# Compression Test



## aseyc (Aug 26, 2020)

Today I decided to do a compression test for no particular reason as the engine runs fine. It was more of a re-assurance process as I do not know the history of the car and the engine however in the back of my mind, I feel this engine should be a bit more powerful simply because I am comparing the 'feel' with my 87 TransAm GTA with a pretty stock 350 which to me the 350 feels a bit more powerful. Both cars have an auto tranny and on the GTA all I did was just a basic hone, fresh rings (0.060 overbore) and edelbrock headers. Not sure if I should be comparing the 400 with the 350 but anyway... Specs of my GTO engine are:

YS Engine
16 Heads
The engine is a numbers matching and was built in June 1968. I am assuming that the heads are also original as their date code is E248 (both) which means May 1968.
According to my PHS documents (where I can verify the engine unit number matches the actual engine), this setup should have a 10.75:1 compression ratio.

Looking at a past post on this forum, someone had said that all 10.5 - 10.75 compression engines should have around 185-210PSI however when I tested mine, all 8x cylinders gave between 148 - 151PSI. I did the test with all 8x plugs out, cranking for 5x rotations with a cold engine.

Unfortunately, I didn't open the engine (yet) and so I don't know if it is bored or not. The odometer reads 35000 so I am assuming it has at least 135000 and I am also 100% sure that the engine was at a certain point opened as I found some photos in the car of when this was being done. No details as to what was actually done to the engine. I would really like to know who the previous owner was but I guess its like trying to find a needle in a haystack especially now that the car is so far from its mother country....

Should I be worried with these compression readings? Should the 400 be comparable to an 87 Chevy 350 in terms of agility? I mean 87 was not really the best times for engines as most of them were quite de-tuned due to emissions....

BTW forgot to say that the GTO has a Holley 6210-3 which is apparently a 650CFM carburetor.

Any thoughts and opinions are very much welcome.


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## aseyc (Aug 26, 2020)

Forgot to add that the car has Ram Air Manifold not the standard ones and if it matters, it also has an Accel HEI ignition not the standard coil + points system.


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## BearGFR (Aug 25, 2008)

I'd recommend repeating the test with the engine warm, and if you didn't already - make sure you perform the test with the throttle blocked wide open. Otherwise you're forcing the engine to pull air through the same passages it does at idle, and that can definitely cause lower readings. Another thing you can do is after you've gotten a good reading on a cylinder, squirt a >small< amount of oil into the cylinder and take another reading. If the reading increases significantly, that tells you that the rings are worn. 

Something else that might be hurting you, performance-wise, is that 650 cfm carburetor. All QJets, one of which would have been original to the car, were capable of at least 750, some capable of more.

Bear


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## aseyc (Aug 26, 2020)

Very interesting. Thanks for your recommendations. I will warm up the engine for 5 minutes or so and repeat at least a single cylinder to see it it made a difference (while holding the throttle open).
Re. the carb, while I was writing this post, to be honest I double checked the p/n of the Holley as I was convinced it was a 750CFM but to my surprise, the one I have as I wrote is a 650.. I guess I have to start looking for a 750. Tempted to go Fitech but the modifications necessary in the fuel delivery system put me off a bit.


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## BearGFR (Aug 25, 2008)

Yeah, plumbing for that big return line can be a pain.


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## armyadarkness (Dec 7, 2020)

A 650 is arguably to small for well maintained 400.

An 87 GTA had a tuned port LT1. They were great motors and the GTA was much lighter... also more aerodynamic, and I suspect a much more efficient drivetrain. My Jeep Grand Cherokee feels faster than my GTO, so I have no doubt that an 87 GTA would. In fact, I think an 87 TA with a manual would likely walk all over most GTO's from the 60's


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## aseyc (Aug 26, 2020)

The 87 Transam is actually an L98 with TPI and the L98 are not known to be the most powerful SBC...

Needless to say, I have already started looking at carb options for the GTO . And it looks like there is a common consensus across a lot of people that Q-Jets would be the way to go.


