Not as quick since rebuild !

[VettePaul]

Guy's. I have noticed since my engine rebuild that it doesn't feel as quick off the mark. When I'm Highway driving at say 70mph and hit the gas up to 100mph there is a lot more power than I used to have but from the lights if I floor it, it's like your waiting for it all to happen. I had it tuned on a rolling road yesterday and he said that although my compression readings were all pretty much the same they were lower than before I changed the cam, heads and rings etc. meaning that was why I am losing some low-end grunt. All the readings were around 130/135 psi. Is this right? VettePaul

 

[JHL]

What cam did you use Paul, when I installed the RPM cam in mine it killed the bottom end. It was a dog to drive around town and had nothing below 3000rpm. I also had this problem with a Trans-Am, did the cam, intake and so on, the car was faster in the quarter but it`s 0 - 60 was slower, it only wakened up half track. Maybe you could think about a higher stall torque converter for your next mod. I assume that at 70mph you are at around 2000 - 2500rpm ??? then a converter with a stall in that sort of range would give you a bit of get up and go off the line. Another possibility it some better rear end gears, maybe 3.55 or 3.70`s

You said in a previous post that you used "running in" oil was this a synthetic or a normal oil. I have heard it said that rings will just not seat well if synthetic is used. I had a quick look in a chevy book I have and they quote 145 - 150psi. I know it goes against some of the other previous replies but I still think your initial oil problems were due poor ring seating, some times it can take a few miles to sort it`s self out and for everthing to settle down. If the readings are consistant for all 8 cylinders I wouldn`t worry to much, just rack up a few miles and then check it again and see if it has improved.

J.

 

[VettePaul]

Hi John. The cam is a federal mogal job which when I phoned them to ask a couple of questions they told me it was a marine cam and anything marine spells performance(that's what they said). On speaking to forum members in the past I have been told that it is in fact the same spec as an L-82 cam. I'm on a bit of a downer really because after all the work thru the winter I thought it may have really livened up. It's interesting about the torque converter but I didn't really want to start messing around so soon after getting it all back together. It maybe something that I'll look into at a later date.
BTW, the running in oil was just a normal oil, glad to get the stuff out. That was causing me some agro ! Paul.

 

[JHL]

Paul,

I would have thought that an L82 cam would have plenty of grunt with the standard gears and converter. Have you done anything with the ignition. Again with my car I found that the cam liked a lot of timing. I was running with 16*btdc and had some light springs on the advance wieghts to get the advance in a bit quicker. Even with the standard HEI you should be able to bump the initial timing to 12*btdc. Having it set up like this made a huge difference on my car it seemed the more initial timing the better. Maybe somewhere inbetween this and the standard set up would get you going. Anywhere will sell you a small kit to do this, it`s cheap and fairly easy to sort out. I know how you feel though I threw £4k at mine and although it was faster I succeded in turning a nice smooth daily driver into a bit of a dog that toasted a clutch in less than 500 miles and wasn`t nice to drive at all.

I`m sure there is a lesson to be learned somewhere. Hopefully the new cam will prove to be a bit better and I will get some use out of it.

J.

 

[VettePaul]

Yes, I know all about throwing money at it John. My timing was set the other day at 14degrees. I did put one of those kits in a couple of years back. You get 3 different rates of springs. If I remember rightly I put the strongest ones in. Should I have gone the opposite and put the weakest in ? Cheers, Paul.

 

[resto75]

just a thought

I don't know much about cams but I do know that at boat normally runs up over 3000 rpm constantly for long periods when you are just out crusing around. On the other hand your car rarely goes much over 3000 rpm except for short bursts and only for a minute or less. Therefore I dont think a cam for marine use would be any good for a car. It might work for a oval racer or something like that which keeps the rpms up, but not for a street car.

Just my 2 cents

 

[MoeJr]

If it is in fact an L82 cam then that would explain your weak bottom end grunt. The duration at .050 is 224 Int./Exh. and it has 92 degrees of overlap so it is bleeding off some pressure at low rpm hence the lower Compression readings and there for is lacking in the low end power you desire. I however used this cam in a customers 74 L48 Convertible and with a chambered exhaust.HEI and 2200-2400 Stall Converter it gets up and goes quite nicely now. The converter is what made the difference. That cam likes over 9:1 CR and its design dates to the 69 350/350 Motor which was 10.5:1 CR. Back then it would belled off the pressure down low in the RPM range to keep pinging away. Hope this helps.

 

[Jack]

do try a lightER spring (or two)

/ Paul ... You get 3 different rates of springs. If I remember rightly I put the strongest ones in. Should I have gone the opposite and put the weakest in ? Cheers, Paul.

Paul:
The lighter springs permit the spark advance to occur earlier than with heavier springs ... earlier spark advance is a good thing when looking for acceleration. Mid-to-Light springs are routinely employed by rodders. Some use both light springs ... or one light & one mid ... or two mids. But ... Don't go so light that it routinely spark-rattles/pings upon acceleration ... or steady cruising. Use of weakER springs should help cure bogging.

