Quench virus compression ratio.
Thread Starter
Race Director
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From: Sunshine Coast, Queensland, Australia
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Quench virus compression ratio.
I read a fair bit on the "Correct " quench. most say 35 - 40 thousands. And we all know that compression makes power.
But isn't it true that compression makes power at higher RPM'S?
My question is. how much effect does the actual quench affect tuning and power, and at what, if not all RPM's?
I'm fairly certain that the quench is less than ideal in my engine. But also fairly certain it never was.
What can be expected of my engine and the tuning there of. by getting the quench into the 35 - 40 thou range?
And in so doing, from what I've read so far, it should raise my compression approximately a half point. (going off information in this forum from a few years back).
All of this involves tearing down a perfectly good running engine, but which I plan to do anyway as I am just not happy with current performance.
Some of the regulars on here have seen me post a number of threads and questions on the tuning of my engine.
And no, no matter how much I tune, recurve the distributor. try leaner, richer settings. more accelerator pump, less pump.
I'm not satisfied.
More info needed.
looking for an education here guys. true benefits of reducing Quench in a street driven engine.
But isn't it true that compression makes power at higher RPM'S?
My question is. how much effect does the actual quench affect tuning and power, and at what, if not all RPM's?
I'm fairly certain that the quench is less than ideal in my engine. But also fairly certain it never was.
What can be expected of my engine and the tuning there of. by getting the quench into the 35 - 40 thou range?
And in so doing, from what I've read so far, it should raise my compression approximately a half point. (going off information in this forum from a few years back).
All of this involves tearing down a perfectly good running engine, but which I plan to do anyway as I am just not happy with current performance.
Some of the regulars on here have seen me post a number of threads and questions on the tuning of my engine.
And no, no matter how much I tune, recurve the distributor. try leaner, richer settings. more accelerator pump, less pump.
I'm not satisfied.
More info needed.
looking for an education here guys. true benefits of reducing Quench in a street driven engine.
Le Mans Master
Joined: May 2007
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From: North tonawanda NY
I know the smaller quench area reduces the likelyhood of detonation when running higher compression combinations. I use steel shim gaskets to give my 355 .39 quench. I have 10.15:1 compression and it runs great on 89 octane. I have never tried 87. I did this because my block was never decked and the aftermarket flat top pistons (like many) sit down in the hole a bit compared to the stock ones making the quench worse than stock without the thinner gasket as far as I understand.
I imagine it also effects timing, much in the same way as a flat top piston vs a domed piston.
I imagine it also effects timing, much in the same way as a flat top piston vs a domed piston.
Last edited by augiedoggy; Jul 23, 2021 at 07:36 AM.
Le Mans Master
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The difference in power is not significant. Tight quench makes the engine "less sensitive" to fuel octane requirements. Engines that are not octane sensitive now will not see a measurable difference if they tighten up thier quench.
Example: If you used to require 93 octane to prevent detonation, you might be able to run regular unleaded after tightening your quench.
Example: If you used to require 93 octane to prevent detonation, you might be able to run regular unleaded after tightening your quench.
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A cam change might might be more beneficial. What cam are you running now? Scr? Dcr? Quench height can be adjusted as easily as changing to a thinner head gasket which will raise compression ratio a bit and hence, some performance gain. But a different cam can do wonders.
Race Director
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From: SW Florida.
you worry a bout quench when building a motor or at least when you have to pull the heads. there is not enough gain to be had to play go fish. only option is thinner gasket or new heads without doing the whole engine. thinner gasket will help quench and raise CR a bit. if your CR is low, it is justifiable. if CR is where you want itor a bit high already, raising CR to improve detonation seems backwards-land.
Last edited by derekderek; Jul 23, 2021 at 09:06 AM.
Le Mans Master
Joined: Aug 2017
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From: Cool Northern Michigan
Your best tools to use right now is not sockets and ratchets. Use a calculator, pens, paper.
Find a GOOD compression ratio calculator on the net. Find one that goes deep into decimal points and not just a rough idea or guesstimate.
I think Jeepsters or maybe Wallace Engines have the formula to help you.
Enter all the data asked for by the forms, to the best of your knowledge. Be very specific. One decimal point in the wrong place makes a huge difference.
For example: Head Gasket volume. Piston depth in the hole. Piston top in C.C. Intake Valve closing event, etc.