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## armyadarkness (Dec 7, 2020)

Fuel injection on these cars is no big deal, if you buy the FI fuel tank, with all of the fixins. Carb guys will argue that a carb makes the same power, but yeah, that's when it's tuned by an expert on a dyno. FI rules the roost for tuning, mileage, and ease. And, when you consider how much it might cost to tune the ignition and carb on a non-EFI car, the price is even comparable. Especially now that American Holleys are made from overseas parts and cost eight million dollars


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## armyadarkness (Dec 7, 2020)

aseyc said:


> The 87 Transam is actually an L98 with TPI and the L98 are not known to be the most powerful SBC...
> 
> Needless to say, I have already started looking at carb options for the GTO . And it looks like there is a common consensus across a lot of people that Q-Jets would be the way to go.


Whoops, yeah, I meant the tuned port part. Maybe they werent 'the most powerful", but they were definitely well suited for that car. Even the 305 was fairly fun, the 350 much more so. I had a 91 Camaro, 305 TBI when I bought my first HO 67 GTO, and with nothing but bolt on goodies, it gave the GTO a run for its money.


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## armyadarkness (Dec 7, 2020)

aseyc said:


> The 87 Transam is actually an L98 with TPI and the L98 are not known to be the most powerful SBC...
> 
> Needless to say, I have already started looking at carb options for the GTO . And it looks like there is a common consensus across a lot of people that Q-Jets would be the way to go.


Qjets are the recommended carb, but a good one will run at least $800. The Summit/ Jegs/ Amazon/ Parts Chain rebuilds, are VERY hit or miss, and widely discouraged.


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## aseyc (Aug 26, 2020)

Do you run FI on your car yourself? Perhaps anything in particular I should be looking at? I am well acquainted in FI so it would surely be easier for me to get the grasps of it.


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## armyadarkness (Dec 7, 2020)

No I don't. I would and likely will, but I'm looking to convert to a manual trans, first. Only thing I can say is that you should get the ready made EFI fuel tank... Ive done several of them.

That being said, I'd get an Edelbrock 800 AVS2 carb, and call it a day! It's a dramatically improved version of the Carter, which was an OEM GTO carb, cheap, and it's super easy to tune.

This is assuming that you don't want to spend $1000 for a quality Qjet.


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## Mine'sa66 (Oct 30, 2019)

As mentioned, be sure your throttle plate is wide open.
What you're really looking for with a compression test is relative difference between cylinders. If they're all within about 5% of each other you're probably good.
That being said, if you do a wet test.....again as mentioned, if you pour some light oil (marvel, ATF) into the cylinder and find you gain a significant amount, then you do have issues.
Pretty rare though that all cylinders would experience the exact same amount of loss, thus the relative difference is what you really want to measure.


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## PontiacJim (Dec 29, 2012)

BearGFR said:


> I'd recommend repeating the test with the engine warm, and if you didn't already - make sure you perform the test with the throttle blocked wide open. Otherwise you're forcing the engine to pull air through the same passages it does at idle, and that can definitely cause lower readings. Another thing you can do is after you've gotten a good reading on a cylinder, squirt a >small< amount of oil into the cylinder and take another reading. If the reading increases significantly, that tells you that the rings are worn.
> 
> Something else that might be hurting you, performance-wise, is that 650 cfm carburetor. All QJets, one of which would have been original to the car, were capable of at least 750, some capable of more.
> 
> Bear


X2 on what *Bear *has said.

If the engine has been apart, quite possible you have dished pistons to get the compression down. You would need one of the bore scopes or one you can attach to your hand held phone - *armyadarkness* can tell you what he has. This is the only way you could confirm if you have factory "eyebrow" valve relief flat top pistons or something with a dish or large valve reliefs used to drop compression. Being consistent across the board would suggest you may have a piston to lower compression, but the tests *Bear* went through will confirm this, or not, if compression come up.

It is also possible that your timing has been retarded to help with any detonation IF you have the stock compression and are using pump gas. What gas and/or boost octane additive are you using if any?

650CFM in my opinion, along with the rest, is too small. You should have a responsive and snappy bottom end RPM range and then it'll fall off in power due to the engine wants a little more air/fuel. Always go with a vacuum secondary carb for street use. You did not say what manifold?

The factory intake would be a Q-jet spreadbore and to use a 650CFM (Holley?) you would need a spacer/adapter and that could kill some bottom end throttle response if an open hole adapter is used, versus one that takes advantage of the dual plane manifold that factory used. So if you still have the factory intake, then a rebuilt Q-jet would be the most logical choice. *SMI* seems to be a suggested company to get a good Q-jet from.