Also, visually verify that your distributor's mechanical advance weights-cams (and vacuum advance if you have it) can move freely throught their range ... rust & shmutz can bind them up (especially if they've sat for a while)... and vac cans will leak. If your problem seems to worsen as the engine gets hotter ... consider a new ignition module (inexpensive).
Jack:gap

 

[VettePaul]

Thanks Guy's. Looks like I've got a whole bunch of work if I want to solve this one.
MoeJr, your right, it does have 224 Int/Ex duration. Not sure about the overlap though.
Jack, on the dyno the other day the vacuum advance was moving freely throughout the range.
Rather than mess around with the torque converter would it not be better to just change the cam ? VettePaul.

 

[Tom Bryant]

Paul,

Looks like everybody is on the right page here. Most of the Corvette flat tappet cams were designed for higher compression ratio engines and Zora liked them to be strong on the top end for competition use. This sacrificed bottom end but that didn't matter on the track. As was suggested you need to experiment with the lighter advance springs for one thing. More importantly you need to get your RPM up faster. That will require lower rear gears or higher stall speed on the torque converter. My choice would be a different cam.

The best way to select a cam, unless you are well experienced in the subject, is to let the pros do it for you. Raw numbers for lift, duration, lobe seperation and centerline don't tell the whole story today in this era of assemetrical lobe profiles. Email Crane, Comp Cams or whoever you like. Give them your complete combination; everything from the air cleaner to the rear end ratio including carb, intake, heads and mods, valve size, compression ratio. piston type, exhaust, headers or not, trans, converter ect. Also be honest about how you are going to be driving the car 90% of the time. They will recommend a cam that will match your combination and driving style that you will be happy with. This is their business. They won't steer you wrong as their reputation depends on you being thrilled with the results.

There are several good threads on the forums about camshaft selection if you do the search.

Good luck,
Tom

 

[VettePaul]

Tom. I really didn't want to get this deep so soon after finishing rebuild but what you say makes sense. I think I'll experiment with the different springs and then maybe live with it till winter. Thanks a lot, Paul.

 

[JHL]

Hi again Paul,

If memory serves me right I ended up using one light spring and one medium spring, can`t remember the colours but this worked best for me. Unfortunately I had the same problem with the RPM cam, got carried away reading to many books and one Summit catalogue to many. I eventually had to bite the bullitt and change tha cam. In your case though it`s not to wild and I would think about the converter or gears. It depends on where you want to be at with your Vette.

I technical question for you guys.

I was under the impression that measuring the individual cylinder pressure would not be affected by the cam that was fittted no matter how wild as at some point in the stroke both valves would be shut and the piston up in the cylinder being checked giving a peak reading. The gauge I have used some time ago appeared to be like this and after checking each cylinder had to be reset to zero. This being different from measuring the either the static or dynamic CR. Or can cylinder pressure vary depending on where the piston is in the cylinder when both the valves are fully closed. Can anybody clarify or expand on this one ??? I`m curious now.

J.

 

[Tom Bryant]

That sounds right John. Quite often it is necessary to mix springs to get the right advance rate.

On the cylinder pressure issue, I know we covered that in detail once but I can't find it right now. Briefly, without writing a book and ignoring my lawn mowing further (deck belt broke and I am waiting for the mower and me to cool down. :eyerole ) compression ratio is set mechanically by your combination of parts but cylinder pressure is very much affected by when the valves are both closed during the compression stroke. The sooner that both valves close when the piston is moving up on the compression stroke the more volumn will be compressed and the higher the cylinder pressure. This is the idea behind the assemetrical lobe profiles that were developed starting in the 70s in low compression engines. Open the valves fast and close them fast with very little overlap, trapping a greater amount of fuel/air mixture in the cylinders causing higher cylinder pressure at tdc. This makes the engine think it has a higher compression ratio while keeping the lift and duration reasonable.

This is what I was referring to earlier. Two cams with the same numbers can have totally different characteristics due to the shape of the lobe or area under the curve as it is often referred to.

Here is what David Reher of Reher-Morrison Racing Engines fame had to say about compression testers and cylinder pressure in the premier issue of Drag Racing Action.

"An old-fashioned compression test isn't as high-tech as a leakdown test but I believe that it provides more useful information. A leakdown test essentially tells you how big the piston ring end gaps are. If you took the top compression rings out of an engine, the tapered face 2nd rings would still produce excellent leakdown numbers but they wouldn't seal the cylinders very effectively under actual operating conditions.

"In the real world, cylinder pressure is what makes power. If you've pinched a top ring, for example, a compression test will spotlight the problem instantly, while a leakdown test usually won't. When you perform a compression test all of the cylinders should produce similar readings. The numbers on the guage are irrelevant, but it's important that the compression readings be consistant among all of the cylinders. It's unlikely that you will have eight bad holes; you can usually isolate the problem quickly to one or two cylinders."

Interesting words from an engine builder with 7 NHRA Pro Stock National Championsip trophys on the shelf and who knows how many Sportsman class wins.

You should hear what he has to say about rod length and rod ratios. But that's for another time.

Tom

 

[JHL]

Hey Tom,

I have all night, it`s pitch dark here so I won`t be cutting any grass.

:L :L

Excellent sum up.
Thanks
J.