OK here it goes. Im in the middle of a motor swap project and want to have all my information and parts in order. Im using a 93 350 4 bolt truck block (I know low compression and flat tappet but its a budget thing....) a speed density TPI setup and T5 trans in an 87 BMW 3 series. Now before I purchase any more performance parts I need to know what I can get away with using. I plan on using either aftermarket or L98 aluminum heads with bigger valves and port job, whatever I can afford really, a decent size cam and aftermarket intake. Now, the question is: with a 64 CC dished head with port and polish and obvious performance upgrades, what is the highest or recommended compression ratio for the TPI's? I plan on going with just about the biggest cam possible since its not a daily driver, vacuum shouldnt be a problem either since Im going with a hydroboost setup. I need to figure out what pistons Ill need so I can get stuff ordered and get it installed before winter since Im without a garage! Any info would be greatly appreciated! Thanks in advance.

 

Big cams and TPI don't play nice together. The LONG runners of the TPI system (about 24" from plenum to intake valve) mean it's only going to help you fill the cylinders in the 2500-4500 RPM range. Everywhere above and below that point it's not helping amy more, and definitely hurting things the higher the RPMs go. You don't want to be fighting city hall. You won't like the results.

So, forget the big cam. You need something to COMPLEMENT the TPI intake to get the most out of it. That usually means something with about 210-220* intake duration, little more on the exhaust and an LSA in the ~114* range. Try to shove a gigunda cam in it and you'll hate it- it'll be a pig everywhere.

Compression.... you should know that GM switched pistons on the L98 somewhere around 1990-1991 from dished (old pickup truck pistons) to hypereutectic flattops (I'm talking about the F-body L98 here). They didn't tell anyone of the change and they didn't change the advertised compression specs in the brochures. Nonethless, those who have torn into later model L98s will confirm this is true. As such, the factory changed the compression from about 9.1:1 on the early motors to about 9.8:1 on the late motors with no other significant changes to the engine. Should tell you something right there.

You can jack the compression ratio even higher if you want- it won't affect the basic functioning of the TPI system. It's really a matter of how high the octane of the fuel you plan to use can put up with. Certainly, an 11:1 TPI motor with a little performance cam in it isn't going to be happy running on pump premium.

 

Need to think this out ,TPI intake only makes power to 4500 so a "big" cam is pointless. There are bigger than stock cams for TPI's but are not radical given the intake flow restriction

For the money you spend on rework would be better to get aftermarket ones.Alum L98 heads are 58cc.

Compression is not really related to intake.
More related to what gas you are going to run

 

Well I know I have to upgrade the pistons so I want to keep within a budget but also add some ponies. On my behalf Im not looking to get 500 HP out of this setup but merely wake it up. So what Im gathering is that a 10.5:1 would be good on premium pump but possibly gain a few HP. I will most certainly take the advice on the cam since I have no real experience with these setups. I think with a decent bottom end and good heads along with the calibration and air/fuel upgrades it will be great for the application. The car is stripped out to the point of race seats and a fiberglass dash and probably weighs in at 2000 lbs loaded so even at 300HP that thing should be a rocket lol.