You will not only find your true C.R. but more importantly you need to know your Dynamic Compression Ratio (DCR). That is what makes or breaks a worthy engine.
You could have 11:1 C.R but if the DCR is too low the performance will suffer.
What is dynamic? We all know standard compression (aka static) is measured with piston in bottom of bore to the top of bore. Simple enough. Squeeze & measure.
BUT WAIT !. The piston is rising and the Intake Valve is still open! What will happen? That's dynamic. DCR
You are shooting for a DCR of between 8.0 and 8.5.
Find your DCR first.
Find a GOOD compression ratio calculator on the net. Find one that goes deep into decimal points and not just a rough idea or guesstimate.
I think Jeepsters or maybe Wallace Engines have the formula to help you.
Enter all the data asked for by the forms, to the best of your knowledge. Be very specific. One decimal point in the wrong place makes a huge difference.
For example: Head Gasket volume. Piston depth in the hole. Piston top in C.C. Intake Valve closing event, etc.
You will not only find your true C.R. but more importantly you need to know your Dynamic Compression Ratio (DCR). That is what makes or breaks a worthy engine.
You could have 11:1 C.R but if the DCR is too low the performance will suffer.
What is dynamic? We all know standard compression (aka static) is measured with piston in bottom of bore to the top of bore. Simple enough. Squeeze & measure.
BUT WAIT !. The piston is rising and the Intake Valve is still open! What will happen? That's dynamic. DCR
You are shooting for a DCR of between 8.0 and 8.5.
Find your DCR first.
Last edited by HeadsU.P.; Aug 4, 2021 at 08:09 AM.
Le Mans Master
Joined: May 2007
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From: North tonawanda NY
I used both the jeepsters and wallace calculators and found a dependency between the two. The DCR does have a lot to do with things and is one of the factors in deciding what cam to use since my iron heads are more sensitive to detonation. a half a point of compression was worth about 16hp on my combo according to the numbers and dyno software. YMMV
Last edited by augiedoggy; Jul 23, 2021 at 10:00 AM.
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Le Mans Master
Joined: May 2007
Posts: 5,025
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From: North tonawanda NY
you worry a bout quench when building a motor or at least when you have to pull the heads. there is not enough gain to be had to play go fish. only option is thinner gasket or new heads without doing the whole engine. thinner gasket will help quench and raise CR a bit. if your CR is low, it is justifiable. if CR is where you want itor a bit high already, raising CR to improve detonation seems backwards-land.
Last edited by augiedoggy; Jul 23, 2021 at 10:04 AM.
Instructor
Joined: Feb 2007
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From: McKinney TX
I built a 355 with the intent to approximate a 70's era LT-1. I used the original LT-1 pistons, decked the block to place the piston top right at the deck and used AFR 190cc heads with 68cc chamber size. By numerous calculations, I run a 10.1 static compression ration. I always liked the sound of a long duration cam so I installed a Crane CCH-296-2 cam. The combination was a true dog. I ran mid 15's in the quarter (same as a stock 250 HP 63 I once had). I could shift at 7000 (engine was balanced) but no power. I swapped the cam to a Crane HMV-278-2 and it was a completely different car. I now shift at 6000, have huge torque, and now enjoy driving the car. I can't provide a comparative quarter mile time as that would depend on my launch and I haven't developed the skill (nor have I tried) to limit the tire spin I get with the new cam.
The original cam had a dynamic compression ration of 7.26 (as best I've been able to calculate) while the new cam is 7.62. I haven't touched the engine since 2005 so I'm sure there's more to be gained with an even better cam. The suggestion I'm making is: look into a cam swap to increase power. Higher compression ratios allow more capable cam profiles, but you may be able to do much with a simple cam change.
The original cam had a dynamic compression ration of 7.26 (as best I've been able to calculate) while the new cam is 7.62. I haven't touched the engine since 2005 so I'm sure there's more to be gained with an even better cam. The suggestion I'm making is: look into a cam swap to increase power. Higher compression ratios allow more capable cam profiles, but you may be able to do much with a simple cam change.
Last edited by Gearhead74; Jul 23, 2021 at 11:03 AM.
Team Owner
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No compression makes more power across all rpms because your engine is more thermally efficient.