Other Services - Rebuilding & Modifying - Rochester Quadrajet


SMI Carburetor offers carburetors for Other Services - Rebuilding & Modifying - Rochester Quadrajet




www.smicarburetor.com





If you have an aluminum aftermarket intake with squarebore flange, then go with the AVS2 800 CFM as *armyadarkness* has suggested. The AVS2 is an improved version of the AVS which has it's lineage with the well known/proven AFB carb. Annular primary boosters and vacuum secondaries with an adjustable air flap like the Q-jet. Like all things, some like them, some hate them. Summit has them and shipping is free over $100.00. You may have to dial it in, but you typically need to play with any new carb you install.









Edelbrock 1913 Edelbrock AVS2 Series Carburetors | Summit Racing


Free Shipping - Edelbrock AVS2 Series Carburetors with qualifying orders of $99. Shop Carburetors at Summit Racing.




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No comparison between the GTA and '68. First off, gear ratio's in the transmission - 700R4: 1st gear 3.06, and 2nd gear 1.63 in the GTA versus the '68 1st gear TH-400 of 2.48 and 2nd gear of 1.48 so off the line the GTA will seem faster and more responsive. Then throw in a rear gear comparison, GTA 350 looks to be 3.27 versus the '68 which standard gearing is listed as 3.55 for non A/C and 3.23 with A/C. So advantage to the GTO if no A/C and if an opional ratio was not chosen with the car. So the GTA gets the GTO out of the hole, BUT, as many learned, the 400CI & 455CI (and most Pontiac engines) are designed around torque. No shame in getting pulled off at the starting line, but at the end of the finish line, that's when they saw tail lights from a GTO. The GTO may not "feel" faster, but the torque curve can be so smooth and broad that the GTO is indeed faster.

Carb, TBI, or EFI, or......? Read this article and you decide:






Choosing an Electronic Fuel Injection Retrofit for Your Street Rod or Muscle Car


Fuel injection is here to stay and we delve into the benefits of such retrofit systems for your classic muscle car or street rod in our EFI retrofit buyer's guide!




www.speedwaymotors.com


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## aseyc (Aug 26, 2020)

Wow! What a wealth if knowledge you are!! You even researched on the GTA 😱

Plan moving forward is this.
Will warm up engine and do the compression test again.

Will get a bore scope to try and see if I have dished pistons. Will try to take some photos.

In terms of fuel, I use pump 98 RON. Have a SC mustang which used to experience early detonation and I had to retard ignition when I used normal 92RON however this 98 solved it.

Intake is standard cast iron. The part number is 9794234. There must be some adapter plate although to be honest I am not seeing it as you can see from the photo.


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## aseyc (Aug 26, 2020)

Intake to carb photo


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## O52 (Jan 27, 2019)

Its a spread bore Holley. QJ replacement.






Holley 0-6210 650 CFM Quadrajet™ Style Spreadbore Carburetor


Holley Replacement Carburetors are engineering to improve performance and keep you emission legal. The 0-6210 is a Model 4165™ Spread Bore designed to fit several applications. See application chart below in tech resources for specific details.




www.holley.com


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## aseyc (Aug 26, 2020)

That what I thought but no harm in double checking. Cant understand the reason why a 650CFM carb is marketed as a QJet replacement which was originally 750CFM.


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## PontiacJim (Dec 29, 2012)

There is also this one which has the mecnical secondaries:






Holley 0-80555C 650 CFM Classic Holley Carburetor - Spreadbore Quadrajet™ Style


Do you like the tunability Holley's modular carbs, but need to simply replace a Q-jet? The spread bore bolt pattern of the 0-80555C lets you do just that. It has vacuum secondaries and electric choke, so it is useable on a wide variety of vehicles and its single fuel inlet that is in the...




www.holley.com





Holley used to offer a 800CFM spreadbore with mechanical secondaries. I bought one around 1981-82 to use on my Chevy 409. Thought it might provide better gas mileage that the 2x4 set-up. I had to use an adapter. I lost HP in a big way and gas mileage was not much improved. So off it came after about a week and I set it on the shelf. Now long gone.

