Dougs951S

I know there has been a good bit of coverage on this topic, but just once more, I want to get a sample of opinions concerning optimum cam timing for a largish turbo 2.5 liter car. I know Shawn did a really good job of obtaining dyno charts showing the effects of both retarding and advancing the cam, but that was on a very torquey 3 liter engine. I have a cam key kit, and I'm going to be pulling the head soon to rebuild it so that seems like the perfect time to put in the offset key if I decide to do it. I see peak tq ~4.5k rpm but the motor rarely sees >6k on the street. Should I bother putting in the offset key to shift the curve down somewhat, or will it not be worth the top end loss on a car that is above 2.8k rpm very nearly all of the time?

Paulyy

the key will move the curve to the left. bringing the peak torque lower.
But don't you think your turbo is to big for the street?
Even if you're pushing 400rwhp. changing gears in a race will slow you down because the response of the turbo needing to build boost again.

Dougs951S

Pauly, it IS a tad large for a street driven car but given the entire car's setup, the lag is acceptable. The car is driven 95% highway and 5% being beaten on in the twisties. As it currently sits, I rarely use my car's full power band which extends well past 7k rpm, so I feel like I could give up some top end in exchange for a more acceptable torque curve. I'm just not sure what exactly I'd be given up. I love the insane, stupid rush of power when the car spools up, it feels like the car doubles its torque output inside of 1000 rpm, (it probably does) its nearly unusable but it plasters a grin on my face which has to count for something. I don't want to kill the car's power above 5.5k.

Paulyy

I doubt you'll have a power increase past 6k with a stock cam. Even with my turbo, increasing power above 6200 wasn't happening. and i'm probably using 3/5ths of my turbos capabilities.
and with stock internals revving past 6500-6700 isn't a good idea either.

If i was in your shoes. I'd be doing everything i can to get that turbo spooling sooner and increasing bottom end torque.

lee101315

How about lower gearing for a better low end response? I understand your car is driven mostly on the highway, but if youre complaining about lag and you wont go smaller on the turbo, a S2 trans with a NA 5th gear should do the trick. Even a 968 transmission has lower 1st-3rd gears than a 951, but its a bit expensive.

If not, the 3 liter thing is the only thing you can do if you MUST have a big turbo+long gears, and expect to get normal throttle response.

Have you ever driven a Subaru STI or an EVO? You fly through the first couple of gears much easier with that type of gearing. Me? I swapped a 911 engine into my boxster for my daily driver, so I can tell you theres nothing like displacement increase for low end throttle response. Even with all that power, it behaves nothing like a 951 after 75mph.

refresh951

You may want to try pulling some ignition timing down low to improve spool up. I have messed with this and had some success.

Dougs951S

Good tip Shawn, I actually thought that my conservative ignition table may be partly to blame, looks like that isn't the case. I'm really not complaining about lag, that wasn't my point at all. The car is always above 75 mph and a drop to 4th puts me in my power band. Pauly, the cam is not stock. The fact that I am on stock rods is exactly why I don't make a habit of revving past 6k. An S2 trans with a tall 5th is definitely in the works, if I could ever find one for a reasonable price with LSD. Looks like you guys are not too terribly worried about what I may take off the top, so it looks like when the head comes off, in goes the 4* advance cam key. Shawn, I've been pouring over your high flow head thread and even though I havn't had much to add, I've been taking notes. In a way, my head needing a rebuild is convenient because now is a good time to build it up to work better with my cam and the bigger displacement to come in the not-too-distant future. Thanks gentleman.

Edit: Not the place for this I know, I'll start a thread about this later on when I get closer to making it happen but I wanted to see what you guys thought about a high compression turbo engine. My car is my semi-dd which means I have another vehicle, but it only gets driven when the 951 doesnt work. When the time does come to build my stroker engine, I don't want my car down for the duration of the build so I was planning on swapping in an early 9.5:1 compression N/A motor and building up a separate megasquirt map for it with less boost and more conservative ignition to run until my stroker was done. I bet a 9.5:1 motor on say, 10-12 lbs of boost would still be a lot of fun and have pretty good response. I sort of like the idea of a high comp turbo engine, I'm shooting for ~8.8-9:1 on my stroker motor. Any thoughts on this?

lee101315

It sounds like a lot of trouble, id rather bump up the compression slightly by shaving the head .020" and enjoy the slight bump off boost.

If you want to go through all the trouble of high compression+turbo, why not swap in a newer direct injection engine, like the 2.0 turbo from VW? Direct injection avoids detonation by injecting fuel into the cylinder, as opposed to letting it atomize in the intake manifold and be compressed. I can tell you from experience that the MK5 GTI can be brought to about 270 with the stock turbo (and about 300hp with a K04 turbo) and has very little lag.

Dougs951S

I have thought about several possible turbo engine swaps, but ultimately its not nearly as expensive or troublesome to build a stroker and I figure if I'm going to bother, a mild bump in compression would be nice. As for swapping in the N/a motor, well thats just because they are cheap and plentiful and that way I could keep driving the car, its not like I think its better than a stock 951 rotating assembly. If I ever did decide to swap in a motor, it would almost have to be a larger displacement block for it to be worth it in my eyes. I wonder if the porsche 3.6 V6 would fit?

blown 944

why do the swap at all? Just use the NA engine for the stroker.

Paulyy

man, 1 project at a time

If not. build your ideal motor and not worry about other stuff. more stuff you do prior to your end goal is just going to cost you more.

lee101315

Forget about Porsche swaps. I always wanted to stick a 4g63 in there...you get an early 6 bolt motor, bolt in a set of cams and a huge turbo, and rev it to the moon with 30psi....

Dougs951S

Hold on...my brain did a stupid. Good point, not much sense in dropping in the n/a motor when I can just rebuild my turbo head real quick and get it running, and then use the n/a block to build up a stroker. Guess I didnt think that one through *shrug*

blown 944

LOL, I will be doing this at some point in time. But more like 40 psi

Dougs951S

I just wish those 4G motors didn't always sound like a bunch of rocks inside a washing machine..not like a 951 sounds all that much better come to think!