Here's What 'Compression Ratio' Actually Means And Why It Matters (jalopnik.com)
Quench is the blow down area between the piston and head. Blowing the A/F mixture towards the spark plug. Tighter quench keeps the fuel atomized to burn faster. It affects tuning by using less timing advance. Modern fast burn chambers use quench and swirl to increase combustion speed and thats why they say 32 degrees max mech advance. The perfect ignition actually retards the timing as the rpm climbs really high. Because as rpm go up the A/F mix is so highly atomized. So you have all in at 3000 - 3200 and at 6500 rpm you decrease the timing at a rate of about 2 degrees per 500 rpm. Your typical race gasoline motor is more like 22 degrees at 9000 rpm.
Here's What 'Compression Ratio' Actually Means And Why It Matters (jalopnik.com)
Le Mans Master
Joined: Apr 2011
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From: Hermosa
With a good quench you get good turbulence in the combustion chamber. This is good for fuel atomization, flame propagation (faster burning) and detonation prevention. It allows you to reduce timing advance and thereby negative work by the piston producing more power. It's not JUST CR that makes more power.
You can use a lower octane fuel if you don't develop high cylinder pressures (high density altitudes, low CR etc) whereas you may have needed a higher octane before to prevent detonation.
You could run more dynamic CR as well without detonation if you want to with a cam change.
It's going to run better all around with better response and power everywhere due to better fuel atomization and a more homogeneous mixture at ignition.
This article was written for motorcycles but applies to all engines and is an interesting read on quench and detonation.
https://www.motorcycle.com/how-to/wr...tion-3420.html
You can use a lower octane fuel if you don't develop high cylinder pressures (high density altitudes, low CR etc) whereas you may have needed a higher octane before to prevent detonation.
You could run more dynamic CR as well without detonation if you want to with a cam change.
It's going to run better all around with better response and power everywhere due to better fuel atomization and a more homogeneous mixture at ignition.
This article was written for motorcycles but applies to all engines and is an interesting read on quench and detonation.
https://www.motorcycle.com/how-to/wr...tion-3420.html
Last edited by REELAV8R; Jul 23, 2021 at 01:43 PM.
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Thanks for all the replies so far. I am currently running a Crane Xtreame Energy XE268H-10 Cam, 268 - 280 duration, 110 separation. with 1.6-1 rockers. Budget E Street Edlebrock heads, 185 runners and 64cc chambers. Flat top hyperunetic pistons. never decked the block.
Maybe cam? Put this in about 3 years ago. maybe 5 thousand miles on it.
plan on changing the rams horn manifolds to the larger 2 1/2 inch ones. Fel Pro shim head gaskets and install a med rise single plane manifold. (I am running Throttle body EFI and thinking my duel plane intake is a compromise with tuning).
So far just rounding up gaskets and trying to find a good price on manifolds shipped to Australia.
Trying to get as much info as I can before taking it apart.
Budget is VERY tight.
Maybe cam? Put this in about 3 years ago. maybe 5 thousand miles on it.
plan on changing the rams horn manifolds to the larger 2 1/2 inch ones. Fel Pro shim head gaskets and install a med rise single plane manifold. (I am running Throttle body EFI and thinking my duel plane intake is a compromise with tuning).
So far just rounding up gaskets and trying to find a good price on manifolds shipped to Australia.
Trying to get as much info as I can before taking it apart.
Budget is VERY tight.
Last edited by 4-vettes; Jul 23, 2021 at 05:28 PM.
Old Pro Solo Guy
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What is the current performance?
And what about it do you not like?
A well performing engine combo is all about having a plan or "target" and then the entire drivetrain combo is optimized toward that target. It's way more involved than quench.
It is Quench plus DCR, rpm range, torque curve, head flow, intake flow, exhaust flow ratio, cam duration, trans gearing, rear gearing etc. etc. etc. I would not even put quench that close to the top of the list. I'd put DCR and cam at #1 &2.
An internal combustion engine/drivetrain combo can only be optimized for about a 3-4000 rpm range. Where do you want it? Just one item being significantly off can make it a "dog" and it doesn't matter which item.
Pair GKulls 7k rpm motor with a 2.7 rear and you and it won't be happy unless you are going 150mph! Oh that's right, that's what he does!
Last edited by leigh1322; Jul 23, 2021 at 06:06 PM.