That was my only experience with a Holley. I am not a fan of Holley, but they are certainly good carbs when set-up correctly for an engine, just like any carb. You could play around with the 650 and see if you could dial it in a little better, but I still feel a good 750 CFM Q-jet may perform better with a little more air flow. The Q-jet may also need to be dialed in once installed. So it could be half a dozen of one, and half a dozen of the other. So maybe play around a bit with the Holley just because you have it, and then if not satisfied, consider the Q-jet.


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## armyadarkness (Dec 7, 2020)

I tried to buy a Holley for the Vette, about a month into the pandemic. They had none. NONE! I had to put my name on a list for the Qjet replacement, and if you didnt put your name on, then you didnt get one. They didn't like when I questioned why an American made carb, was waiting for parts from overseas.

Anywho, two months later, I overpaid for my Holley and I got it in the mail. It looked SOOOOOOOOOOO cheap, compared to back in the day. Like a Chinese reproduction... and, when I went to operate it, it was jammed. Closer inspection revealed a bent shaft, and I could tell that it had been assembled that way. There was no box damage, and a QC tag, too.

I called them up and it took several weeks to reach a human on the phone. They wanted to send another, and that was all. I wasn't going to give them my business, twice. If I want to gamble, Ill go to Atlantic City.


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## aseyc (Aug 26, 2020)

On this carb discussion if i don’t manage to get some more power by setting up the Holley I will most probably go for an fitech system.

But putting that aside, I did the bore scope test and from what I can see, i dont have dished pistons. Will now try the compression test with a warm engine and revert back. Was hoping I could see a marking if the piston was overbore marked on the piston face but didn’t see anything.

not the best picture quality either….


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## aseyc (Aug 26, 2020)

Did the test with engine at 180F. Practically the same PSI. 154[ish]. Tried putting some oil and it went up to 162[ish]. Both tests with full throttle.

What would be your opinions on this? Rockers not properly adjusted would cause this? Maybe they are too tight?


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## O52 (Jan 27, 2019)

Those pistons are 'generic'. In other words, with the multiple valve reliefs, they fit the 389 and 400 with the early or late heads. As such they do add some extra volume to the compression chamber. 
I would check compression one more time after a full day cruising around the island. 180* water temperature doesn't necessarily mean the block/pistons are fully warmed up and expanded. And if the compression remains the same it isn't the end of the world. Those numbers show a well used, but perfectly acceptable engine. They all have the same reading which is good.


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## aseyc (Aug 26, 2020)

Will do as you say. Will go for a Sunday morning cruise and get it up to temperature and check again. I will still eventually have the engine opened and I might put in a fresh set of rings and perhaps a light hone.
As I said I would really like a bit of a lumpy idle so will have to get into the engine.
Car will be completely dismantled and that would be the right time.
Until then I will keep on buying parts every once in a while. Will hold off on engine parts until I get to open it up to see what i find.


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## PontiacJim (Dec 29, 2012)

aseyc said:


> Did the test with engine at 180F. Practically the same PSI. 154[ish]. Tried putting some oil and it went up to 162[ish]. Both tests with full throttle.
> 
> What would be your opinions on this? Rockers not properly adjusted would cause this? Maybe they are too tight?


Your compression is probably correct. Those 8-valve relief pistons will reduce compression. I found what looks to be the match, Sealed Power, but no specs on how many CC's those vavle reliefs provide. I found one reference in a post, and they said 13-14 CC's. A set of Iconn pistons with similar reliefs were 10 CC's, so 14 CC's may be a good number to go with.

So using the Wallace Compression Calculator and plugging in a few numbers based on you engine/pistons, I get a compression ratio of 9.06. Could be a tad bit more or less, but not by much. So near 9.0 compression will work on pump gas, may 91 octane depending on cam, and will feel a little sluggish if the best cam is not selected to match the compression. A big cam will make the engine a dog, certainly at lower RPM's. So this may explain some of the lack of get-up and go as compared to the GTA.

Don't fret, as 9.0 is a good number for a street engine and can be made to really pull hard. You just want to match parts and that includes a good cam grind to best take advantage of the lower compression.









