I am getting ready to tear into my 91 and overhaul it.I am curious about the biggest possible lift I can go with..?..I am going to bore it over 40 and port the heads and stick a cam and some springs on it.
If anybody has done any good engine mods (stroking,boring,CAMS,exc.) on a tpi and kept all the computer stuff (besides emission) and got real good hp ratings, please let me know...
Thanks

 

Yeah, I'm curious too for future referance.. I'm thinking the largest cam w/ stock block and componets, w/out throwing everything out of wack.

 

Gosh Mjones! Your avatar is...thought provoking. Is that a T-Top or convertable?

 

Pretty cool huh? curves on the vette and chicks are close.... hooters and wings are conversation and match.

 

You can likely go with the LPE 219 cam; for sure the TPIS ZZ9 will work.

To get the most from a cam port and work the stock #113 heads.

If your exhaust isn't restrictive, with tuning you can expect around 30chp from heads work and 30chp from a good cam.

 

That would be a "Pervertible" not convertible

 

OK, well I am going to port, polish,and surface my heads and hog them out for 202s...I am realy thinking about going with a stroker.either a 383 or 396..Probably a 383...from what i hear they are more reliable for daily driving...If anybody on hear has built a 383 tpi and you have had good results please give me some good hints...I plan to start on it next weekend.....anyway thanks guys

 

When you start on the stock heads concentrate on the pocket area as this will yield most flow. Be careful to not change the basic bowl shape as it's already designed for good swirl.

All you need to go with 2.02/1.60 valves is new seats. Then carefully blend the seat bottoms into the bowl. With those valves you should do a little unshrouding near the intake valve, but be careful of getting too close to the water jacket. Also smooth the spark plug boss.

Finish up with a 5-angle valve job and you will get enough flow increase about 30chp gain over stock heads. And you will retain good low rpm velocity for low end torque.

 

You could also try the XE276HR. Specs are 224/230 @.050 .503"/.510" lift with 1.5 rockers, and 112 LSA. With 1.6 rr's you could get .536"/.544".

 

OK if i go with a .500 or bigger lift cam what kind of tuning will be needed to keep a good idle??

 

It isn't the lift that causes tuning issues. It is the duration of the cam that is the culprit and also the lobe separation angle which when combined determines the amount of overlap in a cam. Overlap is when both intake and exhaust valves are open which causes the lumpity lump sound in a cammed up engine. More overlap means more lump, and the more lump you have the harder its going to be to tune in a TPI application, especially if its a 90 and 91 modely year.

 

tj, what is difference from 89 to 90/91?

 

 

i mean they(l98) are all the same basic engines except for some improvements during there run before lt1. so if you put the same cam/heads on earlier one like 88/89 and do the same on a 90/91 what is the difference in tune? sorry for the confusion?

 

The difference is in the control strategy of the two different ECMs. The 86 thru 89 ECMs were a 1227165 and in 1990 they changed to the 1227727 ECM which was also relocated to the under hood location above the battery.

The earlier 7165 ECMs is what is called a MAF strategy which means it controls fueling based on MASS AIR FLOW into the engine. The later 7727 ECMs are known as SPEED DENSITY strategy. This means that the ECM controls fueling based on engine speed and manifold pressure. Both ECMs relie on look up tables for its fuel requirements. The 7165 ECM looks to the MAF (mass air flow meter) tables for fuel. The MAF sensor tables are often either frequency output from the sensor or a voltage output. Either way, voltage or frequency a given constant = x grams of air to the ECM. which in turn supplies x amount of fuel based on the MAF sensor output.The 7727 ECM uses a VE (volumetric efficiency) look up table for its fuel needs, that table is based on Engine RPM and engine manifold pressure for its x and y coordinates. The both ECMs also relies on a o2 sensor for its final control element. Just as all engine management systems do to date.

The MAF straetgy is much more forgiving to changes. For instant if one installs a popular 219 cam into a 89 C4, he can start and run the engine often without any tuning. It may idle rough but it will run. In later 90 model it is not nearly as forgiving as a MAF car. Changing out the cam means that the idle characturistics has changed in a way that the ECM may not be able to offset. The idle vacuum usually goes up so the ECM thinks there is additional load on the engine and therefore fuels for that load. This often causes a rich condition to a point where the ECM can't control becuase its at its control limit. That is where a custom tune has to come into play.

In 1992 and up to current model year Corvettes and for that matter all GM cars. GM has changed back to MAF control strategy. One reason is that OBD2 regulations demand it and other is that MAF control is much more accurate. From the early MAF strategies to today, the amount of coding in the ECMs to todays PCMs has increased over 100 fold For instance in the 89 model year prom code the binary files are only 16k in size, the later SD controllers are 32k and the 94/95 OBD1 cars were 128k and the later 96 and up cars are 512k in file size. These PCMs are so smart these days its really amazing that they can do all that they do.