their motor to reduce pollution. For gasoline engines one way to improve efficiency hence reduce pollution is by increasing the compression ratio. Why greenies should push for lower octane fuels?

[Mr9146]

Steve and Aaron said all that needs to be said.

If I were to add anything to the discussion it's this ... small motors benefit from COMPRESSION!