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## aseyc (Aug 26, 2020)

And how do you get to a good cam? This is a new experience to me. I guess i will need to first ensure what pistons I have, measuring chamber and deck. Check valves and clearances then give an this information to companies like comp cams and see what they come out with?


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## PontiacJim (Dec 29, 2012)

aseyc said:


> And how do you get to a good cam? This is a new experience to me. I guess i will need to first ensure what pistons I have, measuring chamber and deck. Check valves and clearances then give an this information to companies like comp cams and see what they come out with?


It is all about building cylinder pressure. 9.0 compression, in my opinion, works well with a cam having a 112 Lobe Separation Angle (LSA). Many, like Comp Cams, use a 110 LSA. I am not a fan of a 110 LSA as it builds cylinder pressure - so this can increase your chances of having detonation issues unless you step up into higher octane gasses or additives, essentially you are raising compression pressures through the operation of the cam. The 110 LSA cams work great with low compression engines like 7.5-8-5 or so as this is where you want to build more cylinder pressures to enhance the low piston/engine compression. Going more compression, let's say 9.5 and up, then 114-116 LSA may be a better bet as it can bleed off some cylinder pressure at lower RPM's which can reduce the chances of detonation with the higher compression.

Cam lift should be selected on head CFM flow. You don't want a big lift cam of .500" when the heads only flow enough CFM's for .430" lift.

Duration is selected as to where you want your power band to be most effective. The more duration a cam has, the higher you have to spin RPM's to take advantage of it. You don't want a RAIV can if your engine won't spin to 6,200 RPM's without breaking, and you don't mind the 1,200 RPM idle and a sluggish bottom end. On the other hand, too low of a duration will give up power as it will die well before the engine useable RPM's - so you are throwing away HP in the upper middle power band.

There is more that goes into all this, but this is part of my meaning in selecting the best/correct cam to match engine compression and it's other parts so as to make a complete package that works together.

So my generic suggestion for 9.0 compression is a cam with a 112 LSA, around a 280 duration, and .450-.480" lift (with matching valve springs). Start getting past .480" lift and you want to check valve to piston clearance to ensure the valves do not kiss the piston tops and/or the valve reliefs are deep enough - which is usually not an issue with aftermarket pistons as they are made for bigger lift cams in mind.

Most of the cam manufacturers will state the RPM band at which the cam works and what compression is needed as well as other driveline components to work with the cam.


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## BearGFR (Aug 25, 2008)

aseyc said:


> Did the test with engine at 180F. Practically the same PSI. 154[ish]. Tried putting some oil and it went up to 162[ish]. Both tests with full throttle.
> 
> What would be your opinions on this? Rockers not properly adjusted would cause this? Maybe they are too tight?


That Holley 650 isn't big enough to feed that engine and no amount of "setting it up" can correct that.

Also, we know from those aftermarket pistons that the engine has been apart before, so perhaps it has a non-stock can also, and that can affect the compression test at cranking speed.

Having the numbers come up with a squirt of oil tells you that the rings at least might be worn. Oil won't result in higher numbers if pressure is leaking past the valves.

Also, on a stock Pontiac valve train with factory rocker studs, nuts, rockers there's no such thing as either "too loose" or "too tight" because the factory system is not adjustable.

If someone at some point has tried to adjust them like a ::cough:: Chevy then they will most certainly have loosened up as soon as the engine was run afterwards.

The only 'adjustment' on the factory system is to torque all the rocker nuts down to 20 lb. ft. so that they're all seated onto the bottleneck of each stud.

That's another possibility for your lackluster performance. If someone has changed "some parts" in the past and didn't know that, it's possible that the engine now requires an adjustable system that uses poly-lock rocker nuts and possibly different length pushrods.



Bear


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## aseyc (Aug 26, 2020)

Will eventually eliminate all this unknown by dismantling the engine. Then I will see what is the best way forward.

And yes I thought it was like and SBC 😄


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## BearGFR (Aug 25, 2008)

Ok, now that you know, it would be worth removing the valve covers and having a look. If you have the factory system, and perhaps a previous owner made the same mistake, it's very possible that they've loosened up and that's the cause of both your performance problem and apparent low compression. 

I'd expect them to be making noise if that were the case, but it's easy to verify, and would only cost you some time and maybe a set of gaskets. 

Bear


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