Inliners International Forums General Discussion Engines Which Knock Sensor for a Chevy 250ci inline 6.

I will soon replace my 1965 Nova's 194 ci engine with a reworked 250 bored 0.030” over (3.905”). The head is from the original 194 with combustion chambers opened to 66cc. The deck has been milled to provide quench (squish) of 0.041” (thickness of the head gasket). The dished pistons give 9.3:1 static compression ratio, and a mild Crower cam produces a Dynamic CR of 8.825:1.

I plan TBI via Megasquirt. A Knock Sensor will be used because of the rather high CR. Since that was never part of a pre-70's engine, I've been doing a lot of research, but still have questions.

The Knock Sensor will go in the block drain, unless I am convinced otherwise. The rationale is; that's where GM does it on their V8 engines (until recent high performance ones). Also, I was told by my machinist, that the block drain works well because the “Sonar” effect transmits knock impulses from any and all cylinders.

As I understand it, the knock frequency is a function of bore size. I believe a 3.905” bore should put the frequency at 5700 Hz using http://www.phormula.co.uk/KnockCalculator.aspx . Results of this link match my hand done calculations exactly, and it is very easy to use.

Based on that calculation, I searched for a matching sensor that would screw into the NPT drain. I found a sensor that resonates between 4800 – 5600 Hz, and another from 5600 – 6400 Hz and others that are too far from 5700 Hz to be considered.

The 4800-5600 sensor fits 1996-1999 Chevrolet C2500 Suburban 5.7 L V8 (and others) – 4.0” bore.
The 5600-6400 sensor fits 1996-2005 Buick Park Avenue 3.8 L V6 (and others) – 3.8” bore.

My first reaction is to select the 5600-6400 unit because 5700 is within that range. However, my bore is almost exactly half way between the Chevy and the Buick, so I'm not 100% sold. Notice how the bigger the bore, the lower the frequency (like an organ pipe).

My questions are: 1) Which of the two sensors have others used successfully, or perhaps another one I didn't find. 2) If someone has a better location than the drain plug hole, where did you mount it. Naturally, I welcome any comments, especially from someone who has done something similar and discovered what works and what doesn't.

Try one from this 5.7L engine, it should work based on your formula....


GM LS1 engine from the Chevrolet Corvette C5
The Generation III 5.7 L shares little other than similar displacement, external dimensions, and rod bearings, with its predecessor (LT1). It is an all-aluminum 5,665 cc (5.665 L; 345.7 cu in) pushrod engine with a bore of 3.898 in (99.0 mm) and a stroke of 3.62 in (92 mm).

Great idea. I don't know much about the LS1. I was under the impression it had two Knock Sensors in the valley. Do you know if they are NPT? I know some have fairly fine metric threads, but like I said, I don't know much about the LS1. If they will work with the L6, the bore sounds like it would be almost a perfect match.

I think they do fit into the valley area, but as I understand, they all work off of the same principle. I don't know if they make metric pipe threads or not, but it should't be too hard to adapt, just drill and tap another hole somewhere else in the block.

I may be able to go one better. I need to verify this, but I believe the LS1 sensor has M10 X 125 threads. If that's true, then I may be able to make a bushing with male 1/4" fine pipe threads to screw into the block, and metric female threads for the sensor. If that's possible, it would keep me from drilling holes in my engine (which I'd rather not do).

I need to verify one more thing. I think non pipe thread sensors must be threaded all the way into the block so that the base of the sensor is in contact with a flat surface. If that's the case, drilling a hole in the block becomes a little more challenging. A place with a smooth, flat surface must be selected in a location suitable for knock detection with a low incidence of other noises. Boy this is really fun.

Thanks for helping me brainstorm. I appreciate your input.

Making a bushing would be a quick fix if thats something you have the ability to do. Spotfacing a flat place is a little more of a challenge and expense if you don't already have tooling to do that with. Keep us posted, it might be another alternative for others wishing to do the same mods.

Thanks for the encouragement. Your suggestion caused me to look a little harder. The Corvette sensor would have been difficult to fit, but I found another sensor with the same resonant frequency, plus it is the same pipe thread as the 250's block drain. It fits 1992-1992 G20 Chevy Van 5.0L and a few others. I like the fact that it will just screw in without bushings or machining. I pasted the product description below.

Amazon lists it as: Delphi AS10014 Ignition Knock Sensor. $34.79.


DELPHI KNOCK SENSOR -- 38.1 mm diameter, 21.84 mm hex size, 5500-6400 Hz frequency, 100 K Ohms resistance, 1/4-18 NPTF dry seal thread size; A high quality, direct fit OE replacement knock sensor.

Oh, by the way, I plan to photo document the build. I plan to register a domain for the project much like I did for a previous one. If you are interested it is at http://www.nwseven.info

I think the little saying that Einstein coined above was meaning what we call today "thinking outside the box". Your approach seemed to be just that, in an attempt to solve your problem.

What TBI will you use?

Since I'm working for economy rather than max power, I've created an adapter for the stock intake manifold. I'll put a photo on my web site as soon as I register the domain.

The TBI I'm planning is a 1 barrel from a Pontiac Sunbird 2.8L. It is also found on a number of smaller displacement GM cars of the mid to late '80s. The TBI bore is about the same as the Rochester carburetor throat. Since the TBI has no venturi, it should flow a lot more air. BTW, they are easy to find in U-Pull-It yards for about $25, complete with Throttle Position Sensor. While I was pulling mine, I grabbed the air cleaner for another $10. It should give a somewhat stock appearance, which is another of my goals.

The stock Sunbird injector is too small for a Chevy 250, but I've replaced it with an injector from a 454 ci truck engine (interchangeable). It should flow enough to support the Horsepower the 250 will produce. If I'm wrong, I'll probably switch up to a GM two barrel, and make a different adapter.

Wow. I've never been compared to Albert before. Some people have called me smart, but usually they follow that with another word -- as in; "You're a Smart A--"

But seriously, Thanks for the kind words. We'll see if thinking outside the box makes my engine run, or proves to be an academic exercise.