88 350 tpi formula

iTrader: (9)

I wanted to know who knows what the differance is betweeen the two? I know they show up as two differant part numbers and you should swap them when you do the tpi 305-tpi350 swap. I am really confused because it looks like everyone with a 350 tpi 5-speed or 6-speed is using the 305- t-5 bin as a starting point and adjusting fuel and spark.

The way I see it the 305 knock sensor should be used then right?

or is the 350 one less sensitive due to the larger bore of the eng.?

I have done searches but, no one really says what the differance is. just to use the 350 one due to it having a differant knock setup. this is for a 88 in mind but, should hold true for all the MAF setups

Vader

Actually, it should be the other way around. A 350 has a larger diameter cylinder bore, and necessarily larger cast sleeves for the cylinders. This makes the water jacket space between the cylinders' outer walls and outer block case smaller. That would tend to transfer the shock waves to the KS more efectively, so the KS sensitivity would have to be lower to generate a relatively equal signal. Conversely, a 305 has a smaller cast cylinder jacket, and more coolant space between the outer cylinder wall and outer block case wall. This tends to "kill" some of the shock wave from detonation, so the 305 KS would have to be more sensitive. Using a 305 KS in a 350 can generate excessive knock counts.

Another thing to be aware of is that later models used a different sensor, since the output from the KS was wired directly to the ECM inputs, rather than to a separate EST control as on older models. The signal level is higher on the later sensors, and the impedance across the bridge in the sensor is higher to generate a larger signal. If you are using a later (S-D) ECM, you need to use one of the later sensors.

In your case, an early style 350CID KS is probably what you'll be needing. The ECM looks only at the number of counts to determine knock and attack rate. There is no real difference in programming (at least from a detonation perspective) for a 305 versus a 350. The real difference is in the sensor, and what level of shock will produce a signal that is above the recognition threshold of the ECM inputs (or in your case, EST input) to be counted as a knock.

88 350 tpi formula

iTrader: (9)

vader, thats along the lines if what i am thinking now I am running a sbc 400 and I get these knock singnals that are very hard to track down they are not always at the sam rpms and are not always the same amount. some times I get 2 deg. timming pulled and other times I get as much as 20 pulled I have been checking for other issues like other things causing knock singnals but, I am still thinking its the 305 sensor with 211,XXX miles on it and should be using a 350 knock sensor. but, since I am using a sbc 400 and the bores are even larger yet pluse the cyls are siameased I think this could cause some issues as well.

I thought I was running into real knock counts at first I even tried race gas and additives ect, still get them. my compresstion ratio is about 10:1 actually slightly higher but, lots of people run that with iron heads with out problems. can a plug with a slightly higher heat range cause spark knock? faulty plug wires? the plugs look very good but, they look like they maybe running a little hot (very very hard to tell as I compare them to a chart) any other car I would say they are fine.

is there any way to make it less sensitive? pipe dope? I have everything for the 730 ecm swap but, I want to get this ironed out first before I go switching stuff around and end up with more issues to figure out.

one last thing I just thought of is I have forged pitions from what I understand these can make noise in certant applications

Vader

A few randome thoughts:

1. Forged pistons can have excess skirt clearance until they are warmed, since most of them are cam ground (not truly round). Once at operating temperature, there should be no real difference.
2. A 400 case might be inherently a bit "noisy" because of the design, reinforcement, and bore diameter. A 305 KS in a 400 could be a really serious mismatch.
3. Changing the ECM may require you to change the KS anyway, since my understanding is that the later sensors are calibrated differently, and the ECM expects a different resistance from the Wheatstone bridge in the sensor in "quiet" mode.
4. Hot spark plugs are just another hot spot in the chamber, and therefore another potential for preignition. It certainly wouldn't hurt to experiment with one or even two steps colder plugs.
5. I run 10.8:1 CR on an iron-headed LT1, but have no idea of knock counts from the two sensors. I've never monitored for that with my AutoTap/laptop. That's on 89 octane gasohol, too, with a 185° thermostat.