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OK, I'm not trying to build a race car ,looking for good performance on the street. almost never take it over 5K RPM.
currently performance is poor below 3K RPM. runs well while cruising and light throttle, but poor under heavy throttle. Very poor 2,500 - 3,000 RPM. Then goes hard 3K and up.
3.36 rear gear. 255/60-15 tires. 4 speed T-10 manual transmission.
Thoughts?
currently performance is poor below 3K RPM. runs well while cruising and light throttle, but poor under heavy throttle. Very poor 2,500 - 3,000 RPM. Then goes hard 3K and up.
3.36 rear gear. 255/60-15 tires. 4 speed T-10 manual transmission.
Thoughts?
Old Pro Solo Guy
Joined: Dec 2017
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From: Marlton NJ
Sounds like mid to hi 9s for CR. OK
DCR low 8s. OK
Heads sound OK.
Cam is 224/230. Hmm
Kind of large for what you are trying to do. Should kick in hard at 2500 tho.
Long first gear, only 7.8 overall.
You'll be going 30 mph already by the time you hit 3000.....hmm
The poor WOT below 3000 leads me to think about timing curve.
What's that like?
Little smaller cam would help, little more 1st gear. But timing could be critical. Is cam straight up or advanced? Advancing it 4 degrees ( if not already) should help DCR .3. or so. A 10 degree smaller cam should do even more. 214? But you still have a long 1st gear. But at least that would help it pull hard by 2000 / 20mph.
I am still worried about the timing. ??
DCR low 8s. OK
Heads sound OK.
Cam is 224/230. Hmm
Kind of large for what you are trying to do. Should kick in hard at 2500 tho.
Long first gear, only 7.8 overall.
You'll be going 30 mph already by the time you hit 3000.....hmm
The poor WOT below 3000 leads me to think about timing curve.
What's that like?
Little smaller cam would help, little more 1st gear. But timing could be critical. Is cam straight up or advanced? Advancing it 4 degrees ( if not already) should help DCR .3. or so. A 10 degree smaller cam should do even more. 214? But you still have a long 1st gear. But at least that would help it pull hard by 2000 / 20mph.
I am still worried about the timing. ??
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Timing, 18 degrees idle no vacuum, 11 degrees on vacuum can. car runs 13.5 inches of vacuum at idle. can tips in at about 9(best memory). all in by my 13.5 for certain. Timing starts to move about 1,200 curve is smooth and progressive till 3,100 RPM. 35 Degrees all up (no vacuum). spent a TON of time on this. getting Timing in earlier made it run much worse. with my 4 valve pocket pistons, standard Fel-Pro head gasket, thinking compression between 9.2 and 9.5 . different calculators come up with different answers. Shim head gaskets should bring me up closer to 10-1.
Cam is in straight up. Crane claims power band to be 1,600 to 5,800.
Cam is in straight up. Crane claims power band to be 1,600 to 5,800.
Last edited by 4-vettes; Jul 24, 2021 at 01:20 AM.
Safety Car
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OK, I'm not trying to build a race car ,looking for good performance on the street. almost never take it over 5K RPM.
currently performance is poor below 3K RPM. runs well while cruising and light throttle, but poor under heavy throttle. Very poor 2,500 - 3,000 RPM. Then goes hard 3K and up.
3.36 rear gear. 255/60-15 tires. 4 speed T-10 manual transmission.
Thoughts?
currently performance is poor below 3K RPM. runs well while cruising and light throttle, but poor under heavy throttle. Very poor 2,500 - 3,000 RPM. Then goes hard 3K and up.
3.36 rear gear. 255/60-15 tires. 4 speed T-10 manual transmission.
Thoughts?
Define poor performance under 3k .. Because it means different things to different people. And could in fact be a tuning issue . After all its a basic set up might make 350hp .. So it should drive pretty close to a stock 350 but with more power as you run through the RPM
Last edited by diehrd; Jul 24, 2021 at 09:45 AM.
Le Mans Master
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From: Hermosa
Is this flat tappet cam? What are all the specs of the cam? What EXACTLY is your CR if you know? Exhaust system, true duals? manifolds or headers?
Is this an automatic?
Ideal quench is good, but it can’t fix low CR or too big of a cam.
Is this an automatic?
Ideal quench is good, but it can’t fix low CR or too big of a cam.
Last edited by REELAV8R; Jul 24, 2021 at 10:38 AM.